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

Little information on temperature tolerances was found. The assessment is based on the reported global distribution. The majority of records of Neopentadactyla mixta occur in the British Isles, although its distribution extends along the North and West European coasts, from the Faeroe Islands, the west coast of Norway (Molde), the Barents Sea to the Bay of Biscay (Southward & Campbell, 2006; Mazik et al., 2015; OBIS, 2016; Samyn et al., 2021). SS.SCS.CCS.Nmix is widely distributed throughout the Shetland Islands, Port Erin Bay marine nature reserve and Ramsey Bay marine nature reserve (Garratt et al., 2022a; 2022b; Riley et al., 2024). Neopentadactyla mixta has been recorded in areas with sea surface temperatures of 0 to 20°C, with the majority of records between 10 to 15 °C (OBIS, 2025). Based on this evidence, it is likely to tolerate a chronic change in temperature in UK waters.

Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed to acute temperature changes due to thermal effluents. It is likely to withdraw into the sediment, away from the thermal plume, and be protected by the temperature of the interstitial waters.  Only long-term acute change (greater than the benchmark) is likely to adversely affect the population. In winter months, it is probably too deep to be affected by significant decreases in temperature as it burrows to a depth of 30 to 60 cm into the substratum (Smith & Keegan, 1985). Smith & Keegan (1985) suggested that light or winter temperature might be one cue for seasonal torpor, but noted that winter turbulence and increased turbidity, due to water movement, may also induce Neopentadactyla mixta to overwinter at depth.

Sensitivity assessment. Therefore, if exposed to a short-term acute change i.e. 5°C for a month, it will probably withdraw into the sediment and be unable to feed, resulting in a temporary loss of condition. Overall, a resistance of 'High' is suggested, with a resilience of 'High' so that the biotope is assessed as 'Not sensitive' at the benchmark level.

Temperature decrease (local)

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

Evidence

Little information on temperature tolerances was found. The assessment is based on the reported global distribution. The majority of records of Neopentadactyla mixta occur in the British Isles, although its distribution extends along the North and West European coasts, from the Faeroe Islands, the west coast of Norway (Molde), the Barents Sea to the Bay of Biscay (Southward & Campbell, 2006; Mazik et al., 2015; OBIS, 2016; Samyn et al., 2021). SS.SCS.CCS.Nmix is widely distributed throughout the Shetland Islands, Port Erin Bay marine nature reserve and Ramsey Bay marine nature reserve (Garratt et al., 2022a; 2022b; Riley et al., 2024). Neopentadactyla mixta has been recorded in areas with sea surface temperatures of 0 to 20°C, with the majority of records between 10 to 15 °C (OBIS, 2025). Based on this evidence, it is likely to tolerate a chronic change in temperature in UK waters.

Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed to acute temperature changes due to thermal effluents. It is likely to withdraw into the sediment, away from the thermal plume, and be protected by the temperature of the interstitial waters.  Only long-term acute change (greater than the benchmark) is likely to adversely affect the population. In winter months, it is probably too deep to be affected by significant decreases in temperature as it burrows to a depth of 30-60 cm into the substratum (Smith & Keegan, 1985). Smith & Keegan (1985) suggested that light or winter temperature might be one cue for seasonal torpor but noted that winter turbulence and increased turbidity, due to water movement, may also induce Neopentadactyla mixta to overwinter at depth.

Sensitivity assessment. Therefore, if exposed to a short-term acute change i.e. 5°C for a month, it will probably withdraw into the sediment and be unable to feed, resulting in a temporary loss of condition. Overall, a resistance of 'High' is suggested, with a resilience of 'High' so that the biotope is assessed as 'Not sensitive' at the benchmark level.

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

Echinoderms are restricted to the marine environment and one of the only stenohaline phyla in the animal kingdom (Russell, 2013).  Although some species can acclimatise to hypo/hypersaline conditions, Russell (2013) did not mention Neopentadactyla mixta amongst them. Smith (1983) noted that hypo or hypersaline water caused the animal to withdraw its tentacles. Neopentadactyla mixta has been recorded in sea surface salinity 30 to 35 psu (OBIS, 2025).

Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed due to hypo/hypersaline effluents. Roberts et al. (2010b) reported that hypersaline effluents from desalination plants disperse with tens of metres of the discharge point but reported widespread alteration in seagrass and soft sediment communities in poorly flushed environments. Echinoderms and ascidians were amongst the most sensitive to hypersaline brine in the studies examined (Roberts et al., 2010b).  While hypersaline effluents are likely to sink to the seabed, and potentially penetrate into the sediment, the water movement characteristic of this biotope is likely to disperse the effluent and limit the effect to the immediate vicinity of any discharge point.

Sensitivity assessment.  An increase in salinity above 40 psu is likely to be detrimental to Neopentadactyla mixta and interrupt feeding, but if prolonged for a year (see benchmark) may result in the death of individuals in the vicinity of the discharge. Therefore, a precautionary resistance assessment of 'Medium' is suggested but with Low confidence. Resilience is probably 'Medium', so sensitivity is assessed as 'Medium'.

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

Echinoderms are restricted to the marine environment and one of the only stenohaline phyla in the animal kingdom (Russell, 2013).  Although some species can acclimatise to hypo/hypersaline conditions, Russell (2013) did not mention Neopentadactyla mixta amongst them. Smith (1983) noted that hypo or hypersaline water caused the animal to withdraw its tentacles. Neopentadactyla mixta has been recorded in sea surface salinity 30 to 35 psu (OBIS, 2025).

Sensitivity assessment.  The biotope and Neopentadactyla mixta are only recorded from ‘full’ marine conditions. A reduction is salinity to reduced (18 to <30 psu) for a year is likely to reduce feeding or drive Neopentadactyla mixta into the sediment, where it cannot feed. Its seasonal torpor lasts from September to March each year, during which it loses condition significantly; it is unlikely to survive for a year without feeding.  Therefore, a resistance of 'None' is suggested but with Low confidence. Resilience is probably 'Medium', so sensitivity is assessed as 'Medium'.

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

Neopentadactyla mixta occurs in maerl beds and coarse gravel sediments, both of which are associated with water flow either due to tidal streams (moderately strong to weak, Connor et al. 2004) or wave mediated water movement (exposed to moderately wave exposed).  For example, the beds of Neopentadactyla mixta in coarse sediments examined by Konnecker & Keegan (1973) were found in tidal currents of up to 2.5 knots (ca 1.28m/s).  Nevertheless, artificially increased current beyond the calm weather, spring tide, maximum of ca 1.5 m/s caused Neopentadactyla mixta to stop feeding a withdraw into its burrow, as did bombardment with dislodged sediment (Smith & Keegan 1985). Similarly, a heavy gale in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days (Smith & Keegan, 1985).  The species regularly undertakes a ca six month long torpor period, during which it loses condition and lipid energy stores.  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food availability coupled with increased turbulence experienced in winter at their study site.  An increase in water flow may also modify the sediment, causing a loss of the sediment from the surface and mobilisation of the bed, although these sediments routinely bear mega-ripples caused by current flow and storms.  However, a decrease in flow will probably result in deposition of fine sediments and detritus, resulting in a change in sediment type, and a complete change in the biological community,

Sensitivity assessment.  Water flow (due to tidal flow or wave action) is an important structuring factor in habitats dominated by Neopentadactyla mixta (e.g. SS.SCS.CCS.Nmix and SS.SMp.Mrl.Pcal.Nmix), maintaining an open matrix of maerl or coarse sediment, removing fine sediments, allowing oxygenation deep within the sediment and providing adequate food supply to suspension feeders such as Neopentadactyla mixta. In areas of weak flow the biotope probably experience higher wave action, while in areas of moderate wave exposure, tidal flow is probably more important.  However, a change in water flow of 0.1-0.2 m/s is probably of limited effect in the biotopes normal range of <0.5 to 1.5 m/s, especially if low flow occurs in wave exposed areas. Therefore, a resistance of High is suggested, with a resilience of High so that the biotope is probably Not sensitive at the 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 pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

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

Evidence

Neopentadactyla mixta occurs in maerl beds and coarse gravel sediments, both of which are associated with water flow either due to tidal streams (moderately strong to weak, Connor et al. 2004) or wave mediated water movement (Exposed to Moderately exposed). Smith & Keegan (1985) noted that a  heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days.  The species regularly undertakes a ca six month long torpor period, during which it loses condition and lipid energy stores.  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food availability coupled with increased turbulence experienced in winter at their study site. 

Sensitivity assessment.  Water flow (due to tidal flow or wave action) is an important structuring factor in habitats dominated by Neopentadactyla mixta (e.g. SS.SCS.CCS.Nmix and SS.SMp.Mrl.Pcal.Nmix), maintaining an open matrix of maerl or coarse sediment, removing fine sediments, allowing oxygenation deep within the sediment and providing adequate food supply to suspension feeders such as Neopentadactyla mixta.  An increase in wave action may also modify the sediment, causing a loss of the sediment from the surface and mobilisation of the bed, although these sediments routinely bear mega-ripples caused by current flow and storms.  However, a decrease in wave action (in areas of low water flow) will probably result in deposition of fine sediments and detritus, resulting in a change in sediment type, and a complete change in the biological community. However, a change in nearshore wave height of >3% but <5%) is unlikely to be significant. Therefore, a resistance of High is suggested, with a resilience of High so that the biotope is probably Not sensitive at the benchmark level.  

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.

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.

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.

Radionuclide contamination

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

Evidence

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

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 but evidence is presented where available.

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

Neopentadactyla mixta probably needs coarse sediments to survive, as the open matrix provided by coarse sediments or maerls at depth, together with water flow, ensures that the water is oxygenated at depth in the sediment. Neopentadactyla mixta reduces its metabolism and oxygen consumption from 0.11 ml O₂/ gm dry wt. to 0.03 ml O₂/ gm dry wt. during its overwinter torpor (Smith & Keegan, 1985). Therefore, it might be able to survive lower oxygen levels overwinter than in spring, summer and autumn.

Lawrence (1996) reported mass mortality of echinoderms in the Gulf of Trieste due to hypoxia caused by a strong thermocline combined with high pelagic productivity and eutrophication. The brittlestar Ophiura quinquemaculata was killed with a few days, holothurians including Ocnus planci (as Cucumaria planci), starfish Asteropecten sp. and the remaining brittlestars were killed within a week. Echinoderms were shown to be intolerant of the effects of algal blooms, resulting in mortalities of the sea urchins Echinus esculentus and Paracentrotus lividus, and the holothurian Labidoplax digitata amongst other echinoderms, probably due to hypoxia caused by death of the algal bloom algae (Boalch, 1979; Forster, 1979; Griffiths et al., 1979; Lawrence, 1996). Diaz & Rosenberg (1995, Figure 5) suggested that shrimp and crustaceans were lost as oxygen levels dropped below ca 0.75 ml/l and that the macroinfauna was reduced below ca 0.4ml/l.

Vaquer-Sunyer & Duarte (2008) suggested a median sublethal oxygen concentration of 1.22 mg O₂/l (± 0.25) for a number of echinoderms reviewed in their study.  Echinoderms were neither the most or the least sensitive of the taxonomic groups examined. 

Sensitivity assessment.  Neopentadactyla mixta may be more resistant of decrease oxygen levels while in it winter torpor. No information on juveniles was found. However, the species has a preference of coarse, mobile, deposits in areas of moderate to strong water flow (Konnecker & Keegan, 1973; Keegan et al., 1985). This suggests that it prefers well oxygenated habitats.  The evidence from the Gulf of Trieste also suggests that echinoderms are sensitive to hypoxia. Therefore, a resistance of Low is suggested based on expert judgement. Resilience is probably Medium so that the biotope is assessed as Medium sensitivity at the benchmark level.

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

Nutrient enrichment can lead to increase in algal growth and algal blooms, whose subsequent death results in hypoxia or even anoxia. Nutrient enrichment can also result in increased bacterial growth in sediments, that also result in hypoxia. However, this biotope occurs in well flushed habitats so that only continuous of extreme enrichment is likely to be detrimental. But no direct evidence was found, therefore there is 'Insufficient evidence' on which to base an assessment at present.

Organic enrichment

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

Evidence

Organic enrichment due to sewage and other effluents has been implicated in the loss of maerl beds and a complete shift in their resident communities.  For example in Brittany, numerous maerl beds were affected by sewage outfalls and urban effluents, resulting in increases in contaminants, suspended solids, microbes and organic matter with resultant deoxygenation (Grall & Hall-Spencer, 2003).  This resulted in increased siltation, higher abundance, and biomass of opportunistic species, loss of sensitive species and reduction in biodiversity.  Grall & Hall-Spencer (2003) note that two maerl beds directly under sewage outfalls were converted from dense deposits of live maerl in 1959 to heterogeneous mud with maerl fragments buried, under several centimetres of fine sediment, with communities dominated by only a few species by 1997.  Similarly, changes in sediment community structure from diverse communities to communities dominated by opportunistic deposit feeders is well documented (Pearson & Rosenberg 1978; Diaz & Rosenberg 1995).

Sensitivity assessment. Although the evidence available could not be compared directly with the benchmark, the evidence suggests that organic enrichment could lead to a complete change in the community and loss of Neopentadactyla mixta populations.  In addition, while this biotope is not characterized by maerl, the coarse sediment provides a similar open matrix, and would probably respond to organic enrichment in a similar manner. However, it is not possible to compare the reported evidence to the benchmark level of impact.  Therefore, a resistance of 'Low' is suggested.  A resilience of 'Low' is suggested as the habitat would need to recover before the species could return.  Sensitivity is, therefore, assessed as 'High'. 

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 sedimentary substrata were replaced with rock substrata the biotope would be lost, as it would not longer be a sedimentary habitat and would no longer support Neopentadactyla mixta or other infauna or epifauna.

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

Physical change (to another sediment type)

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

Evidence

Neopentadactyla mixta is recorded from coarse sand and gravel sediments. Könnecker & Keegan (1973) reported that it had a preference for gravel type substrata on the west coast of Ireland and that the highest densities of individuals occurred in loose, mobile deposits. Keegan et al. (1985) reported that it occurred in coarse sediments in moderate to strong tidal stream but that it was less common in other deposits. Connor et al. (1997a) reported that Neopentadactyla mixta occurred in biotopes from gravel, algal gravel (maerl) and coarse clean sand, while this biotope (CCS.Nmix is only recorded from sandy gravel habitats (Connor et al., 2004). Therefore, a change in sediment type to fine sediments e.g. to fine sands, sands with gravel or muddy sands would result in a loss of the biotope as described by the habitat classification. Neopentadactyla mixta probably needs coarse sediments to survive, as the open matrix provided by coarse sediments or maerls at depth, together with water flow, ensures that the water is oxygenated at depth in the sediment. This is probably especially important as Neopentadactyla mixta overwinters for ca six months at depth (30-60 cm).  A change is sediment type to 100% gravel wouls also result in loss of the biotope as described by the classification. Neopentadactyla mixta may also be lost, presumably, because the higher water flow associated with gravel habitats would preclude feeding.

Sensitivity assessment.  Therefore, a resistance of None is recorded. As the change is permanent, resilience is Very low and sensitivity is assessed as High.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Neopentadactyla mixta lives in the sediment is a ‘U-shaped’ posture with its oral tentacles raised above the surface and its anus just below the surface of the sediment (Smith & Keegan, 1985).  It is usually found in this position in its burrow 15-25 cm deep in the sediment (Könnecker & Keegan, 1973).  However, in the winter months (ca September to March) its burrows into the sediment to a depth of 30-60 cm.  It maintains this depth, even if the surface of the sediment is eroded or accreted (Smith & Keegan, 1985).

Sensitivity assessment. In spring to autumn, extraction of the sediment to 30 cm is likely to remove the majority of the resident population but in winter, the majority of the population would survive as long as suitable substratum remained after extraction.  Therefore, a resistance of None is recorded to represent to worst case scenario.  Resilience is probably Medium so that sensitivity is 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

The burrow of Neopentadactyla mixta in spring/autumn is 15 to 25 cm deep, and 30 to 60 cm deep during its winter torpor (Smith & Keegan, 1985).  Therefore, it is unlikely to be directly impacted by surface abrasion.  For example, in long-term studies of scallop dredging and subsequent recovery (Hall-Spencer & Moore 2000a, 2000b) deep burrowing species including Neopentadactyla mixta were not impacted and their abundance changed little over the four-year period.  It should be noted however that no information on juveniles was available. 

Therefore, a resistance of High is suggested. Resilience is probably also High (as there is no impact to recover from) so that biotope is assessed as Not Sensitive. 

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

In long-term studies of scallop dredging and subsequent recovery (Hall-Spencer & Moore 2000a, 2000b) deep burrowing species including Neopentadactyla mixta were not impacted and their abundance changed little over the four year period.  However, experimental hydraulic blade dredging removed and damaged deep-burrowing species, including small numbers of Neopentadactyla mixta (Hauton et al. 2003), and affected the maerl bed to a depth of 9 cm.  Hydraulic dredging in coarse sand and gravel may have similar effects.

Overall, penetrative gear may adversely affect Neopentadactyla mixta populations and a resistance of 'Medium' is suggested. Resilience is likely to be 'Medium', so sensitivity is assessed as 'Medium'.

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

This biotope occurs in well flushed areas subject to moderately strong to weak flow and/or wave exposed or moderately wave exposed conditions.  Neopentadactyla mixta is a passive suspension feeder. It holds its tentacles into the water column and particles of food and detritus stick to the sticky mucus on the tentacles while filamentous algae lodge amongst the tentacles. The tentacles are them placed into the mouth and the food consumed (Smith, 1983). It feeds on unicellular and filamentous algae, diatoms, dinoflagellates, the exoskeletons of planktonic crustaceans and other organic material (Smith, 1983). Therefore, an increase in suspended sediment may increase food availability while an increase in turbidity may reduce phytoplankton abundance.  

Smith & Keegan (1985) noted that a  heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days (Smith & Keegan, 1985).  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food quality of the seston in winter months coupled with increased turbulence experienced in winter at their study site.  Perhaps poor food quality was due to lack of phytoplankton in the winter months. Smith & Keegan (1983) also noted that in strong flow the tentacles became heavily turbated but were still held into the water column.

Sensitivity assessment.  No direct measure of turbidity normally experienced by this biotope was found. Suspension feeders require good or constant water flow and a supply of seston. So an increase in suspended sediment could provide extra food. However, in turbid conditions, the suspension feeding apparatus may become clogged or overwhelmed by particulates and the animal stop feeding.  The evidence of winter torpor in Neopentadactyla mixta may suggest that it avoids a natural increase in turbidity in the more stormy winter months, and/or avoids organic particulates in winter in preference for more energy rich phytoplankton in spring to autumn.  Therefore, a resistance of 'Medium' is suggested to represent the potential loss of feeding and food quality if the turbidity was to increase (e.g. from clear to intermediate; see benchmark) but with Low confidence. Resilience is probably 'Medium', so sensitivity is assessed as 'Medium'.

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

This biotope occurs in well flushed areas subject to moderately strong to weak flow and/or wave exposed or moderately wave exposed conditions.  Neopentadactyla mixta is a passive suspension feeder. It holds its tentacles into the water column and particles of food and detritus stick to the sticky mucus on the tentacles while filamentous algae lodge amongst the tentacles. The tentacles are them placed into the mouth and the food consumed (Smith, 1983). It feeds on unicellular and filamentous algae, diatoms, dinoflagellates, the exoskeletons of planktonic crustaceans and other organic material (Smith, 1983). Therefore, an increase in suspended sediment may increase food availability while an increase in turbidity may reduce phytoplankton abundance.  

Smith & Keegan (1985) noted that a  heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days (Smith & Keegan, 1985).  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food quality of the seston in winter months coupled with increased turbulence experienced in winter at their study site.  Perhaps poor food quality was due to lack of phytoplankton in the winter months. Smith & Keegan (1983) also noted that in strong flow the tentacles became heavily turbated but were still held into the water column.

Sensitivity assessment.  No direct measure of turbidity normally experienced by this biotope was found. Suspension feeders require good or constant water flow and a supply of seston. So an increase in suspended sediment could provide extra food. However, in turbid conditions, the suspension feeding apparatus may become clogged or overwhelmed by particulates and the animal stop feeding.  The evidence of winter torpor in Neopentadactyla mixta may suggest that it avoids a natural increase in turbidity in the more stormy winter months, and/or avoids organic particulates in winter in preference for more energy rich phytoplankton in spring to autumn.  Therefore, a resistance of 'Medium' is suggested to represent the potential loss of feeding and food quality if the turbidity was to increase (e.g. from clear to intermediate; see benchmark) but with Low confidence. Resilience is probably 'Medium', so sensitivity is assessed as 'Medium'.

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

Neopentadactyla mixta lives in the sediment is a ‘U-shaped’ posture with its oral tentacles raised above the surface and its anus just below the surface of the sediment (Smith & Keegan, 1985).  It is usually found in this position in its burrow 15-25 cm deep in the sediment (Könnecker & Keegan, 1973).  However, in the winter months (ca September to March) its burrows into the sediment to a depth of 30-60 cm.  It maintains this depth, even if the surface of the sediment is eroded or accreted (Smith & Keegan, 1985).

Sensitivity assessment. The tentacular crown can expand to 140 cm² (Smith & Keegan, 1985) and probably extends to ca 4-5 cm above the substratum (expert opinion). However, the deposit of 30 cm of fine sediment would probably discourage Neopentadactyla mixta from feeding and it would probably withdraw into its burrow. Fine sediment will also penetrate the surface of the sediment in the affected area, significantly reducing water flow, and increasing the possibility of anoxia within the sediment.  If the smothering sediment remained, it would result in a complete shift of the community and loss of the Neopentadactyla mixta population.  Smith & Keegan (1985) noted that a heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days.  However, in the areas of water movement in which these habitats occur it is unlikely that the smothering sediment would persist, depending on the local hydrography.  As Neopentadactyla mixta can survive ca 6 months without feeding (Konnecker & Keegan, 1973; Smith & Keegan, 1985) it is likely that resistance is High and resilience is also High. Therefore, the biotope is assessed as Not Sensitive at the benchmark level.

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. Neopentadactyla mixta is a passive suspension feeder in which small particulates stick to its mucus-covered tentacles.  It seems logical that microplastics could also stick to its tentacles and be ingested, where they occur in these habitats. However, no evidence was found.

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. There have been no studies examining the effect of EMFs on macroalgae. 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.  

Sensitivity assessment. Given the lack of data at the level of individual biotopes, resistance and resilience to EMFs cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'. 

Underwater noise changes

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

Evidence

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

Introduction of light or shading

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

Evidence

Neopentadactyla mixta exhibits a diurnal feeding pattern. On the west coast of Ireland, individuals began to withdraw into the sediment about an hour after sunrise, and had all withdrawn within 2-3 hours and remained in the sediment for 1-2 hours before emerging again over a 4 hour period (Könnecker & Keegan, 1973). Yet Könnecker & Keegan (1973) also reported that they did not show an immediate response to strong white light.  Smith & Keegan (1985) suggested that light may not be the cause of the diurnal behaviour.

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.  A change in incident light or shading from artificial structures may not affect feeding behaviour in Neopentadactyla mixta.  However, 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 - this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed or propagules.  But seed or propagule dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure (Death for collision pressure definition).

Evidence

Not relevant to seabed habitats. 

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature (Visual disturbance pressure definition). 

Evidence

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

Genetic modification & translocation of indigenous species

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

Evidence

No evidence of genetic modification, breeding, or translocation was found.

Introduction of microbial pathogens

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

Evidence

No evidence was available on the effect of microbial pathogens.

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

Scallops may be targeted in coarse sand and gravel habitats. Their removal may result in the physical effects discussed under ‘abrasion’ and ‘penetration’ pressures above. However, there are no clear relationships between the dominant important characterizing species Neopentadactyla mixta  and other characterizing species.  Therefore, a resistance of High is suggested so that resilience is also High and the biotope is assessed as Not sensitive.

Removal of non-target species

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

Evidence

Scallops may be targeted in coarse sand and gravel habitats. Their removal may result in the physical effects discussed under ‘abrasion’ and ‘penetration’ pressures above. If the dominant important characterizing species Neopentadactyla mixta was removed as bycatch and its abundance reduced significantly, then the biotope would be lost. However, experimental hydraulic blade dredging removed and damaged deep-burrowing species, including small numbers of Neopentadactyla mixta (Hauton et al. 2003b), and affected a maerl bed to a depth of 9 cm.  Hydraulic dredging in coarse sand and gravel may have similar effects.  Due to the depth of it burrow, if only a few individual or juvenile Neopentadactyla mixta are vulnerable as bycatch, then a resistance of Medium is suggested. Resilience is likely to be Medium so that sensitivity is assessed as Medium.

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; 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 from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; 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 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, which provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species 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, 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.  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 (Tillin et al., 2020). 

Sensitivity assessment. The above evidence suggests that Crepidula fornicata could colonize coarse sediment habitats in the subtidal, typical of this biotope. Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. Therefore, if Crepidula colonized this biotope, it would probably 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. Neopentadactyla requires well-oxygenated open substrata and is likely to be excluded from silted, smothered sediments. This is a high to moderate energy habitat, in which storms may mobilise the sediment (Smith & Keegan, 1985), which may mitigate or prevent colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). Crepidula fornicata has the potential to colonize this habitat, especially where water movement is mediated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. However, Crepidula reduced the density of suspension feeders and mobile Crustacea in coarse sediment even at low densities (De Montaudouin & Sauriau, 1999). 

Therefore, resistance is assessed as 'Medium' in examples where wave action is high and subject to storms, but 'Low' in areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'High' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope, so the confidence in the assessment is 'Low' and further evidence is required. 

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009).  

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 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 hours) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009).

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 were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can 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). 

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA, the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the sand ridges that bounded the site, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). In addition, evidence found that the species can also not 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, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffith et al., 2009). However, some studies on Georges Bank, USA and Sandwich, Massachusetts, observed colonies were able to survive partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow the sandy bottom (cited Gittenberger, 2007). In the Netherlands, the coastal zone is composed of mud and sand, with only shells as hard substrata. Didemnum sp. remained rare until 1996, when populations quickly expanded, and it became a dominant invasive species because of an increase in available hard substrata for colonization after a cold winter between 1995 and 1996 caused a decrease in the abundance of many marine animals (Gittenberger, 2007). Thus, Didemnum vexillum was able to colonize and establish in mud and sand habitats where hard substrata were present.  

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. 

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). 

Sensitivity assessment: This biotope is potentially suitable for the colonization by Didemnum vexillum due to the presence of hard substrata, including gravel and clean shells (Valentine et al., 2007a; 2007b; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Tillin et al., 2020; Cinar & Ozgul, 2023). The biotope occurs within the depth range suitable for the invasive species. However, the exposed to moderately exposed wave conditions may be unsuitable for Didemnum vexillum.  Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’. The confidence in the assessment is 'Low' due to the lack of direct evidence.

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Barón, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Barón, 2010; Padilla, 2010). Magallana gigas is recognised as a beneficial and important species in aquaculture worldwide (Padilla, 2010). It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011, 2012; Humphreys et al., 2014, cited in Alves et al., 2021; Hansen et al., 2023). 

Since its introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSIP, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergström et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024).

Shipping activity has also been associated with the introduction of Magallana gigas in the northeastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019). It was also suggested that some Magallana gigas populations were established in southwest England from France, possibly via fouling on ships (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019). 

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann, 2012; Kochmann et al., 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020) because its larvae require hard substrata for successful settlement and development (McKinstry & Jensen, 2013; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020). Invasive populations of Magallana gigas have been found wave-exposed rocky shores to wave-sheltered soft sediment environments, and it has been described as a habitat generalist (Troost, 2010; Kochmann, 2012; Kochmann et al., 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). They are unlikely to occur under boulders as they require access to the water column (Tillin et al., 2020). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).  

Although shorelines comprised mainly of mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone, otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (McKinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020). The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present, developing more quickly once there is a sufficient amount (Troost, 2010). Bergström et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%).  

Zwerschke et al. (2018) found at intertidal rocky sites and sites with gravel around the UK, Ireland and northern France, densities of Pacific oysters more than 10 m² had a different macrofaunal assemblage structure than sites with low density or no Magallana gigas. Their results showed a greater abundance of species such as barnacles in mud, rock, and gravel sites when Pacific oysters were superabundant (oyster density more than 99 /m²). However, a decrease in abundance of kelp, Fucus vesiculosus and periwinkle Littorina sp. was observed on the rocky shore sites colonized by the oysters (Zwerschke et al., 2018).

Sensitivity assessment. This biotope could be suitable for the colonization of Magallana gigas due to the presence of hard substrata and mixed sediment, including gravel and shell (Kochmann, 2012; Kochmann et al., 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). However, the majority of the evidence indicates that habitats that occur at depths of more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’. The confidence in the assessment is 'Low' due to the lack of direct evidence.

Wireweed, Sargassum muticum

Evidence

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

Wakame, Undaria pinnatifida

Evidence

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

Other INIS

Evidence

No other relevant INIS were identified.