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

The important or key characteristic species are widely distributed in the North East Atlantic, with Ampharete falcata distributed from southwest England and the Kattegat to Spitsbergen (Sui & Li, 2020). Outside of the Atlantic, in the Yellow Sea and East China Sea, Ampharete falcata seems to be restricted to water depths less than 400 m (Sui & Li, 2020). The similar species Ampharete lindstroemi and Ampharete santillani are also found throughout the North East Atlantic (Parapar et al., 2018), and Ampharete acutifrons occurs from the Arctic to the Mediterranean (Hayward & Ryland, 1995b). The bivalve Parvicardium ovale occurs from Iceland to the Mediterranean and Canary Islands and is also found in the Black Sea (Hayward & Ryland, 1995b; Kolyuchkina et al., 2020).

Schückel et al. (2010) investigated the temporal variability of macrofauna communities in the northern North Sea in relation to changes in temperature and/or changes in hydrography. The authors observed a significant negative correlation between total macrofaunal abundance, including that of Ampharete falcata, and mean surface temperature (SST). Mean surface temperatures recorded varied between 13.4 and 16.6°C. Mean bottom temperatures remained more stable and varied between 6.7 and 7.6°C. The authors observed a strong decrease in mean abundance as a result of increased SST, where increased SST mainly enhanced stratification and contributed to a decrease in food availability for the benthic community at the site.

Parvicardium ovale was recorded in Norway in waters with annual temperatures varying between 1 and 11°C (Gulliksen & Bahr, 2001). Holte et al. (2005) investigated the variations in soft-bottom macrofauna from stratified Norwegian basins. Pavicardium ovale occurred at the study sites, which experienced temperatures between 0.5 and 14°C. Kolyuchkina et al. (2020) investigated the role of abiotic environmental factors on the vertical distribution of macrozoobenthos off the northeastern Black Sea coast. They found that near-bottom water temperature was a critical factor in distinguishing between macrozoobenthic groups (19 to 32% for Bivalvia). Similar to Parvicardium ovale, Parvicardium simile occurred throughout the study site, where the temperature at depths of 10 to 20 m in the summer period was above 20°С and dropped to 11 to 13°С between 20 and 25 m (the zone of the thermocline) (Kolyuchkina et al., 2020). At depths of 50 to 70 m, the water temperature was 8 to 10°С and the occurrence of Parvicardium simile was 100% (Kolyuchkina et al., 2020). Along the entire coastline of the Black Sea, at depths of about 60 m, water temperature did not rise above 9°С (Kolyuchkina et al., 2020). Kolyuchkina et al. (2020) noted how the communities recorded in this study (2020) between 50 and 70 m were similar to those observed in 2013 and the middle of the 20th century. At 50 to 70 m deep, sites are characterized by a constant near-bottom temperature and homogeneous sediments, which is likely why communities have remained relatively stable (Kolyuchkina et al., 2020). However, Kolyuchkina et al. (2020) noted that many species typical for the zone below the thermocline may exist at higher temperatures and solitary specimens are recorded at lower depths, but outside the optimal zone, they are competitively excluded by more adapted heat-loving species and, as a result, do not reach high quantitative parameters.

Sensitivity assessment. The characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4°C to 19°C (Huthnance, 2010). Although no information was found on the maximum temperature tolerated by the characterizing species, it is likely that Ampharete falcata and Parvicardium ovale are able to resist a long-term increase in temperature of 2°C. However, the biotope occurs in the margins of stratified seas (Connor et al., 2004), so an increase in temperature could result in enhanced stratification and consequently in restriction of food availability for the biotope community. Although stratification is not an instantaneous process, it may develop over time scales of hours in coastal waters (Nunes Vaz, 1990), so a short-term increase of 5°C may result in some adverse implications for the characterizing species. Resistance is therefore assessed as ‘Medium’ (loss <25%), but with a Low confidence. Resilience is likely to be ‘High’, so the biotope is considered to have ‘Low’ sensitivity to an increase in temperature at the pressure 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

The important or key characteristic species are widely distributed in the North East Atlantic, with Ampharete falcata distributed from southwest England and the Kattegat to Spitsbergen (Sui & Li, 2020). Outside of the Atlantic, in the Yellow Sea and East China Sea, Ampharete falcata seems to be restricted to water depths less than 400 m (Sui & Li, 2020). The similar species Ampharete lindstroemi and Ampharete santillani are also found throughout the North East Atlantic (Parapar et al., 2018), and Ampharete acutifrons occurs from the Arctic to the Mediterranean (Hayward & Ryland, 1995b). The bivalve Parvicardium ovale occurs from Iceland to the Mediterranean and Canary Islands and is also found in the Black Sea (Hayward & Ryland, 1995b; Kolyuchkina et al., 2020).

Schückel et al. (2010) investigated the temporal variability of macrofauna communities in the northern North Sea in relation to changes in temperature and/or changes in hydrography. The authors observed a significant negative correlation between total macrofaunal abundance, including that of Ampharete falcata, and mean surface temperature (SST). Mean surface temperatures recorded varied between 13.4 and 16.6°C. Mean bottom temperatures remained more stable and varied between 6.7 and 7.6°C. The authors observed a strong decrease in mean abundance as a result of increased SST, where increased SST mainly enhanced stratification and contributed to a decrease in food availability for the benthic community at the site. Also, cold-temperate species, such as Ampharete falcata, are favoured by thermoclines, and water mixing at different depths may affect small spatial patterns of macrofaunal species independent of increasing SST. Méndez (2007) investigated the distribution of deep-water polychaete fauna in the Gulf of California according to varying environmental parameters, including depth, temperature, and dissolved oxygen. The authors found that dominant species, including Ampharete spp. occurred within limited ranges of environmental parameters, not occurring at temperatures below 2°C.

Parvicardium ovale was recorded in Norway in waters with annual temperatures varying between 1 and 11°C (Gulliksen & Bahr, 2001). Holte et al. (2005) investigated the variations in soft-bottom macrofauna from stratified Norwegian basins. Pavicardium ovale occurred at the study sites, which experienced temperatures between 0.5 and 14°C. Kolyuchkina et al. (2020) investigated the role of abiotic environmental factors on the vertical distribution of macrozoobenthos off the northeastern Black Sea coast and found that near-bottom water temperature was a critical factor in distinguishing between macrozoobenthic groups (19 to 32% for Bivalvia). Similar to Parvicardium ovale, Parvicardium simile occurred throughout the study site, where the temperature at depths of 10 to 20 m in the summer period was above 20°С and dropped to 11 to 13°С between 20 and 25 m (the zone of the thermocline) (Kolyuchkina et al., 2020). At depths of 50 to 70 m, the water temperature was 8 to 10°С and the occurrence of Parvicardium simile was 100% (Kolyuchkina et al., 2020). Along the entire coastline of the Black Sea, at depths of about 60 m, water temperature did not rise above 9°С (Kolyuchkina et al., 2020). Kolyuchkina et al. (2020) noted how the communities recorded in this study (2020) between 50 and 70 m were similar to those observed in 2013 and the middle of the 20th century. At 50 to 70 m deep, sites are characterized by a constant near-bottom temperature and homogeneous sediments, which is likely why communities have remained relatively stable (Kolyuchkina et al., 2020). However, Kolyuchkina et al. (2020) noted that many species typical for the zone below the thermocline may exist at higher temperatures and solitary specimens are recorded at lower depths, but outside the optimal zone, they are competitively excluded by more adapted heat-loving species and, as a result, do not reach high quantitative parameters.

Sensitivity assessment. The characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4 to 19°C (Huthnance, 2010). Although no information was found on the minimum temperature tolerated by the characterizing species, it is likely that Ampharete falcata and Parvicardium ovale are able to resist a long-term decrease in temperature of 2°C or 5°C in the short-term. Furthermore, the biotope occurs in the margins of stratified seas (Connor et al., 2004), so a decrease in temperature could result in reduced stratification and consequently enhanced food availability for the biotope community. Resistance and resilience are therefore assessed as ‘High’, and the biotope is 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

SS.SMu.OMu.AfalPova is found within fully marine subtidal locations and it is highly unlikely that the biotope would experience hypersaline conditions. Although the study did not involve hypersaline conditions, Zettler et al. (2007) compared indices of ecological quality of benthic biodiversity along salinity gradients in the Baltic Sea, and suggested that Parvicardium ovale was not able to deal with wide variations in salinity.

Sensitivity assessment: There is little direct evidence of the effects of hypersaline conditions on the characterizing species of this biotope, Ampharete falcata and Parvicardium ovale. However, it is unlikely that the biotope community would be able to resist an increase in salinity to >40 psu, resulting in mortality of the characterizing species. Resistance is therefore assessed as 'Low' (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably 'High', so sensitivity is therefore assessed as 'Low'.

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

SS.SMu.OMu.AfalPova is found within fully marine subtidal locations, and it is highly unlikely that the biotope would experience conditions of hyposalinity. However, it is likely that key components of the biotope community would not be resistant to a decrease in salinity. For example, Zettler et al. (2007) compared indices of ecological quality of benthic biodiversity along salinity gradients in the Baltic Sea, where salinities varied from 1.5 to 27.8 psu. Parvicardium ovale was considered not able to deal with the wide variations in salinity that occur in the study site. However, Parvicardium ovale was recorded in the Baltic Sea at near bottom salinity of 12-18 PSU (Gogina et al., 2010).

Sensitivity assessment: Based on the evidence presented, it is likely that characterizing species Parvicardium ovale would resist a decrease in salinity from full to reduced conditions. No direct evidence of the effects of hyposaline conditions on characterizing species Ampharete falcata was found. Based on the species offshore preferences of between 30-90 m (Heath, 2005), it is unlikely that Ampharete falcata would be adapted, and the community is unlikely to resist a decrease in salinity at the pressure benchmark level. As Ampharete is considered a key characterizing and structuring species of this biotope, loss of this species would result in the biotope being lost. Resistance is therefore assessed as Low (loss of 25-75%) but with low confidence. Once prior conditions resume, resilience is probably 'High', so sensitivity is, therefore, assessed as Low.

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. The most damaging effect of increased flow rate (above the pressure benchmark) could be the erosion of the substratum as this could eventually lead to loss of the habitat. Increased water flow rates are likely to change the sediment characteristics in which the species live, primarily by re-suspending and preventing deposition of finer particles (Hiscock, 1983). Tube building species Ampharete falcata is likely to depend on the weak (<0.5 m/s) tidal streams that characterize the biotope (Connor et al., 2004), as it allows development of the fine sediment habitat. 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.

Cooper et al. (2007) investigated recovery of the seabed following marine aggregate dredging on the south east coast of England. The maximum spring tidal current velocity was ca. 1.3 m/s. The authors confirmed that the sediment became coarser with increased dredging intensity and that the seabed appeared extremely uneven, with these effects appearing to persist at least eight years after cessation of dredging activities. Ampharete spp. were recorded from the study site characterized by high intensity of dredging activities soon after cessation, possibly because of the opportunistic life habits of this taxon. On the other hand, Parvicardium ovale has been recorded in Norway in waters with current velocities up to 1.7 m/s (Gulliksen & Barh, 2001).

Furthermore, the biotope occurs near the margins of stratified seas (Connor et al., 2004). Changes in water flow are likely to affect the mixing of waters in the area and influence the extent of the stratified waters, consequently having an impact on the biotope. Increased flow rates may enhance mixing and allow expansion of the biotope’s boundaries where suitable substrata occur.

Sensitivity assessment: Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, silts and clays require greater critical erosion velocities because of their cohesiveness. SS.SMu.OMu.AfalPova occurs in weak tidal streams (0.5 m/s) in localised areas on a crucial point on a depositional gradient between areas of tide-swept mobile sands and quiescent stratifying muds (Connor et al., 2004). Based on the evidence presented, it is likely that characterizing species Parvicardium ovale would resist changes in water flow, and no direct evidence of the maximum flow velocity tolerance of characterizing species Ampharete falcata was found. Although changes in water flow (above the benchmark) would be likely to change the sedimentary regime in the biotope and consequently have implications for tube-building Ampharete falcata, the cohesive nature of the sandy muds that characterize the biotope is likely to provide some protection to changes in water flow at the pressure benchmark. Resistance and resilience are therefore assessed as High, and the biotope is 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

Changes in emergence are Not Relevant to the biotope, which is restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

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

Evidence

Potentially the most damaging effect of increased wave heights would be the erosion of the fine sediment substratum as this could eventually lead to loss of the habitat. Furthermore, changes in wave exposure may influence the supply of particulate matter for tube building and feeding activities of the characterizing species. Decreases in wave exposure may influence the supply of particulate matter because wave action may have an important role in re-suspending the sediment that is required by the species to build its tubes. Food supplies may also be reduced, affecting growth and fecundity of the species.

Sensitivity assessment: No direct evidence of the specific tolerances of the characterizing species to changes in wave exposure was found. The depth of the biotope (>50 m) is likely to protect the biotope from anything other than the most severe change in wave action. Hiscock (1983) suggested that a Force 8 Gale could result in oscillatory wave-induced water flow at 80 m of 0.09 m/s or ca 0.4 m/s at 50 m. A change in significant wave height of 3-5% is roughly equivalent to a change from force 3 to 4. Therefore, it is unlikely to be significant in deep water biotopes. Resistance and resilience are therefore assessed as High, and the biotope is considered Not Sensitive at the benchmark level, but with low confidence.

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.

There is no information on the resistance of the key species in the biotope. Experimental studies with various species suggests that polychaete worms are quite tolerant of heavy metals (Bryan, 1984).

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

The biotope is considered to be 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

The biotope is considered to be 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

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

The biotope is dominated by opportunistic small bivalves and polychaetes near margins of stratified seas, which suggests that the biological community is likely to be adapted to the adverse effects of stratification. Variations in environmental conditions may cause the stratification boundaries to shift, potentially resulting in episodes of hypoxia in the biotope.

Ampharete spp. are present in the deep trough of the Gulf of St. Lawrence, Canada, where hypoxic zones are found, with mean densities of around 227 individuals per m² at oxygen saturations of 23% (~2.3 mg/l), and the genus is known to be tolerant of low oxygen conditions (Zettler & Pollehne, 2023). Zettler & Pollehne (2023) concluded that the stability of the environmental conditions and the absence of stress, where the constant supply of oxygen, even at low concentrations, seems to be more important than the absolute oxygen concentration. Méndez (2007) investigated the distribution of deep-water polychaete fauna according to varying environmental parameters, including dissolved oxygen. The author found that dominant species, including Ampharete spp. occurred within limited ranges of environmental parameters, not occurring at oxygen levels below ca 0.8 mg/l.

In 2005, in the waters off western Ireland, a large dinoflagellate, Karenia mikimotoi, bloom led to the mass mortality of both benthic and pelagic marine life due to oxygen depletion, cellular toxicity, and physical smothering (O’Boyle et al., 2016). Donegal Bay was one of the areas to experience the worst effects of the bloom due to its hydrographic characteristics (seasonally stratified, weak residual flow), and hypoxic conditions (2.2 mg/l O₂) were directly observed in the Bay post-bloom collapse (O’Boyle et al., 2016). During observations, the shore was characterized by the presence of large numbers of dead cockles, Parvicardium ovale, in various states of decay, and some were heavily decomposed, while others were still alive but distressed (O’Boyle et al., 2016). However, Gogina et al. (2010a) suggested that Parvicardium ovale showed a strong positive correlation with dissolved oxygen. Holte et al. (2005) recorded Parvicardium ovale from stratified Norwegian basins with minimum oxygen values in deep water as low as 2.2 mg/l. Rosenberg et al. (1991) exposed benthic species from the North East Atlantic to oxygen concentrations of around 1 mg/l for several weeks, including species of small bivalves. After 11 days in hypoxic conditions, bivalve individuals were still alive, although individuals showed increased stretching of the siphon out of the sediment. It is possible that Parvicardium ovale would deal with low oxygen in a similar way.

Sensitivity assessment. Direct evidence regarding the effect of de-oxygenation on the key species in the biotope or the biotope as a whole is limited. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. The evidence suggests that Ampharete spp. are tolerant of low oxygen conditions, while O’Boyle et al. (2016) observed Parvicardium ovale, in various states of decay, or still alive but distressed, in hypoxic conditions of 2.2 mg/l O₂. Hence, a reduction in oxygen levels to less than 2 mg/l for a week (the benchmark) might adversely affect the Parvicardium ovale population within the biotope. Therefore, Resistance to de-oxygenation at the pressure benchmark level is assessed as ‘Medium’. Resilience is assessed as ‘High’, and sensitivity as ‘Low’ but with 'Low' confidence.   

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

Increased nutrients are most likely to affect the abundance of phytoplankton, which may include toxic algae (OSPAR, 2009). This primary effect resulting from elevated nutrients will impact other biological elements or features (e.g. toxins produced by phytoplankton blooms or de-oxygenation of sediments) and may lead to ‘undesirable disturbance’ to the structure and functioning of the ecosystem. With enhanced primary productivity in the water column, organic detritus that falls to the seabed may also be enhanced, which may be utilized by the deposit feeders in the community.

Parvicardium exiguum and Ampharete grubei are both found in areas rich in silt and organic content (Lastra et al., 1993; Holme, 1949). Hiscock et al. (2005a) suggested that Ampharete spp. might be favoured by nutrient enrichment, but the overall species diversity is likely to decline.

Sensitivity assessment. The community, and hence the biotope, may change to one dominated by nutrient enrichment resistant species, in particular polychaete worms. However, limited evidence on the effects of nutrient enrichment on the characteristic species was found. Therefore, the evidence is ‘Insufficient’ to form the basis of an assessment.

Organic enrichment

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

Evidence

Borja et al. (2000) included Ampharete spp. and Parvicardium ovale in their Ecological Group (I) “Species very sensitive to organic enrichment and present under unpolluted conditions (initial state)”. These results confirm suggestions that Parvicardium ovale has a strong negative correlation with total organic content (Gogina et al., 2010a). Martinez-Garcia et al. (2019) studied the benthic impacts of fin fish aquaculture at Guardamar del Segura in the southeast of Spain, using polychaetes to assess the ecological status. Both sandy and muddy sediments were examined. However, sandy experimental units had a greater sensitivity for the detection of organic matter enrichment, and the species with the most pronounced responses was Ampharete lindstroemi (Martinez-Garcia et al., 2019). Ampharete lindstroemi demonstrated significant differences in the factor ‘organic enrichment’, with drastic reductions in abundance under fish farms, and the species group Ampharete was the most likely to disappear under fish farms (Martinez-Garcia et al., 2019).

In the Rance Basin, Brittany, France, monitoring following the construction of a Tidal Power plant (built between 1963 and 1966) was undertaken to understand the ecological changes to the area. The power plant remained in operation for 45 years. Long-term changes in the soft sediment communities were observed between the years of 1976 and 2020; The polychaete Ampharete baltica (found in fine sandy sediments) declined between 1976 and 1995 in the downstream part of the Rance basin, with one possible explanation due to an increase in organic matter enrichment, which the species is sensitive to (Brébant et al., 2025).

Holte et al. (2005) investigated the variations in soft-bottom macrofauna from stratified Norwegian basins. Parvicardium ovale showed preferences for open areas and was recorded in the highest abundances at sites where total organic carbon (TOC) was relatively low (10 mg/g) compared to sites where it was absent, with TOC as high as 69 mg/g. However, Parvicardium ovale has been recorded in intertidal mudflats supporting high primary production in France, with an annual total primary production input of 466 gC/m²/yr, of which only 157 gC/m²/yr was exported (Leguerrier et al., 2003). These results suggested that the community under study had approx. 300 gC/m²/yr available, but it was not clear what the exact proportion of organic matter reached the seafloor.

Sensitivity assessment. SS.SMu.OMu.AfalPova is found in areas of fine sediment where organic content will generally be higher than in coarse sediments. Parvicardium exiguum and Ampharete grubei are both found in areas rich in silt and organic content (Lastra et al., 1993; Holme, 1949). However, the evidence presented is not directly comparable to the pressure benchmark. It is, however, likely that some mortality of the characterizing species is likely to occur as a result of organic enrichment. Resilience is therefore assessed as ‘Medium’ (<25% loss), but with low confidence. Resilience is likely to be ‘High’, and the overall sensitivity of the biotope is judged as ‘Low’.

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

Ampharete spp. and Parvicardium spp. are widespread subtidal species occurring in high numbers in fine sandy, muddy, silty sediments (Blake, 2017; Kolyuchkina et al., 2020; Park & Shin, 2025). If the sediment that characterizes this biotope were replaced with rock substrata, this would represent a fundamental change to the physical character of the biotope. The characterizing species would no longer be supported, and the biotope would be lost and/or re-classified.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very Low’, given the permanent nature of this pressure. 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

SS.SMu.OMu.AfalPova is only recorded from cohesive sandy muds (Connor et al., 2004), and the characterizing species Ampharete falcata exhibits specific preferences for fine sediment substrata that provide the material for tube-building activities. Other Ampharete spp. also occur in fine sandy, muddy sediments and are influenced by sedimentary factors (Blake, 2017; Park & Shin, 2025). In the Baltic Sea, Parvicardium ovale has been shown to have a strong positive correlation with medium grain size (Gogina et al., 2010a). In the Black Sea, Parvicardium simile is observed in silty, sandy sediments, and the ratio of silt to sand was a critical factor in distinguishing its distribution (Kolyuchkina et al., 2020).

Sensitivity assessment. In SS.SMu.OMu.AfalPova, a change in Folk class (based on the Long, 2006 simplification) would mean a change from mud and sandy mud to sand or muddy sand, or to gravelly mixed sediment. The characterizing species are unlikely to be resistant to such a change in sediment type. The biotope is likely to be lost, so resistance is therefore assessed as ‘None’ and resilience as ‘Very Low’, given the permanent nature of this pressure. The biotope is 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, expose underlying sediment which may be anoxic and/or of a different character 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 unsuitable conditions. Newell et al. (1998) state that removal of 0.5 m of sediment is likely to eliminate benthos from the affected area. Some epifaunal and swimming species may be able to avoid this pressure. Removal of 30 cm of sediment will remove species that occur at the surface and within the upper layers of sediment, such as the characterizing species of this biotope. For example, dredging operations were shown to affect large infaunal and epifaunal species, decrease sessile polychaete abundance and reduce the numbers of burrowing heart urchins (Eleftheriou & Robertson, 1992). For example, Cooper et al. (2007) investigated recovery of the seabed following marine aggregate dredging on the south-east coast of England. The authors confirmed that the sediment became coarser with increased dredging intensity and that the seabed appeared extremely uneven, with these effects appearing to persist at least 8 years after cessation of dredging activities. Ampharete spp. were recorded from the study site characterized by high intensity of dredging activities soon (<1 year) after cessation of dredging.

Sensitivity assessment: Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope, so resistance is assessed as None. Newell et al. (1998) indicate that local hydrodynamics (currents and wave action) and sediment characteristics (mobility and supply) strongly influence the recovery of soft sediment habitats. The biotope occurs in low-energy environments, so resilience is therefore judged as Medium (see resilience section). Sensitivity has been 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

This biotope (SS.SMu.OMu.AfalPova) can be affected by fishing activity in areas such as the northern Irish Sea, where the community may also contain Nephrops norvegicus (Connor et al., 2004). Ball et al. (2000a) investigated the long- and short-term consequences of a Nephrops trawl fishery on benthos and the environment of the Irish Sea, comparing samples taken before and after fishing activity. Of the two inshore and offshore sites analysed, both occurred in similar fine sand and silt-clay sediment. Their offshore station occurred at a depth and in sediment type similar to that of this biotope, and a sparse benthic macrofauna was observed, dominated by small polychaetes and a few crustaceans and bivalves. Ampharete falcata was among the species present at the control site but absent from the fished grounds at both inshore and offshore sites. However, Ampharete acutifrons was among the species found at the fished grounds, but not at the control sites. Overall, the authors found that the numbers of species, species richness and biomass had all dropped after trawling. In comparison, Kaiser et al. (2006) undertook a meta-analysis of different fishing gears on a range of habitats. The authors concluded that the footprint of the impact and the recovery of communities varied with gear and habitat types. For example, mud habitats were shown to have substantial initial impacts by otter trawling, but the effects tended to be short-lived, with an apparent long-term positive post-trawl disturbance response from the increase of small-bodied fauna.

Furthermore, SS.SMu.OMu.AfalPova occurs in cohesive sandy muds (Connor et al., 2004). Abrasion events caused by a passing fishing gear or scour by objects on the seabed surface are likely to have marked impacts on the substratum and cause turbulent resuspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards from digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures).

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

Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). In general, species with large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Fauna that inhabit or construct tubes, such as the characterizing species Ampharete falcata, are likely to be particularly vulnerable to damage or disturbance by beam trawls (Kaiser & Spencer, 1996). Furthermore, Parvicardium ovale 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 the small bivalves.

Sensitivity assessment. The characterizing species are considered likely to be damaged and removed by abrasion, particularly the turf of Ampharete falcata, as these protrude from the muddy sediments and are not physically robust. Therefore, resistance to abrasion is assessed as ‘None’. Resilience of the biotope is likely to be ‘Medium’ given that the low energy of the biotope may impede immediate recolonization by the characterizing species (see resilience). The biotope is therefore considered to have ‘Medium’ sensitivity to abrasion or disturbance of the surface of the seabed.

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

This biotope (SS.SMu.OMu.AfalPova) can be affected by fishing activity in areas such as the northern Irish Sea, where the community may also contain Nephrops norvegicus (Connor et al., 2004). Ball et al. (2000a) investigated the long- and short-term consequences of a Nephrops trawl fishery on benthos and the environment of the Irish Sea, comparing samples taken before and after fishing activity. Of the two inshore and offshore sites analysed, both occurred in similar fine sand and silt-clay sediment. Their offshore station occurred at a depth and in sediment type similar to that of this biotope, and a sparse benthic macrofauna was observed, dominated by small polychaetes and a few crustaceans and bivalves. Ampharete falcata was among the species present at the control site but absent from the fished grounds at both inshore and offshore sites. However, Ampharete acutifrons was among the species found at the fished grounds, but not at the control sites. Overall, the authors found that the numbers of species, species richness and biomass had all dropped after trawling. In comparison, Kaiser et al. (2006) undertook a meta-analysis of different fishing gears on a range of habitats. The authors concluded that the footprint of the impact and the recovery of communities varied with gear and habitat types. For example, mud habitats were shown to have substantial initial impacts by otter trawling, but the effects tended to be short-lived, with an apparent long-term positive post-trawl disturbance response from the increase of small-bodied fauna.

Furthermore, SS.SMu.OMu.AfalPova occurs in cohesive sandy muds (Connor et al., 2004). Abrasion events caused by a passing fishing gear or scour by objects on the seabed surface are likely to have marked impacts on the substratum and cause turbulent resuspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards from digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures).

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

Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). In general, species with large body size, low dispersal, late maturation and long lifespan are considered sensitive to physical disturbance (Bolam et al., 2014; Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Fauna that inhabit or construct tubes, such as the characterizing species Ampharete falcata, are likely to be particularly vulnerable to damage or disturbance by beam trawls (Kaiser & Spencer, 1996). Furthermore, Parvicardium ovale 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 the small bivalves.

Sensitivity assessment. The characterizing species are considered likely to be damaged and removed by abrasion, particularly the turf of Ampharete falcata, as these protrude from the muddy sediments and are not physically robust. Therefore, resistance to abrasion is assessed as ‘None’. Resilience of the biotope is likely to be ‘Medium’ given that the low energy of the biotope may impede immediate recolonization by the characterizing species (see resilience). The biotope is therefore considered to have ‘Medium’ sensitivity to abrasion or disturbance of the surface of the seabed.

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 biotope occurs in sheltered areas, in fine sediments, subject to high suspended sediment loads. Therefore, the important characteristic species are unlikely to be impacted by an increase in suspended sediments. Suspended sediment and siltation of particles are likely to be important for tube building in Ampharete falcata, so a decrease in suspended solids may reduce the material available for tube building. For most benthic deposit feeders, food is suggested to be a limiting factor for body and gonad growth, at least between events of sedimentation of fresh organic matter (Hargrave, 1980; Tenore, 1988). Consequently, increased organic matter in suspension that is deposited may enhance food supply on the seabed. A decrease in the suspended sediment and hence siltation may reduce the flux of particulate material to the seabed. Since this includes organic matter, the supply of food to the biotopes would probably also be reduced.

Sensitivity assessment: An increase in suspended solids at the pressure benchmark level is unlikely to affect the characterizing species of this offshore biotope. Although a decrease in suspended matter in the biotope could result in limitation of material for tube-building activity of Ampharete falcata, the species is not likely to suffer mortality due to its ability to also use sand for tube building. Resistance and resilience of the biotope are assessed as High, so the biotope is considered Not Sensitive to a change in suspended solids at the pressure benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

The biotope occurs in sheltered areas, in fine sediments, subject to high suspended sediment loads. Therefore, the important characteristic species are unlikely to be impacted by an increase in suspended sediments. Suspended sediment and siltation of particles are likely to be important for tube building in Ampharete falcata, so a decrease in suspended solids may reduce the material available for tube building. For most benthic deposit feeders, food is suggested to be a limiting factor for body and gonad growth, at least between events of sedimentation of fresh organic matter (Hargrave, 1980; Tenore, 1988). Consequently, increased organic matter in suspension that is deposited may enhance food supply on the seabed. A decrease in the suspended sediment and hence siltation may reduce the flux of particulate material to the seabed. Since this includes organic matter, the supply of food to the biotopes would probably also be reduced.

Sensitivity assessment: An increase in suspended solids at the pressure benchmark level is unlikely to affect the characterizing species of this offshore biotope. Although a decrease in suspended matter in the biotope could result in limitation of material for tube-building activity of Ampharete falcata, the species is not likely to suffer mortality due to its ability to also use sand for tube building. Resistance and resilience of the biotope are assessed as High, so the biotope is considered Not Sensitive to a change in suspended solids at the pressure benchmark level.

Smothering and siltation rate changes (heavy)

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

Evidence

Smothering by 30 cm of sediment is likely to lead to the death of some of the organisms in the biotope. Ampharete falcata is a small polychaete worm up to 18 mm in length (Heath, 2005). The populations of this tube-dwelling polychaete will probably be unable to feed or respire, resulting in some mortality, although 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). It is unclear whether the small bivalve Parvicardium ovale would be able to migrate vertically through the deposited sediment to re-establish burrow openings.

Sensitivity assessment: No direct evidence was found regarding the ability of the characterizing species to deal with this pressure. However, some mortality of both characterizing species, Ampharete falcata and Parvicardium ovale, is likely to occur as a result of a ‘heavy’ deposition of fine sediment in a discrete event. Resistance is therefore assessed as Low (loss 25-75%), with low confidence. However, resilience is likely to be High and the biotope is considered to have Low sensitivity at the pressure 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.

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 Ampharete falcata or Parvicardium ovale. However, one study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity, indicative of a stress response, were reported. However, there is Insufficient evidence on which to base an assessment of the likely sensitivity of this biotope to EMFs. 

Underwater noise changes

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

Evidence

Ampharete falcata, Parvicardium ovale and some of the other species in the biotope may respond to vibrations from predators or excavation by retracting their palps into their tubes. However, the characterizing species are unlikely to be affected by noise pollution, so the biotope is assessed as Not relevant.

Introduction of light or shading

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

Evidence

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 50 m (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. 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

The biotope occurs in deep water where available light is very low. Of the characterizing species, Parvicardium ovale lives infaunally and Ampharete falcata has only two simple eyespots, so have no or poor visual perception and unlikely to be affected by visual disturbance such as shading. Therefore, this pressure is probably 'Not relevant' to this biotope.

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 key characterizing species in the biotope 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

A record of the parasitic polychaete Heamatocleptes terebellidis, normally associated with lamella-worm Terebellides stroemmii, has also been recorded from Ampharete falcata in the Irish Sea (O’Reilly, 2016). The authors recognized that the parasite is likely to be under-recorded but probably widely distributed due to its clandestine habits, as it has only been recorded from Sweden, where it was originally recorded. The parasite is known to live inside the coelom of Terebellides stroemmii, and other polychaetes. No further information of the potential impacts on the polychaetes’ health and survival were provided.

Sensitivity assessment: No direct evidence of the biotopes being affected by the introduction of microbial pathogens was found on which to assess this pressure.

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

It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. Even in areas where Nephrops norvegicus is present, trawlers are likely to avoid clogging of the nets by the high densities of Ampharete tubes. This pressure is therefore considered Not relevant.

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

SS.SMu.OMu.AfalPova can be affected by fishing activity in areas such as the northern Irish Sea, where the community may also contain Nephrops norvegicus (Connor et al., 2004). Species living in deep subtidal mud habitats are considered to be more vulnerable to physical disturbance as they are adapted to stable conditions (Pommer et al., 2016). Similarly, sessile epifauna and species that live at or near the sediment surface are likely to be more vulnerable to physical disturbance than deep-burrowing or mobile species (Pommer et al., 2016). Fauna that inhabit or construct tubes, such as the characterizing species Ampharete falcata, are likely to be particularly vulnerable to damage or disturbance by beam trawls (Kaiser & Spencer, 1996). Furthermore, Parvicardium ovale 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 the small bivalves.

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of bycatch. Species in this biotope, including the characterizing species Ampharete falcata and Parvicardium ovale, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Loss of the Ampharete falcata turf, along with the dense community of Parvicardium ovale, would alter the character of the biotope, resulting in re-classification, and would alter the physical structure of the habitat, resulting in the loss of the ecosystem functions such as secondary production performed by these species.

Sensitivity assessment. Removal of the characterizing species would result in the biotope being lost or re-classified. Thus, the biotope is considered to have a resistance of ‘None’ to this pressure and to have ‘Medium’ resilience, resulting in the sensitivity being judged as ‘Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

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

In the Rance basin, Brittany, France, monitoring following the construction of a Tidal Power plant (built between 1963 and 1966) was undertaken to understand the ecological changes to the area. The power plant remained in operation for 45 years, and between the years of 1976 and 2020, long-term changes in the soft sediment communities were observed, mainly a decline in the polychaete Ampharete baltica was recorded between 1976 and 1995 in the downstream part of the Rance basin (Brébant et al., 2025). Brébant et al. (2025) noted how populations of Ampharete baltica, which are mainly found in fine sandy bottoms, are sensitive to organic matter enrichment; however, another possible explanation for their decline may also have been strongly influenced by the proliferation of Crepidula fornicata, which invaded the downstream part of the Rance basin in the late 1980s.

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

Sensitivity assessment

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

The carpet sea squirt, Didemnum vexillum

Evidence

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

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build 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 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). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

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

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in 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 the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures, such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

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

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

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

Sensitivity assessment

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

The Pacific oyster, Magallana gigas

Evidence

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

Wireweed, Sargassum muticum

Evidence

This biotope is circalittoral, subject to sedimentation, both of which probably exclude macroalgae. Hence, it is unlikely to be colonized by Sargassum. Therefore, this INIS is probably 'Not relevant' in this biotope.

Wakame, Undaria pinnatifida

Evidence

This biotope is circalittoral, subject to sedimentation, both of which probably exclude macroalgae. Hence, it is unlikely to be colonized by Undaria. Therefore, this INIS is probably 'Not relevant' in this biotope.

Other INIS

Evidence

Although several non-native species of polychaete and mollusc have invaded British waters, there are none that are likely to affect SS.SMu.OMu.AfalPova. Although there is always the potential for this to occur. Hence, there is currently 'No evidence' for the effects of other invasive species on this biotope.