The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature between 5 and 15°C (Hughes, 1998a). Although unusually warm summers or cold winters may change the temperatures outside this range, benthic burrowing species will be buffered from extremes by their presence in the sediment.

Maxmuelleria lankesteri is recorded from the Irish Sea, Clyde Sea, Scottish sea lochs, the Kattegat and Skagerrak and north-west Spain (Hughes, 1998a). Calocaris macandreae is abundant in muddy sediments around the British east and west coasts, extending from Scandinavia to West Africa and the Mediterranean (Ingle & Christiansen 2004; cited in Pinn & Atkinson, 2010). 

Callianassa subterranea is recorded from the Norwegian coasts of the North Sea, south through the Bay of Biscay to the Mediterranean (OBIS, 2016). In the North Sea, Callianassa subterranea lives in water temperatures that vary between 6 and 15°C (Rowden et al., 1998). Under warming scenarios in the North Sea, Callianassa subterranea is expected to largely decline in some areas (from 100 to 20%) but also increase in others (from 49 to 86%) by 2099 (Weinert et al., 2021). A second study modelling the shift in Callianassa subterranea distribution in the North Sea also observed a decrease in the south and an increase in the north by 2099, leading to predicted increases in bioturbation (Weinert et al., 2022).

Nephrops norvegicus is distributed from Iceland to the eastern Mediterranean at temperatures between 6 and 17°C (Eriksson et al., 2013; Johnson et al., 2013). Hernroth et al. (2012) exposed individuals from a population found in the Skagerrak to temperature elevations of 4°C above normal for the area for four months. No signs of oxidative stress were observed, and mortality rates were not affected. However, temperature is considered a major contender as a causal factor in the larval phenology shift of the species due to its importance in embryonic development. McGeady, Lordan & Power (2020) used a model to predict the larval release date of Nephrops norvegicus based on temperature-dependent incubation rates in the Irish Sea and found a 17.2-day earlier shift between 1982 and 1995 (day of year 119.8 ± 2.2) and 2000 and 2010 (day of year 102.6 ± 1.8) due to a 0.9°C increase in temperature. Previous experiments have demonstrated a 50% reduction in embryonic development duration because of a 10°C increase in temperature. The Nephrops norvegicus planktonic larvae stage is estimated to take between one to two months at temperatures of 8.5 to 13°C (McGeady, Lordan & Power, 2020). In addition, at warmer temperatures (13 to 14°C), muscle and cholinolytic necrosis of the cuticle of Nephrops norvegicus, induced by bacteria, was observed (Aguzzi et al., 2023). Overall, increases in temperature for Nephrops norvegicus may reduce habitat suitability for the species, as well as impact growth, condition, and reproductive output, and their low adult mobility limits their capacity to shift to more favourable grounds, leaving them vulnerable to localised environmental degradation (Conville, 2025).

Virgularia mirabilis is found throughout the UK continental shelf and other regions of the northeast Atlantic, as well as in the warmer waters of the Mediterranean (Bastari et al., 2018; Downie et al., 2021). Mean bottom temperature is a key predictor variable of Virgularia mirabilis distribution and has a temperature range of 7.8 to 12.9°C, but also exists in areas which experience warmer mean temperatures (Downie et al., 2021).

Sensitivity assessment. Short-term acute changes in temperature and long-term chronic changes in temperature at the pressure benchmark are considered unlikely to adversely affect this biotope, as global distribution suggests Nephrops norvegicus, Calocaris macandreae and Callianassa subterranea can potentially adapt to a wide range of temperatures experienced in both northern and southern waters. While Maxmuelleria lankesteri has a more limited distribution, it would probably be able to avoid locally acute changes in temperature as they burrow deeply into the sediment. Therefore, resistance and resilience are assessed as ‘High’. This biotope is, therefore, considered to be ‘Not sensitive’. However, long-term increases in temperature may prove detrimental, especially for Nephrops.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature between 5 and 15°C (Hughes, 1998a). Although unusually warm summers or cold winters may change the temperatures outside this range, benthic burrowing species will be buffered from extremes by their presence in the sediment.

Maxmuelleria lankesteri is recorded from the Irish Sea, Clyde Sea, Scottish sea lochs, the Kattegat and Skagerrak and north-west Spain (Hughes, 1998a).

Calocaris macandreae is abundant in muddy sediments around the British east and west coasts, extending from Scandinavia to West Africa and the Mediterranean (Ingle & Christiansen 2004; cited in Pinn & Atkinson, 2010). 

Callianassa subterranea is recorded from the Norwegian coasts of North Sea, south through the Bay of Biscay to the Mediterranean (OBIS, 2016). In the North Sea, Callianassa subterranea lives in water temperatures that vary between 6 and 15°C (Rowden et al., 1998). 

Nephrops norvegicus is distributed from Iceland to the eastern Mediterranean at temperatures between 6 and 17°C (Eriksson et al., 2013; Johnson et al., 2013). Hernroth et al. (2012) exposed individuals from a population found in the Skagerrak to temperature elevations of 4°C above normal for the area for four months. No signs of oxidative stress were observed, and mortality rates were not affected. However, temperature is considered a major contender as a causal factor in the larval phenology shift of the species due to its importance in embryonic development. McGeady, Lordan & Power (2020) used a model to predict the larval release date of Nephrops norvegicus based on temperature-dependent incubation rates in the Irish Sea and found a 17.2-day earlier shift between 1982 and 1995 (day of year 119.8 ± 2.2) and 2000 and 2010 (day of year 102.6 ± 1.8) due to a 0.9°C increase in temperature. Previous experiments have demonstrated a 50% reduction in embryonic development duration because of a 10°C increase in temperature. The Nephrops norvegicus planktonic larvae stage is estimated to take between one to two months at temperatures of 8.5 to 13°C (McGeady, Lordan & Power, 2020).

Virgularia mirabilis is found throughout the UK continental shelf and other regions of the northeast Atlantic, as well as in the warmer waters of the Mediterranean (Bastari et al., 2018; Downie et al., 2021). Mean bottom temperature is a key predictor variable of Virgularia mirabilis distribution and has a temperature range of 7.8 to 12.9°C, but also exists in areas which experience warmer mean temperatures (Downie et al., 2021).

Sensitivity assessment. Short-term acute changes in temperature and long-term chronic changes in temperature at the pressure benchmark are considered unlikely to adversely affect this biotope as global distribution suggests Nephrops norvegicus, Calocaris macandreae and Callianassa subterranea can potentially adapt to a wide range of temperatures experienced in both northern and southern waters. While Maxmuelleria lankesteri has a more limited distribution, it would probably be able to avoid locally acute changes in temperature as they burrow deeply into the sediment. Resistance is, therefore, assessed as 'High' and resilience as ‘High’. This group is therefore considered to be ‘Not sensitive’. 

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

No evidence was found to assess the salinity tolerance of Calocaris macandreae, although it is found in fully marine conditions, and no records from estuaries or brackish water were found. Little evidence was found in relation to Virgularia mirabilis salinity tolerance, but salinity was noted as an influencing variable in its distribution (Bastari et al., 2018). Virgularia mirabilis has been observed between salinities of 27 and 34‰ off the west coast of Scotland, experiencing a peak in its modelled habitat suitability index at both low, 27‰, and high, 33.5 to 34‰, salinities (Greathead et al., 2014 cited in Bastari et al., 2018). Similarly, Callianassa subterranea and Maxmuelleria lankesteri are only recorded from full (30 to 35) saline conditions. Thompson & Ayers (1989) noted that Nephrops larvae were found at salinities of 34 to 35 ppt in the wild. Johnson et al. (2013) noted that Nephrops was generally restricted to full salinity waters, considered stenohaline, and was recorded by OBIS from 31.8 to 38.8 psu. Farmer (1975) reported that Nephrops occurred at salinities of 29 to 30 ppt in the Kattegat and 35.8 to 38.7 ppt in the Adriatic. However, Höglund (1942; cited in Farmer, 1975) suggested that the absence of Nephrops norvegicus in the Baltic Sea was due to its intolerance of very low salinities.

The effects of low salinity exposure and emersion were tested to simulate the conditions experienced by discarded Nephrops in the Kattegat area as these are transported through the halocline (Harris & Ulmestrand, 2004). Nephrops exposed to 15 psu (for <2 hours) suffered mortalities of 25 to 42% overall. Exposed animals gained mass rapidly as water was absorbed and showed delayed or absent responses to stimulation following return to waters of 33 psu (Harris & Ulmestrand, 2004; Johnson et al., 2013). In addition, Nephrops was reported to survive at 28 psu but experience 50% mortality at 25 psu and 100% mortality at 21 psu (Harris & Ulmestrand, 2004).

An increase in salinity at the benchmark level would result in a salinity of >40 psu, and as hypersaline water is likely to sink to the seabed, the biotope may be affected by hypersaline effluents. Ruso et al. (2007) reported that changes in the community structure of soft sediment communities due to desalination plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b).

Sensitivity assessment. An increase in salinity from full to >40 psu is probably detrimental to the important characteristic species of the biotope. Hypersaline water would probably sink to the seabed and potentially into the sediment via burrows. Although there is no direct evidence of the effects of hypersaline water, the stenohaline nature of the community suggests that hypersaline conditions may cause mortality. Therefore, a resistance of Low is recorded but at Low confidence. Resilience would probably be Low, so that sensitivity may be High, albeit with Low confidence.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

No evidence was found to assess the salinity tolerance of Calocaris macandreae, although it is found in fully marine conditions, and no records from estuaries or brackish water were found. Little evidence was found in relation to Virgularia mirabilis salinity tolerance, but salinity was noted as an influencing variable in its distribution (Bastari et al., 2018). Virgularia mirabilis has been observed between salinities of 27 and 34‰ off the west coast of Scotland, experiencing a peak in its modelled habitat suitability index at both low, 27‰, and high, 33.5 to 34‰, salinities (Greathead et al., 2014 cited in Bastari et al., 2018). Similarly, Callianassa subterranea and Maxmuelleria lankesteri are only recorded from full (30 to 35) saline conditions.

Thompson & Ayers (1989) noted that Nephrops larvae were found at salinities of 34 to 35 ppt in the wild. Johnson et al. (2013) noted that Nephrops was generally restricted to full salinity waters, considered stenohaline, and was recorded by OBIS from 31.8 to 38.8 psu. Farmer (1975) reported that Nephrops occurred at salinities of 29 to 30 ppt in the Kattegat and 35.8 to 38.7 ppt in the Adriatic. However, Höglund (1942; cited in Farmer, 1975) suggested that the absence of Nephrops norvegicus in the Baltic Sea was due to its intolerance of very low salinities.

The effects of low salinity exposure and emersion were tested to simulate the conditions experienced by discarded Nephrops in the Kattegat area as these are transported through the halocline (Harris & Ulmestrand, 2004). Nephrops exposed to 15 psu (for <2 hours) suffered mortalities of 25-42% overall. Exposed animals gained mass rapidly as water was absorbed and showed delayed or absent responses to stimulation following return to waters of 33 psu (Harris & Ulmestrand, 2004; Johnson et al., 2013). In addition, Nephrops was reported to survive at 28 psu but experience 50% mortality at 25 psu and 100% mortality at 21 psu (Harris & Ulmestrand, 2004).

Sensitivity assessment. A decrease in salinity from full to reduced (18 to 30 psu) is likely to be detrimental to most of the important characteristic species in the biotope. Callianassa subterranea, Calocaris macandreae and Maxmuelleria lankesteri are only recorded from full (30-35) saline conditions. The above evidence shows that short, acute reductions in salinity result in mortality in Nephrops, but also that reduced salinity results in mortality. Therefore, a reduction in salinity for a year is likely to either cause the mobile species to move out of the affected area or cause significant mortality. Therefore, a resilience of ‘Low’ is recorded. Resilience would probably be ‘Low’, so that sensitivity may be ‘High’. It is noted that many Scottish sea lochs in which this biotope is recorded may experience variable salinity conditions, but the hyposaline conditions probably do not occur at the depths this community occurs.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

This biotope occurs in weak to very weak (negligible) flow, in deep, low-energy environments. A further decrease in water flow is unlikely. Nephrops has been shown to walk in line with water flow between 0.07 and 0.2 m/s in flume experiments (Newell et al., 1988), but no other direct evidence of the effects of changes in water flow was found. Virgularia mirabilis distributions correspond with areas that experience low current and wave velocity, which is a key distribution predictor for the species, but it can tolerate velocity up to 1.1 m/s  (Downie et al., 2021).

Increased flow has the potential to modify the sediment, especially at the surface. A significant increase in water flow may winnow away the mud surface or even remove the mud habitat and hence the biotope if prolonged. An increase of 0.2 m/s may begin to erode the mud surface where the site is already subject to flow (e.g. weak flow at the seabed), based on sediment erosion deposition curves (Wright et al., 2001). However, given the depth of mud that characterizes the biotope, only the surface of the mud may be removed within a year. Hence, the deep burrowing community may remain intact, but the surface infauna and erect epifauna may decrease in abundance. Therefore, a resistance of ‘Medium’ is recorded but with Low confidence. Resilience is probably ‘Medium’, and the sensitivity of the biotope is assessed as ‘Medium’.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Changes in emergence are Not relevant to the biotope, which is restricted to circalittoral below 10 metres. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

SS.SMu.CFiMu.MegMax occurs in low-energy environments, extremely sheltered to sheltered from wave exposure (Connor et al., 2004), a prerequisite for the fine mud sediments in which the community is found (Hughes, 1998). In addition, the biotope is found to considerable depths, at which, wave action is unlikely to be significant. Therefore, a decrease in wave exposure is 'Not relevant'. Any activity or climatic effect that increased wave action or storminess could have a significant effect on the shallower examples of the biotope, due to removal or modification of the sediment. However, a change of 3-5% in significant wave height (the benchmark) is unlikely to be significant. Therefore, the biotope is probably 'Not sensitive' (resistance and resilience are High) at the benchmark level.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

In Norwegian fjords, Rygg (1985) found a relationship between species diversity in benthic fauna communities and sediment concentrations of the heavy metals Cu, Pb, and Zn. Cu, in particular, showed a strong negative correlation and the author suggested a cause-effect relationship. Those species not present at sites where Cu concentrations were greater than ten times higher than the background level, such as Calocaris macandreae, Amphiura filiformis and several bivalves including Nucula sulcata and Thyasira equalis, were assessed as non-tolerant species. The tolerant species were all polychaete worms. Therefore, increased heavy metal contamination in sediments may change the faunal composition of the community and decrease overall species diversity. Some burrowing crustaceans, brittlestars, and bivalves may disappear from the biotope and lead to an increasing dominance of polychaetes.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This biotope is recorded as This pressure is Not assessed but evidence is presented where available.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

In the fish farming industry, a range of synthetic compounds has been used treat parasitic infestations in the past. Ivermectin, an anti-louse treatment, was shown to be highly toxic to sediment-dwelling polychaetes (Black et al., 1997; Thain et al., 1997), epibenthic shrimps (Burridge & Haya, 1993) and also thought to be toxic to burrowing megafauna (Hughes, 1998a). The pesticide carbamyl (1-naphthol n-methyl carbamate; trade name Sevin®) has been used to control populations of thalassinidean mud-shrimps in areas important for oyster cultivation (Feldman et al., 2000).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

Burrowing fauna probably have a major influence on the distribution of radionuclides within the sediments (Kershaw et al., 1983). Maxmuelleria lankesteri burrows may act as sinks for surface-derived radionuclides and there is probably little return of deeply-buried material to the sediment surface (Hughes et al., 1996). Plutonium is reported to accumulate in the linings of Maxmuelleria lankesteri burrows, this emphasises the role of bioturbation in the incorporation of radionuclides in deeper sediments (Kershaw et al., 1984). Communities similar to this biotope containing abundant burrowing megafauna and sea pens were found to exist in areas heavily contaminated by radionuclides, in particular near Sellafield, Cumbria, due to the activities of the British Nuclear Fuels Plc reprocessing plant (Hughes & Atkinson, 1997) but no information on the level of radiation was provided.  No reports on the effects on the fauna themselves were found. Therefore, the biotope is may be resistant of such effluent but there is insufficient evidence to assess this pressure against the benchmark.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Crustaceans are thought to be amongst the most susceptible group to the effects of hypoxia (Diaz & Rosenberg, 1995; Vaquer-Sunyer & Duarte, 2008; Eriksson et al., 2103). Large, active animals with high respiratory demands are likely to be most affected by oxygenation changes, whereas burrow-dwelling fauna may be less susceptible to hypoxic conditions. The oxygen concentrations in Nephrops grounds were reported to be between 6 and 9.4 mgO₂/l, and in Scottish sea lochs have been reported to range from 1 to 9.2 mgO₂/l (Gilibrand et al., 2006; Eriksson et al., 2013). Oxygen depletion can occur naturally (warm temperatures combined with prolonged stratification of the water column); however, it can also be caused by human activities such as sewage disposal, cage aquaculture or eutrophication.

Diaz & Rosenberg (1995) suggested that Nephrops norvegicus was sensitive to hypoxia. Nephrops norvegicus is sensitive to extreme hypoxia (0.6 mg O₂/l), but in moderately hypoxic conditions (1 mg O₂/l) can compensate by increasing haemocyanin levels (Baden et al., 1990). In the laboratory, juvenile Nephrops behaviour and mortality changed with lowered oxygen concentrations; energetically costly activities were reduced, and activity in general declined. In normoxia, juveniles initially walked and then burrowed, but when exposed to hypoxia, they were mainly inactive with occasional outbursts of escape swimming. To increase oxygen availability, the juveniles were observed to raise their bodies on stilted legs (similar to adults in hypoxic conditions), but an oxygen saturation of 25% (<2.5 mg O₂/l) was lethal within 24 h (Eriksson & Baden, 1997). Burrowing behaviour was tested in post-larvae at an oxygen saturation of >80% (<8 mg O₂/l) for one week. The difference in time taken to complete a V-shaped depression or a U-shaped burrow was measured and showed a strong negative relationship between post larval age and burrowing time, but all individuals made a burrow. Eriksson & Baden (1997) suggested that juveniles were, therefore, more sensitive to hypoxia than adults.

In moderately hypoxic conditions, 38 to 43% saturation (3.8 to 4.3 mg O₂/l), adult Nephrops norvegicus compensate by increasing production of haemocyanin (Baden et al., 1990). In the laboratory, this compensation lasted one week, so at the level of the benchmark, the species would not be killed. However, in severe hypoxia, <20% saturation (<2 mg/l), Nephrops became less active and raised their bodies on their legs (Baden et al., 1990). During laboratory studies at <15% O₂ (1.5 mg O₂/l), specimens of Nephrops norvegicus stopped feeding even though there was available food, suggesting that hypoxia induces starvation (Baden et al., 1990).  At 12% (<1.2 mg O₂/l) oxygen saturation, some specimens of Nephrops norvegicus began to 'tiptoe'. They supported themselves by elevating the body from the substratum with their claws and telson. The lobsters remained elevated until they became tired, sluggish, and barely moved when touched for 2 to 3 days, after which they died (Baden et al., 1990). At <10% (<1 mg O₂/l), adult Nephrops died within a few days (Baden et al., 1990; Eriksson et al., 2013).

Nephrops leave their burrows at <50% O₂ (at 1 m above the substratum) or <15% O₂ (at 0.5 m above the substratum) (Eriksson et al., 2013). Catches of Nephrops norvegicus were found to be high in hypoxic conditions, probably because the animals are forced out of their burrows (Eriksson & Baden, 1997). Baden et al. (1990) reported that the Nephrops biomass declined from 10.8 kg/hour to zero from October 1984 to September 1989 in the southeast Kattegat, an area affected by 1- to 3-month periods of low oxygen concentrations (< 2ml/l = 2.8 mg/l) during the 1980s. Both Eriksson et al. (2013) and Conville (2025) suggested that increased temperatures due to climate change may increase hypoxic stress on benthos as median lethal oxygen concentration increases with temperature.

The bioturbative activity of burrowing megafauna improves ventilation and oxygenation of burrows. Thalassinidean mud-shrimps are very resistant to oxygen depletion, and many species can survive extended periods of anoxia of 50 to 60 hr at 10°C (Anderson et al., 1994). Callianassa subterranea burrows are often hypoxic or even anoxic (Hughes, 1998a). In laboratory experiments, the species survived for up to five days under anoxic conditions at 6°C (Powilleit & Graf, 1996). The species has several adaptations that allow it to survive in low oxygen environments: a low rate of oxygen consumption, large gill areas, and a respiratory pigment with a high oxygen affinity (Astall et al., 1997; Taylor et al., 2000). However, anoxic, sulphide-rich waters upwelling in a Norwegian fjord (Christiansen & Stene, 1998; cited in Hughes, 1998a) killed Callianassa subterranea.

Calocaris macandreae also inhabits burrows subject to severe hypoxia. In the laboratory, Calocaris macandreae was highly tolerant of anoxia, with a LT₅₀ of 43 hr (although some specimens survived for 49.5 hrs), as it exhibited anaerobic metabolism at severe hypoxia (PO₂ <7 Torr, or ca 0.4 mg O₂/l) (Anderson et al., 1994). Calocaris macandreae have also been known to switch from an aerobic to anaerobic metabolism below its threshold (0.085 mg/l = 0.1 ml/l) and is one of the most tolerant species worldwide (Zettler & Pollehne, 2023).

Mud shrimps are among the few species to survive the low oxygen partial pressures and high sulphide levels in the vicinity of fish cages in sea lochs (Atkinson, 1989). In Caol Scotnish, Loch Sween, bacterial mats of Beggiatoa were reported in the immediate vicinity of salmon cages in 1987. The burrowing megafauna (Maxmuelleria lankesteri, Callianassa subterranea and Jaxea nocturna) were abundant in unimpacted areas. However, by 1988, the bacterial mats covered most of the seabed in the basin, the sediment was close to anoxic, and the burrows of megafauna were restricted to small areas free of Beggiatoa. After the removal of salmon cages in 1989, some recovery was apparent by 1990, with more burrows apparent, although the size of the individuals of Maxmuelleria lankesteri and Callianassa subterranean suggested that they had survived the loch basin during the peak of enrichment (Atkinson, 1989; Hughes, 1998a). Little other evidence on the tolerance of Maxmuelleria lankesteri to deoxygenation was found. However, it is thought to be tolerant of hypoxic conditions, as burrow conditions often become hypoxic (Hughes, 1998a).

In experiments exposing benthic invertebrates to decreasing oxygen levels, Amphiura chiajei only left its protected position in the sediment when oxygen levels fell below 0.54 mg O₂/l (Rosenberg et al., 1991). This escape response increases its risk to predators. Mass mortality in a superficially similar species of ophiuroid, Amphiura filiformis, from the south-east Kattegat has been observed during severe hypoxic events (< 0.7 mg/l), while the abundance of Amphiura chiajei remained unchanged at the same site and time (Rosenberg & Loo, 1988).

Sensitivity assessment. A decrease in oxygenation to 2 mg/l or below for a week (the benchmark) is likely to result in mortality of Nephrops due to an increase in predation or fishing mortality, or direct mortality due to severe hypoxia. The mud shrimp fauna and Maxmuelleria lankesteri may survive severe hypoxia or even anoxia for a short period or one week (the benchmark). Amphiura chiajei abundance in the biotope may also not be adversely affected until exposed to severe hypoxic conditions. However, a reduction in the abundance of Nephrops is unlikely to change the character of the biotope, as the other important characteristic species are likely to remain. Therefore, a resistance of ‘Medium’ is suggested. As the Nephrops population would probably recover within 2 to 10 years, resilience is likely to be ‘Medium’, and, hence, sensitivity is assessed as ‘Medium’.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Burrowing megafauna flourish in areas where the sediments are naturally high in organic matter and hence nutrients, such as sheltered sea lochs (Hughes, 1998a). For example, Nephrops norvegicus was present in high densities in Loch Sween, Scotland, where the organic content was about 5% and as high as 9% in some patches (Atkinson, 1989). 

Callianassa subterranea is found in sediments with a range of organic content. In the soft, organically enriched sediments (typical organic carbon values of 3.6 - 7.8%) of Loch Sween, a sea loch in Scotland, Callianassa subterranea was present as a significant megafaunal burrower (Atkinson, 1989). The maximum depth of the species burrows has been recorded as 86 cm, which Nickell & Atkinson (1995) suggest is an underestimate, indicating a nutritional requirement for sub-surface organic matter. In addition, the water in burrows can hold higher concentrations of phosphate and ammonium compared to the overlying water (Tiano et al., 2019). In the North Sea, where sediments have a low organic content, Rowden & Jones (1997) found Callianassa subterranea had to construct much more complex burrows to support their energetic costs. The same behaviour was recorded for Callianassa subterranea populations in Korea, where more sediment reworking was undertaken by individuals in areas where nutrient content was low, and less when high (Seo & Koo, 2021).

An increasing gradient of organic enrichment (e.g. in the vicinity of point sources of organic-rich effluent or sewage sludge dump sites) results in a decline in the suspension feeding fauna and an increase in the number of deposit feeders, in particular, polychaete worms (Pearson & Rosenberg, 1978). The effects of organic enrichment on burrowing megafauna and other infauna depended on the degree of enrichment and any resultant hypoxia, which depended on the sediment type and local hydrology. For example, in a survey of Garoch Head sludge dumping grounds, Firth of Clyde, the burrowing megafauna (Nephrops norvegicus, Callianassa subterranean, Calocaris macandreae, Lumpenus lampraetiformis and Cepola rubsecens) were abundant where organic content was <4% but absent where the organic content exceeded 6% (Smith, 1988, cited in Hughes, 1998a). Calocaris macandreae did not extend as far into the gradient as Nephrops norvegicus or Lumpenus lampraetiformis (Smith, 1988, cited in Hughes, 1998a). In Caol Scotnish, Loch Sween, bacterial mats of Beggiatoa were reported in the immediate vicinity of salmon cages in 1987. The burrowing megafauna (Maxmuelleria lankesteri, Callianassa subterranea and Jaxea nocturna) were abundant in unimpacted areas. But by 1988, the bacterial mats covered most of the seabed in the basin, the sediment was close to anoxic, and the burrows of megafauna were restricted to small areas free of Beggiatoa. After the removal of salmon cages in 1989, some recovery was apparent by 1990, with more burrows apparent, although the size of the individuals of Maxmuelleria lankesteri, Callianassa subterranean, suggested that they had survived the loch basin during the peak of enrichment (Hughes, 1998a).

Hoare & Wilson (1977) noted that Virgularia mirabilis was absent from part of the Holyhead Harbour heavily affected by sewage pollution. However, the species was abundant near the head of Loch Harport, Skye, close to a distillery outfall discharging water enriched in malt and yeast residues and other soluble organic compounds (Nickell & Anderson, 1977; cited in Hughes, 1998a), where the organic content of the sediment was up to 5%. Virgularia mirabilis was also present in Loch Sween in Scotland in sites where organic content was as high as 4.5% (Atkinson, 1989). Virgularia mirabilis tolerance to pollution seems variable. For example, while studying the effects of salmon aquaculture effluents on Virgularia mirabilis populations in southern Norway, although lethal and sub-lethal effects were observed, some colonies were found below salmon farms that had been active for over 20 years, suggesting that the species was able to recover and persist in the presence of excess nutrients (Taormina et al., 2024).

Sensitivity assessment. The community associated with this biotope is characterized by nutrient and organically-enriched muddy habitats. Further increases in enrichment may modify the community, but many of the component species would probably survive, and the biotope would remain recognizable. Hence, the biotope is probably ‘Not sensitive’ to nutrient enrichment.  

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Burrowing megafauna flourish in areas where the sediments are naturally high in organic matter and hence nutrients, such as sheltered sea lochs (Hughes, 1998a). For example, Nephrops norvegicus was present in high densities in Loch Sween, Scotland, where the organic content was about 5% and as high as 9% in some patches (Atkinson, 1989). 

Callianassa subterranea is found in sediments with a range of organic content. In the soft, organically enriched sediments (typical organic carbon values of 3.6 - 7.8%) of Loch Sween, a sea loch in Scotland, Callianassa subterranea was present as a significant megafaunal burrower (Atkinson, 1989). The maximum depth of the species burrows has been recorded as 86 cm, which Nickell & Atkinson (1995) suggest is an underestimate, indicating a nutritional requirement for sub-surface organic matter. In addition, the water in burrows can hold higher concentrations of phosphate and ammonium compared to the overlying water (Tiano et al., 2019). In the North Sea, where sediments have a low organic content, Rowden & Jones (1997) found Callianassa subterranea had to construct much more complex burrows to support their energetic costs. The same behaviour was recorded for Callianassa subterranea populations in Korea, where more sediment reworking was undertaken by individuals in areas where nutrient content was low, and less when high (Seo & Koo, 2021).

An increasing gradient of organic enrichment (e.g. in the vicinity of point sources of organic-rich effluent or sewage sludge dump sites) results in a decline in the suspension feeding fauna and an increase in the number of deposit feeders, in particular, polychaete worms (Pearson & Rosenberg, 1978). The effects of organic enrichment on burrowing megafauna and other infauna depended on the degree of enrichment and any resultant hypoxia, which depended on the sediment type and local hydrology. For example, in a survey of Garoch Head sludge dumping grounds, Firth of Clyde, the burrowing megafauna (Nephrops norvegicus, Callianassa subterranean, Calocaris macandreae, Lumpenus lampraetiformis and Cepola rubsecens) were abundant where organic content was <4% but absent where the organic content exceeded 6% (Smith, 1988, cited in Hughes, 1998a). Calocaris macandreae did not extend as far into the gradient as Nephrops norvegicus or Lumpenus lampraetiformis (Smith, 1988, cited in Hughes, 1998a). In Caol Scotnish, Loch Sween, bacterial mats of Beggiatoa were reported in the immediate vicinity of salmon cages in 1987. The burrowing megafauna (Maxmuelleria lankesteri, Callianassa subterranea and Jaxea nocturna) were abundant in unimpacted areas. But by 1988, the bacterial mats covered most of the seabed in the basin, the sediment was close to anoxic, and the burrows of megafauna were restricted to small areas free of Beggiatoa. After the removal of salmon cages in 1989, some recovery was apparent by 1990, with more burrows apparent, although the size of the individuals of Maxmuelleria lankesteri, Callianassa subterranean, suggested that they had survived the loch basin during the peak of enrichment (Hughes, 1998a).

Hoare & Wilson (1977) noted that Virgularia mirabilis was absent from part of the Holyhead Harbour heavily affected by sewage pollution. However, the species was abundant near the head of Loch Harport, Skye, close to a distillery outfall discharging water enriched in malt and yeast residues and other soluble organic compounds (Nickell & Anderson, 1977; cited in Hughes, 1998a), where the organic content of the sediment was up to 5%. Virgularia mirabilis was also present in Loch Sween in Scotland in sites where organic content was as high as 4.5% (Atkinson, 1989). Virgularia mirabilis tolerance to pollution seems variable. For example, while studying the effects of salmon aquaculture effluents on Virgularia mirabilis populations in southern Norway, although lethal and sub-lethal effects were observed, some colonies were found below salmon farms that had been active for over 20 years, suggesting that the species was able to recover and persist in the presence of excess organics (Taormina et al., 2024).

Sensitivity assessment. The community associated with this biotope is characterized by nutrient and organically-enriched muddy habitats. Further increases in enrichment may modify the community, but many of the component species would probably survive, and the biotope would remain recognizable. Hence, the biotope is probably ‘Not sensitive’ to organic enrichment.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

The characterizing species in this biotope all require muddy, silty, and clay-based soft seabed types to burrow into (Muir et al., 2017; Tiano et al., 2019; Bastari et al., 2018; Gutow et al., 2020; Aguzzi et al., 2023; Loulidi et al., 2025). If sedimentary substrata were replaced with rock substrata, the biotope would be lost, as it would no longer be a sedimentary habitat and would no longer support sea pens and burrowing megafauna.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’ or ‘None’ (as the pressure represents a permanent change) and the sensitivity of this biotope is assessed as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Species creating permanent burrows typically have specific sediment requirements, relating to the maintenance of burrow structures. Callianassa subterranea creates a complex lattice of galleries at 30 to 40 cm below the surface in the fine muds but burrows less deeply (9 to 23 cm) in coarse sediments (Rowden & Jones, 1995; Hughes, 1998a). Calocaris macandreae creates burrows with a total depth of 21 cm in muddy sediments with a high silt content, but is not found in sandy sediments (Buchanan, 1963; Hughes, 1998a). The large echiuran Maxmuelleria lankesteri burrows up to 80 cm into the sediment and is found in fine muds and muddy sands in deep water (10 to 80 m) (Nickell et al., 1995; Hughes, 1998a). The sea pen Virgularia mirabilis is adapted to soft, muddy and gravelly sediments which allows it to retract into the seabed to help it avoid physical damage (Bastari et al., 2018; Loulidi et al., 2025).

Nephrops norvegicus burrows to 20 to 30 cm and is found in soft mud sediments (Hughes, 1998a). Nephrops has been shown to be more frequent in sandy muds than muds off the southwest and southeastern grounds off Portugal (Leotte et al., 2005; abstract only). In coarse, sandy sediments, population density is low because of the instability of the sediment and the tendency of burrows to collapse. In medium-grained mud sediments, Nephrops are able to construct stable burrows and population density peaks. In very fine-grained, soft muds, Nephrops excavate extensive burrow complexes, and competition for space is a limiting factor on population density (Afonso-Dias, 1998).

The important characterizing burrowing megafauna occur in a relatively restricted range of sediment types, related to the burrowing life habit (and feeding for Calocaris macandreae). The species are, therefore, considered to have ‘Low’ resistance to a change in sediment type of one Folk class for a year. However, resilience is ‘Very low’ or ‘None’ (as the pressure represents a permanent change). Sensitivity is, therefore, assessed as ‘High’.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

No direct evidence was found to assess the impacts of this pressure. Callianassa subterranea creates a complex lattice of galleries at 30-40 cm below the surface in the fine muds but burrows less deeply (9-23 cm) in coarse sediments (Rowden & Jones, 1995; Hughes, 1998a).  Calocaris macandreae creates burrows with a total depth of 21 cm in muddy sediments with as high silt content (Buchanan, 1963; Hughes, 1998a); Maxmuelleria lankesteri burrows up to 80 cm into the sediment (Nickell et al., 1995; Hughes, 1998a); and Nephrops norvegicus burrows to 20-30 cm and is found in soft mud sediments (Hughes, 1998a). Based on burrow depths (maximum depth 21 cm for Calocaris macandreae and 30 cm for Nephrops norvegicus) extraction (of 30 cm of sediment) is likely to disturb and remove the majority of the population of Calocaris macandreae and Nephrops norvegicus, together with a proportion of the Callianassa subterranea population within the affected area. A proportion of the Maxmuelleria lankesteri population may remain, depending on their depth within the sediment.  Resistance is assessed as None (removal of >75% of individuals) and recovery is assessed as Low. Sensitivity is, therefore, assessed as High. Confidence in the quality of evidence for this assessment is Low as it is based on expert judgement, informed by life habit of the species assessed.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). In general, species with large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Large bioturbating or bio-irrigating species may be especially sensitive, and their loss may affect the community (Widdicombe et al., 2004; Pommer et al., 2016).

Hinz et al. (2009) noted that different studies on the effects of otter trawl disturbance in muddy sediments gave mixed results and that the effect on abundance, biomass and diversity at a community level was largely inconsistent between studies. For example, experimental studies on short-term effects showed modest changes in the benthic communities (e.g. Tuck et al., 1998) and meta-analysis suggested that otter trawling on muddy sediments had one of the least negative impacts on the benthos (Kaiser et al., 2006). However, other studies showed that areas of seabed protected by wrecks from Nephrops trawls had higher abundance and biomass of benthos (Ball et al., 2000), while Smith et al. (2000) showed significantly lower abundance, biomass and species richness of benthos in high-intensity trawling lanes. Hinz et al. (2009) suggested that the differences in results were the result of differences in statistical analysis, prior fishing intensity and duration of the studies. Hinz et al. (2009) reported that chronic otter trawling from a Nephrops fishery had significant negative effects on the benthic macrofauna. Hinz et al. (2009) concluded that while the initial impact of otter trawl on muddy sediments was modest, the long-term disturbance could lead to profound changes in the benthic communities, especially epifauna and shallow burrowing infauna.

Nephrops norvegicus fisheries could, therefore, affect burrowing megafauna and sea pen biotopes. Nephrops is a commercially targeted species that is harvested by static and mobile gears. Information on the European fisheries for this species is summarised by Ungfors et al. (2013). It is difficult to conduct stock assessments on Nephrops, which can only be harvested selectively by trawls and static gears. Nephrops cannot be aged directly. European Nephrops fisheries are managed as Functional Units (FUs), which are smaller than the usual ICES sub-regions due to the limited dispersal abilities of Nephrops. The estimates of abundance, and hence the recommended maximum sustainable yield (MSY), the related Biomass trigger points and fishing mortality (F_MSY), estimated harvest rates and ICES recommended limits on landings and by-catch vary between FUs (Ungfors et al., 2013; Marine Scotland, 2016). For example, harvest rates (ratio of total catch to absolute abundance) vary from ca 5 to 25% between 2007 and 2015 in the Farn Deeps, to ca 5 to 30% between 2005 and 2015 in South Minch (Marine Scotland, 2016). Marine Scotland (2016) suggested that the abundance of most stocks in the North Sea has declined to the MSY Biomass trigger point but remained above the F_MSY trigger point. However, in West Scotland, most stocks are above the Biomass trigger point but fluctuate around the F_MSY (Marine Scotland, 2016). Nevertheless, landings of Nephrops in 2014 were 13,700 tonnes in the North Sea and 12,800 in West Scotland (Marine Scotland, 2016).

Nephrops trawls catch specimens that are foraging at the surface. Trawl-caught Nephrops females were reported to have fewer eggs on average than creel-caught females from the same area during an experimental study, and it was likely that the eggs may be lost due to physical abrasion (Chapman & Ballantyne, 1980). The proportion of eggs lost to abrasion ranged from 11 to 22% in samples taken from the Clyde and West of Kintyre (Chapman & Ballantyne, 1980). The entrances to Nephrops burrows are likely to be damaged by abrasion. Although Aguzzi et al. (2023) reported that, due to the burrowing nature of Nephrops (with a burrow system of 20 to 30 cm in depth), this helps individuals to avoid haul capture when not on the surface. However, Marrs et al. (1998) reported that burrows were re-established within two days, providing that the occupant had remained unharmed (Marrs et al., 1998). Nevertheless, burrow density was lower in frequently trawled areas of Loch Fyne except in areas protected from trawling by submarine obstructions (Howson & Davies, 1991; Hughes, 1998a). In addition, Nephrops have some resilience to being caught with a documented high discard survival rate, ranging from 57 to 67% in the winter and 40 to 47% in the summer (Fox et al., 2020); Nephrops survival is best explained by temperature, tow duration, and its carapace length (Morfin et al., 2025). Bottom trawling is a stressful process for Nephrops, but the physiological recovery of caught survivors (that were held in water tanks) is recorded as being quick, occurring after 6 hours and before 24 hours, with higher survival in spring (68.4 ± 7.1%) than in autumn (33.8 ± 7.8%), likely due to the higher temperatures registered after summer months (Barragán-Méndez et al., 2020).

Video studies have found that only a low proportion (circa 5%) of Nephrops approached creels to enter them (Bjordal, 1986; Adey, 2007, cited in Ungfors et al., 2013). Factors that govern emergence will influence catch rates, as only individuals that have emerged from burrows will be caught by trawl hauls. The degree of emergence from burrows for feeding or mating appears to be mainly governed by light intensities and therefore depends on factors such as time of day and season, and varies between populations at different depths (Katoh et al., 2013; Aguzzi et al., 2023).  Experimental trawling (Bell et al., 2008) to evaluate catch rates showed that catchability varied between vessels in the same area and that catch rates were strongly linked to tidal cycles, with increased catch rates at spring rather than neap tides. Catch rates differ between genders (Ungfors et al., 2013); berried females tend to stay within burrows and are rarely caught in trawls (Aguzzi and Sarda 2008, cited in Katoh et al., 2013). The population of Nephrops in the Irish Sea, in mud between the Isle of Man and the Irish coast, may be an exception. The area is subject to a near-surface gyre that retains larvae in the vicinity of the adults, so that while the population is self-sustaining, it may be vulnerable to over-exploitation as it is unlikely to receive recruitment from the surrounding area (Hill et al., 1996b, 1997b; Hughes, 1998a). Hughes (1998a) noted that areas that were unsuitable for trawling due to rocky outcrops or other obstacles were often exploited by creeling. Hughes (1998a) suggested that trawling could reduce the density of Nephrops in confined sea lochs but cited Atkinson’s observation that the resilience of Nephrops populations to trawling is enhanced by the fact that juveniles and egg-bearing females remain in their burrows and are not caught by trawls. Studies have observed Nephrops norvegicus populations before and after the impacts from fishing practices, and in the northeastern Mediterranean, four years after the implementation of a deep-sea no-take reserve, Nephrops norvegicus populations increased in abundance, biomass, body size, and trophic level in the no-take reserve (Vigo, 2023; Vigo et al., 2023).

Calocaris macandreae is suggested to rarely venture onto the surface (Nash et al. 1984). Bergmann et al. (2002) noted that small numbers of Calocaris macandreae were bycatch in Nephrops trawls in the Clyde Sea. Comparisons between grab samples collected at trawled and untrawled sites in the Oslofjord, a northern branch of the Skagerrak in the North Sea, showed that Calocaris macandreae were depleted at trawled sites. The mean abundance of Calocaris macandreae was 41.5 individuals per m² (ca ±9.91) in non-trawled areas and 14.5 individuals per m² (ci.±4.99) in trawled areas (Olsgard et al., 2008). Trawled areas were visited by otter trawlers targeting Pandalus montagui between 50 and 100 times per year, and based on the size of the trawls and the boat speed, each part of these areas was trawled on average 2 to 3 times per year (Olsgard et al., 2008). It is not clear whether the impact is cumulative, with decreases in the population occurring incrementally or if the first pass removes the most vulnerable individuals, and those that remain are either new recruits or individuals that are more resistant due to factors such as burrow depth. However, Pommer et al. (2016) did not find a significant difference in the abundance of Calocaris macandreae with trawling intensity in the Kattegat. Duineveld et al. (2007) reported a higher species diversity and abundance of mud shrimps (Callianassa subterranea and Upogebia deltura) with a fisheries exclusion zone in the North Sea than in the surrounding area. While studying the impacts of demersal and seine fisheries in the North Sea, Jager, Witbaard & Kroes (2018) noted how Callianassa subterranea was able to recover, observing an increase in abundance from approximately 40 ind./m² in 1982 to 319 ind./m² in 2000.

One of the main threats Virgularia mirabilis faces is from intense demersal fishing. However, their ability to rapidly retract helps them avoid damage from fishing gear (Bastari et al., 2018; Downie et al., 2021). For example, Virgularia mirabilis colonies are still visible at seabed sites post-fishing and were not recorded as bycatch despite being subjected to trawling activities (Angiolillo et al., 2023; Buhl-Mortensen et al., 2023). While no clear negative relationship was observed between Virgularia mirabilis and trawling, other sea pens, such as Funiculina quadrangularis, do seem highly sensitive to fishing activity, and the sea pen Pennatula phosphorea has been recorded decreasing along a gradient of increasing trawling pressure (Downie et al., 2021). Although the predicted UK distributions of Virgularia mirabilis did not appear to be adversely affected by demersal fishing, it should be noted that the models used by Downie et al. (2021) only reflect the presence or absence of sea pens, offering no insight into density or condition.

Bolam et al. (2014) suggested that spoon worms (Echiura) of the family Bonellidae (the family to which Maxmuelleria lankesteri belongs) had a high sensitivity to trawling based on their average sensitivity score from eight traits (mainly body size, morphology, lack of mobility, longevity and low recruitment) in their biological traits analysis. However, the depth of the burrows constructed by characterizing megafauna (mud-shrimps and Maxmuelleria lankesteri) probably protects the species from surface abrasion and fishing activities. For example, Maxmuelleria lankesteri burrows up to 80 cm into the sediment (Nickell et al., 1995; Hughes, 1998a); Callianassa subterranea creates a complex lattice of galleries at 30-40 cm below the surface in the fine muds but burrows less deeply (9-23 cm) in coarse sediments (Rowden & Jones, 1995; Hughes, 1998a); Calocaris macandreae creates burrows with a total depth of 21 cm in muddy sediments with a high silt content (Buchanan, 1963; Hughes, 1998a); and Nephrops norvegicus burrows to 20 to 30 cm and is found in soft mud sediments (Rice and Chapman 1971; Nash et al. 1984; Hughes, 1998a). Based on burrow depths, surface abrasion is unlikely to likely to disturb or remove the majority of the population of Maxmuelleria lankesteri, Calocaris macandreae, Callianassa subterranea, or Nephrops norvegicus within the affected area, although the proboscis of Maxmuelleria lankesteri could be damaged by passing gear at night.

The burrow opening may be damaged (as above), but observations from Loch Sween suggest that they are re-established soon after disturbance (Marrs et al., 1998; Hughes, 1998a). Atkinson (1989) suggested that trawling was unlikely to affect burrowing megafauna (other than Nephrops) to ‘any great extent’. Similarly, Vergnon & Blanchard (2006) and OSPAR (2010) noted that burrowing megafauna (Nephrops and other non-commercial crustaceans) did not show any reduction in total biomass or abundance in highly exploited sites. In their study, Nephrops norvegicus, Munida rugosa, and Liocarcinus depurator dominated highly exploited sites in the Bay of Biscay (Vergnon & Blanchard, 2006).

Sensitivity assessment. The burrowing habit of the important characterizing species probably confers some protection from direct impacts of surface abrasion. Therefore, resistance is assessed as ‘Medium’ (loss of <25% of individuals), as some individuals may be exposed within the direct footprint when on the surface. Most Nephrops populations are reported to be resilient to fishing activity, and the majority of the population will probably remain to support recovery. However, there is the potential for overfishing in populations enclosed by hydrology (e.g. in the Irish Sea) or in sea lochs. Virgularia mirabilis also seems resilient to demersal fishing above/on the seabed, with evidence of populations remaining after trawling activities. In addition, while Callianassa subterranea may recover quickly, Calocaris macandreae may take longer to recover due to its benthic larvae and lower fecundity, and Maxmuelleria lankesteri is long-lived, with stable populations and low recruitment rates. Therefore, resilience is assessed as ‘Medium’, so sensitivity is assessed as ‘Medium’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). In general, species with large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Large bioturbating or bio-irrigating species may be especially sensitive, and their loss may affect the community (Widdicombe et al., 2004; Pommer et al., 2016).

Hinz et al. (2009) noted that different studies on the effects of otter trawl disturbance in muddy sediments gave mixed results and that the effects on abundance, biomass and diversity at a community level were largely inconsistent between studies. For example, experimental studies on short-term effects showed modest changes in the benthic communities (e.g. Tuck et al., 1998) and meta-analysis suggested that otter trawling on muddy sediments had one of the least negative impacts on the benthos (Kaiser et al., 2006). However, other studies showed that areas of seabed protected by wrecks from Nephrops trawls had higher abundance and biomass of benthos (Ball et al., 2000), while Smith et al. (2000) showed significantly lower abundance, biomass and species richness of benthos in high-intensity trawling lanes. Hinz et al. (2009) suggested that the differences in results were the result of differences in statistical analysis, prior fishing intensity and duration of the studies. Hinz et al. (2009) reported that chronic otter trawling from a Nephrops fishery had significant negative effects on the benthic macrofauna. Hinz et al. (2009) concluded that while the initial impact of otter trawl on muddy sediments was modest, the long-term disturbance could lead to profound changes in the benthic communities, especially epifauna and shallow burrowing infauna.

Nephrops norvegicus fisheries could, therefore, affect burrowing megafauna and sea pen biotopes. Nephrops is a commercially targeted species that is harvested by static and mobile gears. Information on the European fisheries for this species is summarised by Ungfors et al. (2013). It is difficult to conduct stock assessments on Nephrops, which can only be harvested selectively by trawls and static gears. Nephrops cannot be aged directly. European Nephrops fisheries are managed as Functional Units (FUs), which are smaller than the usual ICES sub-regions due to the limited dispersal abilities of Nephrops. The estimates of abundance, and hence the recommended maximum sustainable yield (MSY), the related Biomass trigger points and fishing mortality (F_MSY), estimated harvest rates and ICES' recommended limits on landings and by-catch vary between FUs (Ungfors et al., 2013; Marine Scotland, 2016). For example, harvest rates (ratio of total catch to absolute abundance) vary from ca 5 to 25% between 2007 and 2015 in the Farn Deeps, to ca 5 to 30% between 2005 and 2015 in South Minch (Marine Scotland, 2016). Marine Scotland (2016) suggested that the abundance of most stocks in the North Sea has declined to the MSY Biomass trigger point but remained above the F_MSY trigger point. However, in West Scotland, most stocks are above the Biomass trigger point but fluctuate around the F_MSY (Marine Scotland, 2016). Nevertheless, landings of Nephrops in 2014 were 13,700 tonnes in the North Sea and 12,800 in West Scotland (Marine Scotland, 2016).

Nephrops trawls catch specimens that are foraging at the surface. Trawl-caught Nephrops females were reported to have fewer eggs on average than creel-caught females from the same area during an experimental study, and it was likely that the eggs may be lost due to physical abrasion (Chapman & Ballantyne, 1980). The proportion of eggs lost to abrasion ranged from 11 to 22% in samples taken from the Clyde and West of Kintyre (Chapman & Ballantyne, 1980). The entrances to Nephrops burrows are likely to be damaged by abrasion. Although Aguzzi et al. (2023) reported that due to the burrowing nature of Nephrops (with a burrow system of 20 to 30 cm in depth), this helps individuals to avoid haul capture when not on the surface. However, Marrs et al. (1998) reported that burrows were re-established within two days providing that the occupant had remained unharmed (Marrs et al., 1998). Nevertheless, burrow density was lower in frequently trawled areas of Loch Fyne except in areas protected from trawling by submarine obstructions (Howson & Davies, 1991; Hughes, 1998a). In addition, Nephrops have some resilience to being caught with a documented high discard survival rate, ranging from 57 to 67% in the winter and 40 to 47% in the summer (Fox et al., 2020); Nephrops survival is best explained by temperature, tow duration, and its carapace length (Morfin et al., 2025). Bottom trawling is a stressful process for Nephrops, but the physiological recovery of caught survivors (that were held in water tanks) is recorded as being quick, occurring after 6 h and before 24 h, with higher survival in spring (68.4 ± 7.1%) than in autumn (33.8 ± 7.8%), likely due to the higher temperatures registered after summer months (Barragán-Méndez et al., 2020).

Video studies have found that only a low proportion (circa 5%) of Nephrops approached creels to enter them (Bjordal, 1986; Adey, 2007, cited in Ungfors et al., 2013). Factors that govern emergence will influence catch rates, as only individuals that have emerged from burrows will be caught by trawl hauls. The degree of emergence from burrows for feeding or mating appears to be mainly governed by light intensities and therefore depends on factors such as time of day and season, and varies between populations at different depths (Katoh et al., 2013; Aguzzi et al., 2023).  Experimental trawling (Bell et al., 2008) to evaluate catch rates showed that catchability varied between vessels in the same area and that catch rates were strongly linked to tidal cycles, with increased catch rates at spring rather than neap tides. Catch rates differ between genders (Ungfors et al., 2013); berried females tend to stay within burrows and are rarely caught in trawls (Aguzzi and Sarda 2008, cited in Katoh et al., 2013). The population of Nephrops in the Irish Sea, in mud between the Isle of Man and the Irish coast, may be an exception. The area is subject to a near-surface gyre that retains larvae in the vicinity of the adults, so that while the population is self-sustaining, it may be vulnerable to over-exploitation as it is unlikely to receive recruitment from the surrounding area (Hill et al., 1996b, 1997b; Hughes, 1998a). Hughes (1998a) noted that areas that were unsuitable for trawling due to rocky outcrops or other obstacles were often exploited by creeling. Hughes (1998a) suggested that trawling could reduce the density of Nephrops in confined sea lochs but cited Atkinson’s observation that the resilience of Nephrops populations to trawling is enhanced by the fact that juveniles and egg-bearing females remain in their burrows and are not caught by trawls. Studies have observed Nephrops norvegicus populations before and after the impacts from fishing practices, and in the northeastern Mediterranean, four years after the implementation of a deep-sea no-take reserve, Nephrops norvegicus populations increased in abundance, biomass, body size, and trophic level in the no-take reserve (Vigo, 2023; Vigo et al., 2023).

Calocaris macandreae is suggested to rarely venture onto the surface (Nash et al. 1984). Bergmann et al. (2002) noted that small numbers of Calocaris macandreae were bycatch in Nephrops trawls in the Clyde Sea. Comparisons between grab samples collected at trawled and untrawled sites in the Oslofjord, a northern branch of the Skagerrak in the North Sea, showed that Calocaris macandreae were depleted at trawled sites. The mean abundance of Calocaris macandreae was 41.5 individuals per m² (ca ±9.91) in non-trawled areas and 14.5 individuals per m² (ci.±4.99) in trawled areas (Olsgard et al., 2008). Trawled areas were visited by otter trawlers targeting Pandalus montagui between 50 and 100 times per year, and based on the size of the trawls and the boat speed, each part of these areas is trawled on average 2 to 3 times per year (Olsgard et al., 2008). It is not clear whether the impact is cumulative, with decreases in the population occurring incrementally or if the first pass removes the most vulnerable individuals, and those that remain are either new recruits or individuals that are more resistant due to factors such as burrow depth. However, Pommer et al. (2016) did not find a significant difference in the abundance of Calocaris macandreae with trawling intensity in the Kattegat. Duineveld et al. (2007) reported a higher species diversity and abundance of mud shrimps (Callianassa subterranea and Upogebia deltura) with a fisheries exclusion zone in the North Sea than in the surrounding area. While studying the impacts of demersal and seine fisheries in the North Sea, Jager, Witbaard & Kroes (2018) noted how Callianassa subterranea is able to recover, observing an increase in abundance from approximately 40 ind./m² in 1982 to 319 ind./m² in 2000.

One of the main threats Virgularia mirabilis faces is from intense demersal fishing; however, their ability to rapidly retract helps them avoid damage from fishing gear (Bastari et al., 2018; Downie et al., 2021). For example, Virgularia mirabilis colonies are still visible at seabed sites post-fishing and were not recorded as bycatch despite being subjected to trawling activities (Angiolillo et al., 2023; Buhl-Mortensen et al., 2023). While no clear negative relationship is observed between Virgularia mirabilis and trawling, other sea pens, such as Funiculina quadrangularis, do seem highly sensitive to fishing activity, and the sea pen Pennatula phosphorea has been recorded decreasing along a gradient of increasing trawling pressure (Downie et al., 2021). Although the predicted UK distributions of Virgularia mirabilis did not appear to be adversely affected by demersal fishing, it should be noted that the models used by Downie et al. (2021) only reflect the presence or absence of sea pens, offering no insight into density or condition.

Bolam et al. (2014) suggested that spoon worms (Echiura) of the family Bonellidae (the family to which Maxmuelleria lankesteri belongs) had a high sensitivity to trawling based on their average sensitivity score from eight traits, and high scores for body size, morphology, lack of mobility, longevity and low recruitment, in their biological traits analysis. However, the depth of the burrows constructed by characterizing megafauna (mud-shrimps and Maxmuelleria lankesteri) probably protects the species from surface abrasion and fishing activities. For example, Callianassa subterranea creates a complex lattice of galleries at 30-40 cm below the surface in the fine muds but burrows less deeply (9-23 cm) in coarse sediments (Rowden & Jones, 1995; Hughes, 1998a).  Calocaris macandreae creates burrows with a total depth of 21 cm in muddy sediments with a high silt content (Buchanan, 1963; Hughes, 1998a); Maxmuelleria lankesteri burrows up to 80 cm into the sediment (Nickell et al., 1995; Hughes, 1998a); and Nephrops norvegicus burrows to 20-30 cm and is found in soft mud sediments (Rice and Chapman, 1971; Nash et al., 1984; Hughes, 1998a). Based on burrow depths (maximum depth 21 cm for Calocaris macandreae, 30 cm for Nephrops norvegicus and up to 80 cm for Maxmuelleria lankesteri), the potential effect of penetrative gear will depend on the depth to which the gear penetrates the substratum and, hence, gear type. 

The burrow opening may be damaged (as above), but observations from Loch Sween suggest that they are re-established soon after disturbance (Marrs et al., 1998; Hughes, 1998a). Atkinson (1989) suggested that trawling was unlikely to affect burrowing megafauna (other than Nephrops) to ‘any great extent’. Similarly, Vergnon & Blanchard (2006) and OSPAR (2010) noted that burrowing megafauna (Nephrops and other non-commercial crustaceans) did not show any reduction in total biomass or abundance in highly exploited sites. In their study, Nephrops norvegicus, Munida rugosa, and Liocarcinus depurator dominated highly exploited sites in the Bay of Biscay (Vergnon & Blanchard, 2006).

Sensitivity assessment. The burrowing habit of the important characterizing species probably confers some protection from direct impacts, depending on the depth of penetration of the passing fishing gear or other penetrative activity. Penetration of the substratum surface may damage the proboscis of Maxmuelleria lankesteri, but more importantly, remove shallow burrowed specimens of the burrowing megafauna, especially juveniles, and disturb and damage the burrow network within the sediment. Therefore, resistance is assessed as ‘Low’ (loss of 25%-75% of individuals), as some individuals may be exposed within the direct footprint on the surface or in shallow parts of their burrows. Most Nephrops populations are reported to be resilient to fishing activity, and the majority of the population will probably remain to support recovery. However, there is the potential for overfishing in populations enclosed by hydrology (e.g. in the Irish Sea) or in sea lochs. Virgularia mirabilis also seems resilient to demersal fishing above/on the seabed, with evidence of populations remaining after trawling activities, but could still be damaged depending on the depth of fishing gear penetration. In addition, while Callianassa subterranea may recover quickly, Calocaris macandreae may take longer to recover due to its benthic larvae and lower fecundity, and Maxmuelleria lankesteri may take a long time to recover due to its low recruitment potential. Therefore, resilience is assessed as ‘Low’ and sensitivity is assessed as ‘High’.  

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

The eye of Nephrops norvegicus is well adapted to low levels of light at the seabed, and hence changes in clarity are unlikely to interfere with visual perception. Nephrops norvegicus emerge from burrows and, due to adaptations to ambient light, Nephrops in shallower waters emerge from burrows at dawn and dusk, whereas those from deeper waters emerge about midday (Ball et al., 2000b; Aguzzi et al., 2023). Alteration in light intensity due to turbidity may, therefore, alter emergence rhythms. Aréchiga & Atkinson (1975) reported that the burrowing activity of Nephrops norvegicus is restricted to an optimum range of light intensity from about 10,000 to 10 m-c (meter/candles) (equivalent to approximately 10% to 0.001% of natural daylight). In addition, since approximately half (47%) of the diet of Nephrops norvegicus are from suspended particulate organic matter, an increase in suspended solids may be of benefit to Nephrops, particularly in small or medium-sized individuals where significantly more suspension feeding is observed (Santana et al., 2020).

Calocaris macandreae are considered to rarely emerge from the burrow system and to mostly feed on organic material within the burrow deposits. Callianassa subterranea also feeds on the organic matter within its burrow. Maxmuelleria lankesteri does not leave its burrow but extends its proboscis to the surface to feed and is thought to be highly averse to light (Hughes, 1998a). Virgularia mirabilis is documented to tolerate high amounts of suspended matter, up to 5.5 g/m³, and the concentration of winter suspended particulate matter in the water column is an important predictor variable for the species (Downie et al., 2021).

Sensitivity assessment. The biotope occurs in sheltered areas, in fine sediments, and is subject to high suspended sediment loads. Therefore, the important characteristic species are unlikely to be impacted by an increase in suspended sediments. As they dwell in deep burrows (and only emerge for short periods of time, e.g. Nephrops), they are unlikely to be affected by the resultant increase in turbidity and reduction in light. Resistance is, therefore, assessed as ‘High’ based on its burrowing habit, and resilience is ‘High’ (based on no impact to recover from), and the biotope is assessed as ‘Not sensitive’.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

The important characteristic of burrowing megafauna (mud-shrimp and Nephrops) is unlikely to be affected adversely, as they are active burrowers and Nephrops norvegicus, Calocaris macandreae and Callianassa subterranea were reported within the Garroch Head (Firth of Clyde) sludge dumping ground (Smith, 1988; cited in Hughes, 1998a). In Korea, the sediment reworking rate of Callianassa subterranea increased with elevation, suggesting that rapid sediment reworking is necessary to process large volumes of sediment and meet energy demands (Seo & Koo, 2021). In addition, if the deposited sediment occludes burrow openings, then they would be reopened quickly. Observations from Loch Sween suggest that they are re-established soon after experimental disturbance (Hughes, 1998a).

Although Virgularia mirabilis burrows into the sediment and can retract itself, it is not freely mobile like the characterizing crustaceans of this biotope and could be more susceptible to smothering, with pollution being a key threat to the species (Bastari et al., 2018; Downie et al., 2021; Taormina et al., 2024). While studying the effects of salmon aquaculture effluents on Virgularia mirabilis populations in southern Norway, lethal and sub-lethal effects were observed, although some colonies were found below salmon farms that had been active for over 20 years, suggesting that the species is able to recover and persist in the presence of smothering impacts (Taormina et al., 2024). In terms of mortality, Virgularia mirabilis populations suffered the most directly below salmon farms, with sublethal effects visible up to 500 m from the farm, and close to aquaculture localities, particulate organic matter sedimentation can reach eight to 20 times the natural sedimentation rate (Taormina et al., 2024). Typically, the strongest environmental footprint caused by a fish farm is estimated to extend 40 to 150 m away from the farm, but in more dispersive areas, this footprint can be greater, reaching 550 to 600 m (Taormina et al., 2024). Taormina et al. (2024) observed farm effluent footprints reaching 680 m away, but these sites were not exposed to significant amounts of effluents generated by the farm. In addition, Virgularia mirabilis is documented to tolerate high amounts of suspended matter, up to 5.5 g/m³, and the concentration of winter suspended particulate matter in the water column is an important predictor variable for the species (Downie et al., 2021).

Sensitivity assessment. This biotope occurs in deep, sheltered muddy habitats where the accretion rates are potentially high. Therefore, it is probable that deposition of 5 cm of fine sediment will have little effect other than to temporarily suspend feeding and the energetic cost of burrowing. Therefore, a resistance of ‘High’ is suggested, resulting in a resilience of ‘High’ and a sensitivity of ‘Not sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

The important characteristic of burrowing megafauna (mud-shrimp and Nephrops) is unlikely to be affected adversely, as they are active burrowers and Nephrops norvegicus, Calocaris macandreae and Callianassa subterranea were reported within the Garroch Head (Firth of Clyde) sludge dumping ground (Smith, 1988; cited in Hughes, 1998a). In Korea, the sediment reworking rate of Callianassa subterranea increased with elevation, suggesting that rapid sediment reworking is necessary to process large volumes of sediment and meet energy demands (Seo & Koo, 2021). In addition, if the deposited sediment occludes burrow openings, then they would be reopened quickly. Observations from Loch Sween suggest that they are re-established soon after experimental disturbance (Hughes, 1998a).

Although Virgularia mirabilis burrows into the sediment and can retract itself, it is not freely mobile like the characterizing crustaceans of this biotope and could be more susceptible to smothering, with pollution being a key threat to the species (Bastari et al., 2018; Downie et al., 2021; Taormina et al., 2024). While studying the effects of salmon aquaculture effluents on Virgularia mirabilis populations in southern Norway, lethal and sub-lethal effects were observed, although some colonies were found below salmon farms that had been active for over 20 years, suggesting that the species is able to recover and persist in the presence of smothering impacts (Taormina et al., 2024). In terms of mortality, Virgularia mirabilis populations suffered the most directly below salmon farms, with sublethal effects visible up to 500 m from the farm, and close to aquaculture localities, particulate organic matter sedimentation can reach eight to 20 times the natural sedimentation rate (Taormina et al., 2024). Typically, the strongest environmental footprint caused by a fish farm is estimated to extend 40 to 150 m away from the farm, but in more dispersive areas, this footprint can be greater, reaching 550 to 600 m (Taormina et al., 2024). Taormina et al. (2024) observed farm effluent footprints reaching 680 m away, but these sites were not exposed to significant amounts of effluents generated by the farm. In addition, Virgularia mirabilis is documented to tolerate high amounts of suspended matter, up to 5.5 g/m³, and the concentration of winter suspended particulate matter in the water column is an important predictor variable for the species (Downie et al., 2021).

This biotope occurs in deep, sheltered muddy habitats where the accretion rates are potentially high. Therefore, it is probable that deposition of 30 cm of fine sediment will have little effect other than to temporarily suspend feeding and the energetic cost of burrowing. Therefore, a resistance of ‘High’ is suggested, resulting in a resilience of ‘High’ and a sensitivity of ‘Not sensitive’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

No studies have examined the effect of EMFs on Maxmuelleria lankesteri, Calocaris macandreae, Callianassa subterrranea, or Virgularia mirabilis. However, one study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity indicative of a stress response were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Some of the characterizing species associated with this biotope, in particular, the sea pens, may respond to sound vibrations and can withdraw into the sediment. Feeding will resume once the disturbing factor has passed. However, most of the species are infaunal and unlikely respond to noise disturbance at the benchmark level. Therefore, this pressure is probably Not relevant in this biotope.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Nephrops norvegicus emerge from burrows and, due to adaptations to ambient light, Nephrops in shallower waters emerge from burrows at dawn and dusk, whereas those from deeper waters emerge about midday (Ball et al., 2000b; Aguzzi et al., 2023). Alteration in light intensity due to turbidity may, therefore, alter emergence rhythms. Aréchiga & Atkinson (1975) reported that the burrowing activity of Nephrops norvegicus is restricted to an optimum range of light intensity from about 10,000 to 10 m-c (meter/candles) (equivalent to approximately 10% to 0.001% of natural daylight). But Nephrops are more active by day in deeper waters (ca 100 m) (Hughes, 1998a). In the shallow waters of Loch Sween, Maxmuelleria lankesteri only extends its proboscis to the surface to feed at night, and then only for short periods (Hughes et al., 1993). Hughes (1998a) noted this species would behave differently in deep water. Calocaris macandreae is considered to rarely emerge from its burrow system (Hughes, 1998a).

Sensitivity assessment. Light is probably not relevant in the deep examples of this biotope. In shallow examples of the biotope, shading may increase the time available for feeding by Nephrops or Maxmuelleria lankesteri, while an increase in ambient light might decrease feeding. Therefore, a resistance of ‘High’ is recorded. Resilience is ‘High’ (by default), so that the biotope is probably ‘Not sensitive’ at the benchmark level.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed.  But seed dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not relevant to seabed habitats. 

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Most species within the biotope are burrowing and have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. Epifauna such as crabs have well developed visual acuity and are likely to respond to movement in order to avoid predators. However, it is unlikely that the species will be affected by visual disturbance at the benchmark level.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence of genetic modification, breeding, or translocation in sea pens or burrowing megafauna was found.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

The infection of Nephrops norvegicus by a parasitic dinoflagellate of the genus Hematodinium has been known since the mid-1980s (Field et al., 1992). Infected populations have been found in the Irish Sea, Scottish sites in the Clyde Sea area and the west and east coasts. Hematodinium has also been reported in Nephrops in the German Bight, the Skagerrak and Kattegat (Briggs & McAliskey, 2002). Infected animals are recognized by an opaque vivid body colouration, believed to be due to high densities of parasites in the haemolymph. The muscle of infected animals is said to have a bitter taste. The infection causes a general morbidity of the lobster and a reduction in swimming performance. Death usually occurs when the parasite bursts out of the haemolymph (Marrs, pers. comm.). Recovery of Hematodinium infected Nephrops has not been observed to date (Stentiford et al., 2001). Results from the Irish Sea and Scottish surveys show a seasonal pattern to the level of Hematodinium infection in Nephrops norvegicus, with peaks in spring (Stentiford et al., 2001; Briggs & McAliskey, 2002). There was also a marked spatial variability in infection rates in animals in the Irish Sea (Briggs & McAliskey, 2002). The prevalence of infection is higher in immature animals although the reasons for this are still unclear. High mortalities seen during some surveys although these values may be an artefact brought about by the increased catchability of infected Nephrops as swimming performance falls with the severity of infection (Stentiford et al., 2000). This may lead to overestimation of the actual prevalence of infection in fishing stocks. Briggs & McAliskey (2002) report that the disease has been present in populations of Nephrops at least since 1994 and despite inflicting juvenile mortality on the Nephrops stock, recent assessments indicated a stable situation.

Mud shrimp are parasitized by parasitic isopods called bopyrids. The parasite lives in the gills and reduces reproductive output (Hughes, 1998a). Rowden & Jones (1994) reported that 11% of Callianassa subterranea in the southern North Sea were infected.  Other than a reduction in reproduction, no other effects were reported (Hughes, 1998a).

Sensitivity assessment. As Hematodinium could result in increased mortality in Nephrops, a resistance of Medium is recorded. Resilience is probably Medium so that the sensitivity of the biotope is assessed as Medium.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). In general, species with a large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Large bioturbating or bio-irrigating species may be especially sensitive, and their loss may affect the community (Widdicombe et al., 2004; Pommer et al., 2016).

Hinz et al. (2009) noted that different studies on the effects of otter trawl disturbance in muddy sediments gave mixed results and that the effect on abundance, biomass and diversity at a community level was largely inconsistent between studies. For example, experimental studies on short-term effects showed modest changes in the benthic communities (e.g. Tuck et al., 1998) and meta-analysis suggested that otter trawling on muddy sediments had one of the least negative impacts on the benthos (Kaiser et al., 2006). However, other studies showed that areas of seabed protected by wrecks from Nephrops trawls had higher abundance and biomass of benthos (Ball et al., 2000), while Smith et al. (2000) showed significantly lower abundance, biomass and species richness of benthos in high-intensity trawling lanes. Hinz et al. (2009) suggested that the differences in results were the result of differences in statistical analysis, prior fishing intensity and duration of the studies. Hinz et al. (2009) reported that chronic otter trawling from a Nephrops fishery had significant negative effects on the benthic macrofauna. Hinz et al. (2009) concluded that while the initial impact of otter trawl on muddy sediments was modest, the long-term disturbance could lead to profound changes in the benthic communities, especially epifauna and shallow burrowing infauna.

Nephrops norvegicus fisheries could, therefore, affect burrowing megafauna and sea pen biotopes. Nephrops is a commercially targeted species that is harvested by static and mobile gears. Information on the European fisheries for this species is summarised by Ungfors et al. (2013). It is difficult to conduct stock assessments on Nephrops, which can only be harvested selectively by trawls and static gears. Nephrops cannot be aged directly. European Nephrops fisheries are managed as Functional Units (FUs), which are smaller than the usual ICES sub-regions due to the limited dispersal abilities of Nephrops. The estimates of abundance, and hence the recommended maximum sustainable yield (MSY), the related Biomass trigger points and fishing mortality (F_MSY), estimated harvest rates and ICES' recommended limits on landings and by-catch vary between FUs (Ungfors et al., 2013; Marine Scotland, 2016). For example, harvest rates (ratio of total catch to absolute abundance) vary from ca 5-25% between 2007 and 2015 in the Farn Deeps, to ca 5-30% between 2005 and 2015 in South Minch (Marine Scotland, 2016). Marine Scotland (2016) suggests that the abundance of most stocks in the North Sea has declined to the MSY Biomass trigger point but remains above the F_MSY trigger point. However, in West Scotland, most stocks are above the Biomass trigger point but fluctuate around the F_MSY (Marine Scotland, 2016). Nevertheless, landings of Nephrops in 2014 were 13,700 tonnes in the North Sea and 12,800 in West Scotland (Marine Scotland, 2016).

Nephrops trawls catch specimens that are foraging at the surface. Trawl-caught Nephrops females were reported to have fewer eggs on average than creel-caught females from the same area during an experimental study, and it was likely that the eggs may be lost due to physical abrasion (Chapman & Ballantyne, 1980). The proportion of eggs lost to abrasion ranged from 11-22% in samples taken from the Clyde and West of Kintyre (Chapman & Ballantyne, 1980). The entrances to Nephrops burrows are likely to be damaged by abrasion. Although Aguzzi et al. (2023) reported that, due to the burrowing nature of Nephrops (with a burrow system of 20 to 30 cm in depth), this helps individuals to avoid haul capture when not on the surface. However, Marrs et al. (1998) reported that burrows were re-established within two days providing that the occupant had remained unharmed (Marrs et al., 1998). Nevertheless, burrow density was lower in frequently trawled areas of Loch Fyne except in areas protected from trawling by submarine obstructions (Howson & Davies, 1991; Hughes, 1998a). In addition, Nephrops have some resilience to being caught with a documented high discard survival rate, ranging from 57 to 67% in the winter and 40 to 47% in the summer (Fox et al., 2020); Nephrops survival is best explained by temperature, tow duration, and its carapace length (Morfin et al., 2025). Bottom trawling is a stressful process for Nephrops, but the physiological recovery of caught survivors (that were held in water tanks) is recorded as being quick, occurring after 6 hours and before 24 hours, with higher survival in spring (68.4 ± 7.1%) than in autumn (33.8 ± 7.8%), likely due to the higher temperatures registered after summer months (Barragán-Méndez et al., 2020).

Video studies have found that only a low proportion (circa 5%) of Nephrops approached creels to enter them (Bjordal, 1986; Adey, 2007, cited in Ungfors et al., 2013). Factors that govern emergence will influence catch rates, as only individuals that have emerged from burrows will be caught by trawl hauls. The degree of emergence from burrows for feeding or mating appears to be mainly governed by light intensities and therefore depends on factors such as time of day and season, and varies between populations at different depths (Katoh et al., 2013; Aguzzi et al., 2023).  Experimental trawling (Bell et al., 2008) to evaluate catch rates showed that catchability varied between vessels in the same area and that catch rates were strongly linked to tidal cycles, with increased catch rates at spring rather than neap tides. Catch rates differ between genders (Ungfors et al., 2013); berried females tend to stay within burrows and are rarely caught in trawls (Aguzzi and Sarda 2008, cited in Katoh et al., 2013). The population of Nephrops in the Irish Sea, in mud between the Isle of Man and the Irish coast, may be an exception. The area is subject to a near-surface gyre that retains larvae in the vicinity of the adults, so that while the population is self-sustaining, it may be vulnerable to over-exploitation as it is unlikely to receive recruitment from the surrounding area (Hill et al., 1996b, 1997b; Hughes, 1998a). Hughes (1998a) noted that areas that were unsuitable for trawling due to rocky outcrops or other obstacles were often exploited by creeling. Hughes (1998a) suggested that trawling could reduce the density of Nephrops in confined sea lochs but cited Atkinson’s observation that the resilience of Nephrops populations to trawling is enhanced by the fact that juveniles and egg-bearing females remain in their burrows and are not caught by trawls. Studies have observed Nephrops norvegicus populations before and after the impacts from fishing practices, and in the northeastern Mediterranean, four years after the implementation of a deep-sea no-take reserve, Nephrops norvegicus populations increased in abundance, biomass, body size, and trophic level in the no-take reserve (Vigo, 2023; Vigo et al., 2023).

Hughes (1998a) suggested that the mix of megafaunal burrowers creates a continuously shifting mosaic of habitat patches that reflected different disturbances. He suggested that this ‘patchiness’ was a factor in the maintenance of the species diversity of these communities. Although the burrows of different megafaunal burrowers were often interconnected, there was no evidence of any relationship between the species, and the connections were probably accidental. In addition, there was no evidence that any single species of megafaunal burrower was dominant or determined the structure or functioning of the community (Hughes, 1998a).

Sensitivity assessment. The physical effects of Nephrops fisheries are discussed under the ‘abrasion’ and penetration’ pressures above. However, there is no evidence to suggest that the targeted removal of Nephrops from the community would have any biological effect on the community. This conclusion is supported by Atkinson’s (1989) observation that trawling was unlikely to affect other megafaunal burrowers to ‘any great extent’.  Similarly, Vergnon & Blanchard (2006) and OSPAR (2010) noted that burrowing megafauna (Nephrops and other non-commercial crustaceans) did not show any reduction in total biomass or abundance in highly exploited sites. In their study, Nephrops norvegicus, Munida rugosa, and Liocarcinus depurator dominated highly exploited sites in the Bay of Biscay (Vergnon & Blanchard, 2006). In mesocosm experiments, Widdicombe et al. (2004) noted that bioturbation of muddy sediments associated with burrow formation (by Calocaris macandreae) was less important for maintaining biodiversity than the bioturbation of bulldozing species (e.g. Brissopsis lyrifera and Aphrodita aculeata) in the surface 10 cm of the sediment. In addition, Sciberras et al. (2016) concluded that the physical effects of trawling on the biogeochemistry of muds were larger than in sands but that these changes were not mediated by changes in the infauna. Therefore, a resistance of ‘High’ is recorded, so that resilience is also ‘High’ (by default), and the biotope is recorded as ‘Not sensitive’.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The physical effects of fisheries are discussed under the ‘abrasion’ and 'penetration’ pressures above.  Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016).  In general, species with large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Large bioturbating or bio-irrigating species may be especially sensitive and their loss may affect the community (Widdicombe et al., 2004; Pommer et al., 2016).

Hinz et al. (2009) noted that different studies on the effects of otter trawl disturbance in muddy sediments gave mixed results and that the effect on abundance, biomass and diversity at a community level were largely inconsistent between studies. For example, experimental studies on short-term effects showed modest changes in the benthic communities (e.g. Tuck et al., 1998) and meta-analysis suggested that otter trawling on muddy sediments had one of the least negative impacts on the benthos (Kaiser et al., 2006). However, other studies showed that areas of seabed protected by wrecks from Nephrops trawls had higher abundance and biomass of benthos (Ball et al., 2000), while Smith et al. (2000) showed significantly lower abundance, biomass and species richness of benthos in high-intensity trawling lanes. Hinz et al. (2009)  suggested that the differences in results were the result of differences in statistical analysis, prior fishing intensity and duration of the studies. Hinz et al. (2009) reported that chronic otter trawling from a Nephrops fishery had significant negative effects on the benthic macrofauna. Hinz et al. (2009) concluded that while the initial impact of otter trawl on muddy sediments was modest, the long-term disturbance could lead to profound changes in the benthic communities, especially epifauna and shallow burrowing infauna.

However, there is no evidence to suggest that the targeted removal of Nephrops or other megafaunal burrowers from the community would have any biological effect on the community.  This conclusion is supported by Atkinson’s (1989) observation that trawling was unlikely to affect other megafaunal burrowers to ‘any great extent’.  Similarly, Vergnon & Blanchard (2006; OSPAR, 2010) noted that burrowing megafauna (Nephrops and other non-commercial crustaceans) did not show any reduction in total biomass or abundance in highly exploited sites. In their study, Nephrops norvegicus, Munida rugosa and Liocarcinus depurator dominated highly exploited sites in the Bay of Biscay (Vergnon & Blanchard, 2006).  In mesocosm experiments, Widdicombe et al. (2004) noted that bioturbation of muddy sediments associated with burrow formation (by Calocaris macandreae) was less important for maintaining diversity than the bioturbation of bulldozing species (e.g. Brissopsis lyrifera and Aphrodita aculeata) in the surface 10 cm of the sediment.  In addition, Sciberras et al. (2016) concluded that the physical effects of trawling on the biogeochemistry of muds were larger than in sands but that these changes were not mediated by changes in the infauna. 

Hughes (1998) suggested that the mix of megafaunal burrowers creates a continuously shifting mosaic of habitat patches that reflected different disturbances.  He suggested that this ‘patchiness’ was a factor in the maintenance of the species diversity of these communities. Although the burrows of different megafaunal burrowers are often interconnected, there was no evidence of any relationship between the species, and the connections were probably accidental. In addition, there was no evidence that any single species of megafaunal burrower was dominant or determined the structure or functioning of the community (Hughes, 1998).  Therefore, a resistance of 'High' is recorded, so that resilience is also 'High' (by default) and the biotope is recorded as 'Not sensitive'.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the shells of other Crepidula, or other species, e.g., oysters and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’, it preferred shell, especially that of conspecifics. Close examination of the literature (2023) shows that evidence of its colonization and density on circalittoral mud was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds at 60 metres in the English Channel, but circalittoral mud is unlikely to be suitable due to substratum and depth (Hinz et al., 2011). Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on a gravel substratum with boulders. However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogeneous fine sediment and areas dominated by boulders.

Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and, possibly, burial due to deposition, which could mitigate Crepidula density. 

Sensitivity assessment. The circalittoral mud characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata due to substratum (fine mud) and depth (Tillin et al., 2020). Crepidula has been recorded from the lower intertidal to ca 160 m in depth, but is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). In addition, Crepidula requires some hard substratum (stones, gravel or shells) to successfully settle, which are lacking or rare in this fine mud biotope. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is probably ‘Not sensitive’ to colonization by Crepidula.  

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight-hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal, between marinas and shellfish culture sites, is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics and its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a).  Larval release and recruitment typically occur between 14 to 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures, such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus  and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015).

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). Furthermore, in Holyhead marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffiths et al., 2009). 

Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow sandy bottoms (cited Gittenberger, 2007). In the Netherlands, the coastal zone is composed of mud and sand, with only shells as hard substrata. Didemnum sp. remained rare until 1996, when populations quickly expanded, and it became a dominant invasive species because of an increase in available hard substrata for colonization after a cold winter between 1995 and 1996 caused a decrease in the abundance of many marine animals (Gittenberger, 2007). Thus, Didemnum vexillum was able to colonize and establish in mud and sand habitats only where hard substrata were present.

Sensitivity assessment. Didemnum vexillum has not been reported to colonize deep burrowing communities. Nevertheless, Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). However, it is probably excluded from soft mud and sand communities in the absence of suitable hard substrata. Hard substrata are rare or absent in this fine mud biotope (JNCC, 2015) and unlikely to provide a suitable hard substratum for colonization by Didemnum sp. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is probably ‘Not sensitive’ to colonization by Didemnum vexillum, albeit with ‘Low’ confidence due to the lack of direct evidence.  

The Pacific oyster, Magallana gigas

Evidence

The majority of the evidence indicates that muddy sediments and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, this INIS is probably 'Not relevant' in this biotope. 

Wireweed, Sargassum muticum

Evidence

The circalittoral nature and sedimentation of this biotope probably exclude macroalgae. Hence, it is unlikely to be colonized by Sargassum. Therefore, this INIS is probably 'Not relevant' in this biotope.

Wakame, Undaria pinnatifida

Evidence

The circalittoral nature and sedimentation of this biotope probably exclude macroalgae. Hence, it is unlikely to be colonized by Undaria. Therefore, this INIS is probably 'Not relevant' in this biotope.

Other INIS

Evidence

Sternapsis scutata is a non-native polychaete that has extended its range into inshore muddy sediments in the south-west of the UK (Shelley et al., 2008). However, in mesocosm experiments, little effect on biological functioning was detected after the introduction of the polychaete and a doubling of its biomass (Shelley et al., 2008). The red king crab Paralithodes camtschaticus is a voracious, omnivorous benthic predator that has spread from the Barents Sea to the coast of Norway, where it is a threat to shellfisheries and demersal fisheries. It has not been recorded in UK waters to date, although entry to the UK through natural range expansion is possible, but uncertain (GBNNSIP, 2015).

No direct evidence of the effect of these non-native species on sea pen and burrowing megafauna communities was found. However, this assessment should be revisited in light of new evidence.