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

There are no published studies that explore the thermal minima or maxima for Eunicella verrucosa. The only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Eunicella verrucosa is a temperate- to cold-water species that is typically seen from 2 to 60 m deep, but can be found up to 200 m, and it has been recorded in the western Mediterranean and off northwest Africa (Wells et al., 1983 cited in Koomen & Helsdingen, 1996; Chimienti, 2020). Hence, an increase in temperature in UK waters is not likely to negatively affect the species. Yet, in the Mediterranean, Eunicella verrucosa is mostly present below 35 m depth, likely to remain in the presence of cold-enough waters (Chimienti, 2020). In the Mediterranean, Eunicella verrucosa has been recorded in seawater between 13 and 14°C (Rodolfo‐Metalpa et al., 2015). Therefore, temperature, along with seabed slope and wave velocity, are important predictors of distribution in Eunicella verrucosa (Jenkins & Stevens, 2022). Jenkins & Stevens (2022) predicted that the northern range of Eunicella verrucosa was constrained by sea surface and/or sea bottom temperature, and based on seasonal marine thermoclines, minimum winter temperature may be a candidate for limiting the distribution of Eunicella verrucosa in Britain and Ireland. For example, the lowest average seafloor temperatures where Eunicella verrucosa was observed was 9.2°C, the median temperature was 10.5°C, and the highest temperature was 11.4°C (Jenkins & Stevens 2022).

Furthermore, during the 1990s and 2000s, mass mortality events related to high seawater temperature anomalies have been reported within the western Mediterranean basin. A mass mortality event in 1999 affected many gorgonians, although Eunicella verrucosa near Gallinaria Island was ‘little affected’ (Cerrano et al., 2000). ‘Occasional’ mortality was observed in the shallowest populations along the Provence coast (at 37 to 38 m) during a high temperature event in 1999, where sea temperature was 23 to 24°C throughout the water column to 40 m depth (Perez et al., 2000). During the European heatwave of 2003, pink sea fan populations were affected in the Gulf of Genoa but not along the Provence coast, where temperatures were 1 to 3°C above the climatic values (mean and maximum) of the Mediterranean Sea (between 29.6 and 23.3°C at 1 and 20 m respectively) (Garrabou et al., 2009). Although total mortality was not explicitly reported for this species, a certain reduction in population size could be suspected, due to delayed mortality of colonies affected by high levels of injury, as observed in some other Mediterranean gorgonians (e.g. Linares et al.,2005; Coma et al., 2006).

Canessa et al. (2022) studied a population of Eunicella verrucosa off northeast Sardinia, Italy. They observed that approximately half (45 out of 100) of the studied colonies showed varying levels of damage (small damage to dead colonies) from mainly either thermal stress or fishing activity. However, Canessa et al. (2022) noted that thermal stress had a negligible influence on the damage described in this study and, in fact, the population of Eunicella verrucosa observed lived below the depth where these phenomena are usually documented. In addition, corals were more susceptible to diseases when stressed, and Eunicella species have been affected by mass mortality events linked to positive thermal anomalies, and evidence of a disease affecting Eunicella verrucosa has been correlated to high concentrations of Vibrio bacteria, most likely due to the elevated seawater temperature (Chimienti, 2020; Canessa et al., 2022).

Finally, like other temperate and cold-water corals, Eunicella verrucosa could be severely affected by ongoing climate change, with drastic habitat loss that can lead to local extinctions with limited refugia and negative effects on the associated community (Chimienti, 2020; Morato et al., 2020; Egger et al., 2025). Jenkins & Stevens (2022) used models to predict the shift in Eunicella verrucosa distribution due to temperature changes induced by climate change. The models first showed areas of present-day (1951–2000) suitable habitat where colonies have not yet been observed, and areas beyond the Eunicella verrucosa known northern range limit were identified. Secondly, analysis with future layers (2081–2100) of temperature and oxygen concentration predicted a sizable increase in habitat suitability for Eunicella verrucosa beyond these current range limits. This suggests that projected climate change may induce a potential northward range expansion for Eunicella verrucosa. However, successful colonization would also be conditional on other factors such as dispersal and interspecific competition (Jenkins & Stevens, 2022). Conversely, Morato et al. (2020) also modelled the effect of climate change in the North Atlantic by 2100 on suitable habitat for cold-water corals, and predicted a decrease of 28 to 100%, with the largest declines for the scleractinian coral Lophelia pertusa and the octocoral Paragorgia arborea, with declines of at least 79% and 99% respectively.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over nine years. High temperature events caused mass mortality among a number of species. The decline in Pentapora fascialis colony cover between 11 and 22 m depth followed the unusually warm summer in 1999 (temperature at 11 m of 23.87 ± 1.4 °C), with an 86% reduction in live colony portion and the larger colonies were most affected. Gradual recovery took place, with deeper communities recovering to pre-disturbance levels within four years. Pagès-Escolà et al. (2020) studied the restoration of a similar bryozoan, Pentapora foliacea, and observed that the highest growth values for this species were observed during the cold months of the year (October–March), whereas in the warmer season, the growth rates decreased. In addition, necrosis was highest during the warmer months, illustrating the vulnerability of Pentapora foliacea to high temperatures, with high necrotic proportions occurring in the largest colonies.

Caryophyllia smithii is found across the British Isles (NBN, 2015; Coolen et al., 2015) and has been recorded in Greece (Koukouras, 2010). It is therefore unlikely to be significantly affected at the benchmark. However, Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns.

There is limited information available about the tolerance of Axinella dissimilis to increases in temperature. In the British Isles, it has a mainly southern and western distribution. The species is found in warmer waters as far south as Spain. It is replaced in the Mediterranean by the very similar species, Axinella polypoides (Howson & Picton, 1997). Long-term increases in temperature may cause extension of the British Isles populations, and decreases in temperature may result in population shrinkage. Goodwin et al. (2013) noted increases in the abundance of Axinella damicornis and Axinella dissimilis in Northern Ireland over a 20-year period and suggested the increase was due to sea temperature warming (relating to a 0.3 to 0.5°C increase in northern Irish Sea surface temperature between 1850 and 2007). Berman et al. (2013) monitored sponge communities off Skomer Island, UK, over four years, with all characterizing sponges for this biotope assessed. Seawater temperature, turbidity, photosynthetically active radiation and wind speed were all recorded during the study. They concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied. 

Sensitivity assessment

Eunicella verrucosa, Pentapora foliacea, Caryophyllia smithii, and Axinella dissimilis are distributed throughout the UK, with the distribution of some of the species as far south as the Mediterranean. A long-term increase in temperature may result in an increase in abundance of some of the characterizing species and a distribution shift more northwards, but also could cause decreases in habitat suitability, and potentially mass mortality due to high-temperature events (Cerrano et al., 2000; Goodwin et al., 2013; Cocito & Sgorbini, 2014; Chimienti, 2020; Canessa et al., 2022; Morato et al., 2020; Jenkins & Stevens, 2022; Egger et al., 2025). However, the key characteristic species may survive a localized increase in temperature of 2°C or a short (5-day) increase by 5°C. Hence, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

There are no published studies that explore the thermal minima or maxima for Eunicella verrucosa, so the only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Eunicella verrucosa is a temperate- to cold-water species that is typically seen from 2 to 60 m deep, but can be found up to 200 m, and its UK distribution is generally limited to the southwest of the British Isles (Hayward & Ryland, 1990; NBN, 2015; Chimienti, 2020). Distribution of Eunicella verrucosa is likely to be influenced by temperature, as Mediterranean colonies of the species are mostly documented below 35 m, likely to remain in the presence of cold-enough waters (Chimienti, 2020; Jenkins & Stevens, 2022). In the Mediterranean, Eunicella verrucosa has been recorded in seawater between 13 and 14°C (Rodolfo‐Metalpa et al., 2015).

Jenkins & Stevens (2022) predict that the northern range of Eunicella verrucosa is constrained by sea surface and/or sea bottom temperature, and based on seasonal marine thermoclines, minimum winter temperature may be a candidate for limiting the distribution of Eunicella verrucosa in Britain and Ireland. For example, the lowest average seafloor temperature Jenkins & Stevens (2022) observed Eunicella verrucosa in was 9.2°C, the median temperature was 10.5°C, and the highest temperature was 11.4°C. A decrease in temperature is likely to result in mortality. However, a live specimen collected from shallow depths off North Devon in 1973 exhibited growth rings that demonstrated that the colony had survived the 1962/63 cold winter (Hiscock, pers comm.). Also, large colonies were collected (for sale as souvenirs) from Lundy in the late 1960s, suggesting no significant loss in 1962/63 (Hiscock, 2007). Assuming that temperature decrease reduces recruitment, the population size might decline for a year, but recovery would occur following successful recruitment.

Pentapora foliacea is found as far north as the Minch off western Scotland (Lombardi et al., 2010). Patzold et al. (1987) recorded the formation of growth bands in Pentapora foliacea during times of reduced reproduction, which appeared during periods of colder water temperatures. Once established, colonies are most likely able to withstand occasional lower or higher than normal temperatures, but long-term decreases in temperature may cause the distribution range to shrink.

Caryophyllia smithii is found across the British Isles (NBN, 2015; Coolen et al., 2015) and has been recorded in Greece (Koukouras, 2010). Whilst Caryophyllia smithii is a southern species (Fish & Fish, 1996), it occurs in the Shetland Isles (NBN, 2015), whereas CR.HCR.Xfa.ByErSp.Eun is concentrated in the south and west of the UK.

There is limited information available about the tolerance of Axinella dissimilis to decreases in temperature. The British Isles are at the northern distribution limit of Axinella dissimilis (Ackers et al., 1992). Long-term increases in temperature may cause extension of the British Isles populations, and decreases in temperature may result in population shrinkage. Apparent shrinkage of individual sponges (negative growth rate) observed in Lundy in some years was attributed to particularly cold winters, notably between 1985 and 1986 (Hiscock, 1993). Berman et al. (2013) monitored sponge communities off Skomer Island, UK, over four years, with all characterizing sponges for this biotope assessed. Seawater temperature, turbidity, photosynthetically active radiation and wind speed were all recorded during the study. They concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied. Long-term increases in temperature may cause extension of the British Isles populations, and decreases in temperature may result in population shrinkage. 

Sensitivity assessment

Eunicella verrucosa already close to its northern distribution limit would likely suffer mortality in the event of a decrease in temperature (Jenkins & Stevens, 2022). However, it appears to have survived the 1962/3 winter and may have some resistance to temporary changes in temperature. Pentapora foliacea and Caryophyllia smithii could experience reduced growth rates in colder temperatures, but are likely to be less affected by short-term decreases in temperature. Long-term decreases in temperature may cause population shrinkage of the British Isles distributions of Axinella dissimilis (Berman et al., 2013). Apparent shrinkage of individual Axinella dissimilis (negative growth rate) was observed in Lundy in some years and was attributed to particularly cold winters, notably between 1985 and 1986 (Hiscock, 1993).  Resistance is therefore assessed as ‘Medium’, resilience as ‘Very Low’ and sensitivity as ‘Medium’.

Salinity increase (local)

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

Evidence

CR.HCR.XFa.ByErSp.Eun is a circalittoral biotope, and an increase in salinity at the benchmark level would result in a change from full to hypersaline conditions. No records of Eunicella verrucosa, Caryophyllia smithii, or Axinella dissimilis in hypersaline conditions were found. However, Rodolfo‐Metalpa et al. (2015) noted how off the coast of Italy, Caryophyllia smithii was recorded in waters with a salinity of 38.6 (±0.14). Chesher (1975) monitored the species surrounding a desalination outfall with brine effluent at 52‰ salinity, together with variable concentrations of copper and nickel. As a group, gorgonians were noted to survive brief exposure to 4 to 5% effluent; however, long-term survival decreased in relation to proximity to the outfall.  

Whilst no evidence for the characterizing species was found, there is evidence of gorgonian mortality due to hypersaline effluent. Therefore, resistance is probably ‘Low’, resilience is ‘Very Low’, and sensitivity is ‘High’.

Salinity decrease (local)

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

Evidence

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata bryozoans were fairly stenohaline and restricted to full salinity (30 to 35 ppt), noting that reduced salinities result in an impoverished bryozoan fauna. For example, Flustra foliacea appears to be restricted to areas with high salinity (Tyler-Walters & Ballerstedt, 2007; Budd, 2008). Dyrynda (1994) noted that Flustra foliacea and Alcyonidium diaphanum were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Although protected from extreme changes in salinity due to their subtidal habitat, severe hyposaline conditions could adversely affect Flustra foliacea colonies. However, Novosel et al. (2004) described large colonies of Pentapora fascialis growing inside the plumes of marine freshwater springs (3 psu lower than water outside of the channel). Eunicella verrucosa has only been recorded in Full salinity biotopes, while Caryophyllia smithii has been recorded in biotopes from Full to Low salinity (Connor et al., 2004) and would probably tolerate a change at the benchmark level. Rodolfo‐Metalpa et al. (2015) noted how off the coast of Italy, Caryophyllia smithii was recorded in waters with a salinity of 38.6 (±0.14). No information was found for Axinella dissimilis in water with salinity below 30 to 40.

Sensitivity assessment. Pentapora foliacea, Eunicella verrucosa (together with associated species), and Axinella dissimilis would probably be affected adversely by a decrease in salinity at the benchmark level and resistance is ‘Low’, resilience is ‘Very low’, and sensitivity is ‘High’.

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

CR.HCR.XFa.ByErSp.Eun consists mainly of species firmly attached to the substratum, and which would be unlikely to be displaced by an increase in the strength of tidal streams at the benchmark level. Sea fans are found in strong tidal streams, but most likely retract their polyps when current velocity gets too high for the polyps to retain food (Reverter-Gil, Souto & Trigo, 2019). Under proper oceanographic conditions, including strong currents and temperate/cold waters, Eunicella verrucosa can form large colony aggregations known as coral forests (Chimienti, Nisio, & Lanzolla, 2020). For example, the presence of Eunicella verrucosa in both the Balearic Sea and the Strait of Sicily in the Mediterranean may be explained by the intense geostrophic circulation of water masses and a complex seafloor topography that both regions exhibit, and due to the presence of islands and seamounts, this generates additional mesoscale eddies and convergent fronts (Canessa et al., 2022). Tidal streams exert a steady pull on the colonies and are therefore likely to detach only very weakly attached colonies. Colonies rely on water flow to bring food and to remove silt (Hiscock, 2007). Jenkins & Stevens (2022) noted how seabed slope, temperature at the seafloor, and wave orbital velocity were important predictors of distribution in Eunicella verrucosa, and that specifically, wave orbital velocity was more important than tidal velocity for bringing in fresh nutrients and oxygen, both for polyps to feed on and for exporting waste products.

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to the high mass transport of water, such as the Menai Strait and tidal rapids, generally support large numbers of bryozoan species (Moore, 1977a). Although bryozoans are active suspension feeders, feeding currents are probably fairly localized, and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability and an increased risk of siltation (Tyler-Walters, 2005). 

Caryophyllia smithii is a suspension feeder, relying on water currents to supply food (Hiscock, 1983). These taxa, therefore, thrive in conditions of vigorous water flow, e.g. around Orkney and St Abbs, Scotland, where Alcyonium digitatum dominated biotopes may experience tidal currents of 3 and 4 knots (approximately 1.5 m/sec) during spring tides (De Kluijver, 1993; Coolen et al., 2015). The life cycle of Caryophyllia smithii includes a larval planktotrophic stage with a duration of 8 to 10 weeks, and during this time, the released larvae float freely in the water column and are transported in the direction of net water movement, which is driven by tidal currents and wind. These residual currents in the North Sea, UK, range between 0.02 and 0.08 cm/s (Coolen et al., 2015). Caryophyllia smithii, in particular, is described as favouring sites with a high tidal flow (Bell & Turner, 2000; Wood, 2005; Coolen et al., 2015). Rodolfo‐Metalpa et al. (2015) noted how off the coast of Italy, Caryophyllia smithii was recorded in waters with a tidal current of 24 cm/s (± 15).

Riisgard et al. (1993) discussed the low energy cost of filtration for sponges and concluded that passive current-induced filtration may be of insignificant importance for sponges. However, water movement is probably required to ensure the supply of food (particulates and dissolved organic matter) as well as oxygen. The sponges Axinella spp. were recorded in biotopes that experienced very weak to moderate flow (0 to 1.5 m/s) (Connor et al., 2004). Eunicella verrucosa, Caryophyllia smithii and Pentapora foliacea have been recorded in biotopes ranging from very weak to strong water flow (0 to 3 m/s) (Connor et al., 2004). In Norway, both Phakellia ventilabrum and Axinella infundibuliformis were primarily observed at sites with a relatively slower horizontal current velocity (0.02 to 0.03 m/s) (Dunlop et al., 2020). However, this biotope occurs in wave-exposed conditions, and although ameliorated by depth, wave action might be a more important source of water movement than tidal streams.

Sensitivity assessment. CR.HCR.XFa.ByErSp.Eun is found in moderately strong water flow (1 to 3 knots) (Reverter-Gil, Souto & Trigo, 2019; Coolen et al., 2015) and, whilst a significant decrease could result in less favourable conditions for Eunicella verrucosa, a change at the benchmark level (0.1 to 0.2 m/s) is unlikely to affect the characterizing species.  Resistance is therefore ‘High’, resilience is ‘High’, and the biotope is ‘Not Sensitive’.

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 this biotope as it is restricted to fully subtidal/circalittoral conditions - 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

Eunicella verrucosa occurs in biotopes that are extremely wave-exposed (Connor et al., 2004). Bunker (1986) reported that Eunicella verrucosa was most abundant in moderately exposed locations. However, dead sea fans have been recorded washed up along Chesil Beach (UK) following winter storms (Hatcher and Trewhella, 2006 cited in Wood, 2015b). Jenkins & Stevens (2022) noted how seabed slope, temperature at the seafloor, and wave orbital velocity were important predictors of distribution in Eunicella verrucosa, and that specifically, wave orbital velocity is more important than tidal velocity for bringing in fresh nutrients and oxygen, both for polyps to feed on and for exporting waste products.

Caryophyllia smithii has been recorded in very sheltered to extremely exposed biotopes (Connor et al., 2004). Pentapora foliacea is recorded as occurring in biotopes experiencing moderate to extreme wave exposure (Connor et al., 2004). However, extreme wave action (storms) has been noted to cause widespread destruction of colonies (Cocito et al., 1998a). Significant increases in wave exposure may, therefore, cause damage to colonies. Hiscock (2003) suggested that ‘prolonged Easterly gales in 1985’ might account for the loss of Axinella dissimilis specimens at Lundy. Nevertheless, following severe gales in 2013/2014, the abundance of sea fans, Pentapora fascialis, and associated branching sponges on stable rocky habitats appeared much as always (Hiscock, pers. comm.).

Sensitivity assessment. CR.HCR.XFa.ByErSp.Eun occurs across a number of wave exposures, from moderate to extremely exposed. A decrease in wave exposure, e.g. due to artificial barriers, may be detrimental as the biotope is dependent on strong water flow. While storms may cause mortality, a 3 to 5% change in significant wave height is unlikely to result in any impact in a wave-exposed biotope. Resistance is ‘High’, resilience is ‘High’, and the biotope is ‘Not Sensitive’ 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

Chan et al. (2012) studied the response of the gorgonian Subergorgia suberosa to heavy metal-contaminated seawater from a former coastal mining site in Taiwan. Cu, Zn, and Cd each showed characteristic bioaccumulation. Metallic Zn accumulated but rapidly dissipated. In contrast, Cu easily accumulated but was slow to dissipate, and Cd was only slowly absorbed and dissipated. Associated polyp necrosis, mucus secretion, tissue expansion, and increased mortality were reported in Subergorgia suberosa exposed to water polluted with heavy metals.However, this pressure is Not assessed.

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 pressure is Not assessed but evidence is presented where available.

CR.HCR.XFa.ByErSp.Eun is a sub-tidal biotope complex (Connor et al., 2004). Oil pollution is mainly a surface phenomenon and its impact on circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014).

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones that experience high exposure and show correspondingly high mortality, as are bottom-dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981). There is little information on the effects of hydrocarbons on bryozoans. Ryland & Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. in Milford Haven or St. Catherine's Island, south Pembrokeshire, although it did alter the breeding period. Banks & Brown (2002) found that exposure to crude oil significantly impacted recruitment in the bryozoan Membranipora savartii. No evidence for Eunicella verrucosa was found, although White et al. (2012) reported on deep water gorgonian communities, including Swiftia pallida six months after the Deep Water Horizon oil spill. Stress in the gorgonians was observed including excessive mucus production, retracted polyps and smothering by brown flocculent material (floc).

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.

Bryozoans are common members of the fouling community and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur less than 165 m from the effluent source.

Radionuclide contamination

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

Evidence

No evidence

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

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l (ca 2.66 mg/l) (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l.

No evidence was found concerning the effects of hypoxia on Eunicella verrucosa. However, as a species that lives in fully oxygenated waters in conditions of flowing waters, it is expected that it would be intolerant to decreased oxygen levels. Jenkins & Stevens (2022) used models to predict the shift in Eunicella verrucosa distribution due to temperature and oxygen changes induced by climate change. Analysis with future layers (2081–2100) of temperature and oxygen concentration predicted a sizable increase in habitat suitability for Eunicella verrucosa beyond these current range limits. This suggests that projected climate change may induce a potential range expansion northward for Eunicella verrucosa, but areas such as southwest England, Brittany (northwest France) and the Channel Islands are still predicted to remain as suitable habitat. However, successful colonization would also be conditional on other factors such as dispersal and interspecific competition (Jenkins & Stevens, 2022).

Little information on the effects of oxygenation on bryozoans was found. Sagasti et al. (2000) reported that epifaunal communities, including the dominant bryozoans, were unaffected by periods of moderate hypoxia (ca 0.35 to 1.4 ml/l) and short periods of anoxia (<0.35 ml/l) in the York River, Chesapeake Bay, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. 

Bell (2002) reported that an oxycline at Lough Hyne (<5% surface concentration) limited vertical colonization by Caryophillia smithii.

No evidence was found for Axinella dissimilis. Hiscock & Hoare (1975) reported an oxycline forming in the summer months (June to September) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation below ca 10 m. No Tethya aurantia, Kirchenpaueria pinnata, Hymeniacidon pereleve, or Polymastia boletiformis were recorded at depths below 10 to 11 m. Gunda & Janapala (2009) investigated the effects of variable oxygen levels on the survival of the marine sponge, Haliclona pigmentifera. Under hypoxic conditions (1.5 to 2.0 ppm), Haliclona pigmentifera with intact ectodermal layers and subtle oscula survived for 42 ± 3 days. Sponges with prominent oscula, foreign material, and damaged pinacoderm exhibited poor survival (of 1 to 9 days) under similar conditions. Complete mortality of the sponges occurred within 2 days under anoxic conditions (<0.3 ppm O²).

Bell et al. (2024) studied the stability of shallow water sponges at Lough Hyne, Ireland, and concluded that changes to the deeper subtidal sponge assemblages were possibly driven by local processes associated with deeper water, potentially related to the seasonal oxythermocline (development of a colder, oxygen-poor layer in the deeper areas from northern hemisphere spring) that forms within Lough Hyne. This low-oxygen layer is thought to have a strong influence on the ecology and biology of organisms in the deeper areas of the lough, with a marked decline in the biodiversity of sponges and other organisms below approximately 25 m (Bell et al., 2024). However, explicit testing of Lough Hyne sponges’ oxygen tolerance found sponges to be resilient to short-term oxygen stress, with the focus now being on the presence of hydrogen sulphide as the main driver of change (Bell et al., 2024).

Sensitivity assessment. Despite limited evidence, Eunicella verrucosa, Axinella dissimils, and Caryophyllia smithii are unlikely to tolerate hypoxic events given their preference for moderate water movement and based on general information for marine invertebrates. Resistance is ‘Low’, resilience is 'Very Low’, and sensitivity is ‘High’.

Nutrient enrichment

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

Evidence

Whilst little information on Pentapora spp. was found, O’Dea & Okamura (2000) found that annual growth of the bryozoan Flustra foliacea in western Europe has substantially increased since 1970. They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998). Echavarri-Erasun et al. (2007) described the effects of deep water sewage discharge on the relative abundance of rocky reef communities. Species typical of hard substrata (including Caryophyllia smithii and bryozoans) increased in total richness and abundance near the outfall. Whilst Eunicella verrucosa could be at risk of competition from algae in shallow waters due to nutrient enrichment, CR.HCR.XFa.ByErSp.Eun is a circalittoral biotope and flora are not considered in this review.  

Nutrient enrichment could result in greater exposure to microbial pathogens, leading to disease and loss of characterising species. Microbial pathogens are considered below, along with evidence of bacterial damage to Eunicella verrucosa. 

There seems to be 'Insufficient evidence' for a nutrient enrichment sensitivity assessment of this biotope using the weight of evidence approach.

Organic enrichment

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

Evidence

Cocito et al. (2013) demonstrated the ability of Eunicella verrucosa and other gorgonians to feed on both suspended organic matter and zooplankton.  Whilst little information on Pentapora spp. was found, O’Dea & Okamura (2000) found that annual growth of the bryozoan Flustra foliacea in western Europe has substantially increased since 1970.  They suggest that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).   Novosel et al. (2004) described large colonies of Pentapora fascialis growing inside the plume of marine freshwater springs.  The plumes had significantly higher concentrations of NO₃^-, SiO₄, NH₄⁺, NO₂^- and PO₄^3-.  Echavarri-Erasun et al. (2007) described the effects of deep water sewage discharge on the relative abundance of rocky reef communities.  Species typical of hard substrata (including Caryophyllia smithii and bryozoans) increased in total richness and abundance near the outfall.  No evidence was found for Axinella dissimilis.  

Sensitivity assessment. All characterizing species are sessile filter feeders and the evidence suggests that all tolerate, or increase in abundance when exposed to organic enrichment in the circalittoral.  Resistance is ‘High’ resilience is ‘High’ and the biotope is ‘Not sensitive’.

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

If rock were replaced with sediment, this would represent a fundamental change to the physical characteristics of the biotope, and the species would be unlikely to recover. The biotope would be lost. Eunicella verrucosa, Pentapora foliacea, Caryophillia smithii, and Axinella sponges require a hard substratum to attach to, such as rock, steel, and other coralligenous formations (Pikesley et al., 2016; Chimienti, 2020; Fabri et al., 2022; Jenkins & Stevens, 2022). During visual surveys of these species, substrata may not always be obvious due to a thin sediment veneer commonly observed on top of the seabed (Sheehan et al., 2017; Chimienti, Nisio, & Lanzolla, 2020).

Sensitivity assessment. Resistance to this pressure is considered ‘None’, and resilience is ‘Very low’. Sensitivity has been 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

‘Not relevant’ to biotopes occurring on bedrock.

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

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

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

Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increased when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018). Heavy mobile gears could also result in movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998), and sensitivity of Eunicella verrucosa to abrasion events has been assessed as High in previous reviews, largely due to its slow growth rate and fragility to physical damage (MacDonald, 1996; Hall et al., 2008; Tillin et al., 2010; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Egger et al., 2025). For example, strandings of Eunicella verrucosa have been observed on the southwest coast of England, where the coral appeared to be entangled in lost fishing gears, as well as in domestic marine litter (Sheehan et al., 2017; Chimienti, Nisio, & Lanzolla, 2020). In addition, physical contact with fishing gear scrapes has been noted to favour and increase the development of epibionts on gorgonian corals (Canessa et al., 2022). Epibionts substantially modify host–environment interactions (e.g., transference of energy or matter), eventually reducing their fitness, and large masses of epibionts lead to a burdening of the colonies and greater mechanical stress (Canessa et al., 2022). The response of Eunicella verrucosa colonies to physical stress and epibionts was studied in the Catalan Sea, Spain. Canessa et al. (2022) observed that Eunicella verrucosa in unprotected areas, which experienced fishing damage, experienced epibiosis at least four times higher than colonies in protected areas; 10 to 30% compared to 4 to 10% respectively.

Fishing gear such as bottom trawling, bottom longlines, trammel nets, and gillnets have all been observed to adversely affect Eunicella verrucosa (Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020). For example, Eunicella verrucosa was the most common coral bycatch species (32%) over two fishing seasons (summer-autumn and spring) in southern Portugal (Dias et al., 2020). In Lyme Bay, UK, before the exclusion of bottom trawling, Eunicella verrucosa would occur as bycatch during fishing operations. The populations of Eunicella verrucosa and other benthic taxa in Lyme Bay have benefited since the trawling ban in 2008 (Kaiser et al., 2018; Chimienti, Nisio, & Lanzolla, 2020). Sheehan et al. (2013) noted that within three years of closing an area in Lyme Bay, UK, to fishing, some recovery of Eunicella verrucosa had occurred, with a marked increase compared to areas that were still fished. However, recovery of slow-growing, long-lived, sessile epifauna is expected to take decades. Kaiser et al. (2018) specifically studied the recovery of sessile epifauna following the exclusion of towed mobile fishing gear in Lyme Bay, UK. Their estimates suggest that no recovery occurred within the timescale of the study, and that some biogenic habitats (particularly sponges and soft corals) could require up to, or more than, 20 to 30 years before signs of recolonization and recovery may occur. The maximum recovery time modelled was 51 years for yellow branched sponges, while Eunicella verrucosa and Pentapora foliacea increased in abundance, but had not fully recovered, with their projected recovery time being 17 to 20 years (Kaiser et al., 2018). Therefore, recovery rates of biota depend on life-history factors and habitat-specific requirements, with the longer-lived species that require specific habitats and have low dispersal potential taking longer to recover (Kaiser et al., 2018).

A 15-year review of the Lyme Bay trawling ban by Renn et al. (2024) highlighted definitive evidence of recovery, in terms of increased species richness, with key sessile taxa (Pentapora foliacea and Phallusia mammillata) showing signs of early recovery between 2008 and 2013. In terms of exploited species, between 2008 and 2019, fish experienced a 430% increase in taxon richness and 370% increase in total abundance inside the Marine Protected Area (MPA), but invertebrates (crab, lobster, cuttlefish, and whelk) exhibited no signs of recovery (Renn et al., 2024). Renn et al. (2024) concluded that the evidence of recovery recorded in Lyme Bay broadly aligned with the wider literature by detecting early stages of recovery within the first few years of MPA establishment. However, full recovery is thought to occur over decadal timescales, and measuring full recovery rates in-situ remains a priority for future research in Lyme Bay. Coma et al. (2006) reported ongoing recovery in Eunicella singularis populations in the Mediterranean four years following a mass-mortality event.  Although not recovered,

Other studies suggest that Eunicella verrucosa may be more resistant to abrasion pressures. Eno et al. (2001) conducted experimental potting on areas containing fragile epifaunal species in Lyme Bay, south-west England. Divers observed that pink sea fans flexed and bent before returning to an upright position under the weight of pots. Although relatively resistant to a single event, long-term deterioration or the effects of repeated exposure were not clear (Eno et al., 2001). Observation of pots suggested that they were dragged along the bottom when wind and tidal streams were strong. However, little damage to epifauna was observed. Eunicella verrucosa were patchily distributed in areas subject to potting damage, but the study could not determine whether this was due to damage from potting (Eno et al., 2001). A further four-year study on potting in the Lundy Marine Protected Area detected no significant differences in Eunicella verrucosa between areas subject to commercial potting and those where this activity was excluded (Sheehan et al., 2013). However, Tinsley (2006) observed flattened sea fans that had continued growing, with new growth being aligned perpendicular to the current, so even colonies of Eunicella verrucosa that are damaged can survive. 

Healthy Eunicella verrucosa are able to recover from minor damage and scratches to the coenenchyme (Tinsley, 2006), and the coenenchyme covering the axial skeleton will re-grow over scrapes on one side of the skeleton in about one week (Hiscock, pers. comm, cited in Hiscock, 2007). While Hinz et al. (2011a) reported that abundance and average body size of Eunicella verrucosa were not significantly affected by scallop dredging intensity, there is evidence of Eunicella verrucosa detached by mobile gear (Hiscock, pers. comm.). In addition, Eunicella verrucosa that has been accidentally collected in fishing activities have been proven to survive if returned quickly to the sea when it settles on cobbles and is less damaged when removed (Enrichetti et al., 2019).

Pentapora foliacea, Caryophyllia smithii, and Axinella sponges are likely to be affected by physical disturbances. Some large Pentapora foliacea individuals were observed to be badly smashed by potting (Eno et al., 2001), and fishing gear has been shown to adversely affect emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018). Hiscock (2014) identified Axinella dissimilis as being very susceptible to towed fishing gear. Hinz et al. (2011a) studied the effects of scallop dredging in Lyme Bay, UK and found that the presence of the erect sponge Axinella dissimilis was significantly higher at non-fished sites (33% occurrence) compared to fished sites (15% occurrence). However, Pentapora foliacea, as well as other vulnerable benthic taxa such as Eunicella verrucosa, Phallusia mammillata, and Axinella sponges, showed signs of recovery once demersal towed fishing equipment was excluded from an area of sea (Pikesley et al., 2016; Chimienti, Nisio, & Lanzolla, 2020; Kaiser et al., 2018). For more information, see above under recovery of Eunicella verrucosa.

Sensitivity assessment. Sessile epifauna such as Eunicella verrucosa is likely to be severely damaged by heavy gears, such as scallop dredging (MacDonald et al., 1996, Hiscock, pers. comm.; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Egger et al., 2025). However, some studies suggest that the species may be more resistant, particularly to low-intensity lighter abrasion pressures, such as pots and associated anchor damage (Eno et al., 1996). Pentapora foliacea, Caryophyllia smithii, and Axinella dissimilis are also likely to be damaged by towed fishing gear (Hinz et al., 2011a; Hiscock, 2014; Sheehan et al., 2017; Kaiser et al., 2018). Taking all the evidence into account, resistance is ‘Low’. Resilience is ‘Very Low’ and sensitivity is ‘High’.

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

The species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure. This pressure is thought ‘Not Relevant’ to hard rock biotopes.

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

While high levels of suspended solids may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007) and are probably tolerant of increases in suspended sediment (Hiscock et al., 2004b). However, Pikesley et al. (2016) noted that the turbidity of coastal waters in parts of the southwest UK may preclude Eunicella verrucosa from colonizing certain areas like South Pembrokeshire or the Gower Peninsula, where otherwise suitable habitat was available. Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but occurred at sites described as ‘moderately silted’. Bilan et al. (2023) studied the vulnerability of cold-water corals to sediment resuspension from bottom trawling in the Mediterranean and found that cup coral and octocoral did not exhibit symptoms of distress, whereas colonial scleractinian corals and black coral experienced substantial polyp mortality in enhanced suspended sediment concentration treatments.

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus.

Populations of Caryophyllia smithii were studied at three sites of differing sedimentation regimes in Lough Hyne, Ireland (Bell & Turner, 2000). The height, length, width and density of individuals were measured along with the depth of accumulated sediment on the rock substratum at each site. Calyx size was largest at the site of least sedimentation and smallest at the site of most sedimentation. In contrast, the height of individuals was greatest at the site of most sedimentation and smallest at the site of least sedimentation. The height of individuals correlated with the level of surrounding sediment. Caryophyllia smithii was more abundant in areas with higher sedimentation (Bell & Turner, 2000).

Axinella dissimilis is mainly found on upward-facing clean or silty rock, and whilst it tends to prefer clean oceanic water, it is tolerant of silt (Ackers et al., 1992).

Sensitivity assessment. CR.HCR.XFa.ByErSp.Eun occurs on bedrock in moderate water flow in the circalittoral and is unlikely to experience highly turbid conditions. From the evidence presented above, the characterizing species would probably tolerate some siltation, and a change at the benchmark level is unlikely to cause mortality. Resistance is ‘High’, resilience is ‘High’, and the biotope is ‘Not sensitive’ at the benchmark level.

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

Eunicella verrucosa forms large colonies which branch profusely up to 30 cm in height (Picton & Morrow, 2005). While high levels of suspended solids may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007). Although it is thought that smothering causes mortality (Hiscock et al., 2004b). Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but occurred at sites described as ‘moderately silted’. However, Eunicella verrucosa seems to tolerate heavy silting, as Canessa et al., (2022) noted how Eunicella verrucosa colonies of northeast Sardinia, Italy, occurred mainly on sub-horizontal rocks characterized by heavy silting between 30 and 215 m.

Colonies of Pentapora fascialis can reach a height of 30 cm in the British Isles (Hayward & Ryland, 1979). Partial mortality due to siltation has been recorded in the Mediterranean (Cocito et al., 1998a), although recovery was observed in all but one colony (which fragmented into two smaller colonies).

Caryophyllia smithii is a small (approx. <3 cm height from the seabed) species and would therefore likely be inundated in a “light” sedimentation event. Coolen et al. (2015) noted how low abundance of Caryophyllia smithii is typically observed at locations with low tidal current strength and high sedimentation. For example, in the Skomer Island, UK, Marine Conservation Zone, higher numbers of Caryophyllia smithii were observed on vertical walls, likely due to less surface sediment accumulating there (Lock et al., 2025). However, Bell & Turner (2000) reported Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7 mm ± 0.5 mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 5 cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope.  Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover.

Hiscock & Jones (2004) reported that Axinella dissimilis (as Axinella polypoides) grew up to a height of ca 30 cm. Axinella sponges are commonly found alongside Eunicella verrucosa and can therefore tolerate a similar/high level of smothering (Canessa et al., 2022). Ackers et al. (1992) described Axinella dissimilis as preferring clean oceanic water but tolerating silt. No evidence of smothering of axinellids was found. However, Pineda et al. (2017b) exposed three phototrophic (due to symbiotic algae) and two heterotrophic sponges from New Zealand to repeated deposition events and sediment cover over 80 to 100% of sponge surface to a depth of ca 0.5 mm for up to 30 days in laboratory conditions. All five species survived with minimal physiological effects. However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment; 16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina, respectively.

Tidal fluctuations, mixing by internal waves, and storms (particularly in shallower waters) are natural ways in which sediments are periodically resuspended within oceans, and help to keep deep-sea sponges fed with organic material (Samuelsen et al., 2022). However, one understood source of sedimentation within the marine environment is from offshore oil and gas activities, mainly via drilling (Vad et al., 2018). Vad et al. (2018) studied the impacts of oil and gas drilling on deep-sea sponges and observed that physical disruption and increased sedimentation during well drilling and infrastructure installations could locally diminish benthic communities by more than 90% in terms of megafaunal density within sponge grounds. Major reductions in sponge densities and reduced diversity were seen close to drilling activity, within 100 to 200 m, and persisted for several years (Vad et al., 2018).

Vad et al. (2018) concluded that effects on deep-sea sponge grounds from such physical disturbance were still detectable up to 10 years post-drilling, and this slow, partial recovery was inversely related to the distance to the well and the time after drilling, resulting from the long-lived nature, slow growth rates and low reproduction rates of most deep-sea organisms. Furthermore, if oil and gas drilling used synthetic and water-based muds, the decrease in community diversity and abundance was detected up to 1,000 m away from the release (Vad et al., 2018). Functional changes in benthic communities, associated with a loss of suspension-feeding species and an increase in deposit feeders, have also been detected at drill release sites (Vad et al., 2018). Conversely, Durden et al. (2023) also observed the effects of industrial sedimentation on sponge communities; however, once sedimentation accumulated on sponges, it cleared mostly from them gradually over time, but sometimes sharply. Yet, sponges never returned to their original state, and this partial recovery likely involved a combination of active and passive removal of the sediment.

Sensitivity assessment. Smothering by 5 cm would cover the majority of Caryophyllia smithii and the smallest examples of the other characterizing species and could result in limited mortality. Caryophyllia smithii has been reported as quite tolerant of temporary burial, and the biotope occurs in moderate water flow, and the sediment would likely be removed rapidly. Resistance was assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’ at the benchmark level.

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

Eunicella verrucosa forms large colonies which branch profusely up to 30 cm in height (Picton & Morrow, 2005). While high levels of suspended solids may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007). Although it is thought that smothering causes mortality (Hiscock et al., 2004b). Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but occurred at sites described as ‘moderately silted’. However, Eunicella verrucosa seems to tolerate heavy silting, as Canessa et al. (2022) noted how Eunicella verrucosa colonies of northeast Sardinia, Italy, occurred mainly on sub-horizontal rocks characterized by heavy silting between 30 and 215 m.

Colonies of Pentapora fascialis can reach a height of 30 cm in the British Isles (Hayward & Ryland, 1979). Partial mortality due to siltation has been recorded in the Mediterranean (Cocito et al., 1998a), although recovery was observed in all but one colony (which fragmented into two smaller colonies). 

Caryophyllia smithii is a small (approx. <3 cm height from the seabed) species and would therefore likely be inundated in a “heavy” sedimentation event. Coolen et al. (2015) noted how low abundance of Caryophyllia smithii is typically observed at locations with low tidal current strength and high sedimentation. However, Bell & Turner (2000) reported Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7mm ± 0.5mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 5cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope. Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover.

Hiscock & Jones (2004) reported that Axinella dissimilis (as Axinella polypoides) grew up to a height of ca 30 cm. Axinella sponges are commonly found alongside Eunicella verrucosa and can therefore tolerate a similar/high level of smothering (Canessa et al., 2022). Ackers et al. (1992) described Axinella dissimilis as preferring clean oceanic water but tolerating silt. No evidence of smothering of axinellids was found. Pineda et al. (2017b) exposed three phototrophic (due to symbiotic algae) and two heterotrophic sponges from New Zealand to repeated deposition events and sediment cover over 80 to 100% of sponge surface to a depth of ca 0.5 mm for up to 30 days in laboratory conditions. All five species survived with minimal physiological effects. However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment; 16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina, respectively.

Tidal fluctuations, mixing by internal waves, and storms (particularly in shallower waters) are natural ways in which sediments are periodically resuspended within oceans, and help to keep deep-sea sponges fed with organic material (Samuelsen et al., 2022). However, one understood source of sedimentation within the marine environment is from offshore oil and gas activities, mainly via drilling (Vad et al., 2018). Vad et al. (2018) studied the impacts of oil and gas drilling on deep-sea sponges and observed that physical disruption and increased sedimentation during well drilling and infrastructure installations can locally diminish benthic communities by more than 90% in terms of megafaunal density within sponge grounds. Major reductions in sponge densities and reduced diversity were seen close to drilling activity, within 100 to 200 m, and persisted for several years (Vad et al., 2018).

Vad et al. (2018) concluded that effects on deep-sea sponge grounds from such physical disturbance were still detectable up to 10 years post-drilling, and this slow, partial recovery was inversely related to the distance to the well and the time after drilling, resulting from the long-lived nature, slow growth rates and low reproduction rates of most deep-sea organisms. Furthermore, if oil and gas drilling used synthetic and water-based muds, the decrease in community diversity and abundance was detected up to 1,000 m away from the release (Vad et al., 2018). Functional changes in benthic communities, associated with a loss of suspension-feeding species and an increase in deposit feeders, have also been detected at drill release sites (Vad et al., 2018). Conversely, Durden et al. (2023) also observed the effects of industrial sedimentation on sponge communities; however, once sedimentation accumulated on sponges, it cleared mostly from them gradually over time, but sometimes sharply. Yet, sponges never returned to their original state, and this partial recovery likely involved a combination of active and passive removal of the sediment.

Sensitivity assessment. Smothering by 30 cm of sediment would likely bury the majority of characterizing species, with only those individuals on boulders and vertical surfaces escaping burial. The biotope occurs in high-energy environments, and it is likely that the sediment would be removed. However, the damage to the resident community would depend on the time taken for the deposited sediment to be removed. Therefore, resistance was assessed as ‘Low’ as the worst-case scenario. Hence, resilience is probably ‘Very Low’, and sensitivity is assessed as ‘High’.

Litter

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

Evidence

Physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018). Sensitivity of Eunicella verrucosa to abrasion events has been assessed as High in previous reviews, largely due to its slow growth rate and fragility to physical damage (MacDonald, 1996; Hall et al., 2008; Tillin et al., 2010; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Egger et al., 2025). Both Sheehan et al. (2017) and Giusti et al. (2019) highlight how in addition to the direct damage from fishing, ghost fishing may also be responsible for some Eunicella verrucosa mortality, either through direct damage or making them more vulnerable to removal from their anchorage to the sea floor, particularly during storms.

For example, during 2015, strandings of Eunicella verrucosa were observed on the southwest coast of England, where the coral appeared to be entangled in lost fishing gears, as well as in domestic marine litter, and almost all of the tangled bundles of marine debris contained a central dead, black or brownish skeletal remains of Eunicella verrucosa (Sheehan et al., 2017; Chimienti, Nisio, & Lanzolla, 2020). Divers have often encountered plastic fishing gear, fishing line, and other marine debris, such as plastic bags, amongst living coral gardens on rocky reefs off the coasts of southwest England and have become snagged and subsequently overgrown by Eunicella verrucosa (Sheehan et al., 2017). When colonies are broken, such as through being severed by fishing line, the corals then lie flat on the seafloor and eventually die; the pink or white outer coenenchyme rots, leaving the black internal skeleton visible (Sheehan et al., 2017; Giusti et al., 2019). Alternatively, Eunicella verrucosa entangled with marine debris could have formed after gorgonians were detached from the seabed, for example, due to damage from gill nets, and then picked up debris as they travel along the seabed with the currents (Sheehan et al., 2017).

In addition, physical contact with fishing gear scrapes (lost lines entangled in colonies) has been noted to favour and increase the development of epibionts on gorgonian corals (Canessa et al., 2022). Epibionts substantially modify host–environment interactions (e.g., transference of energy or matter), eventually reducing their fitness, and large masses of epibionts lead to a burdening of the colonies and greater mechanical stress (Canessa et al., 2022). The response of Eunicella verrucosa colonies to physical stress and epibionts was studied in the Catalan Sea, Spain. Canessa et al. (2022) observed that Eunicella verrucosa in unprotected areas, which experienced fishing damage, experienced epibiosis at least four times higher than colonies in protected areas, 10 to 30% compared to 4 to 10% respectively.

There are no records of ghost fishing affecting Pentapora fascialis, Caryophyllia smithii, or Axinella sponges. However, epifaunal communities are vulnerable to damage from fishing gear, and are likely still vulnerable to being dislodged or damaged through lost fishing gear, and possibly certain types of marine litter.

Sensitivity assessment. Ghost fishing by discarded fishing gear, lines and pots could cause severe damage to the community, especially the tall erect epifauna, where discarded lines may catch the upright epifauna and increase drag, especially in stormy weather (Sheehan et al., 2017; Giusti et al., 2019). Fishing lines can cause lesions to the gorgonian coenenchyme, leading to greater aggregates of epibionts, which can eventually cause the branch to rupture (Bo et al., 2014; Canessa et al., 2022). Taking all the evidence from ghost fishing and discarded lines into account, resistance is ‘Low’. Resilience is ‘Very Low’ and sensitivity is ‘High’.

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 Eunicella verrucosa, Pentapora fascialis or Caryophyllia smithii. 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

Stanley et al. (2014) studied the effects of vessel noise on fouling communities and found that the bryozoans Bugula neritina, Watersipora arcuate and Watersipora subtorquata responded positively.  More than twice as many bryozoans settled and established on surfaces with vessel noise (128 dB in the 30–10,000 Hz range) compared to those in silent conditions.  Growth was also significantly higher in bryozoans exposed to noise, with 20% higher growth rate in encrusting and 35% higher growth rate in branching species.

Sensitivity assessment. Whilst no evidence could be found on the effects of noise or vibrations on the characterizing species, it is unlikely that these species would be adversely affected by noise. Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive'. 

Introduction of light or shading

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

Evidence

It is well understood that light influences the spawning of tropical corals, along with other environmental cues such as solar insolation, day length, and temperature (Davies et al., 2023; Egger et al., 2025). However, for temperate and intermediate-water species, some of these cues may be absent or differ significantly. Many cold-water corals live beyond the reach of moonlight, but Eunicella verrucosa spans approximately 10 to 200 m in depth, including depths where moonlight remains detectable (Egger et al., 2025). Egger et al. (2025) is the first study that provides the first description of the spawning and early life ecology of Eunicella verrucosa, and through lab-based studies with artificial moonlight, found that the spawning of Eunicella verrucosa was less pronounced, occurring over 2 to 3 consecutive days to 6 to 7 days after the full moon. Egger et al. (2025) concluded that Eunicella verrucosa spawning may be influenced by the lunar patterns as observed in other temperate gorgonians like Paramuricea clavata but likely relies on a combination of additional cues to regulate the exact timing of spawning (Egger et al., 2025).

No evidence was found for the effect of light on Pentapora fascialis and Caryophyllia smithii. However, it was reported that axinellid sponges prefer vertical or shaded bedrock to open light surfaces (Jones et al., 2012). Nevertheless, whilst no evidence could be found for the effect of light on these species of this biotope, we know that within the first 200 m of ocean depth, light, both natural and artificial, would reach the seabed. In addition, shading of light or the introduction of light within the first 50 m could have an effect on marine organisms, such as triggering early coral spawning or affecting the opening and reproduction rhythm of bivalves (Charifi et al., 2023; Davies et al., 2023; Smyth et al.,2021). Below 200 m, it is unlikely that these species would be impacted, as the light level that reaches beyond this point is very low and unsuitable for photosynthesis.

Sensitivity assessment. Given the rapid expansion of the evidence base but the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Sensitivity is therefore recorded as ‘Insufficient evidence’.

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: barriers and changes in tidal excursion are not relevant to biotopes restricted to open waters.

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.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

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

Evidence

Not relevant

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 for the characterizing species could be 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 first recorded incidence of cold-water coral disease was noted in Eunicella verrucosa, in south-west England in 2002 (Hall-Spencer et al., 2007). Video surveys in south-west England from 2003 to 2006 of 634 separate colonies at 13 sites revealed that disease outbreaks were widespread, and 9% of colonies had tissue necrosis. Coenenchyme became necrotic in diseased specimens, leading to tissue sloughing and exposing skeletal gorgonin to settlement by fouling organisms. Sites where necrosis was found had significantly higher incidences of fouling. No fungi were isolated from diseased or healthy tissue, but significantly higher concentrations of bacteria occurred in diseased specimens. Vibrio isolated from Eunicella verrucosa did not induce disease at 15°C, but, at 20°C, controls remained healthy, and test gorgonians became diseased, regardless of whether Vibrio was isolated from diseased or healthy colonies. Bacteria associated with diseased tissue produced proteolytic and cytolytic enzymes that damaged Eunicella verrucosa tissue and may be responsible for the necrosis observed. Monitoring at the site where the disease was first noted showed new gorgonian recruitment from 2003 to 2006; 5 of the 18 necrotic colonies videoed in 2003 had died and become completely overgrown, whereas others had continued to grow around a dead central area (Hall-Spencer et al., 2007). In addition, corals are more susceptible to diseases when stressed. Eunicella species were affected by mass mortality events linked to positive thermal anomalies, and evidence of a disease affecting Eunicella verrucosa was correlated to high concentrations of Vibrio bacteria, most likely due to the elevated seawater temperature (Chimienti, 2020). Furthermore, damaged corals, such as those with injury from tissue abrasion via fishing gear, can lead to infection and disease, particularly in tropical corals, where a four-fold higher level of coral disease was observed outside of a marine no-take reserve (Sheehan et al., 2017). No evidence of disease in the Pentapora fascialis or Caryophyllia smithii could be found.

Gochfeld et al. (2012) found that diseased sponges hosted significantly different bacterial assemblages compared to healthy sponges, with diseased sponges also exhibiting a significant decline in sponge mass and protein content. Sponge disease epidemics can have serious long-term effects on sponge populations, especially in long-lived, slow-growing species (Webster, 2007). Numerous sponge populations have been brought to the brink of extinction, including cases in the Caribbean with 70 to 95% disappearance of sponge specimens (Galstoff, 1942), the Mediterranean (Vacelet, 1994; Gaino et al.,1992). Decaying patches and white bacterial film were reported in Haliclona oculata and Halichondria panicea in North Wales, 1988-89 (Webster, 2007). Specimens of Cliona spp. have exhibited blackened damage since 2013 in Skomer. Preliminary results have shown that clean, fouled and blackened Cliona all have very different bacterial communities. Blackened Cliona are effectively dead and have a bacterial community similar to marine sediments. The fouled Cliona have a very distinct bacterial community, which may suggest a specific pathogen caused the effect (Burton, pers comm; Preston & Burton, 2015). 

Sensitivity assessment. Sponge diseases have caused limited mortality in some species in the British Isles (although no evidence was found for axinellid sponges), mass mortality and even extinction have been reported further afield. Based on this evidence, together with mortality linked to disease in Eunicella verrucosa, resistance is assessed as ‘Medium’, resilience as ‘Very Low’ sensitivity as ‘Medium’. Confidence is 'Low'.

Removal of target species

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

Evidence

Eunicella verrucosa has historically been harvested as a curio by divers and was collected until recently in the British Isles (Bunker, 1986; Wells et al., 1983 cited in Koomen & Helsdingen, 1996), however, it is now protected under schedule 5 of the Wildlife and Countryside Act 1981 and harvesting is illegal.  No evidence of harvesting of the other characterizing species was found.  

Hiscock (2003) stated that the greatest loss of Axinella dissimilis at Lundy might have been due to collecting during scientific studies in the 1970s. No indication of recovery was evident. Axinella damicornis was harvested in Lough Hyne during the 1980s (for molecular investigations) and the populations were reduced to very low densities, which subsequently recovered very slowly, although they are now considered to be back to their original densities (Bell, 2007).

Sensitivity assessment. The characterizing Eunicella verrucosa and the sponge Axinella dissimilis are sessile, epifaunal and would have no resistance to harvesting.  Resistance has been assessed as ‘None’, resilience as ‘Very Low’ and sensitivity is, therefore ‘High’.

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

Removal of the characteristic epifauna due to by-catch is likely to remove a proportion of the biotope and change the biological character of the biotope. For example, Eunicella verrucosa and, in particular, Axinella dissimilis, are sessile epifauna and are likely to be severely damaged by heavy gears, such as scallop dredging (MacDonald et al., 1996; Hinz et al., 2011).  

This biotope may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species in this biotope. The unintentional removal of the important characterizing species will result in loss of the biotope. Therefore, 

Sensitivity assessment. Therefore, resistance is ‘Low’, resilience is ‘Very Low’ and sensitivity is ‘High’

The American slipper limpet, Crepidula fornicata

Evidence

This biotope is classified as circalittoral and therefore no algal species have been considered. Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or 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., 2011b; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al., (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011b). However, Hinz et al., (2011b) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al., (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and 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. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1 to 0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space and many are fast growing and opportunistic, so may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment. The circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata due to the extremely wave exposed to moderately wave exposed conditions, in which wave action and storms may mitigate or prevent the colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011b). In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required. 

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-meditated 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°C to 20°C and slow or cease below 9°C 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°C to 22°C) and the lowest average temperatures were recorded in July (9°C 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).

In contrast, Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al., (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009), stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (Mckenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls the species can experience higher current velocities, which is enough to cause dislodgement (Reinhardt et al., 2012).  

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). This  biotope occurs on bedrock, which could provide a suitable hard substratum for colonization by Didemnum sp. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope experiences very weak to moderately strong water flow (0 to 1.5 m/s) but moderate to extreme wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of corals, bryozoans, and sponges. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

The Pacific oyster, Magallana gigas

Evidence

The majority of the evidence indicates that infralittoral rock 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 depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. 

Wakame, Undaria pinnatifida

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. 

Other INIS

Evidence

Several invasive bryozoans are of concern including Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b), although 'No evidence' of these affecting CR.HCR.XFa.ByErSp.Eun was found.