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 distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001), suggesting that some species are intolerant of temperature changes.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). In the Yellow and East China Seas, Thyasira tokunagai is found in habitats with low bottom temperatures ranging from 8 to 14°C, particularly at depths >50 m in the Yellow Sea (Xu et al., 2022). Xu et al. (2022) used a species distribution model to predict the effect of warming on macrobenthos in the Yellow and East China Seas and found that Thyasira tokunagai would likely contract its range and showed northward movement trends under future climates. Thyasira tokunagai would mainly lose its habitat suitability in the shallow areas of the Yellow Sea (Xu et al., 2022). Wilson (1981) investigated the temperature tolerances of six bivalve species from Dublin Bay. The author concluded that species variations in tolerance to increased temperature varied seasonally and with distribution along tidal height. Lethal temperatures for all six bivalve species in the study varied greatly and were, in most cases, well above 20°C. The maximum sea surface temperatures around the British Isles rarely exceed 20°C (Hiscock, 1998).

In the northern North Sea, foraminiferan-dominated biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria are present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low temperature range (Stephen, 1923), with temperatures oscillating between 6 and 8°C (McIntyre, 1961). Further south, a greater range of bottom temperatures occurs, and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community, which contains Thyasira flexuosa and Crithionina granum, lives in ‘varying Boreal water’, with temperatures varying between 7 and 13°C (McIntyre, 1961).

Sensitivity assessment. Available information suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. This pressure is not assessed due to a lack of evidence.

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 distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001). This suggests that they are intolerant of temperature changes.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). In the Yellow and East China Seas, Thyasira tokunagai is found in habitats with low bottom temperatures ranging from 8 to 14°C, particularly at depths >50 m in the Yellow Sea (Xu et al., 2022). Thyasira sarsii and Thyasira obsoleta have been recorded in sub-Arctic fjords, Northern Norway; a rare single individual Thyasira obsoleta (no established populations) was recorded in Saltfjord, where bottom water conditions were approximately 7.0°C and 35.3 psu, while a low abundance of Thyasira sarsii was recorded in Skjerstadfjord, where the bottom waters were colder and less saline (4.9°C and 33.8 psu) (Kokarev et al., 2024). Thyasira obsoleata is asymbiotic, feeding on particulate matter, while Thyasira sarsii is symbiotic, relying on sulphur-oxidizing bacteria and hydrogen sulphide conditions typical of these fjord conditions (Kokarev et al., 2024).

In the northern North Sea, foraminiferan-dominated biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria are present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low temperature range (Stephen, 1923), with temperatures oscillating between 6 and 8°C (McIntyre, 1961). Further south, a greater range of bottom temperatures occurs, and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community, which contains Thyasira flexuosa and Crithionina granum, lives in ‘varying Boreal water’, with temperatures varying between 7 and 13°C (McIntyre, 1961).

Sensitivity assessment. Available information suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. This pressure is not assessed due to a lack of evidence.

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

The Saccammina genus is found in environments with a wide salinity range of 28 to 32 psu, and Ponomarenko, Krechik & Dorokhova (2020) confirm these findings and show that this genus can tolerate salinities as low as 10.3 psu. In addition, Ponomarenko, Krechik & Dorokhova (2020) noted that in the south-eastern Baltic Sea, Saccammina were observed in their highest numbers in areas near-bottom water salinity (11.8 to 14.4 psu in the Gdansk Deep, the deepest part of the Gdansk Basin, and >18 psu in the Bornholm Basin), likely due to the proximity of the inflow source (of dense, high-saline, oxygenated North Sea water), which presents favourable environmental conditions for most of foraminiferal genera.

The Haplophragmoides genus is also observed to have a variable salinity tolerance. In a salt marsh in Suncheon Bay, South Korea, Haplophragmoides are distributed broadly throughout it, experiencing salinity below approximately 11 psu (Jeong et al., 2016). The presence of Haplophragmoides spp. is usually associated with lower salinities (Hayward et al., 2004 cited in Jeong et al., 2016), and Haplophragmoides wilberti is well documented as an obligate brackish water inhabitant around New Zealand (Hayward and Hollis 1994 cited in Jeong et al., 2016). In Kelantan Delta, Peninsular Malaysia, mangrove swamps experience a salinity range of 4 to 25 psu, and Haplophragmoides wilberti is dominant in low salinity waters (mean 7 psu), but were observed throughout the mangrove’s full salinity range, whereas Haplophragmoides manilaensis were associated with higher salinity waters (18 to 20 psu) (Suriadi et al., 2025). In the Florida Everglades, Haplophragmoides wilberti are observed in salinities of 16 to 18 psu, likely having a salinity preference of 10 to 20 psu and is clearly excluded from salinities lower than 6 psu, but occur at a wide range of salinities worldwide (Verlaak & Collins, 2021). Haplophragmoides wilberti is described as a low salinity species but occurs in New Zealand mangroves between 3 and 20 psu, in a North Carolina salt marsh between 19 and 36 psu, and throughout mangroves in Trinidad (Guilbault & Patterson, 2000, Hayward & Hollis,1994, Kemp et al., 2009, and Saunders,1958 cited in Verlaak & Collins, 2021).

Thyasira sarsii and Thyasira obsoleta have been recorded in sub-Arctic fjords, Northern Norway; a rare single individual Thyasira obsoleta (no established populations) was recorded in Saltfjord, where bottom water conditions were approximately 7.0°C and 35.3 psu, while a low abundance of Thyasira sarsii was recorded in Skjerstadfjord, where the bottom waters were colder and less saline (4.9°C and 33.8 psu) (Kokarev et al., 2024). Thyasira obsoleta is asymbiotic, feeding on particulate matter, while Thyasira sarsii is symbiotic, relying on sulphur-oxidizing bacteria and hydrogen sulphide conditions typical of these fjord conditions (Kokarev et al., 2024). According to OBIS (2025), Thyasira spp. is recorded from 5 to 40 psu, but most records occur between 30 and 35 psu.

This circalittoral biotope is found in full salinity (30-35 ppt) and has not been recorded from locations with brackish waters. It is probably highly intolerant of an increase in salinity. In the northern North Sea, foraminiferan-dominated biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria are present where salinities remain fairly constant, between 35.20 and 35.26 ppm (McIntyre, 1961). However, the Atlantic community, which contains Thyasira flexuosa and Crithionina granum (foraminifera), occurs in waters where salinity varies between 33.86 and 34.33 (McIntyre, 1961). This suggests that the community is dependent on a relatively constant salinity.

Sensitivity assessment. Although the available information suggests that both Saccammina and Haplophragmoides have a wide salinity tolerance, limited evidence exists on the effects of salinity >40 psu on the characteristic species. Thyasira spp. are common and dominant at full salinity. This biotope occurs in full salinity, so an increase in salinity would represent hypersaline conditions. Although unlikely in the deep-water characteristic of the biotope, hypersaline conditions would probably be detrimental. However, no relevant evidence was found. Therefore, the evidence is ‘insufficient’ to form the basis of an assessment. 

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

The Saccammina genus is found in environments with a wide salinity range of 28 to 32 psu, and Ponomarenko, Krechik & Dorokhova (2020) confirm these findings and show that this genus can tolerate salinities as low as 10.3 psu. In addition, Ponomarenko, Krechik & Dorokhova (2020) noted that in the south-eastern Baltic Sea, Saccammina were observed in their highest numbers in areas near-bottom water salinity, likely due to the proximity of the inflow source (of dense, high-saline, oxygenated North Sea water), which presents favourable environmental conditions for most of the foraminiferal genera.

The Haplophragmoides genus is also observed to have a variable salinity tolerance. In a salt marsh in Suncheon Bay, South Korea, Haplophragmoides are distributed broadly throughout it, experiencing salinity below approximately 11 psu (Jeong et al., 2016). The presence of Haplophragmoides spp. is usually associated with lower salinities (Hayward et al., 2004 cited in Jeong et al., 2016), and Haplophragmoides wilberti is well documented as an obligate brackish water inhabitant around New Zealand (Hayward and Hollis 1994 cited in Jeong et al., 2016). In Kelantan Delta, Peninsular Malaysia, mangrove swamps experience a salinity range of 4 to 25 psu, and Haplophragmoides wilberti is dominant in low salinity waters (mean 7 psu), but were observed throughout the mangrove’s full salinity range, whereas Haplophragmoides manilaensis were associated with higher salinity waters (18 to 20 psu) (Suriadi et al., 2025). In the Florida Everglades, Haplophragmoides wilberti are observed in salinities of 16 to 18 psu, likely having a salinity preference of 10 to 20 psu and is clearly excluded from salinities lower than 6 psu, but occur at a wide range of salinities worldwide (Verlaak & Collins, 2021). Haplophragmoides wilberti is described as a low salinity species but occurs in New Zealand mangroves between 3 and 20 psu, in a North Carolina salt marsh between 19 and 36 psu, and throughout mangroves in Trinidad (Guilbault & Patterson, 2000, Hayward & Hollis,1994, Kemp et al., 2009, and Saunders,1958 cited in Verlaak & Collins, 2021).

Thyasira spp. can inhabit waters of reduced salinity, with 25 to 30 psu being optimal. However, adults exposed to lower than optimal salinities produced non-viable or slow-developing eggs (Jackson, 2007). Thyasira sarsii and Thyasira obsoleta have been recorded in sub-Arctic fjords, Northern Norway; a rare single individual Thyasira obsoleta (no established populations) was recorded in Saltfjord, where bottom water conditions were approximately 7.0°C and 35.3 psu, while a low abundance of Thyasira sarsii was recorded in Skjerstadfjord, where the bottom waters were colder and less saline (4.9°C and 33.8 psu) (Kokarev et al., 2024). Thyasira obsoleata is asymbiotic, feeding on particulate matter, while Thyasira sarsii is symbiotic, relying on sulphur-oxidizing bacteria and hydrogen sulphide conditions typical of these fjord conditions (Kokarev et al., 2024). According to OBIS (2025), Thyasira spp. is recorded from 5 to 40 psu, but most records occur between 30 and 35 psu.

Sensitivity assessment. This circalittoral biotope is found in full salinity (30-35 ppt) and has not been recorded from locations with brackish waters, and so is probably intolerant of a decrease in salinity. In the northern North Sea, foraminiferan-dominated biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria are present where salinities remain fairly constant, between 35.20 and 35.26 ppm (McIntyre, 1961). However, the Atlantic community, which contains Thyasira flexuosa and Crithionina granum (foraminifera), occurs in waters where salinity varies between 33.86 and 34.33 (McIntyre, 1961). This suggests that the community is dependent on a relatively constant salinity. However, the above evidence also suggests that Saccammina and Haplophragmoides have a wide salinity tolerance and would likely not be severely affected by a decrease in salinity from full (30 to 35) to reduced (18 to 30). Overall, resistance has been assessed as ‘Low’ to represent the possible loss in abundance of the remaining species within the community. Hence, resilience is assessed as ‘Medium’, and sensitivity 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

This biotope occurs in fine soft mud and deep waters where tidal flows are weak (<0.5 m/s) (JNCC, 2015). In the Espírito Santo Basin, Southeastern Brazil, hydrodynamic conditions are one of the main drivers controlling the bathymetric distribution of benthic foraminiferal assemblages (de Almeida, de Mello & Bastos, 2023). Concentrations of both Psammosphaera and Saccammina have been observed increasing with depth in the south-eastern Baltic Sea along with less intense hydrodynamic activity (Ponomarenko, Krechik & Dorokhova, 2020). In a salt marsh in Suncheon Bay, South Korea, Haplophragmoides are exposed to tidal flows of 26.57 cm/s (Jeong et al., 2016).

Following an increase in water flow rate at the pressure benchmark, the surface sediments and epifaunal foraminifera may be subject to some winnowing, but the cohesive nature of subtidal muds will limit the impact. The lower substratum inhabited by mature specimens of Thyasira sp., infaunal foraminifera, polychaetes and other species are likely to remain unchanged.

Sensitivity assessment. Since the majority of characterizing species are likely to persist, resistance has been assessed as ‘Medium’ as some removal of epifaunal foraminifera may occur. On return to normal water flow rates, resilience is assessed as ‘High’ as water transport from periodic storm events may restore populations of adult foraminifera. Biotope sensitivity is, therefore, assessed as 'Low’.

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

Not relevant to sublittoral 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

As this biotope occurs in circalittoral habitats, it is not directly exposed to the action of breaking waves. In the Espírito Santo Basin, Southeastern Brazil, hydrodynamic conditions are one of the main drivers controlling the bathymetric distribution of benthic foraminiferal assemblages (de Almeida, de Mello & Bastos, 2023). Concentrations of both Psammosphaera and Saccammina have been observed increasing with depth in the south-eastern Baltic Sea along with less intense hydrodynamic activity (Ponomarenko, Krechik & Dorokhova, 2020). Thyasira gouldii lives in rather wave-sheltered areas at the heads of sea lochs (Jackson, 2007). Increases in wave exposure may disrupt the sediment in which they live, cause continual displacement and physical damage to the shells, which are thin and fragile. The characterizing Thyasira spp. and associated polychaete species that burrow are protected within the sediment, but the characterizing foraminiferans would be exposed to oscillatory water flows at the seabed. No specific evidence was found to assess this pressure.

Sensitivity assessment. The biotope is considered to be ‘Not sensitive’ to changes in wave height, at the pressure benchmark, based on the depth of the habitat.

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

Kim et al. (2023) observed the long-term effects of chromium on the benthic environment in the East Sea-Byeong, Korea, an area of sea that is used as an ocean dumping site for various waste and pollutants. They were able to show that the chromium content in red mud surpassed the effect range median from the Sediment Quality Guidelines (370 mg/kg) at 758 mg/kg, however, Thyasira tokunagai dominated the study station (82.6% of total abundance), highlighting it as an opportunistic and contamination-stress-resistant species. In addition, Thyasira tokunagai abundance positively correlated with heavy metals, including mercury (Hg), chromium (Cr), copper (Cu) and cadmium (Cd) (Kim et al., 2018; 2023). Furthermore, Thyasira tokunagai abundance consistently increased over time (post-2010), which may contribute to the improvement of habitat conditions for the benthic environment (Kim et al., 2023). However, abundance was significantly negatively correlated with high concentrations of manganese (Mn), suggesting Mn is a potentially toxic metal to Thyasira spp. (Kim et al., 2018). Lebedeva et al. (2018) reported that Thyasira gouldi accumulated elevated total mercury (Hg) from Grofjorden, Svalbard.

Thyasira spp. have been recorded in impacted sites around oil and gas platforms in the North Sea, where elevated concentrations of heavy metals and total hydrocarbons were recorded up to 500 m from these platforms (Chen et al., 2024). Thyasira spp. were frequently recorded in association with post-drilling contamination, for example, in drill-cutting piles in the North Sea (Henry et al., 2017) and under heavy tailings sedimentation (Ramierz-Llodra et al., 2015).

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

Thyasira spp. have been recorded at increased abundance in impacted sites around oil and gas platforms in the North Sea, where elevated concentrations of heavy metals and total hydrocarbons were recorded up to 500 m from these platforms (Chen et al., 2024), indicating high resistance to hydrocarbon contamination.

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

No evidence.

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

Activated carbon (AC) can be applied to the seabed as a thin-layer cap to sediments contaminated with mercury and dioxins to reduce their bioavailability. While effective in sequestering pollutants, the application of AC mixed with clay can have substantial ecological impacts on benthic communities, particularly suspension feeders. In the Grenland fjords in southern Norway, Raymond et al. (2021) noted that Thyasira equalis was unaffected by AC capping. It was suggested that this could be due to their symbiotic relationship with chemosynthetic bacteria, allowing them to tolerate reduced food availability.

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

Laboratory experiments indicate that foraminifera migrate up and down in the sediment in response to changes in oxygen and food supplies (Gooday, 2019). Epifaunal and shallow infaunal species are generally intolerant of low oxygen concentrations and tend to feed on relatively fresh organic matter, whereas deep infaunal foraminifera species are more tolerant of oxygen depletion and less dependent on labile organic matter (Nomaki et al., 2005, 2006 cited in Gooday, 2019). Deep-infaunal species tend to exhibit an abundance peak near the level at which oxygen (and nitrate) disappear from the sediment, although they also present in oxygenated levels above this depth, as well as in deeper anoxic sediments (Koho and Piña-Ochoa, 2012 cited in Gooday, 2019). Overall, foraminifera exhibit greater tolerance of oxygen deficiency than most metazoan taxa, except for nematodes and some annelids, although prolonged anoxia combined with sulfidic conditions will lead to mortality, such as in the deeper part of the Black Sea and other anoxic basins. (Moodley et al.,1998 and Levin, 2003 cited in Gooday, 2019). Where hypoxia is permanent, oxygen probably only becomes an important limiting factor for foraminifera at concentrations well below 1 mg/l, but some species are abundant at levels of 0.1 mg/l or less, and a few apparently live in permanently anoxic sediments (Gooday, 2019).

The genus Saccammina displays some tolerance to anoxic events. In the northeastern Tethys, Kopet-Dagh basin, development of salinity-stratified water can induce bottom water oxygen depletion, in which Saccammina are observed (Kalanat et al., 2017). Foraminifera, in particular Uvigerinids, play a notable role in carbon cycling in oxygen minimum zones (OMZs). On the Pakistan margin of the Arabian Sea, where a strong OMZ is developed, foraminifera (>300-mm size fraction) were mainly responsible for processing organic carbon at 300 m depth, where oxygen levels were lowest, while the metazoan macrofauna took over this role at deeper sites, where oxygen levels were higher (Woulds et al., 2007 cited in Gooday, 2019).

Dando & Spiro (1993) found that numbers of Thyasira equalis and Thyasira sarsi decreased rapidly following the de-oxygenation of bottom water in the deep basin of Gullmar fjord in 1979-80. However, in the deep trough of the Gulf of St. Lawrence, Canada, where hypoxic zones are found, Thyasira are present, with mean densities increasing to around 700 individuals /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.

Between 1979-1993, Heteromastus filiformis and Thyasira equalis were dominant species in Byfjorden, Raunefjorden, and Sørfjorden, Norway (Johansen et al., 2018). From the mid-1990s to 2016, Heteromastus filiformis increased in abundance, and species dominance shifted from Heteromastus filiformis and Thyasira equalis to Heteromastus filiformis and Paramphinome jeffreysii. This change coincided with dissolved oxygen levels depleting to a hypoxic state, sediment organic matter increasing by 2%, and a ~1°C increase in bottom temperature.

Sensitivity assessment: The evidence suggests that infaunal foraminifera are tolerant of low oxygen conditions or even anoxia, while shallow infaunal or epifaunal foraminifera are intolerant of low oxygen conditions (Gooday, 2019). For example, the genus Saccammina displays some tolerance to anoxic events, but no evidence for other characteristic genera was found. Thyasira spp have also been shown to be tolerant of low oxygen levels (e.g. ca 2.3 mg/l) (Zettler & Pollehne, 2023), but removed by periodic anoxia (Dando & Spiro, 1993). Therefore, a proportion of the characterizing species in these biotopes could be lost due to deoxygenation at the benchmark level, depending on species and resistance is assessed as ‘Medium’. Resilience is probably ‘High’, and therefore the biotope’s sensitivity to this pressure is likely to be ‘Low’.

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

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. Wang et al. (2026) studied the effect of phosphate enrichment on benthic foraminifera over a dissolved inorganic phosphate (DIP) gradient of 0.31 to 50.0 μmol/L for 3 and 6 months. Sediment samples were taken from Jiaozhou Bay, China, and a pronounced compositional shift along the phosphate gradient was observed, with assemblages transitioning from Ammonia-dominated under low-DIP to Psammosphaera-dominated under high-DIP (Wang et al., 2026). Wang et al. (2026) concluded that phosphate stress reshapes benthic foraminiferal communities by favouring stress-tolerant taxa, and since the abundance of Psammosphaera strongly correlated with nutrient gradients, the species is a robust bioindicator of phosphate enrichment.

In a report to identify seabed indicator species to support implementation of the EU habitats and water framework directives, Thyasira spp. were reported as likely to be favoured by nutrient enrichment (Hiscock et al., 2005). In the East Sea-Byeong, Korea, an area of sea is used as an ocean dumping site for various waste and pollutants, however, during surveys between 2009 and 2015, Thyasira tokunagai was the dominant species observed (numbering 5,034 ind/m² in 2014), accounting for 82.6% of the total abundance of all identified species (Kim et al., 2018). In addition, total nitrogen was significantly correlated with benthic community metrics, including species richness and abundance, and Thyasira tokunagai was also the dominant benthic molluscan in sediments rich in organic matter and with a low manganese content (Kim et al., 2018). Enrichment from pulp mills is believed to have been the cause of the death of two populations of Thyasira gouldii in west Scottish sea lochs. However, Thyasira flexuosa has been recorded at densities of up to 4,000/m² in enriched areas (Dando & Southward, 1986).

Sensitivity assessment. 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

Many of the species present are deposit feeders, characteristic of organically enriched areas. An input of organic matter at the pressure benchmark is likely to provide a food subsidy to these species. Thyasira spp. are characteristic of organically enriched offshore sediments (Connor et al., 2004; JNCC, 2022) and have been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979). For example, in the East Sea-Byeong, Korea, an area of sea is used as an ocean dumping site for various waste and pollutants, however, during surveys between 2009 and 2015, Thyasira tokunagai was the dominant species observed (numbering 5,034 ind/m² in 2014), accounting for 82.6% of the total abundance of all identified species (Kim et al., 2018). In addition, total nitrogen was significantly correlated with benthic community metrics, including species richness and abundance, and Thyasira tokunagai was also the dominant benthic molluscan in sediments rich in organic matter and with a low manganese content (Kim et al., 2018).

Birchenough & Frid (2009) analysed the succession of the macrobenthic community in the three years following cessation of sewage sludge disposal off the Northumberland coast, UK, after 18 years of dumping. The authors reported a continued localized increase of individuals and species in the disposal area, followed by a decline in the two sites close to the disposal site (less than 1 km). The control stations did not show this fluctuation in species abundance other than what was expected because of seasonal variations. Particularly relevant was the increase in abundance of the bivalve Thyasira flexuosa. Other studies have also identified elevated Thysira flexuosa abundances in polluted or semi-polluted areas, mainly in fine sediments with high organic content (Pearson & Rosenberg, 1978; López-Jamar et al., 1987; Parra, 2002, cited in Birchenough & Frid, 2009). In Bonne Bay, Newfoundland, Thyasira cf. gouldi were most abundant at sites with higher organic matter content and least abundant where organic matter content was lowest (Batstone & Dufour, 2016).

Organic input from deep-water fish farms can have severe effects on bivalves. Thyasira equalis were more abundant at reference sites than they were at sites in which deep-water fish farming took place (Valdemarsen et al., 2015). Thyasira equalis represented up to 10% of the abundance at the low-current reference site and up to 5% of the abundance at the moderate-current reference site. In both farm sites, this species was absent, indicating high sensitivity to organic enrichment.

Thyasira spp. are also frequently observed at cold methane seeps, where they exist in high sulphide concentrations and hypoxic conditions (Savard et al., 2021; Somoza et al., 2021). Rare specimens of Thyasira sp. have also been reported in association with odontocete bones from Miocene whale fall communities (Danise et al., 2016).

Between 1979 and 1993, Heteromastus filiformis and Thyasira equalis were dominant species in Byfjorden, Raunefjorden, and Sørfjorden, Norway (Johansen et al., 2018). From the mid-1990s to 2016, Heteromastus filiformis increased in abundance, and species dominance shifted from Heteromastus filiformis and Thyasira equalis to Heteromastus filiformis and Paramphinome jeffreysii. This change coincided with dissolved oxygen levels depleting to a hypoxic state, sediment organic matter increasing by 2%, and a ~1°C increase in bottom temperature.

In the development of the AMBI index to assess disturbance (including organic enrichment), both Borja et al. (2000) and Gittenberger & van Loon (2011) assigned Thyasira flexuosa to their Ecological Group III (defined as species tolerant to excess organic matter enrichment). These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slightly unbalanced situations). 

Sensitivity assessment. The evidence presented suggests that the majority of the characterizing and associated species in the biotopes are likely to be able to utilize additional organic load as food and are present in enriched habitats. Biotope resistance is therefore assessed as ‘High’ and resilience as ‘High’, so that the biotope is assessed as ‘Not sensitive’.

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

The biotope is characterized by the sedimentary habitat (JNCC, 2015), a change to an artificial or rock substratum would alter the character of the biotope, leading to reclassification and the loss of the sedimentary community, including the characterizing bivalves, polychaetes and echinoderms that live buried within the sediment.

The foraminifera Psammosphaera, Saccammina, and Haplophragmoides are all documented to be present on fine-grained, sandy or muddy sediment (Ponomarenko, Krechik & Dorokhova, 2020; Suriadi et al., 2025), and sediment properties are one of the main drivers controlling the bathymetric distribution of benthic foraminiferal assemblages (de Almeida, de Mello & Bastos, 2023).

Sensitivity assessment. Based on the loss of the biotope, resistance is assessed as ‘None’, recovery is assessed as ‘Very low’ (as the change at the pressure benchmark is permanent and sensitivity 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

The change referred to at the pressure benchmark is a change in sediment classification (based on Long, 2006) rather than a change in the finer-scale original Folk categories (Folk, 1954). For muddy sediments, resistance is assessed based on a change to either mixed sediments or sand and muddy sands. The characterizing foraminifera Crithionina might prefer sediment environments with smaller particle sizes (Ze et al., 2025). The characterizing Thyasira spp. prefer fine sediments including mud, muddy sand and sandy mud (Jackson, 2007; Martin et al., 2019). In Bonne Bay, Newfoundland, Thyasira cf. gouldi were less abundant in Neddy’s Harbour, a site characterized by a greater percentage of coarser sediments (86% sand and 14% silt-clay), low organic matter content and subject to anthropogenic activities such as dredging (Batstone & Dufour, 2016).

Sensitivity assessment. A change in Folk class from mud to sand or muddy sand would probably not eliminate the characterizing Thyasira spp, and the habitat may still be suitable for foraminiferans, however, some grades of mixed sediments with low fractions of fine sediments would probably be unsuitable. Resistance is therefore assessed as ‘Low’ (loss of 25-75%), and resilience is considered 'Very low’ given the permanent nature of this pressure. Sensitivity is therefore assessed as ‘High’.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock) (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) stated that removal of 0.5 m of sediment was 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 is likely to remove species that occur at the surface and within the upper layers of sediment, such as the characterizing species of this biotope. For example, Thyasira species are found 2 to 8 cm below the sediment surface (Dando & Southward, 1986). 

Sensitivity assessment. Resistance is assessed as ‘None’ as the extraction of the sediment will remove the characterizing and associated species present. Resilience is assessed as ‘Medium’ as foraminiferans may require longer than two years to re-establish (see resilience section), and sediments may need to recover (where exposed layers are different). Biotope sensitivity is therefore assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Little evidence was found to assess this pressure. However, Jackson (2007) stated that Thyasira spp. are small bivalves with thin, fragile shells likely to be damaged and result in mortality within the population, depending on the force of impacts. Specifically, Xu et al. (2022) noted that Thyasira tokunagai has a fragile shell, unlike other bivalves, which makes it vulnerable to predation and therefore, would make it vulnerable to abrasion damage. Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine.

Sensitivity assessment. Abrasion may result in some damage and mortality, resistance is therefore assessed as 'Medium' and resilience as 'High', biotope sensitivity is therefore assessed as 'Low'.

Penetration or disturbance of the substratum subsurface

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

Evidence

Activities that disturb the surface and penetrate below the surface would remove/damage epifauna and infaunal species, such as the characterizing species within the direct area of impact.

The shells of Thyasira spp. are thin and fragile, and penetration and disturbance of the sediment is likely to lead to damage and mortality within the population (Jackson, 2007; Xu et al., 2022). Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months. The direct mortality (percentage of initial density) of Thyasira flexuosa from trawling was estimated as 0-28%, based on samples taken with a Day grab before and 24 hours after trawling (Ball et al., 2000a). Gilkinson et al. (1998) found that otter trawling that disturbed the sediment displaced small bivalves but that these were unharmed. Sediment penetration may therefore disturb and displace foraminiferans, but these may survive due to their small size and robust tests.

Sensitivity assessment. A proportion of the characterizing species in these biotopes is likely to be lost or severely damaged, depending on the scale of the activity (see abrasion pressure). Therefore, a resistance of ‘Low’ (>75% loss) is suggested based on Thyasira spp. Resilience is probably ‘High’ (where foraminiferan populations undergo only small declines), and therefore biotope sensitivity to this pressure is likely to be ‘Low’.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale, e.g. from clear to intermediate for one year (Suspended sediment pressure definition).

Evidence

No evidence. Changes in suspended solids are considered unlikely to affect the characterizing species and resistance is assessed as 'High' and resilience as 'High' so that the biotope is considered to be 'Not sensitive'.

Smothering and siltation rate changes (light)

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

Evidence

No evidence. Changes in suspended solids are considered unlikely to affect the characterizing species and resistance is assessed as 'High' and resilience as 'High' so that the biotope is considered to be 'Not sensitive'.

Smothering and siltation rate changes (heavy)

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

Evidence

Bijkerk (1988, results cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface. This suggests that the characterizing species Thyasira spp. may be able to re-borrow through similar overburdens. Thyasira flexuosa have ‘highly extensible feet’ (Dando & Southward, 1986), allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Trannum et al. (2010) investigated the effects of water-based drill cuttings on benthic macrofaunal communities. While adding natural sediment up to 2.4 cm had no detectable impact, deposition of drill cuttings in layers of 0.3 to 2.4 cm significantly reduced macrofaunal abundance, biomass, taxa richness, and diversity, indicating that factors beyond physical burial, such as altered nutrient content, toxicity, and oxygen depletion, may influence benthic responses. The abundances of Thyasira spp. declined significantly with increasing thickness of the water-based drill-cuttings layer. At 2.4 cm of burial, mollusc abundances were ~3 individuals /0.09 m².

Sensitivity assessment. Epifaunal foraminifera may not be able to burrow to the surface, and at least a proportion of the population may be lost. However, little information on foraminiferan biology was found, and so, in the absence of information, tolerance has been assessed as 'None', albeit with very low confidence. Loss of the characterizing species of foraminifera would mean that the biotope is no longer SS.SMu.OMu.ForThy and so resistance is assessed as 'None'. Resilience is assessed as 'Medium', and sensitivity is assessed as 'Medium'.

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 foraminiferans or Thyasira spp. 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

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 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. 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 off-shore biotopes.

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

Not relevant.

Genetic modification & translocation of indigenous species

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

Evidence

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

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

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996).

Little information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found. A viral infection of the mutualist bacterium living on the gills of Thyasira gouldii has been suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007, references therein),

Sensitivity assessment. No direct evidence of the biotopes being affected by the introduction of microbial pathogens was found 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

No species within the biotope are targeted by commercial or recreational fishers or harvesters. 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

Removal of the characterizing species would reduce the ecological services provided by these species such as secondary production and nutrient cycling.

Sensitivity assessment. Species within the biotope are relatively sedentary or slow moving although the infaunal position of Thyasira spp. may protect this genus from removal. Biotope resistance is therefore assessed as ‘Low’ and resilience as ‘High’ as the habitat is likely to be directly affected by removal but Thyasira spp.  will recolonize rapidly.  Some variability in species recruitment, abundance and composition is natural and therefore a return to a recognisable biotope should occur within 2 years. Repeated chronic removal would, however, impact recovery.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the shells of other Crepidula, or other species, e.g., oysters and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds at 60 metres in the English Channel (Hinz et al., 2011). However, 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. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders. Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’, it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on foraminifera-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Foraminifera and bivalves are filter feeders, so larval predation is probably high, which may prevent colonization by Crepidula.

The slipper limpet Crepidula fornicata was reported to alter the substratum due to the build-up of its pseudofaeces and faeces, as it spatially competes and lowers recruitment of Pecten maximus and Aequipecten opercularis in the Bay of Brest, France (Frésard & Boncoeur, 2006). Although adult scallops that settle amongst Crepidula fornicata beds are not affected, juvenile scallops cannot settle in areas of high Crepidula fornicata density (Chauvaud et al. 2003, cited in Frésard & Boncoeur 2006), and the Crepidula fornicata beds threatened the sustainability of the ongoing scallop restocking program in the area. It should be noted that Crepidula fornicata is a southern species, and it is unlikely that this species could survive far north. No evidence was available to quantify the potential effect on Thyasira spp., however, there is evidence that Crepidula fornicata had an adverse effect on scallop beds in France and suggests the potential for impact in the UK.

Sensitivity assessment. The deep circalittoral muddy sediment characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata, although a lack of wave action might allow limited colonization than more exposed sites. Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). and has been recorded from the lower intertidal to ca 160 m in depth, but it 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). Therefore, colonization of Crepidula would be limited to low densities in deeper examples of the biotope (50 to 100 m). However, no evidence was found on the effect of Crepidula populations on foraminifera-dominated habitats, Thyasira spp., or circalittoral habitats. At present, there is 'Insufficient evidence' to suggest that the deep circalittoral muddy biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required.

The carpet sea squirt, Didemnum vexillum

Evidence

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

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

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

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

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

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

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

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

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

Dijkstra & Nolan (2011) reported that the scallops Placopecten magellanicus, overgrown with the invasive tunicate Didemnum vexillem, showed changes in escape potential. Scallops covered by Didenmnum vexillum became exhausted more quickly and were not able to swim as far in either the horizontal or vertical direction as the control sea scallops without Didemnum vexillum encrustation. The authors conclude that the expansion of Didemnum vexillum into scallop habitat may increase the vulnerability of sea scallops to predation and limit their ability to access food-rich habitats. No evidence was available to quantify the potential effect on Thyasira spp., but as it has been recorded growing on other bivalves, it is therefore a possible threat to Thyasira species.

Didemnum vexillum Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope occurs on muddy sediment, which would not provide a suitable hard substratum for colonization by Didemnum sp., however, it may colonize on the characterizing foraminiferans or Thyasira species. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope experiences very weak water flow (negligible) and no wave exposure. The effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of the characterizing foraminiferans or Thyasira species. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

The Pacific oyster, Magallana gigas

Evidence

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

Wireweed, Sargassum muticum

Evidence

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

Wakame, Undaria pinnatifida

Evidence

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

Other INIS

Evidence

The red king crab (Paralithodes camtschaticus) has become an invasive species in northwestern Europe since its introduction into Russia from the northern Pacific in the 1960s. While there are currently no confirmed observations of this species in UK waters, its European range has spread southward into Norway, and it is on Natural England’s alert list, meaning that it is likely to arrive in the UK and poses a high risk of impact (Natural England, 2009; GBNNSIP, 2015). Nuculid bivalves, which include Thyasira spp., have been shown to increase in abundance after red king crab invasion (Oug, Sundet & Cochrane, 2018). It is possible that these species indirectly benefit from king crab foraging activities, which target larger benthic species while also reworking sediments and creating niches for opportunistic species to proliferate.

While it is unknown if some characteristic species of this biotope may be negatively affected by red king crab invasions, others show increases in abundance. This shift in community composition could result in a change from one biotope to another. However, there is currently Insufficient evidence for a sensitivity assessment for this biotope to red king crab invasion.