The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the Mediterranean Sea and Indian Ocean (Farke, 1979). Therefore, the species must be capable of tolerating higher temperatures than it experiences in Northern Europe. For example, Covazzi Harriague et al. (2007) reported Aphelochaeta marioni occurring in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at 24°C. Also, Aphelochaeta marioni occurring in Richards Bay Harbour, South Africa, is found in temperature ranges from 17.2 to 27.1°C at a depth range of 0.2 to 16.1 meters (Izegaegbe, Vivier & Mzimela, 2020). In addition, Aphelochaeta marioni numbers in the harbour were high during spring (11,653), winter (13,008) and summer (6,356), but low during autumn (5,932), and analysis of environmental data suggested that a number of variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment (Izegaegbe, Vivier & Mzimela, 2020). Furthermore, Aphelochaeta marioni lives infaunally, and so is likely to be insulated from rapid temperature change. An increase in temperature would be expected to cause some physiological stress but no mortality.

Deeper burrowing oligochaetes are protected from fluctuations in temperature by the overlaying sediments, which dampen changes if poorly drained (Giere & Pfannkuche, 1982), and Tubificoides spp. are found from the Atlantic coast of southern Spain (intertidal), the southeast coast of Norway (0.3 to 0.5 m deep) and the west coast of Sweden (0.5 to 2 m deep) (Kvist & Erséus, 2018). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in temperatures between 8.19 and 17.58°C (Lange et al., 2020). Bamber & Spencer (1984) observed that Tubificoides spp. were dominant species in an area affected by thermal discharge in the River Medway estuary, southeast England. Sediments were exposed to the passage of a temperature front of approximately 10^oC between heated effluent and estuarine waters during the tidal cycles. Tubificoides benedii has increased in abundance in mudflat habitats in Jade Bay, North Sea, between 1930 and 2009 (Schueckel & Kroencke, 2013). Climate warming, as well as decreasing nutrient loads and species introductions, have occurred in the region since the 1970s, suggesting the species may adapt to temperature increases at the benchmark level. Furthermore, in Jade Bay, Singer et al. (2017) used modelling to predict future (2009 to 2050) small-scale spatial distribution patterns of seven macrofauna species, including Tubificoides benedii. The future spatial distribution was predicted in response to expected climate change-induced (1) sea-level rise and (2) water temperature increase. They assumed an adequate water temperature increase of 1.5°C for the upcoming forty-year time interval. Between 2009 and 2050, the present and future prediction maps revealed a significant range gain for two macrofauna species (Macoma balthica and Tubificoides benedii), whereas the range sizes of five macrofauna species remained relatively stable across space and time (Singer et al., 2017). Tubificoides benedii is considered an opportunist that is adapted to rapid environmental fluctuations and harsh conditions in estuaries (Gogina et al., 2010), suggesting resistance would be high to this pressure at the benchmark level.

Sensitivity assessment

Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). Most organisms in the biotopes are distributed to the north and south of Britain and Ireland and are unlikely to be affected adversely by long-term temperature change. In addition, subtidal and especially infaunal species are likely to be protected from acute temperature change. Furthermore, the dominance of Tubificoides spp. when exposed to a heated effluent suggests that this genus would be highly resistant to an increase in temperature at the pressure benchmark. Resistance and resilience of the biotopes are therefore assessed as ‘High’, and the biotopes are judged as ‘Not Sensitive’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the western Baltic Sea, South Atlantic Ocean and North Sea (Farke, 1979). Therefore, the species must be capable of tolerating low temperatures. Aphelochaeta marioni lives buried in sediment and is therefore well insulated from decreases in temperature. In the Wadden Sea, the population was apparently unaffected by a short period of severe frost in 1973 (Farke, 1979). Kędra et al. (2010) reported Aphelochaete marioni occurring in the Svalbard Archipelago where temperatures below zero may be experienced in the winter. A decrease in temperature would be likely to cause some physiological stress but no mortality. Another similar bristleworm species, Aphelochaeta palmeri, is found in the Antarctic Ocean and is adapted to freezing conditions. The temperatures Aphelochaeta palmeri were found in ranged from −0.29° to −1.09°C at 25 meters deep (in Rod Bay and Adelie Cove, respectively), from −1.23° to −1.85°C at 70 meters deep (in Adelie Cove and Central Bay, respectively), and from −1.89° to −1.95°C at 140 meters deep (in Adelie Cove and Central Bay, respectively) (Buschi et al., 2024).

Deeper burrowing oligochaetes are protected from fluctuations in temperature by the overlaying sediments, which dampen changes if poorly drained (Giere & Pfannkuche, 1982), and Tubificoides spp. are found from the Atlantic coast of southern Spain (intertidal), the southeast coast of Norway (0.3 to 0.5 m deep) and the west coast of Sweden (0.5 to 2 m deep) (Kvist & Erséus, 2018). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in temperatures between 8.19 and 17.58°C (Lange et al., 2020). Most littoral oligochaetes, including tubificids, can survive freezing temperatures and can survive in frozen sediments (Giere & Pfannkuche, 1982). Tubificoides benedii (studied as Peloscolex benedeni) recovered after being frozen for several tides in a mudflat (Linke, 1939).

Sensitivity assessment

Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). Based on the evidence presented, Aphelochaeta marioni, Tubificoides benedii and Nephtys hombergii are likely to be able to resist a long-term decrease in temperature of 2°C and may resist a short-term decrease of 5°C. Temperature may act as a spawning cue, and an acute or chronic decrease may result in some delay in spawning, however, this is not considered to impact the adult populations and may be compensated by later spawning events. Resistance and resilience are therefore assessed as ‘High’, and the biotopes judged as ‘Not Sensitive’.

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper-buried organisms, since the interstitial or burrow water is less affected. However, under longer-term or permanent increases in salinity, sediment waters would also adjust.

Populations of Aphelochaeta marioni inhabit the open coast where seawater is at full salinity. Covazzi-Harriague et al. (2007) reported Aphelochaeta marioni in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at salinities above 38 psu. In Richards Bay Harbour, South Africa, Aphelochaeta marioni is found in salinity ranges from 18.9 to 34.8 at a depth range of 0.2 to 16.1 meters (Izegaegbe, Vivier & Mzimela, 2020). Although limited freshwater input reaches the harbour due to its isolation from any major river drainage, some salinity fluctuations were observed due to inflow from surrounding wetlands, particularly via the Bhizolo (from the Mhlathuze Papyrus swamps) and Mzingazi (overflow from Lake Mzingazi) canals; the strong tidal influence will, however, result in salinities remaining at close to marine levels throughout most of the year (Izegaegbe, Vivier & Mzimela, 2020). The analysis of environmental data by Izegaegbe, Vivier & Mzimela (2020) suggested that a number of variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment.

Farke (1979) studied the effects of changing salinity on Aphelochaeta marioni (studied as Tharyx marioni) in a microsystem in the laboratory. Over several weeks, the salinity in the microsystem was increased from 25 to 40 psu and no adverse reaction was noted. However, when individuals were removed from the sediment and displaced to a new habitat, they only dug into their new substratum if the salinities in the two habitats were similar. If the salinities differed by 3 to 5 psu, the worms carried out random digging movements, failed to penetrate the sediment and died at the substratum surface after a few hours. This would suggest that Aphelochaeta marioni can tolerate salinity changes when living infaunally but may not resist an increase in salinity when removed from its habitat.

Another similar bristleworm species, Aphelochaeta palmeri, is found in the Antarctic Ocean where salinity ranged from 33.74 to 34.52 at 25 meters (in Rod Bay and Adelie Cove, respectively) and remained stable at around 34.69 at both 70- and 140- meters deep in each area, except for Adelie Cove at 70 metres where it decreased to 34.32 (Buschi et al., 2024).

Oligochaete-dominated biotopes are recorded from a range of salinity regimes, from full (LS.LSa.MoSa.Ol; LS.LSa.MoSa.Ol.FS), variable (SS.SMu.SMuVS.CapTubi) to low (SS.SMu.SMuVS.LhofTtub) habitats (Connor et al., 2004). The species characterizing these biotopes are likely to vary. Giere & Pfannkuche (1982) identified how species change over a hypothetical salinity gradient, with marine stenohaline species present at full salinities replaced by more euryhaline oligochaete species including Tubificoides benedii and Tubificoides pseudogaster. Studies in the Rhine delta have found that Tubificoides benedii is tolerant of a range of salinities (Verdonschot et al., 1982). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in salinities between 6.02 and 118.33‰ (Lange et al., 2020).

Sensitivity assessment

Based on the fact that all species occur in greater salinities than those that occur in these biotopes, the biological assemblages associated with the biotopes are considered to have ‘High’ resistance and ‘High’ resilience and is therefore considered to be ‘Not Sensitive’ to an increase in salinity at the pressure benchmark level.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper-buried organisms, since the interstitial or burrow water is less affected. However, under longer-term or permanent increases in salinity, sediment waters would be expected to also adjust.

Aphelochaeta marioni thrives in estuaries and is, therefore, likely to be tolerant of decreases in salinity. It has been recorded from brackish inland waters in the southern Netherlands with a salinity of 16 psu, but not in areas permanently exposed to lower salinities (Wolff, 1973). However, it also penetrates into areas exposed to salinities as low as 4 psu for short periods at low tide when freshwater discharge from rivers is high (Farke, 1979). In the Severn Estuary, Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated, poorly sorted mud with relatively high interstitial salinity (Broom et al., 1991).

In Ria de Aveiro, western Portugal, Tubificoides benedii characterized communities in estuarine sample sites further upstream with lower salinity, suggesting a high resistance to a decrease in salinity (Rodrigues et al., 2011). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in salinities between 6.02 and 118.33‰ (Lange et al., 2020). In the Gulf of Finland (Baltic Sea), Tubificoides pseudogaster have been recorded in salinities from 3.2 to 5.2 ppt (Berezina, Maximov & Vladimirova, 2019).

Overall, the important characterizing species are likely to tolerate a short-term change in salinity from, e.g. variable to low salinity and a long-term change from variable to reduced salinity. The species richness of the biotopes may decline, but the biotopes will probably not be adversely affected.

Sensitivity assessment

Records indicate SS.SMu.SMuVS.AphTubi occurs in areas of full (30 to 35 ppt) and variable (18 to 35 ppt) salinities (Connor et al., 2004). The characterizing species, Aphelochaeta marioni and Tubificoides spp., occur in estuarine areas experiencing low salinities, so are therefore likely to resist a decrease in salinity at the pressure benchmark level. Resistance of the biotopes is therefore assessed as ‘High’ and resilience as ‘High’ (by default), and the biotopes are considered ‘Not Sensitive’ to a decrease in salinity at the pressure benchmark level.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

The hydrographic regime is an important structuring factor in sedimentary habitats. An increase in water flow rate is not likely to affect Nephtys hombergii, Aphelochaeta marioni, Tubificoides spp. and other characterizing species, as they live infaunally. The most damaging effect of increased flow rate could be the erosion of the substratum, as this could eventually lead to loss of the habitat. Orvain et al. (2007) investigated the spatio-temporal variations in intertidal mudflat erodibility in western France and suggested a potential link between Polychaeta and bed erodibility given the high polychaete abundances observed in the study.

Increased water flow rates is likely to change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). The characterizing species prefer habitats with silty/muddy substrata, which would not occur in very strong tidal streams. Coarser sediments are likely to remain in areas of strongest flow velocity (where finer particles have been resuspended) (Coates et al., 2014). Species such as Tubificoides benedii and other opportunistic polychaetes that tolerate coarser particle size will possibly increase in abundance. Aphelochaeta marioni has been recorded in the Wadden Sea with flow rates up to 0.45 m/s (Compton et al., 2013), and in the Westerscheld estuary (SW Netherlands) where spring current velocities vary between 0.1-0.58 m/s (Van Colen et al., 2010a).

Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability. Decreased water movement would result in increased deposition of suspended sediment (Hiscock, 1983). An increased rate of siltation resulting from a decrease in water flow may result in an increase in food availability for the characterizing species, and therefore growth and reproduction may be enhanced, but only if food was previously limiting.

Sensitivity assessment

Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on the Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, clays and silts require greater critical erosion velocities because of their cohesiveness. The biotopes occur most often in weak, moderately strong and strong tidal streams (Connor et al., 2004), suggesting the species characterizing the biotopes can adapt to a range of tidal currents, aided by each species' burrowing traits. A change in water flow rate at the pressure benchmark level of 0.1-0.2 m/s is considered to fall within the range of flow speeds experienced by populations in the middle of their range. Resistance and resilience are, therefore, assessed as High and the biotopes are considered Not Sensitive to a change in water flow at the pressure benchmark level.

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. (Emergence regime change pressure definition).

Evidence

The biotopes occur in the infralittoral so that only the upper extent of shallow examples are likely to be emersed on extreme low tides. All characterizing species would probably survive an increase in emergence. However, the species can only feed when immersed and is therefore likely to experience reduced feeding opportunities. Over the course of a year, the resultant energetic cost is likely to cause some mortality. In addition, increased emergence is likely to increase the vulnerability to predation from shore birds. A decrease in emergence is likely to allow the biotopes to extend their upper limits, where suitable substrata exist.

Opportunistic, deposit-feeding polychaetes and oligochaetes, such as Aphelochaeta marioni and Tubificoides benedii, are likely to tolerate stressful conditions, and often out-compete more sensitive species in intertidal environments due to greater tolerances (Gogina et al., 2010). Nephtys hombergii is sufficiently mobile to rapidly burrow and seek damper substrates during periods when emergence increases. For instance, Vader (1964) observed that the worm relocates throughout the tidal cycle.

For instance, Tubificoides benedii is capable of penetrating the substrate to depths of 10 cm, shows resistance to hypoxia and is often typified as an ‘opportunist’ that is adapted to the rapid environmental fluctuations and harsh conditions in estuaries (Gogina et al., 2010). Highest abundances were predicted by Gogina et al. (2010) to be related to depth with an optimum of 10 m to 20 m. Further case studies were not returned by literature searches on Tubificoides benedii. The studies returned by searches suggest abundance may be limited by a decrease in high water level or a change in time (increase) where substrate is not covered by the sea. An increase in the time the biotopes are covered by the sea is likely to result in increased abundance of Tubificoides benedii.

Sensitivity assessment

Some mortality of the characterizing species is likely to occur because of emergence regime changes. Resistance is therefore assessed as Medium and resilience is likely to be High, so the biotopes are considered to have Low sensitivity to emergence regime changes at the pressure benchmark level.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year (Wave action pressure definition). 

Evidence

Strong wave action is likely to cause damage or withdrawal of delicate feeding and respiration structures, resulting in loss of feeding opportunities and compromised growth for the characterizing species. Decreases in wave exposure may influence the supply of particulate matter because wave action may have an important role in resuspending the sediment. Furthermore, Aphelochaeta marioni characteristically inhabits soft sediments in sheltered areas (Broom et al., 1991), so individuals may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action.

Disturbance of sediment by waves may reduce oligochaete abundance (Giere, 1977), and oligochaetes may be absent from very wave-exposed shores (Giere & Pfannkuche, 1982). 

Sensitivity assessment

Hydrographic regimes are an important structuring factor in sedimentary habitats, and an increase in wave exposure could result in fine sediments being eroded (Hiscock, 1983), resulting in the likely reduction of the habitat and a decrease in food availability. Some erosion will occur naturally, and storm events may be more significant in loss and damage of the substratum than changes in wave height at the pressure benchmark, but these events will be short-lived. Limited zoobenthic biomass has been recorded in areas exposed to strong currents and wave action (Beukema, 2002), however, impacts from this pressure at the benchmark level may be low for the biotopes, as they are limited to sheltered locations. Increases in wave action may therefore remain within the limits of the species' tolerances, but factors such as sediment redistribution may alter the physical biotopes.

The SMuVS.AphTubi biotope occurs in cohesive sandy mud and mud across three wave-exposure categories: sheltered, extremely sheltered, and very sheltered (JNCC, 2022). Therefore, mid-range biotopes would tolerate both an increase and decrease in wave exposure by a single wave-exposure category for one year. However, in the worst-case scenario, examples of the biotope that occur in ‘sheltered’ conditions would experience moderately wave-exposed conditions at the benchmark level. Moderate wave exposure may winnow away the sediment surface of fine sandy mud, resulting in loss of a proportion of the biotope. Therefore, resistance is assessed as ‘Low’ as a precaution. However, resilience is probably ‘High’, and sensitivity is assessed as ‘Low’. The assessment is based on the biotope classification and, hence, expert judgement. While unlikely, a prolonged increase in wave exposure would probably result in loss of the biotope and modification of the underlying sediment.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition).

Evidence

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

Evidence suggests that polychaetes are fairly resistant to the effects of heavy metals (Bryan, 1984). Aphelochaeta marioni is tolerant of heavy metal contamination occurring in the heavily polluted Restronguet Creek (Bryan & Gibbs, 1983), and it is also an accumulator of arsenic (Gibbs et al., 1983). Nephtys hombergii is also recorded in Restronguet Creek. Rygg (1985) classified Tubificoides spp. as highly tolerant species, common at the most copper-polluted stations (>200 mg/kg) in Norwegian fjords.

Taking into account the variable salinity conditions that affect these biotopes (in general, for estuarine animals, heavy metal toxicity increases as salinity decreases and temperature increases: McLusky et al., 1986), it seems possible that some polychaete species, at least in these biotopes, might be adversely affected by high contamination by heavy metals.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

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

Cirratulids seem to be mostly immune to oil spills, probably because their feeding tentacles are protected by a heavy secretion of mucus (Suchanek, 1993). This is supported by observations of Aphelochaeta marioni following the Amoco Cadiz oil spill in March, 1978 (Dauvin, 1982, 2000). Prior to the spill, Aphelochaeta marioni (studied as Tharyx marioni) was present in very low numbers in the Bay of Morlaix, western English Channel. Following the spill, the level of hydrocarbons in the sediment increased from 10 mg/kg dry sediment to 1443 mg/kg dry sediment 6 months afterwards. In the same period, Aphelochaeta marioni increased in abundance to a mean of 76 individuals/m², which placed it among the top five dominant species in the faunal assemblage. It was suggested that the population explosion occurred due to the increased food availability because of the accumulation of organic matter resulting from high mortality of browsers. Six years later, the abundance of Aphelochaeta marioni began to fall away again, accompanied by gradual decontamination of the sediments.

Tubificoides benedii was found in UK waters near oil refineries as the sole surviving member of the macrofauna. Populations were, however, apparently reduced and the worms were absent from areas of oil discharge and other studies indicate sensitivity to oiling (Giere & Pfannkuche, 1982).

McLusky (1982) found that petrochemical effluents, including organic solvents and ammonium salts, released from a point source to an estuarine intertidal mudflat of the Forth Estuary, Scotland, caused severe pollution in the immediate vicinity. Beyond 500 m, the effluent contributed to an enrichment of the fauna in terms of abundance and biomass similar to that reported by Pearson & Rosenberg (1978) for organic pollution; Nephtys hombergii was found in the area with maximum abundance of species and highest total biomass at 500 m from the discharge. It seems likely that significant hydrocarbon contamination would kill affected populations of the species. On return to prior conditions, recolonization is likely via adult migration and larval settlement.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

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

There is little evidence directly relating to the effects of synthetic chemicals on Aphelochaeta marioni. Waldock et al. (1999) reported that the species diversity of polychaete infauna, including Aphelochaeta marioni, in the Crouch Estuary increased in the three years after the use of TBT was banned within the estuary, suggesting that TBT had suppressed their abundance previously. Furthermore, Beaumont et al. (1989) concluded that TBT had a detrimental effect on the larval and/or juvenile stages of infaunal polychaetes.

No evidence concerning the specific effects of chemical contaminants on Nephtys hombergii was found. Boon et al. (1985) reported that Nephtys species in the North Sea accumulated organochlorines but, based on total sediment analyses, organochlorine concentrations in Nephtys species were not correlated with the concentrations in the (type of) sediment in which they inhabited.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

No Evidence is available on which to assess this pressure.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

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) (deoxygenation pressure definition).

Evidence

Connor et al. (1997a) described sediments in which Aphelochaeta marioni is commonly found as usually with a "black anoxic layer close to the sediment surface". In Richards Bay Harbour, South Africa, Aphelochaeta marioni is found in dissolved oxygen ranges from 3.9 to 8.8 mg/l at a depth range of 0.2 to 16.1 meters, including one site with hypoxic conditions, where a very low dissolved oxygen of 0.5 mg/l was recorded (Izegaegbe, Vivier & Mzimela, 2020). Broom et al. (1991) recorded that Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated mud in the Severn Estuary. They found Aphelochaeta marioni to be dominant where the redox potential at 4 cm sediment depth was 56 mV and, therefore, concluded that the species was tolerant of very low oxygen tensions. It is likely that feeding, growth, and reproduction would be impaired under sustained low oxygen conditions.

Oligochaetes, such as Tubificoides spp. vary in their tolerance of hypoxia and associated high sulphide levels. Most enchytraeids and naidids are sensitive to hydrogen sulphide and hypoxia, while tubificids are often more resistant (Giere, 2006). Tubificoides benedii has a high capacity to tolerate anoxic conditions. Its extreme oxygen tolerance is based on an unusually low respiration rate (Giere et al., 1999). Respiration rates of Tubificoides benedii measured at various oxygen concentrations showed that aerobic respiration is maintained even at very low oxygen concentrations (Giere et al., 1999). Birtwell & Arthur (1980) showed that Tubificoides benedii could tolerate anoxia in the Thames Estuary, England (LT₅₀ = 58.8 hours at 20^oC, 26.6 hours at 25^oC and 17.8 hours at 30^oC in experiments with worms acclimated to 20^oC). Tubificoides spp. are found in fine silt, sand, or mud, in hypoxic conditions in southern Spain (intertidal) (Kvist & Erséus, 2018). In the Gulf of Finland (Baltic Sea), Tubificoides pseudogaster were recorded in oxygen concentrations of bottom water between 2.75 and 4.85 ml/l (Berezina, Maximov & Vladimirova, 2019). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in oxygen concentrations between 7.44 and 10.84 mg/l (Lange et al., 2020).

Sensitivity assessment

The characterizing species are likely to only be affected by severe deoxygenation episodes. Resistance to deoxygenation at the pressure benchmark level is likely to be ‘High’. Resilience of the biotopes is likely to also be ‘High’, and the biotopes are therefore considered ‘Not Sensitive’ to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week.

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

Raman & Ganapati (1983) studied the distribution of Aphelochaeta marioni (studied as Tharyx marioni) in relation to a sewage outfall in Visakhapatnam Harbour, Bay of Bengal. Increased nutrients often derive from sewage inputs, and the presence of species such as Aphelochaeta marioni in such situations (for instance, Broom et al., 1991) may reflect tolerance to high nutrients or to de-oxygenated conditions or both. Aphelochaeta marioni was found to be dominant in the 'semi-healthy zone' characterized by low nutrients (nitrate 0.02 mg/l, phosphate 0.88 mg/l). Aphelochaeta marioni was not found in high numbers in the polluted zone close to the sewage outfall, characterized by high nutrients (nitrate 0.042 to 0.105 mg/l, phosphate 2.35 to 3.76 mg/l) (Raman & Ganapati, 1983). This would suggest that Aphelochaeta marioni is intolerant of eutrophication. However, it would be expected that an increase in organic nutrients would lead to increased food availability for the deposit-feeding Aphelochaeta marioni. Furthermore, Dauvin (1982, 2000) recorded an increase in abundance of Aphelochaeta marioni following an oil spill, which resulted in an explosion of plant growth due to high mortality of grazers. Therefore, the available evidence on the resistance of Aphelochaeta marioni to nutrient changes does not allow consistent conclusions to be drawn.

Oligochaetes can dominate assemblages in nutrient-enriched tidal sediments (Gray, 1971; Leppäkoski, 1975; Birtwell & Arthur, 1980). In the Gulf of Finland (Baltic Sea), Tubificoides pseudogaster were recorded in soluble reactive phosphates (P-PO₄³) between 1.45 and 3.97 µmol/l (Berezina, Maximov & Vladimirova, 2019). No other evidence for Tubificoides spp. or Oligochaetes was found.

Sensitivity assessment

The above evidence suggests that nutrient enrichment may reduce the abundance of Aphelochaeta marioni. There is insufficient evidence of Tubificoides spp. in nutrient-enriched conditions. Therefore, in the case of SS.SMu.SMuVS.AphTubi, resistance to nutrient enrichment is assessed as ‘Medium’, but with ‘Low’ confidence, based on the fact that Aphelochaeta marioni was not found in high numbers in the polluted zone close to a sewage outfall (Raman & Ganapati, 1983), but that an increase in organic nutrients would likely lead to increased food availability for the deposit-feeding species. Resilience is assessed as 'Medium', so the biotope is assessed as ‘Medium’.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr (Organic enrichment pressure definition).

Evidence

Kędra et al. (2010) reported Aphelochaeta mariori that occurred in the Hornsundfjord, Svalbard, where primary production has been recorded as 120 gC/m²/yr. Covazzi Harriague et al. (2007) reported Aphelochaeta mariori at sites with organic matter sedimentation as high as 359 mg/m²/hr in the Ligurian Sea, Italy. Furthermore, Markert et al. (2010) compared macrofaunal communities in the Wadden Sea in reefs dominated by Mytilus edulis and Cassostrea gigas and found Aphelochaeta mariori as a dominant species throughout the study site, which suggested the species was unlikely to be affected by organic enrichment.

Tubificoides benedii and Baltidrilus costatus are both very tolerant of high levels of organic enrichment and often dominate sediments where sewage has been discharged, or other forms of organic enrichment have occurred (Pearson & Rosenberg, 1978; Gray, 1971; McLusky et al., 1980). Their tolerance for organic enrichment is attributed to their adaptation to live in and feed on enriched organic deposits (Pearson & Rosenberg, 1978), and their high population densities in such areas are enhanced by the lack of predation and competition. Tubificoides benedii are abundant in mussel beds (mussel relaying may be the source of smothering), which has been attributed to their tolerance of organically rich, deoxygenated sediment (Commito & Boncavage, 1989). Tubificoides benedii has also been found in elevated abundances in areas of organic enrichment around fish farms (Haskoning, 2006). In the Gulf of Finland (Baltic Sea), Tubificoides pseudogaster were recorded in sediment with a particulate organic carbon level between 1.2 and 4.1% (Berezina, Maximov & Vladimirova, 2019).

Sensitivity assessment

The evidence presented suggests the characterizing species may not be affected by organic enrichment at the benchmark level. Resistance and resilience are therefore assessed as ‘High’, and the biotopes are considered ‘Not Sensitive’ to organic enrichment (deposit of 100 gC/m²/yr).

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

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 (Physical change in subtratum type pressure definition).

Evidence

If the cohesive mud and sandy mud that characterize these biotopes were replaced with rock substrata, this would represent a fundamental change to the physical character of the biotopes. The characterizing species would no longer be supported, and the biotopes would be lost and/or reclassified.

Sensitivity assessment

Resistance to the pressure is considered 'None' and resilience 'Very Low'. Sensitivity has been assessed as '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 sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

Aphelochaeta marioni has been recorded from a variety of different sediment types. In the intertidal area of the Wadden Sea, it achieved the highest abundance where the sediment fraction smaller than 0.04 mm in diameter was greater than 10% of the total sediment (Farke, 1979).

In Richards Bay Harbour, South Africa, a study was conducted to examine the status of the macrobenthic community structure. Sediment and benthic samples were collected quarterly (summer, autumn, winter and spring) using a Van Veen grab (area size of 0.0236 m²) between 2016 and 2017 from seven sites in the harbour to represent different habitat types, with one site being a reference site (site 7). Most of the habitat types had sediment compositions ranging from very fine sand to mud, and the implication of this difference in sediment substrata was profound in the benthic abundance and distribution (Izegaegbe, Vivier & Mzimela, 2020). Analyses of the benthic fauna identified 98 taxa, of which the tanaid Halmyrapseudes digitalis, the polychaetes Mediomastus capensis, Aphelochaeta marioni and Prionospio sexoculata, and the brachyuran crab Paratylodiplax blephariskios were numerically abundant, accounting for 79% of the organisms recorded (Izegaegbe, Vivier & Mzimela, 2020). Aphelochaeta marioni specifically made up 17.81% of the organisms recorded (Izegaegbe, Vivier & Mzimela, 2020). In detail, site 2 (8,814 ind.), 3 (10,678 ind.), and 7 (7,331 ind.) had the most Aphelochaeta marioni, which was the intertidal mudflat (Izegaegbe, Vivier & Mzimela, 2020). The other sites being the deep (15 m) muddy channel (made from a mix of mud and sand) (site 1 and 6; 3,771 and 2,712 ind. respectively) and the mid channel (muddy) (site 4; 3,136 ind.), and the sandflat (characterized by fine sand and low organic content, largely due to the influence of wave action and currents) (site 5; 508 ind.) (Izegaegbe, Vivier & Mzimela, 2020).

Tubificoides benedii (studied as Peloscolex benedeni) is found in a range of substratum types from sandy mixed habitats, fine sands, and coarse sands (Giere & Pfannkuche, 1982). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in mixed soft sediments. Sediment at all studied sites was mostly made up of sand (84.17 to 99.12%), followed by mud (0.51 to 14.53%), and then gravel (0.03 to 0.47%) (Lange et al., 2020). Furthermore, as elevation and sediment shifted, so did benthic assemblages, with the tidal flat dominated by Tubificoides benedii compared to the pioneer zone, lower salt marsh, and upper salt marsh (Lange et al., 2020).

Sensitivity assessment

The characterizing species of these biotopes occur in a range of substrata and are likely to resist a change in sediment type by 1 Folk class, for example, sandy mud to muddy sand (based on the Long, 2006 simplification). However, this would probably represent a fundamental change in the character of the biotopes, and a change in the abundance of the characteristic species, resulting in the loss and/or reclassification of the biotopes. Resistance is therefore assessed as ‘None’ and resilience as ‘Very low’ given the permanent nature of the pressure, and the biotopes are considered to have ‘High’ sensitivity to a change in seabed type by one Folk class.

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) (Removal of substratum pressure definition). 

Evidence

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation and will expose underlying sediment, which may be anoxic and/or of a different character or bedrock and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment/water interface, may be exposed to conditions to which they are not suited. Removal of 30 cm of surface sediment will remove the infaunal community and other species present in the biotopes. For instance, Aphelochaeta marioni lives buried in soft sediments, with the majority of individuals found in the upper 4 cm of the sediment (Rayment, 2007a). Removal of the substratum to 30 cm would result in the loss of the characterizing species. Recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Hydrodynamics and sedimentology (mobility and supply) influence the recovery of soft sediment habitats (Van Hoey et al., 2008).

Sensitivity assessment

Extraction of 30 cm of sediment will remove the characterizing biological component of the biotopes. Resistance of the biotopes is assessed as 'None'. Resilience of SS.SMu.SMuVS.AphTubi is likely to be 'Medium' given the low-energy environment where the biotope occurs, whereas the resilience of SS.SMu.SMuVS.NhomTubi is likely to be 'High'. Both biotopes sensitivity are therefore assessed as 'Medium'.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

Evidence

Aphelochaeta marioni is a soft-bodied organism which exposes its palps and cirri at the surface while feeding (Rayment, 2007a). The species lives infaunally in soft sediment, usually within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would be likely to penetrate the upper few centimetres of the sediment and cause physical damage to Aphelochaeta marioni. De Biasi & Pacciardi (2008) compared macrobenthic communities in a commercial fishing ground exploited by otter trawling with an area closed to fishing for over 10 years in the Adriatic Sea. The authors found that polychaetes, including Aphelochaeta spp., were among the species dominating the disturbed areas, which is likely to result from the ability of the species to recolonize disturbed areas rapidly, rather than indicate that the polychaetes are resistant to disturbance of the seabed surface.

In Poole Harbour, UK, a similar result was observed. Local fishermen utilise a unique ‘pump-scoop’ dredge to harvest the introduced Manila clam Ruditapes philippinarum. A Before-After-Control-Impact sampling design was used to assess the impacts of pump-scoop dredging on benthic physical characteristics and community structure in an area where there was no dredging, an area newly opened to dredging and an area subject to high levels of historic dredging (Clarke et al., 2018). A sampling grid was used in each area to best capture any fishing effort in the newly opened area, and core samples were taken to a depth of 30 cm within intertidal mudflats. A significant loss of fine sediments was observed in the site subject to high-intensity dredging, and a significant change in community structure occurred in both dredged sites throughout the study period (Clarke et al., 2018). In the newly opened site, this was characterized by a relative increase in species richness, including increased abundance of annelid worms, notably Hediste diversicolor and Aphelochaeta marioni and a decline in the abundance of the bivalve mollusc Abra tenuis (Clarke et al., 2018). Clarke et al. (2018) concluded that these changes, albeit relatively small, are attributed to physical disturbance as a direct result of pump-scoop dredging, and despite the significant change in community structure at the newly opened site, no difference in the classification of the biotope or ecological quality of either of the dredged sites was observed. 

Tubificoides benedii can be relatively deeply buried (6 to 8 cm; Bolam, 2011) and could avoid direct exposure to abrasion, although sediment disturbance and compaction could damage these soft-bodied species and oligochaetes in general are not found in high abundances in sediments with high levels of disturbance from wave action.

Chandrasekara & Frid (1996) investigated the impacts of trampling on the benthic infauna of Lindisfarne tidal flats. During the five months in summer, about 10,000 pilgrims typically walk along a traditional path through the mudflat to access the holy site of Lindisfarne (approximately equivalent to ca 50 individuals a day). A transect was positioned perpendicular to the footpath. Five quadrats were sampled, the third being positioned on the path centre. Chandrasekara & Frid (1996) found that repeated trampling on the path during the summer had a significant impact on the benthic community. The abundance of several species reduced on the path, while others increased in abundance. Chandrasekara & Frid (1996) noted that some species reduced in abundance on the pilgrim’s path (Capitella capitata and Scoloplos armiger) while several of the dominant taxa (Tubificoides pseudogaster, Clitello arenarius, Enchytraeus buchholtzi, Pygospio elegans and nematodes) significantly increased in abundance in the face of high levels of trampling, probably due to rapid recruitment and growth of more opportunistic species, even though their population experienced mortality. During winter, the benthic community of the path was not significantly different from other samples. The authors suggested the dominant taxa probably also experienced mortality, but that the observed increases in abundance may have been due to several factors: (1) rapid recruitment of adult stages, (2) trampling stimulating bacterial growth on organic matter, thereby providing food for deposit-feeding infauna, and (3) an additional food source from animals killed/injured by trampling (Chandrasekara & Frid, 1996).

Experimental studies on crab-tiling impacts have found that densities of Tubificoides benedii and Tubificoides pseudogaster were higher in non-trampled plots (Sheehan et al. 2010), indicating that these oligochaetes have some sensitivity to trampling. Whomersley et al. (2010) conducted experimental raking on intertidal mudflats at two sites (Creeksea-Crouch estuary, England, and Blackness-Lower Forth estuary, Scotland), where Tubificoides benedii were the dominant species. For each treatment, 1 m² plots were raked twice to a depth of 4 cm (using a garden rake). Plots were subject to either low-intensity treatments (raking every four weeks) or high-intensity treatments (raking every two weeks). The experiment was carried out for 10 months at Creeksea and for a year at Blackness. The high and low raking treatments appeared to have little effect on Tubificoides benedii (Whomersley et al., 2010).

Sensitivity assessment

As a soft-bodied species, the characterizing species are likely to be crushed and killed by an abrasive force or physical blow, and the biotopes will be impacted if damage to seabed surface features is widespread. However, motile and opportunistic fauna, such as Tubificoides benedii, may recover quickly. Resistance is assessed as 'Low', and resilience is assessed as 'High', providing a sensitivity assessment of 'Low'.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat) (Sub-surface penetration pressure definition).

Evidence

Activities that penetrate below the surface would remove a significant proportion of the characterizing species within the direct area of impact (see evidence under ‘abrasion’ above). However, it is expected that the community is likely to be able to re-establish rapidly.

For example, in Poole Harbour, UK, local fishermen utilise a unique ‘pump-scoop’ dredge to harvest the introduced Manila clam Ruditapes philippinarum. A Before-After-Control-Impact sampling design was used to assess the impacts of pump-scoop dredging on benthic physical characteristics and community structure in an area where there was no dredging, an area newly opened to dredging and an area subject to high levels of historic dredging (Clarke et al., 2018). A sampling grid was used in each area to best capture any fishing effort in the newly opened area, and core samples were taken to a depth of 30 cm within intertidal mudflats. A significant loss of fine sediments was observed in the site subject to high-intensity dredging, and a significant change in community structure occurred in both dredged sites throughout the study period (Clarke et al., 2018). In the newly opened site, this was characterized by a relative increase in species richness, including increased abundance of annelid worms, notably Hediste diversicolor and Aphelochaeta marioni and a decline in the abundance of the bivalve mollusc Abra tenuis (Clarke et al., 2018). Clarke et al. (2018) concluded that these changes, albeit relatively small, are attributed to physical disturbance as a direct result of pump-scoop dredging, and despite the significant change in community structure at the newly opened site, no difference in the classification of the biotope or ecological quality of either of the dredged sites was observed. 

Sensitivity assessment

Activities that penetrate below the surface would remove a significant proportion of the characterizing species within the direct area of impact (see evidence under ‘abrasion’ above). Resistance of the biotopes is therefore assessed as ‘Low’, although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint. Resilience is assessed as ‘High’, and sensitivity is assessed as ‘Low’.

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 (Suspended sediment pressure definition).

Evidence

The biotopes are probably exposed to the high levels of suspended sediment characteristic of estuarine conditions. Therefore, the resident species are probably adapted to high suspended sediment levels. Aphelochaeta marioni lives infaunally and is a surface deposit feeder (Rayment, 2007a), therefore relying on a supply of nutrients at the sediment surface. An increased rate of siltation may result in an increase in food availability and therefore growth and reproduction of Aphelochaeta marioni. However, food availability would only increase if the additional suspended sediment contained a significant proportion of organic matter, and the population would only be enhanced if food was previously limiting. A decrease in the suspended sediment would result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for Aphelochaeta marioni. This would be likely to impair growth and reproduction.

Tubificoides benedii displays preference for fine organic-enriched sediments and may, therefore, increase in abundance if suspended solids settle and lead to increased organic enrichment (Gogina et al., 2010).

Sensitivity assessment

Changes in light penetration or attenuation associated with this pressure are not relevant to Nephtys hombergii, Aphelochaeta marioni and Tubificoides benedii. As the species live in the sediment, they are also likely to be adapted to increased suspended sediment (and turbidity). However, alterations in the availability of food or the energetic costs in obtaining food or changes in scour could either increase or decrease habitat suitability for these characterizing species. The following sensitivity assessment relies on expert judgement, utilizing evidence of species traits and distribution; therefore, confidence has been assessed as low. Resistance is 'High' as no significant negative effects are identified and potential benefits from increased food resources may occur. Resilience is also 'High', as no recovery is required under the likely impacts. Sensitivity of the biotopes is therefore assessed as 'Not Sensitive'.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Aphelochaeta marioni lives infaunally in soft sediments and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. Kędra et al. (2010) reported Aphelochaeta mariori to occur in the Hornsundfjord, Svalbard, where sedimentation rates can vary between 0.1 and 35 cm/yr. Furthermore, Do et al. (2012) studied the macrobenthos recovery in the Arcachon Bay (France) following a deposition of sediment up to 10 cm thick that resulted from dredging activities. The authors reported Aphelochaeta marioni as considerably reduced or absent from impacted areas characterized mainly by mud substrata.

Tubificoides benedii displays a preference for fine organic-enriched sediments and may, therefore, increase in abundance if suspended solids settle and lead to increased organic enrichment (Gogina et al., 2010). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in different zones of the tidal flats depending on elevation and sedimentation. The island of Spiekeroog was the focal point of the study, which is a region characterized by sandy and mixed tidal flats passing into salt marshes with a high mud content (Flemming and Davis Jr., 1994 and Flemming and Ziegler, 1995, cited in Lange et al., 2020). Its morphology is subject to semidiurnal tides and storm waves flowing in from the North-West via the tidal inlet “Otzumer Balje” (Hertweck, 1995, cited in Lange et al., 2020). At the lowest elevation, the community became more diverse over time, including marine and terrestrial taxa, and at the middle elevation, the community shifted from a marine species inventory over an opportunist-dominated assemblage to a less diverse community dominated by Enchytraeidae (Oligochaeta) (Lange et al., 2020). The community at the highest elevation mostly remained constant, resembling the second, opportunistic community from the middle elevation that was dominated by the snail Peringia ulvae (Pennant, 1777) and the oligochaetes Enchytraeidae spp. and Tubificoides benedii (d'Udekem, 1855) (Lange et al., 2020). Overall, as elevation and sediment shifted, so did benthic assemblages. Tidal flats were dominated by Tubificoides benedii, being most abundant in the pioneer zone and decreasing as elevation increased up towards the lower and upper salt marsh; thus, demonstrating the negative impacts of increased sediment elevation on macrofaunal biodiversity and functional diversity, originating from reduced inundation and vegetation establishment (Lange et al., 2020).

Bolam (2011) conducted a field experiment to examine the vertical migration of benthic macrofauna in response to overburden by simulated dredged spoil, in the Crouch Estuary, UK. Experimental, in situ, cores were treated with 6 cm or 16 cm of different sediment types for 4 days: low (native defaunated sediment), high org (organically enriched native sediment), high sand (native sediment plus sand), and high/high (native sediment with added sand and organic matter). A large proportion of the Tubificoides benedii in the cores did not migrate through 6 cm, but the 20% that did were restricted to the low organic content treatments. Three of the four species examined (that included Tubificoides benedii) did not migrate up through any of the 16 cm treatments, within the four days of the experiment. Bolam (2011) noted that the sedentary deposit feeders (Streblospio shrubsolii and Tharyx sp.) have poor migration ability; the errant deposit feeder Tubificoides benedii coped to a degree with 6 cm of overburden.

Sensitivity assessment

Polychaete species have been reported to migrate through depositions of sediment greater than the benchmark (5 cm of fine material added to the seabed in a single discrete event) (Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. For example, Bolam (2011) noted that the majority of Tubificoides benedii did not migrate upwards through 6 cm of deposited sediment but did not document any resultant mortality. Therefore, some mortality of the characterizing species is likely to occur. Placement of the deposit will result in a defaunated habitat until the deposit is recolonized. However, alterations in the availability of food or the energetic costs in obtaining food or changes in scour could either increase or decrease habitat suitability for these characterizing species. The following sensitivity assessment relies on expert judgement, utilizing evidence of species traits and distribution and therefore confidence has been assessed as low. Resistance is ‘High’ as no significant negative effects are identified and potential benefits from increased food resources may occur. Resilience is also ‘High’ as no recovery is required under the likely impacts. Sensitivity of the biotopes is therefore assessed as ‘Not Sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Aphelochaeta marioni lives infaunally in soft sediments and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. Kędra et al. (2010) reported Aphelochaeta mariori to occur in the Hornsundfjord, Svalbard, where sedimentation rates can vary between 0.1 and 35 cm/yr. Furthermore, Do et al. (2012) studied the macrobenthos recovery in the Arcachon Bay (France) following a deposition of sediment up to 10 cm thick that resulted from dredging activities. The authors reported Aphelochaeta marioni as considerably reduced or absent from impacted areas characterized mainly by mud substrata.

Tubificoides benedii displays a preference for fine organic-enriched sediments and may, therefore, increase in abundance if suspended solids settle and lead to increased organic enrichment (Gogina et al., 2010). In the Wadden Sea National Park of Lower Saxony (southern North Sea), Germany, Tubificoides spp. have been observed in different zones of the tidal flats depending on elevation and sedimentation. The island of Spiekeroog was the focal point of the study, which is a region characterized by sandy and mixed tidal flats passing into salt marshes with a high mud content (Flemming and Davis Jr., 1994 and Flemming and Ziegler, 1995, cited in Lange et al., 2020). Its morphology is subject to semidiurnal tides and storm waves flowing in from the North-West via the tidal inlet “Otzumer Balje” (Hertweck, 1995, cited in Lange et al., 2020). At the lowest elevation, the community became more diverse over time, including marine and terrestrial taxa, and at the middle elevation, the community shifted from a marine species inventory over an opportunist-dominated assemblage to a less diverse community dominated by Enchytraeidae (Oligochaeta) (Lange et al., 2020). The community at the highest elevation mostly remained constant, resembling the second, opportunistic community from the middle elevation that was dominated by the snail Peringia ulvae (Pennant, 1777) and the oligochaetes Enchytraeidae spp. and Tubificoides benedii (d'Udekem, 1855) (Lange et al., 2020). Overall, as elevation and sediment shifted, so did benthic assemblages. Tidal flats were dominated by Tubificoides benedii, being most abundant in the pioneer zone and decreasing as elevation increased up towards the lower and upper salt marsh; thus, demonstrating the negative impacts of increased sediment elevation on macrofaunal biodiversity and functional diversity, originating from reduced inundation and vegetation establishment (Lange et al., 2020).

Bolam (2011) conducted a field experiment to examine the vertical migration of benthic macrofauna in response to overburden by simulated dredged spoil, in the Crouch Estuary, UK. Experimental, in situ, cores were treated with 6 cm or 16 cm of different sediment types for 4 days: low (native defaunated sediment), high org (organically enriched native sediment), high sand (native sediment plus sand), and high/high (native sediment with added sand and organic matter). A large proportion of the Tubificoides benedii in the cores did not migrate through 6 cm, but the 20% that did were restricted to the low organic content treatments. Three of the four species examined (that included Tubificoides benedii) did not migrate up through any of the 16 cm treatments, within the four days of the experiment. Bolam (2011) noted that the sedentary deposit feeders (Streblospio shrubsolii and Tharyx sp.) have poor migration ability; the errant deposit feeder Tubificoides benedii coped to a degree with 6 cm of overburden.

Sensitivity assessment

The pressure benchmark (30 cm deposit) represents a significant burial event, and the deposit may remain for some time in a sheltered mudflat. Some impacts on some characterizing species may occur, and it is considered unlikely that significant numbers of the population could reposition, based on Bolam (2011). Polychaete species have been reported to migrate through depositions of sediment greater than the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Placement of the deposit will, therefore, result in a defaunated habitat until the deposit is recolonized. However, alterations in the availability of food or the energetic costs in obtaining food or changes in scour could either increase or decrease habitat suitability for these characterizing species. The following sensitivity assessment relies on expert judgement, utilizing evidence of species traits and distribution and therefore confidence has been assessed as low. Resistance is assessed as ‘Low’ and resilience as ‘High’, and the biotopes are considered to have ‘Low’ sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

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 the characterizing species. 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 a 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

None of the species in the biotopes are likely to be sensitive to noise or vibration at the benchmark level, and no information was found concerning intolerance to noise. Species within the biotopes can probably detect vibrations caused by noise and, in response, may retreat into the sediment for protection. However, the characterizing species are unlikely to be affected by noise pollution, and so the biotopes are assessed as Not Sensitive.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

Changes in light availability impact primary production by phytoplankton in the water column and by the microphytobenthos on the sediment surface, which in turn may affect food availability for the characterizing species.

Farke (1979) noted that Aphelochaeta marioni is intolerant to visual disturbance in a microsystem in the laboratory, possibly due to its nocturnal life habits (Farke, 1979). In order to observe feeding and breeding in the microsystem, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Visual disturbance, in the form of direct illumination during the species' active period at night, may therefore result in loss of feeding opportunities, which may compromise growth and reproduction.

Since 2016, research on artificial light at night (ALAN) has expanded considerably in the marine and coastal environment. Light was previously assumed to be of low ecological significance in subtidal and intertidal habitats, but there is now evidence that ALAN is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organisation. Documented responses include disruption of diel and circalunar rhythms, changes in activity and foraging, altered predator–prey interactions, shifts in community composition, and impacts on algal growth and phenology (Davies et al., 2014, 2015; Gaston et al., 2017; Tidau et al., 2021; Lynn et al., 2022; Marangoni et al., 2022; Miller & Rice, 2023; Ferretti et al., 2025). Evidence for benthic habitats and assemblages specifically is beginning to emerge (e.g. Trethewy et al., 2023; Schaefer et al., 2025), but remains limited and fragmented, often focusing on single taxa or short-term experiments. Mortality thresholds, long-term consequences, and responses at the biotope scale are rarely addressed, and there are major gaps around indirect effects such as trophic cascades or habitat modification.

Sensitivity assessment

Although the characterizing species can perceive light, this pressure is not considered relevant as SS.SMu.SMuVS.AphTubi is not directly dependent on sunlight. In addition, 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 (Barrier to species movement pressure definition).

Evidence

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 (Death for collision pressure definition).

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 (Visual disturbance pressure definition). 

Evidence

Most species will respond to the shading caused by the approach of a predator, however, their visual acuity is probably very low. Farke (1979) noted that Aphelochaeta marioni was intolerant of visual disturbance in a microsystem in the laboratory, possibly due to its nocturnal life habits (Farke, 1979). In order to observe feeding and breeding in the microsystem, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Most aquatic oligochaetes, such as Tubificoides spp., have no eyes, although a few have simple ocelli (eyespots) which are light receptors. However, as the characterizing species live in the sediment, the species will most probably not be impacted at the pressure benchmark.

Sensitivity assessment

At the pressure benchmark, resistance is assessed as 'High' and resilience as 'High', and the biotopes are assessed as 'Not sensitive'.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

Evidence

The important characterizing species in the biotopes are not cultivated or likely to be translocated. This pressure is therefore considered Not Relevant.

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) (pathogen or disease pressure definition).

Evidence

Introduced organisms (especially parasites or pathogens) are a potential threat in all coastal ecosystems. Little information was found regarding microbial infection of polychaetes, although Gibbs (1971) recorded that nearly all of the population of Aphelochaeta marioni in Stonehouse Pool, Plymouth Sound, was infected with a sporozoan parasite belonging to the acephaline gregarine genus Gonospora, which inhabits the coelom of the host. No evidence was found to suggest that gametogenesis was affected by Gonospora infection and there was no apparent reduction in fecundity.

Marine oligochaetes host numerous protozoan parasites without apparent pathogenic effects even at high infestation levels (Giere & Pfannkuche, 1982).

Sensitivity assessment

The biotopes are judged to have 'High' resistance to this pressure. By default, resilience is assessed as 'High', and the biotopes are classed as 'Not Sensitive'.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale (targeted removal pressure definition).

Evidence

Nephtys hombergii is directly removed through commercial bait digging and by recreational anglers, and its abundance significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) had occurred (Watson et al., 2007).

Recovery of Nephtys hombergii has been assessed to be very high, as repopulation would initially occur relatively rapidly via adult migration and later by larval recruitment. Dittman et al. (1999) observed that Nephtys hombergii was amongst the macrofauna that colonized experimentally disturbed tidal flats within two weeks of the disturbance that caused defaunation of the sediment. However, if sediment is damaged recovery is likely to be slower, for instance Nephtys hombergii abundance was reduced by 50% in areas where tractor-towed cockle harvesting was undertaken on experimental plots in Burry Inlet, South Wales, and had not recovered after 86 days (Ferns et al., 2000).

Sensitivity assessment

Resistance is 'Low' for SS.SMu.SMuVS.NhomTubi due to direct removal of a characterizing species, Nephtys hombergii, that on commercial scales can remove a large proportion of the population. For instance, Neves de Carvalho et al. (2013) calculated that bait digging activities in the Douro estuary, Portugal, may remove up to 9.9 tonnes of Hediste diversicolor. Resilience is assessed as 'Medium', as regions that are not regularly harvested may recover rapidly, but continued harvesting will impact the population. Sensitivity of SS.SMu.SMuVS.NhomTubi is assessed as 'Medium'. It is important to consider that the spatial extent and duration of harvesting is important to consider when assessing this pressure, as smaller scale extraction may not impact the entire extent of the biotope, but greater scale extraction over a long period would cause longer-term impacts.

On the other hand, it is extremely unlikely that species indicative of sensitivity of SS.SMu.SMuVS.AphTubi would be targeted for extraction, so this pressure is therefore considered 'Not Relevant' for SS.SMu.SMuVS.AphTubi.

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 (non-targeted removed pressure definition).

Evidence

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in the biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Hall & Harding (1997) demonstrated that commercial cockle harvesting by suction dredging had significant effects on soft-sediment infaunal communities. Following dredging, species numbers were reduced by up to 30% and abundances by up to 50%.

Collie et al. (2000) identified that well-established intertidal communities suffered the greatest impact from bottom-towed fishing activities. The review concluded that there were ecologically important impacts from removal of >50% of fauna from bottom-towed fishing activity (dredge and trawls) (Collie et al., 2000). However, the burrowing and tunnelling traits of the characterizing species may provide some resistance to this pressure. Kaiser et al. (2001) carried out experimental hand raking, similar to that used in intertidal cockle fisheries. Both small and large raked plots showed changed communities in comparison to control plots, smaller plots recovered in 56 days, whilst larger plots remained in an altered state.

Collie et al. (2000) found that Nephtys hombergii abundance was negatively affected by fishing activities. Mean response of infauna and epifauna communities to fishing activities was also much more negative in mud and sand communities than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) had occurred (Watson et al., 2007). Similarly, Nephtys hombergii abundance was reduced by 50% in areas where tractor-towed cockle harvesting was undertaken on experimental plots in Burry Inlet, South Wales, and had not recovered after 86 days (Ferns et al., 2000).

Aphelochaeta marioni is a soft-bodied organism which exposes its palps and cirri at the surface while feeding (Rayment, 2007a). The species lives infaunally in soft sediment, usually within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would be likely to penetrate the upper few centimetres of the sediment and cause physical damage to Aphelochaeta marioni.

Sensitivity assessment

Resistance is 'Low' due to direct removal or damage of characterizing species that, on commercial scales, can remove a large proportion of the population. Resilience is assessed as 'Medium' as regions that are not regularly harvested may recover rapidly, but continued harvesting will impact the population. Sensitivity is assessed as 'Medium'. It is important to consider that the spatial extent and duration of areas impacted by fishing gear effort is important to consider when assessing this pressure, as smaller-scale extraction may not impact the entire extent of the biotopes, but greater-scale extraction over a long period would cause longer-term impacts. The type of fishing activity is also important to consider in relation to the type and severity of the impact.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and on the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40), although it prefers ca 30 (Tillin et al., 2020). 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 in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020; Fitzgerald, 2007; Blanchard, 2009; Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and, once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015; Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy spaces between or within shells, resulting in a homogeneous Crepidula-dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²), but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula , with only moderate or low abundances (11 ind./m²), and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm), but while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action, which may be less suitable (Tillin et al., 2020). 

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogeneous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly by burial due to deposition, which could mitigate Crepidula density.

Sensitivity assessment

The above evidence suggests that Crepidula could colonize cohesive mud habitats where shells, stones, or any other hard substrata can be used for larval settlement (Tillin et al., 2020). Crepidula has the potential to colonize and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003), resulting in the loss of the biotope. This is an extremely sheltered to moderately exposed habitat, which is suitable for Crepidula. There is evidence of Crepidula occurring within this biotope, but it is unclear how much Crepidula is recorded and if the slipper limpet will come to dominate. The relative abundance of hard substrata might limit the spread of Crepidula, and the possibility of burial in a high deposition habitat might limit its abundance. Therefore, resistance is assessed as 'Medium' on the basis that Crepidula shells will provide a hard substratum for the expansion of Crepidula populations. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, the biotope is assessed as 'Medium'. Further evidence is required on the presence of Crepidula within this biotope, so the confidence in the assessment is 'Low'.

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). A lack of published descriptions and an incomplete historical record has led to the widespread misidentification of Didemnum vexillum, and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; McKenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan, with its native range possibly extending into continental Asia and the north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

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; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024). Zhang et al. (2020) suggested that in the current climate conditions (based on depth, current, temperature and salinity) Didemnum vexillum had not yet occupied their predicted suitable habitats, and predicted that the Northern Atlantic coast is susceptible to invasion by Didemnum vexillum and that climate change could cause a poleward expansion of Didemnum vexillum.

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 localised 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 from 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

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

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffiths 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 the 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 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, 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 winter 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 winter (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in the 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 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and establishment. 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). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders (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). The extensive mats formed by the invasive species over cobble-pebble substrata can bind or ‘glue’ small pebbles and cobbles together by filling spaces between the sediment particles, which alters the habitat complexity of the seafloor, turning it into a more homogeneous two-dimensional habitat rather than a heterogeneous three-dimensional one (Griffith et al., 2009; Mercer et al., 2009; Lengyel et al., 2009).

In addition, Didemnum vexillum is commonly found associated with artificial hard substrata, being mostly found in harbours and marinas where it covers a variety of maritime structures such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ships' hulls and at aquaculture facilities (Valentine et al., 2007 a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012). Didemnum vexillum was abundant in the marinas at Terschelling, Texel, and Vlieland, in the Wadden Sea (Gittenberger et al., 2015). In the UK, Didemnum vexillum was initially recorded in marinas and adjacent shallow man-made structures (Tillin et al., 2020). In Wales, it was first recorded in Holyhead Marina, then subsequently reported in Plymouth Marina and other marinas around the UK (Griffith et al., 2009; Minchin & Nunn, 2013; Bishop et al., 2015).

Didemnum sp. can colonize both horizontal and vertical surfaces of fouling and benthic communities, commonly occurring on upper horizontal surfaces in benthic habitats (Dijkstra et al., 2007; Tillin et al., 2020). It has been recorded on overhangs or the underside of boulders (Hitchin, 2012) or on the underside of docks, boat hulls, and pontoons (Griffiths et al., 2009; Minchin & Nunn, 2013). In sheltered areas, colonies are lobed and beard-like, forming long tendrils that drop down from the underside of docks or other artificial substrata to establish new colonies if there are suitable substrata available (Valentine et al., 2007a). In areas of stronger current, colonies are low undulating mats (Valentine et al., 2007a,b).

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 & Grosholz, 2015; Vercaemer et al., 2015). Some species have been shown to tolerate overgrowth by Didemnum vexillum. Such as anemones (not specified), which were observed in high densities of 10 to 339 individuals in transects with high percentage cover of Didemnum vexillum (Lengyel et al., 2009). In the Netherlands, the sea anemone Sagartia elegans and Sabella pavonia tubes were not overgrown by Didemnum sp. (Gittenberger, 2007). Botrylloides violaceus can overgrow Didemnum sp. (Gittenberger, 2007), although it was noted to be overgrown in other studies (Valentine et al., 2007a). In addition, Styela clava and Ascidiella aspera survived overgrowth by Didemnum vexillum as long as their siphons remained free (Gittenberger, 2007). However, Gittenberger (2007) stated that the boring sponge Clione celata, the sea anemone Diadumene cincta, Mytilus edulis, Magallana (syn. Crassostrea) gigas, Ostrea edulis, a variety of hydroids, the colonial ascidians Aplidium (Fig. 4) and Diplosoma listerianum and the solitary ascidians Ciona intestinalis start to die on contact with Didemnum sp.  Didemnum vexillum can overgrow bivalve species, such as oysters, scallops and mussels, as the hard shells can provide suitable hard substrata for settlement. It has been described as a ‘shellfish pest’ by the aquaculture industry because it is likely to completely encapsulate bivalves and smother them resulting in death or partially encapsulate and partially smother them resulting in reduced bivalve growth (Auker, 2010; Bullard et al., 2007; Coutts & Forrest, 2007, Valentine et al., 2007a; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020). Didemnum vexillum has been recorded overgrowing mussels in Strangford Lough, Northern Ireland (Minchin & Nunn, 2013) and recorded forming large mats over Blue Mussel beds in the Gulf of Maine, completely covering individuals (Auker et al., 2014).

There are few observations of Didemnum vexillum on soft-bottom habitats, as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA, the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the surrounding sand ridges, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). Evidence also indicated that the species cannot survive being buried or smothered by coarse or fine-grained sediment. Furthermore, in Holyhead Marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons; they were absent from the natural seabed beneath the pontoon, composed of silty mud, and from deeper sections of mooring chains that became immersed in mud at low spring tides (Griffiths et al., 2009).

In contrast to 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. 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 are enough to cause dislodgement (Reinhardt et al., 2012). 

Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020). It is abundant across various shore heights, thriving in both nearshore and offshore sites, particularly in subtidal areas. For example, colonies of Didemnum vexillum were dominant at depths between 45 and 60 m, occupying 50 to 90% of available space in two gravelly areas (more than 230 km²) composed of immobile pebble and cobble pavement on Georges Bank fishing ground, USA (Bullard et al., 2007; Valentine et al., 2007b; Lengyel et al., 2009). In addition, patchy mats have been observed covering approximately 1 to 1.5 km² of the pebble-cobble seabed, which is interspersed with large boulders and 30 m deep in Long Island Sound, USA (Mercer et al., 2009). In an offshore scallop dredge survey, Didemnum sp. was found attached to cobbles and boulders at 10 to 34 m (Vercaemer et al., 2015). An experiment in the Thames River estuary, Connecticut, found a significant difference between Didemnum vexillum growth rates at different depths, with faster growth rates seen in shallow water (1.0 m) compared to deeper depths (4.0 m) (Bullard & Whitlatch, 2009). It was also found that although Didemnum vexillum grew faster in shallow depths during the experiment, it grew well at all depths examined (1.0 m, 2.5 m and 4.0 m) and there was no significant difference in survival between the depths.

Didemnum vexillum tolerates a wide range of environmental conditions, including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 ppt, is commonly found in marine waters around 33 ppt, but is unable to survive in salinities below 20 ppt (Bullard & Whitlatch, 2009; Groner et al., 2011; Tillin et al., 2020). It has been recorded in estuarine conditions and tidal lagoons (Dijkstra et al., 2007; Tillin et al., 2020). In the Lagoon of Venice, Didemnum vexillum is found in waters at 30 PSU. It was absent in low salinity, such as the estuary and around the salt marshes, but well established in the euhaline and tidally well-flushed zones of the Lagoon of Venice (Tagliapietra et al., 2012). Similar results were found in Connecticut and Rhode Island, where Didemnum vexillum was not found in environments with salinity less than 20 ppt (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 and 25.97 ppt (Gittenberger, 2007; Gittenberger et al., 2015). Salinity can influence the growth rates of Didemnum vexillum. For example, in an experiment in the Thames River estuary, Connecticut, Bullard & Whitlatch (2009) found growth rates were significantly higher in high salinity areas (26 to 30 ppt) and although survival at different salinities was not significantly different, the Didemnum vexillum colonies in low (10 to 26 ppt) and medium (15 to 28 ppt) salinities were bloated, discoloured and appeared to be dying. In unpublished data from Bullard & Whitlatch (2009), similar results were found in the laboratory, as most colonies appeared to be dying after one week in 20 ppt and healthy in 30 ppt.

A study on Didemnum vexillum colonies from Holyhead Marina, Isle of Anglesey, found that colony growth within a week was significantly impaired and reduced by two-thirds at lower salinities (27 PSU and 20 PSU), while in ambient Holyhead Marina salinity (34 PSU), the growth increased and surface area doubled (Groner et al., 2011). Mortality was described as negligible in colonies of Didemnum vexillum in ambient salinity (34 PSU) after two weeks. However, mortality increased as salinity decreased. At the end of the two-week experiment, 72% of invasive colonies survived in 27 PSU and 55% of colonies survived in 20 ppt (Groner et al., 2011). When exposed to severe low salinity of 10 PSU for two hours, Didemnum vexillum showed no mortality, which suggested the duration of exposure influences mortality, not the stress intensity (Groner et al., 2011). Colonies of Didemnum vexillum collected from Anglesey, Wales, experienced more mortality under severe hypo-salinity (20 PSU, 38% colonies survived) compared to moderate hypo-salinity (27 PSU, 82% colonies survived) after two weeks, showing severe hypo-salinity creates more stressful conditions for Didemnum vexillum (Lenz et al., 2011). Therefore, Didemnum vexillum can tolerate a short-term severe decline in salinity, but prolonged exposure over two weeks caused chronic stress and increases in mortality.

Didemnum vexillum is a temperate species that can survive a broad temperature range of -2 to 24°C, with an upper survival limit suggested to be 25°C (Bullard et al., 2007; Valentine et al., 2007a; Herborg et al., 2009; Kleeman, 2009; McKenzie et al., 2017; Holt, 2024). It thrives best at 14 to 20 °C, with optimal growth temperature between 14 and 18°C during summer months (May, June, September, October) (Gittenberger, 2007; Kleeman, 2009; McKenzie et al., 2017). Didemnum vexillum has been recorded surviving in 4 to 15°C in Georges Bank and 5 to 22°C in Holyhead (Bullard et al., 2007; Valentine et al., 2007b; Griffith et al., 2009). In New England, colonies tolerate temperatures as low as -2°C (Bullard et al., 2007), but reports from the Netherlands show colonies “die off” when temperatures drop below 5°C during winter months from November to April (Gittenberger, 2007; Herborg et al., 2009). Cold winter 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). Temperature changes are an important factor influencing the seasonal growth cycle and reproduction of Didemnum vexillum (Valentine et al., 2007a).

Once established, Didemnum vexillum can expand rapidly, taking over most available hard substrata. Gittenberger (2007) stated that in the Oosterschelde, Netherlands, Didemnum sp. could cover around 95% of hard substrata, leaving little space for recruitment and growth of other species. On Georges Bank, USA, Didemnum vexillum has altered the benthic community (Lengyel et al., 2009; Tillin et al., 2020). The pebble gravel substrata on Georges Bank are important to the success and survival of haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua), and the settlement of sea scallop larvae (Placopecten magellanicus). Therefore, the invasion of Didemnum vexillum and its ability to change the habitat complexity of the seafloor may, in turn, negatively impact the benthic community (Lengyel et al., 2009). In Georges Bank, Lengyel et al. (2009) analysed photographs of the seabed and suggested that Didemnum vexillum outcompeted other epifaunal and macrofaunal species. Changes were seen in hydroids, the second most abundant epifaunal species at the location, which were overgrown by the invasive tunicate and negatively correlated with the percentage cover of Didemnum vexillum (Lengyel et al., 2009). The number of non-colonial macrofauna was also negatively related to the percentage cover of Didemnum vexillum (Lengyel et al., 2009). Dredge samples revealed clear differences in benthic species composition and revealed a significant difference in species abundance before and after the colonization of Didemnum vexillum (Lengyel et al., 2009). Invasion of Didemnum vexillum also provided a new habitat for species not normally present, such as two polychaete species, Nereis zonata and Harmothoe extenuata, changing the species composition. The increase in abundance of polychaetes Nereis zonata and Harmothoe extenuata was also seen in dredge samples collected from Georges Bank (Valentine et al., 2007b).

In contrast, some studies have suggested that potentially the overgrowth of Didemnum vexillum has little impact on benthic communities. In Long Island Sound, USA, Mercer et al. (2009) found that the total abundance and richness of native epifaunal and infaunal species were either not different or significantly higher in samples taken inside Didemnum vexillum mats compared with samples collected outside the mats. While the mats did lead to subtle changes in community structure and shifts in species dominance, the authors suggested that benthic species may use Didemnum vexillum mats as a novel habitat and species living beneath the mats may use it for shelter and protection from epibenthic predators (Mercer et al., 2009). In addition, dredge samples taken from Georges Bank found 15 polychaete species and seven bivalve species living beneath the Didemnum vexillum mat (Valentine et al., 2007b). The comparisons of 85 benthic megafauna collected from dredge samples before and after Didemnum sp. became abundant in the Georges Bank fishing ground showed slight changes in abundance, but changes to the invertebrate species composition were statistically marginally insignificant (Valentine et al., 2007b).

Sensitivity Assessment

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). SS.SMu.SMuVS.AphTubi occurs on infralittoral muddy, sandy sediment that possibly contains shell debris and stones (SS.SMu.SMuVS.NhomTubi without shell and stone debris), which could provide a suitable hard substratum for colonization by Didemnum sp., as Didemnum vexillum has been recorded on clay boulders (Hitchin, 2012). 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). SS.SMu.SMuVS.AphTubi experiences moderately strong water flow (1 to 3 kn; 0.5 to 1.5 m/s) but extremely sheltered to sheltered wave exposure. SS.SMu.SMuVS.NhomTubi experiences weak to strong water flow (>1 to 6 kn; 0.5 to 3 m/s) but sheltered to moderately exposed wave exposure. The community is dominated by infauna, and it is unclear if Didemnum could colonize the biotope. Therefore, in terms of SS.SMu.SMuVS.AphTubi, 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). In addition, the muddy, sandy sediment would not provide a suitable attachment for oysters. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The infralittoral muddy, sandy sediment, combined with the low levels of light within this biotope, is considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The infralittoral muddy, sandy sediment, combined with the low levels of light within this biotope, is considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

The polychaete Marenzellaria viridis has become established in estuaries in Europe, but a recent paper on its impacts where Tubificoides were abundant did not report on oligochaete impacts (Delefosse et al., 2012). Infaunal non-natives may impact the biotope through sediment disturbance, predation, or competition for resources; however, no examples were found.

The Manila clam (Tapes philippinarium), which was introduced to Poole Harbour for aquaculture in 1998, has become a naturalised population on the intertidal mudflats (occurring at densities of 60 clams/m² in some locations within the harbour (Jensen et al. 2004, cited in Caldow et al. 2007). However, this species is unlikely to affect any of the characterizing species in these biotopes. No other information on other invasive species was found.

Sensitivity assessment

The above evidence suggests that the characteristic species in the biotope have not yet been adversely affected by other INIS-listed species, or that there is limited information available on the topic. Therefore, 'Insufficient evidence' is recorded until further evidence becomes available.