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

Capitella capitata is a cosmopolitan species in coastal marine and estuarine soft sediment systems. Grassle & Grassle (1976) used electrophoretic enzyme analysis to determine that the global population is actually made up of several genetically distinct (and apparently genetically isolated) sibling species whose distributions overlap such that local Capitella capitata populations actually consist of a number of co-occurring sibling species. Within the complex, tolerances may vary and local acclimation is possible. Capitella capitata has also been recorded in extreme environments around hydrothermal vents (Gamenick & Giere, 1997; Donnarumma et al., 2019), which suggests that the species complex would be relatively tolerant to an increase in temperature.

The Capitella capitata complex is capable of spawning throughout the year, under suitable environmental conditions (Warren, 1976; Gilson & Davies, 2020). Gilson & Davies (2020) suggested Capitella capitata complex spawning may have coincided with warmer ambient temperatures during March and May based on a study of how Ascophyllum nodosum canopies modify sediment conditions, such as temperature and organic content, and how this impacts macrofaunal communities.

Experimental evaluation of the effects of combinations of varying salinities and temperature on Capitella capitata were carried out by Redman (1985) and Warren (1977). Redman (1985) found that length of life decreased as follows: 59 weeks at mid-temperature and salinity (15°C, 25 ppt); 43 weeks at high temperature and high salinity (18°C, 30 ppt); 33 weeks at lower temperature and high salinity (12°C, 30 ppt); 17 weeks at high temperature and low salinity (18°C, 20 ppt). Redman (1985) also found that net reproduction (Ro: the mean number of offspring produced per female at the end of the cohort) decreased as follows: 41.75 control; 36.69 under high salinity, high temperature; 2.19 high temperature, low salinity; 2.16 low temperature, high salinity. Therefore, a combination of changes in temperature and salinity may decrease the viability of the population. Warren (1977) used individual worms collected from Warren Point (southwest England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days and subsequently heated in a water bath to experience a rise in temperature of 1°C per 5 min. When the temperature had reached 28°C worms were removed at 0.5°C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 hours was calculated. Larvae were removed from the maternal tube and tested using the same method. The experiments indicated that temperatures above 30°C were most critical; the upper lethal temperature was 31.5°C for adult worms and a little higher for the larvae.

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). Wilson (1981) investigated 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).

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 Tyasira sarsii is symbiotic, relying on sulfur-oxidizing bacteria and hydrogen sulfide conditions typical of these fjord conditions (Kokarev et al., 2024).

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). The biotope, based on the characterizing species, is considered likely to tolerate a 2°C increase in temperature for a year. The experimental studies by Redman (1985) suggest that changes in temperature may reduce the lifespan of Capitella capitata. However, this effect is not considered to alter the character of the biotope as the short life cycle of this species should lead to rapid replenishment of the population. The experiments by Warren (1977) suggest that both the chronic and acute increases in temperature would not exceed the thermal tolerance of Capitella capitata. However, Thyasira spp. may suffer some mortality as a result of an acute increase in temperature, so resistance is assessed as ‘Medium’ (loss <25%). 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

Capitella capitata is a cosmopolitan species in coastal marine and estuarine soft sediment systems. Grassle & Grassle (1976) used electrophoretic enzyme analysis to determine that the global population is actually made up of several genetically distinct (and apparently genetically isolated) sibling species whose distributions overlap such that local Capitella capitata populations actually consist of a number of co-occurring sibling species. Within the complex, tolerances may vary and local acclimation is possible. Wu et al. (1988) collected animals at seawater temperatures of -2°C that harboured mature oocytes indicating reproductive activity even under low temperatures.

Warren (1977) used individual worms collected from Warren Point (southwest England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days prior to testing. The worms were cooled in a water bath to experience a decrease in temperature of 1°C per 5 min. When the final temperature was reached, worms were removed at 0.5°C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 h was calculated. Each experiment was repeated once. Larval Capitella capitata were removed from the maternal tube and tested using the same method. Both adults and larvae were tolerant of low temperatures, 50% of the adults and 65% of the larvae surviving at -1°C.

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 Tyasira sarsii is symbiotic, relying on sulfur-oxidizing bacteria and hydrogen sulfide conditions typical of these fjord conditions (Kokarev et al., 2024).

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). Short-term acute periods of extreme cold and icing conditions are likely to cause stress and some mortality in bivalve populations (Dame, 1996). However, no specific information on temperature tolerances of Thyasira spp. Was found.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4 to 19°C (Huthnance, 2010). The biotope, based on the characterizing species, is considered to tolerate a 2°C decrease in temperature for a year. The experiments by Warren (1977) suggest that both the chronic and acute decreases in temperature would not exceed the thermal tolerance of Capitella capitata. However, characterizing species Thyasira spp. may suffer some mortality as a result of an acute decrease in temperature, so resistance is therefore assessed as ‘Medium’ (<25% loss), but with low confidence. Resilience is likely to be ‘High’, so the biotopes are considered to have ‘Low’ sensitivity to a decrease in temperature at the pressure benchmark level.

Salinity increase (local)

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

Evidence

The biotopes occur in full salinity (30 to 35 ppt) (Connor et al., 2004) and it is highly unlikely that they would experience conditions of hypersalinity and no evidence was found to assess an increase in salinity above full. Avramidi et al. (2022) reported high abundances of Capitella capitata at poorly flushed, fully marine sites with little to no current in the Port of Rotterdam that were exposed to industrial discharge and brine outfalls. The authors suggested that the distribution of Capitella capitata was driven mainly by salinity rather than sediment organic matter in the sites studied.

Sensitivity assessment. No direct evidence was found to assess the effects of changes in salinity. Capitella capitata is recorded from 0 to 40 psu, but most records occur between 30 and 35 psu (OBIS, 2025). Thyasira spp. is recorded from 5 to 40 psu, but most records occur between 30 and 35 psu (OBIS, 2025). This data suggests that the key components of the biotopes communities would not be resistant of an increase in salinity to >40 psu, resulting in mortality of the characterizing species. Resistance is therefore assessed as ‘Low’ (loss of 25 to 75%). Once normal conditions are resumed, resilience is probably ‘High’, so sensitivity is therefore assessed as ‘Low’, with low confidence due to the lack of direct evidence.

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

Warren (1977) used individual worms collected from Warren Point (south-west England) to test response to reduced salinity. Individual Capitella capitata were acclimated to 33‰ for 1 week prior to exposure to salinities of 1.5‰, 5.5‰, 18‰ and 33‰. Larvae removed from the maternal tube were also tested in groups of 10. The results of tolerance tests showed that adult Capitella capitata acclimated at 33‰ were intolerant of reduced salinities below 20‰, all exposed adults died within four days when exposed at 18‰ and within one day at 9‰. The larvae were more tolerant, living for 10 days at 15.5‰ with little apparent ill effect.

Avramidi et al. (2022) reported high abundances of Capitella capitata at poorly flushed, fully marine sites with little to no current in the Port of Rotterdam that were exposed to industrial discharge and brine outfalls. The authors suggested that the distribution of Capitella capitata was driven mainly by salinity rather than sediment organic matter in the sites studied. Capitella capitata has also been recorded to dominate estuarine environments, occurring in both polyhaline (25 to 30 ppt) and euhaline (30 to 40 ppt) zones (Sarathy 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 Tyasira sarsii is symbiotic, relying on sulfur-oxidizing bacteria and hydrogen sulfide conditions typical of these fjord conditions (Kokarev et al., 2024).

Thyasira spp. 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). There is insufficient information regarding the effects of salinity on adults.

Sensitivity assessment. This biotope is found within fully marine subtidal locations (Connor et al., 2004; JNCC, 2022). Capitella capitata is recorded from 0 to 40 psu, but most records occur between 30 and 35 psu (OBIS, 2025). Thyasira spp. is recorded from 5 to 40 psu, but most records occur between 30 and 35 psu (OBIS, 2025). However, Capitella capitata dominated biotopes (e.g. SS.SMu.ISaMu.Cap and SS.SMu.SMuVS.CapTubi) are recorded from reduced (18 to 30 ppt) and low salinity (<18 ppt) (Connor et al., 2004; JNCC, 2022). The evidence presented by Warren (1977) suggests that local adaptation to different salinity ranges plays a role in salinity tolerance. This biotope is, therefore, considered to have 'High' resistance to this pressure based on its recorded distribution, and 'High' resilience (by default), so the biotopes are considered to be ‘Not sensitive’ to a decrease in salinity at the pressure benchmark level.

Water flow (tidal current) changes (local)

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

Evidence

Increases and decreases in water velocity may lead to increased erosion or deposition, respectively. The associated pressures alteration to sediment type and siltation are assessed separately. Experimental increases in near-bed current velocity were achieved over intertidal sandflats by placing flumes on the sediment to accelerate water flows (Zuhlke & Reise, 1994). The increased flow led to the erosion of up to 4 cm depth of surface sediments. No significant effect was observed on the abundance of Capitella capitata and numbers of Tubificoides benedii and Tubificoides pseudogaster were unaffected, as they probably avoided suspension by burrowing deeper into sediments. This was demonstrated by the decreased abundance of oligochaetes in the 0-1 cm depth layer and increased abundance of oligochaetes deeper in sediments (Zuhlke & Reise, 1994). 

 As the characterizing Capitella capitata can live relatively deeply buried and in depositional environments with low water flows (based on habitat preferences) and low oxygenation, they are considered to be not sensitive to decreases in water flow.

Sensitivity assessment. The hydrographic regime, including flow rates, is an important structuring factor in sedimentary habitats. The low energy environments where this biotope occurs are therefore likely to be important in supporting the development of the mud or sandy mud substrata which characterizes the biotope. Where increased or decreased water flows altered the sediment type, this could lead to sediment reclassification and thus change is assessed in the sedimentary change assessment. As muds tend to be cohesive and the surface tends to be smooth reducing turbulent flow, an increase at the pressure benchmark may not lead to increased erosion. The biotopes resistance is assessed as Medium as a precautionary assessment, resilience is assessed as High (following restoration of usual conditions) and sensitivity is 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 subtidal 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 action would be the erosion of the fine sediment substratum as this could eventually lead to loss of the habitat that characterizes the biotopes. Decreased exposure will probably lead to increased siltation and reduced grain size (muddy sediment). Changes in wave exposure may therefore influence the supply of particulate matter for tube building and feeding activities of the characterizing species. Food supplies may also be reduced affecting growth and fecundity of the species.

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

Sensitivity assessment. The biotope occurs offshore where wave exposure is likely to be negligible (Connor et al., 2004), as the effects of wave action are attenuated with depth, the factor is only likely to affect the biotopes where it occurs at depths of less than 60 m in a strong swell or force 8 gale (Hiscock, 1983). An increase or decrease in wave height at the pressure benchmark (3-5% of significant wave height) is, therefore, considered unlikely to be significant. Resistance and resilience are therefore assessed as High, and the biotope is considered to be 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

Experimental studies with various species suggest that polychaete worms are quite tolerant to heavy metals (Bryan, 1984). High numbers of Capitella capitata have been recorded in areas containing high metal concentrations (Petrich & Reish, 1979; Ward & Young, 1982; Olsgard, 1999). The abundance of Capitella capitata in Norway was also been noted to have a significant negative correlation between sediment content of Cu and abundance of the species, with an obvious reduction in abundance at approximately 900 ppm Cu (Olsgard, 1999). Some impacts on population size and reproduction of Capitella capitata as a result of metal pollution, both in the field and the laboratory, have been observed.

Laboratory tests carried out in water may not reflect sediment conditions, where, again, copper toxicity and exposure are determined by a number of parameters, including the degree to which it is adsorbed onto particles selected as food for deposit feeders. A two-year microcosm experiment was undertaken to investigate the impact of copper on the benthic fauna of the lower Tyne Estuary (UK) by Hall & Frid (1995). During a one-year simulated contamination period, 1 mg/l copper was supplied at two-weekly 30% water changes, at the end of which the sediment concentrations of copper in contaminated microcosms reached 411 μg/g. Toxicity reduced populations of the four dominant taxa, including Capitella capitata. When copper dosage was stopped and clean water supplied, sediment copper concentrations fell by 50% in less than four days, but faunal recovery took up to one year, with the pattern varying between taxa. Since the copper leach rate was so rapid, it was concluded that after remediation, contaminated sediments show rapid improvements in chemical concentrations, but faunal recovery may be delayed, with experiments in microcosms showing faunal recovery taking up to a year.

Rygg (1985) classified Capitella capitata as a highly tolerant species, common at the most copper polluted stations (copper >200 mg/kg) in Norwegian fjords. Capitella capitata was also highly abundant in ocean waste dumping sites in Korea, in sediment contaminated by heavy metals and organic matter (Kim et al., 2023).

In addition, Capitella capitata and 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). Both Capitella spp. and Thyasira spp. were also frequently recorded in association with post-drilling contamination, for example, in drill-cutting piles in the North Sea (Henry et al., 2017) and recorded under heavy tailings sedimentation (Ramierz-Llodra et al., 2015).

Field studies have shown that Thyasira spp. can tolerate heavy metal contamination in sediments. At an ocean waste dumping site in Korea, Thyasira tokunagai dominated mollusc assemblages (around 82% of total abundance) and the abundance positively correlated with heavy metals, including mercury (Hg), chromium (Cr), copper (Cu) and cadmium (Cd) (Kim et al., 2018b; 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., 2018b).

Lebedeva et al. (2018) reported that Thyasira gouldi accumulated elevated total mercury (Hg) in from Grofjorden, Svalbard. The heavy metal concentrations (Hg, Cd, Pb, Cu, Zn) in Capitella capitata were significantly higher than those of other studied polychaetes in the Black Sea, and the results showed Capitella capitata was able to accumulate most heavy metals.

Sensitivity assessment. Capitella capitata and Thyasira sp. have been reported from heavy metal contaminated sites in the vicinity of oil platforms, drill-cutting piles, and waste dump sites in numerous sites worldwide. Both species were considered to be tolerant of heavy metal contamination (Rygg, 1985; Kim et al., 2018b; 2023; Chen et al., 2024). However, some studies noted that Capitella populations were reduced by exposure to high concentrations of copper (Hall & Frid, 1995; Olsgard, 1999), while high concentrations of manganese reduced populations of Thyasira spp. Therefore, the worst-case resistance is assessed as ‘Low’, albeit with low confidence due to the limited evidence. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’.

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

Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, in soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. However, high numbers of Capitella capitata have been recorded in hydrocarbon contaminated sediments (Ward & Young, 1982; Olsgard, 1999; Petrich & Reish, 1979), and colonization of areas defaunated by high hydrocarbon levels may be rapid (Le Moal, 1980). After a major spill of fuel oil in West Virginia, Capitella increased dramatically alongside large increases in Polydora ligni and Prionospio sp. (Sanders et al., 1972, cited in Gray, 1979). Experimental studies adding oil to sediments have found that Capitella capitata increased in abundance initially, although it was rarely found in samples prior to the experiment (Hyland et al., 1985). Capitella capitata is able to withstand relatively high hydrocarbon concentrations and may even take advantage of any available space caused by the mortality of other species. Also, Capitella capitata has been shown to update and accumulate organochloride pesticides and polycyclic aromatic hydrocarbons (Mwevura et al., 2020). 

It should be noted that this biotope occurs in organically enriched areas around oil and gas platforms. Capitella capitata and 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). This shows a high resistance to hydrocarbon contamination by both characteristic species.

Johnson et al. (2018) found that Capitella capitata recolonized and dominated an oiled salt marsh impacted by the Deepwater Horizon oil spill, and plantings of Spartina alterniflora drove this colonization and spike in Capitella capitata density, not the slow-release fertilizer used during plantings. This suggests that habitat restoration can enhance recolonization after disturbance.

Sensitivity assessment. The evidence suggests that Capitella capitata abundance increases with increasing concentration of petroleum hydrocarbons and total organic carbon, indicating it can tolerate organic pollution (Wang et al., 2020). In addition, this biotope is characteristic of sediment in the vicinity of oil platforms and drill cuttings. Therefore, the worst-case resistance is assessed as ‘High’, albeit with low confidence. Hence, resilience is assessed as ‘High’, and sensitivity is considered to be ‘Not sensitive’.

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

Méndez (2006) showed that the effects of exposing the deposit feeding polychaete Capitella to sediment spiked with environmentally relevant concentrations of teflubenzuron (a chemical used to control infestations of sea lice) caused mortality in one species of Capitella and reduced the egestion rate of another. Nevertheless, this pressure is Not assessed.

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

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

Capitella capitata exhibits a relatively high tolerance for sediment hypoxia, low dissolved oxygen levels, hydrogen sulphide concentration, and other sediment conditions avoided by many infauna (Henriksson, 1969; Nkwoji et al., 2020; Kolyuchkina et al., 2022; Han et al., 2024). Capitella capitata can also survive in increased Biological Oxygen Demand (BOD) levels in the water column (Behera et al., 2023).

Forbes & Lopez (1990) experimentally demonstrated that reduced oxygen concentrations (pO₂= 20 mm Hg or less; ca. 1.17 mg/l) led to decreased Capitella capitata growth rates and cessation of burrowing and feeding activity even when an abundance of food was provided. The authors hypothesized that animals rely solely on anaerobic metabolism once this threshold is crossed. Mangum & Van Winkle (1973) similarly observed that Capitella capitata oxygen uptake ceased when pO₂ fell to between 0 and 34 mm Hg (ca 1.99 mg/l). The fact that experimental worms lost body mass under these conditions supports the contention that full aerobic metabolism cannot be sustained at very low ambient oxygen conditions despite a very high affinity of Capitella capitata haemoglobin for oxygen. Despite its tolerance, evidence has suggested that higher abundances of Capitella capitata were observed with increasing dissolved oxygen concentrations in the water column and silt concentrations in the sediment (Khatun et al., 2023), indicating that oxygen availability can influence population even within its tolerant range.

A community of fast-growing Capitella capitata was found at depths of 130 to 144 m in permanent hypoxic conditions, defined in the study as less than 50 μmol O₂/l (1.6 mg/l) , in Golubaya Bay (Kolyuchkina et al., 2022). The community had a generation frequency of around 30 to 40 days and a maturity age of 1 to 4 months, suggesting the community was around 1 to 1.5 months old. Kolyuchkina et al. (2022) noted that Capitella capitata’s short life cycle, multiple reproductions and low fertility throughout the year, meant they could recover and restore their populations in a short time without energy costs. This could help it form communities in hypoxic zones.

López-Jamar et al. (1987) stated that Thyasira flexuosa was adapted to living in reduced sediments and was also found in organically enriched sediments. Zettler & Pollehe (2023) observed Thyasira sp. to be tolerant of low oxygen, being characteristic and often abundant in hypoxic zones, with mean densities increasing to around 700 ind/m² at oxygen saturations of 23% (ca 2.3 mg/l). However, Dando & Spiro (1993) found that the numbers of the congeners Thyasira equalis and Thyasira sarsi decreased rapidly following the deoxygenation of bottom water in the deep basin of the Gullmar fjord in 1979-80. The Gullmar fjord is subject to periodic anoxia. During the 1979 to 1980 winter, the basin became azoic due to oxygen deficiency, which likely resulted in the death of bottom fauna (Dando & Spiro, 1993). This decline was linked to sediment sulphur chemistry. Thyasira spp. populations recolonized post 1980. However, densities declined again in low oxygen levels from 1987 to 1989.  

Rosenberg et al. (1991) exposed benthic species from the NE 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 they showed increased stretching of the siphon out of the sediment. In a meta-analysis study of hypoxia, median sub-lethal oxygen concentrations reported in experimental assessments, although no specific data were reported for all the characterizing species of these biotopes, the thresholds of hypoxia for different benthic groups were LC50 1.42 mg/l for bivalves, and sub-lethal (SLC50) of 1.20 mg/l for annelids (Vaquer-Sunyer & Duarte, 2008). 

Sensitivity assessment. Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotope will have varying responses to deoxygenation. Based on the evidence presented, the characterizing species are likely to be affected only by severe deoxygenation episodes. However, some mortality of Thyasira spp. might occur in near anoxic (0% oxygen) conditions. Resistance to deoxygenation is, therefore, assessed as 'Medium'. Resilience of the biotope is likely to be 'High', and the biotope is, therefore, considered to have 'Low' sensitivity to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

Nutrient enrichment

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

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014). Capitella capitata is an opportunist and widely used as a bioindicator of organic enrichment and pollution (Valdermarsen et al., 2015; Bae et al., 2018; Bat et al., 2019; Martin et al., 2019; Wang et al., 2020; Behera et al., 2023; Khatun et al., 2023; Chen et al., 2024).

Dense Capitella capitata populations are frequently located in areas with greatly elevated organic content, such as areas near sewage disposal and industrial wastewater outfalls, in enriched estuaries and ports, below fish farms and mussel long lines, even though eutrophic sediments are often anoxic and highly sulfidic (Grassle & Grassle, 1974; Thom & Chew, 1979; Gray, 1979; Tenore, 1977; Warren, 1977; Tenore & Chesney, 1985; Bridges et al., 1994; Bridges, 1996; Holte & Oug, 1996; Cardell et al., 1998; Karakassis et al., 2000; Haskoning, 2006; Callier et al., 2007; Leopardas et al., 2016; Bae et al., 2018; Martin et al., 2019; Gilson & Davies, 2020; Nkwoji et al., 2020; Behera et al., 2023; Khatun et al., 2023; Rowshan et al., 2023; Asl et al., 2024; Chen et al., 2024; Sharifinia et al., 2025). The presence of an organically enriched seabed and reduced competition likely enabled the increase in Capitella capitata populations.

Thom & Chew (1979) noted that Capitella capitata dominated the community around the combined sewer and storm water discharge in Puget Sound (USA) during winter, and was replaced by a Nebalia pugettensis dominated community during summer, when there was little storm water run-off. In eutrophic sediments close to sewage discharge effluents Capitella capitata was abundant (Cardell et al., 1998). Capitella capitata dominated the fauna up to 10 m from fish farm cages in Mediterranean coastal areas, where organic carbon and nitrogen content of the sediment increased by a factor of 1.5-5 and ATP content by 4 to 28 compared with the control (Karakassis et al., 2000). Mendez et al. (1997) suggested that differences in population dynamics between heavily polluted and less polluted sites may be because the species is able to modify its life cycle depending on environmental conditions.

Evidence has shown that declines in Capitella capitata populations occur when enrichment and pollution are reduced, and this coincides with improvements in benthic community health. Zhang et al. (2021) showed that Capitella capitata was abundant near an Orange County wastewater outfall during periods of high chlorine and organic enrichment. But following improvements in wastewater treatment after 2011, the community health index scores improved, and there was a significant decrease in the Capitella capitata complex, with abundance decreasing from 59.8% in 2010 to less than 1% by 2014 (Zhang et al., 2021). In addition, in a coastal mariculture pond where Sesuvium portulacastrum was planted to reduce dissolved inorganic nitrogen, the once dominant Capitella capitata disappeared from the pond and was replaced (Lui et al., 2024).

Thyasira spp. are characteristic of organically enriched offshore sediments with Capitella capitata (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). Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Thyasira flexuosa to their Ecological Group III - ‘Species tolerant to excess organic matter enrichment; these species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’. Capitella capitata was assigned to Ecological Group V – ‘First-order opportunistic species (pronounced unbalanced situations). These are deposit-feeders, which proliferate in reduced sediments (Gittenberger & Van Loon, 2011).

Field studies have shown that Thyasira spp. can tolerate and thrive in organic enrichment. At an ocean waste dumping site in Korea, Thyasira tokunagai dominated mollusc assemblages (around 82% of total abundance), and abundance increased at sites with high total organic carbon and total nitrogen (Kim et al., 2018b).

Sensitivity assessment. The evidence indicates that increased nutrient and organic matter levels favour Capitella capitata and Thyasira spp., and resistance is therefore considered to be 'High', resilience 'High' (by default), and the biotope is assessed as ‘Not sensitive’. It should be noted that this biotope occurs in organically enriched areas around oil and gas platforms and that a reduction in organic enrichment may reduce habitat suitability for the characterizing species, leading to biotope loss.

Organic enrichment

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

Evidence

Benthic responses to organic enrichment have been described by Pearson & Rosenberg (1978) and Gray (1981). In general, moderate enrichment increases food supply and increases productivity and abundance. Capitella capitata is an opportunist and widely used as a bioindicator of organic enrichment and pollution (Valdermarsen et al., 2015; Bae et al., 2018; Bat et al., 2019; Martin et al., 2019; Wang et al., 2020; Behera et al., 2023; Khatun et al., 2023; Chen et al., 2024).

Dense Capitella capitata populations are frequently located in areas with greatly elevated organic content, such as areas near sewage disposal and industrial wastewater outfalls, in enriched estuaries and ports, below fish farms and mussel long lines, even though eutrophic sediments are often anoxic and highly sulfidic (Grassle & Grassle, 1974; Thom & Chew, 1979; Gray, 1979; Tenore, 1977; Warren, 1977; Tenore & Chesney, 1985; Bridges et al., 1994; Bridges, 1996; Holte & Oug, 1996; Cardell et al., 1998; Karakassis et al., 2000; Haskoning, 2006; Callier et al., 2007; Leopardas et al., 2016; Bae et al., 2018; Martin et al., 2019; Gilson & Davies, 2020; Nkwoji et al., 2020; Behera et al., 2023; Khatun et al., 2023; Rowshan et al., 2023; Asl et al., 2024; Chen et al., 2024; Sharifinia et al., 2025). The presence of an organically enriched seabed and reduced competition likely enabled the increase in Capitella capitata populations. Capitella capitata has also been recorded in extreme environments around hydrothermal vents (Donnarumma et al., 2019).

For example, Bridges (1996) found that sediment treatments composed of marsh mud enriched with sewage or algae were all associated with opportunistic responses in Capitella capitata in the laboratory and field and juveniles produced in the sewage treatment grew 2 times larger than the control. Holte & Oug (1996) found that during periods of influx of organic material Capitella capitata is capable of explosive population growth, often becoming predominant, under high input levels, to the exclusion of nearly all other species. Thom & Chew (1979) noted that Capitella capitata dominated the community around the combined sewer and storm water discharge in Puget Sound (USA) during winter, and was replaced by a Nebalia pugettensis dominated community during summer, when there was little storm water run-off. In eutrophic sediments close to sewage discharge effluents Capitella capitata was abundant (Cardell et al., 1998). Capitella capitata dominated the fauna up to 10 m from fish farm cages in Mediterranean coastal areas, where organic carbon and nitrogen content of the sediment increased by a factor of 1.5-5 and ATP content by 4 to 28 compared with the control (Karakassis et al., 2000). Mendez et al. (1997) suggested that differences in population dynamics between heavily polluted and less polluted sites may be because the species is able to modify its life cycle depending on environmental conditions.

Evidence has shown that declines in Capitella capitata populations occur when enrichment and pollution are reduced, and this coincides with improvements in benthic community health. Zhang et al. (2021) showed that Capitella capitata was abundant near an Orange County wastewater outfall during periods of high chlorine and organic enrichment. But following improvements in wastewater treatment after 2011, the community health index scores improved, and there was a significant decrease in the Capitella capitata complex, with abundance decreasing from 59.8% in 2010 to less than 1% by 2014 (Zhang et al., 2021). In addition, in a coastal mariculture pond where Sesuvium portulacastrum was planted to reduce dissolved inorganic nitrogen, the once dominant Capitella capitata disappeared from the pond and was replaced (Lui et al., 2024).

Benthic fauna underneath floating salmon farm cages in a Scottish sea loch showed marked changes in species number, diversity, faunal abundance and biomass in the region of the fish farm (Brown et al., 1987). Four ‘zones’ of effect were identified: in zone 1, directly beneath and up to the edge of the cages, there was an azoic zone; in zone 2, from the edge of the cages out to 8 m, the sediments were highly enriched and dominated by Capitella capitella. Kutti et al. (2008) studied organic enrichment of sediments below a fish farm in a fjord system (Norway), during periods of high organic loading, production was mostly by Capitella capitata. 

In two laboratory experiments, Méndez (2016) examined the reproduction and development of Capitella species ‘A’ (within the Capitella capitata complex) under different sediment conditions: organically enriched sediment collected from a bay near a fish farm (organic content around 7.3% and 8.23%) and natural sediment (organic content around 1.57% and 2%). Larvae developed rapidly with positive growth rates in organically enriched sediment. Juveniles were observed one day after hatching, and immature females appeared after 52 days. Maturity was reached more quickly, and individuals survived up to 79 days (Méndez, 2016). In natural sediment, with low food availability and low organic content, Capitella species ‘A’ development was slower, with negative growth rates, and maturity was delayed; male genital spines were reduced in size. Juveniles in the natural sediment died at an earlier development stage, with survival limited to 15 to 22 days (Méndez, 2016). Similarly, Méndez (2021) laboratory experiment found that Capitella teleta and Capitella sp. had higher survival, growth rates and reproductive development of juveniles and adults in organically enriched sediment compared to non-organically enriched sediment. These studies demonstrate that organic enrichment enhances growth and reproduction within the Capitella capitata complex.

Valdemarsen et al. (2015) studied the benthic impact of two deep-water salmon fish farms in Hardanger Fjorden, Norway, over an 18-month production cycle. Both farms had similar waste production but slightly different hydrodynamics: a low current farm with bottom currents rarely exceeding 2 cm/s and a more exposed moderate current farm with mean bottom currents between 3 and 5 cm/s and intermittent periods of more than 20 cm/s. The moderate current site was generally less impacted. Capitella capitata consistently dominated the community there, and its abundance increased by more than ten times from June to September, which coincided with a higher deposition of organic waste during peak production. Capitella capitata also dominated communities in the low current farm, but had disappeared by September, suggesting that the sediment conditions were severely impacted and unsuitable to this species, which usually thrives there. Valdemarsen et al. (2015) concluded that hydrodynamics were critically important for the environmental impact of deep-water fish farms, and fish farming needed a slightly higher water current to facilitate sufficient waste dispersal.

Evidence has suggested that Capitella spp. may acquire beneficial epibiosis of giant sulfur-oxidizing bacteria (Thiomargarita sp.) in organic-enriched and sulfidic conditions (Hourdez et al., 2021). This seasonal relationship increases Capitella spp. survival by detoxifying sulfide in the sediment and/or supplying chemosynthetically fixed nutrients (Hourdez et al., 2021).

Thyasira spp. are characteristic of organically enriched offshore sediments with Capitella capitata (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).  Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Thyasira flexuosa to their Ecological Group III - ‘Species tolerant to excess organic matter enrichment; these species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’. Capitella capitata was assigned to Ecological Group V – ‘First-order opportunistic species (pronounced unbalanced situations). These are deposit-feeders, which proliferate in reduced sediments (Gittenberger & Van Loon, 2011).

Field studies have shown that Thyasira spp. can tolerate and thrive in organic enrichment. At an ocean waste dumping site in Korea, Thyasira tokunagai dominated mollusc assemblages (around 82% of total abundance), and abundance increased at sites with high total organic carbon and total nitrogen (Kim et al., 2018b). 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).

In addition, Thyasira spp. are frequently observed at cold methane seeps, where they exist in high sulfide 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)

Sensitivity assessment. The evidence indicates that increased organic matter levels favour Capitella capitata and Thyasira spp., resistance is therefore considered to be High, resilience High (by default), and the biotope is assessed as ‘Not sensitive’. It should be noted that this biotope occurs in organically enriched areas around oil and gas platforms and that a reduction in organic enrichment may reduce habitat suitability for the characterizing species, leading to biotope loss.

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 (Connor et al., 2004),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 Capitella capitata, other polychaetes and oligochaetes and Thyasira spp. that live buried within the sediment.

Sensitivity assessment. Based on the loss of the biotopes, 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

Capitella capitata can survive in a range of habitats including fine sands and areas with boulders, a change in sediment type was not judged to completely reduce habitat suitability for this species. An increase of sediment coarseness to sand would not exclude this species, based on published habitat preferences, but may have population level effects as habitat suitability may be reduced. Recovery would depend on the return of previous habitat conditions.

The characterizing species Thyasira spp. have a range of sediment preferences, including mud, muddy sand, 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 sediment type to mixed or coarser particles could lead to changes in the density of Capitella capitata, other burrowing polychaetes and oligochaetes depending on species specific responses. However, the loss of the muddy sediment that characterizes this habitat would change the character of the biotopes, the characterizing species, with potentially an increase in bivalves or crustaceans and is likely to lead to reclassification. Based on a change in character, the biotopes are considered to have a resistance of 'None' to this pressure, and resilience is assessed as 'Very Low' (as a change at the pressure benchmark is permanent), and biotopes sensitivity is assessed as 'High'.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, exposing underlying sediment which may be anoxic and/or of a different character or bedrock and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment/water interface, may be exposed to conditions to which they are not suited. Removal of 30 cm of surface sediment will remove the polychaete and oligochaete community and other important species present in the biotopes. Recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Hydrodynamics and sedimentology (mobility and supply) influence the recovery of soft sediment habitats (Van Hoey et al., 2008).

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope. Resistance is assessed as None and biotopes resilience is assessed as High. Biotope sensitivity is therefore 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

Capitella capitata is a soft bodied, relatively fragile species inhabitaing mucus tubes close to the sediment surface. Abrasion and compaction of the surficial layer may damage individuals. Capitella capitata and Pygospio elegans have been categorized through literature and expert reviews as AMBI fisheries Group IV- 'A second-order opportunistic species, which are sensitive to fisheries in which the bottom is disturbed. Their populations recover relatively quickly however and benefit from the disturbance, causing their population sizes to increase significantly in areas with intense fisheries' (Gittenberger & Van Loon, 2011). Chandrasekara & Frid (1996) found that in intertidal muds, along a pathway heavily used for five summer months (ca 50 individuals a day), some species including Capitella capitata and Scoloplos armiger reduced in abundance. Bonsdorff & Pearson (1997) found that sediment disturbance forced Capitella capitata deeper into the sediment, although the species was able to burrow back through the sediment to the surface again. 

Thyasira spp. are small bivalves, the shells are thin and fragile and abrasion is likely to lead to damage and mortality within the population depending on the force (Jackson, 2007). 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. 

Sensitivity assessment. Abrasion may damage or kill a proportion of the population of the characterizing Capitella capitata, Thyasira spp. and associated species. Biotope resistance is assessed as Medium and resilience as High, so sensitivity is 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

Rabaut et al. (2008) found that beam trawling on intertidal Lanice conchilega reefs reduced the abundance of Capitella capitata. Ferns et al. (2000), however, found that tractor-towed cockle harvesting had little effect on Capitella capitata, but species that are present at the surface were more badly affected. The tractor dredging removed 83% of Pygospio elegans (initial density 1850/m²). These results are supported by work by Moore (1991) and Rostron (1995) who also found that cockle dredging can result in reduced densities of some polychaete species, including Pygospio elegans.

Bergman & Van Santbrink (2000) estimated the direct mortality of benthic macrofauna caused by the single pass of commercial beam and otter trawls. The results showed that a single pass of a 4 m or 12 m beam trawl or an otter trawl, in shallow sandy areas and deep silty sand areas (with 3-10% silt) in the North Sea caused a mortality of 20-65% of bivalves and 5-40% of gastropods, starfish, small-medium sized crustaceans and annelid worms. The delicate shells of Thyasira spp. are vulnerable to physical damage (e.g. by otter boards), but small size relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993).

Sensitivity assessment. Capitella capitata and other characterizing species of the biotopes are present in the surface layers of sediment and may be damaged, displaced or killed by penetration and disturbance of the sediment. Resistance is assessed as Low and resilience as High, so sensitivity is assessed as Low.

Changes in suspended solids (water clarity)

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

Evidence

An increase in suspended solids with high organic content may benefit deposit feeders, such as characterizing Capitella capitata if these are deposited. Deposit feeders and tube builders rely on deposition of suspended sediment. A decrease in suspended sediment will reduce this supply and therefore may compromise growth and reproduction.

Sensitivity assessment. The biological assemblage characterizing the biotope is infaunal and consists of sub-surface deposit feeders. Increased suspended solids are unlikely to have an impact and resistance is assessed as High and resilience as High, so the biotope is considered to be Not Sensitive. A reduction in suspended solids may reduce deposition and supply of organic matter, resistance to a decrease is therefore assessed as Medium, as a shift between deposition and erosion could result in the net loss of surficial sediments. A reduction in organic matter as suspended solids could also reduce production within the biotope. Resistance is assessed as High, as over a year the impact may be relatively small, following restoration of usual conditions. Biotope sensitivity is therefore assessed as Low. 

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

An increase in suspended solids with high organic content may benefit deposit feeders, such as characterizing Capitella capitata if these are deposited. Deposit feeders and tube builders rely on deposition of suspended sediment. A decrease in suspended sediment will reduce this supply and therefore may compromise growth and reproduction.

Sensitivity assessment. The biological assemblage characterizing the biotope is infaunal and consists of sub-surface deposit feeders. Increased suspended solids are unlikely to have an impact and resistance is assessed as High and resilience as High, so the biotope is considered to be Not Sensitive. A reduction in suspended solids may reduce deposition and supply of organic matter, resistance to a decrease is therefore assessed as Medium, as a shift between deposition and erosion could result in the net loss of surficial sediments. A reduction in organic matter as suspended solids could also reduce production within the biotope. Resistance is assessed as High, as over a year the impact may be relatively small, following restoration of usual conditions. Biotope sensitivity is therefore assessed as Low. 

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

Capitella capitata has been categorized through expert and literature review, as AMBI sedimentation Group IV – “A second-order opportunistic species, insensitive to higher amounts of sedimentation. Although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit. This causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation” (Gittenberger & Van Loon, 2011).  Evidence suggested that Capitella capitata abundance was positively correlated with sedimentary silt and the species could dominate the sublittoral zone following significant siltation (Stolyarov, 2017; Khatun et al., 2023).

The effects of siltation will depend on the amount and rate which particles are added. Sarathy et al. (2022) found that the abundance of Capitella capitata was negatively correlated with silt and sediment total organic carbon, suggesting that although the species can occur in disturbed habitats, its abundance decreases where sediment is silty or rich in organic matter. Capitella capitata is sedentary, and adults are judged unlikely to have any mechanism to escape from large sediment deposits. A deep covering of sediment will prevent feeding. Where inputs are at low rates and similar to background sediments, then adults may be able to extend tubes to reach the surface to feed.

Thyasira flexuosa have highly extensible feet (Dando & Southward, 1986), allowing them to construct channels within the sediment and to burrow to 8 cm depth. This suggests that characterizing bivalve species such as Thyasira spp. are likely to be able to burrow through 5 cm of deposited sediment although it could temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure.

The pressure benchmark (30 cm deposit) represents a significant burial event and the deposit may remain for some time in low energy environments. Capitella capitata populations are likely to be significantly impacted. Some impacts on other oligochaetes may occur and it is considered unlikely that significant numbers of the population could reposition, based on (Bolam, 2011). Placement of the deposit will, therefore, result in a defaunated habitat until the deposit is recolonized.

Sensitivity assessment. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). Bivalve and 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) (Bijkerk, 1988; Powilleit et al., 2009; Maurer et al., 1982).

However, it is not clear whether the characterizing species are likely to be able to burrow through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as ‘Low’ and resilience as ‘High’ and the biotopes are considered to have ‘Low’ sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. There have been no studies examining the effect of EMFs on macroalgae. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms.  

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

Underwater noise changes

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

Evidence

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 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 characterizing species of the biotopes live infaunally, so are likely to have poor or no visual perception and unlikely to be affected by visual disturbance. Visual disturbance is therefore considered '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 the biotopes are not cultivated or 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

Marine oligochaetes host numerous protozoan parasites without apparent pathogenic effects even at high infestation levels (Giere & Pfannkuche, 1982 and references therein). Furthermore, 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). A viral infection of the mutualist bacterium living on the gills of Thyasira gouldii was suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007), but no information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found.

Sensitivity assessment. Based on the lack of evidence for mass mortalities in the biotopes from microbial pathogens, resistance is assessed as High and resilience as High (by default), so that the biotope is assessed as Not Sensitive.

Removal of target species

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

Evidence

No characterizing species within the biotopes 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

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in this biotope, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures).

Sensitivity assessment. Removal of the characterizing species would result in the biotopes being lost or reclassified. Therefore, the biotope is considered to have a resistance of Low to this pressure and to have High resilience, resulting in the sensitivity being judged as Low.

The American slipper limpet, Crepidula fornicata

Evidence

Invasion by the slipper limpet Crepidula fornicata may lead to shallower examples of this biotope to revert to SS.SMx.SMxVS.CreMed, suggesting high intolerance as the original biotope would be lost. It should be noted that experimental relaying of mussels on intertidal fine sand sediments increased fine sediment proportions and led to colonization by Capitella capitata (Ragnarsson & Rafaelli, 1999), so that sediment modification by bivalves may not render habitats unsuitable for Capitella capitata.

Sensitivity assessment. Reclassification of the biotope following invasion would result in the loss of the biotope. However, Crepidula is typically found around the low water mark and the shallow sublittoral to 60 m (Rayment, 2007), so the depth at which this biotope occurs is likely to offer some protection against invasion. Resistance is therefore assessed as High, and resilience as High (by default) and the biotope is considered Not Sensitive to the introduction of INIS.

The carpet sea squirt, Didemnum vexillum

Evidence

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

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

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

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

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

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

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

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

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). 

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

However, some studies on Georges Bank, USA, and Sandwich, Massachusetts observed colonies were able to survive partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow the sandy bottom (cited Gittenberger, 2007). In the Netherlands, the coastal zone is composed of mud and sand, with only shells as hard substrata. Didemnum sp. remained rare until 1996, when populations quickly expanded, and it became a dominant invasive species because of an increase in available hard substrata for colonization after a cold winter between 1995 and 1996 caused a decrease in the abundance of many marine animals (Gittenberger, 2007). Thus, Didemnum vexillum was able to colonise and establish in mud and sand habitats where hard substrata were present.

Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020). It is abundant across various shore heights, thriving in both nearshore and offshore sites, particularly in subtidal areas. For example, colonies of Didemnum vexillum were dominant at depths between 45 to 60 m, occupying 50 to 90% of available space in two gravelly areas (more than 230 km²) composed of immobile pebble and cobble pavement on Georges Bank fishing ground, USA (Bullard et al., 2007; Valentine et al., 2007b; Lengyel et al., 2009). In addition, patchy mats have been observed covering approximately 1 to 1.5 km² of the pebble cobble seabed, which is interspersed with large boulders and 30 m deep in Long Island Sound, USA (Mercer et al., 2009). In an offshore scallop dredge survey, Didemnum sp. was found attached to cobbles and boulders at 10 to 34 m (Vercaemer et al., 2015). 

Sensitivity assessment: This biotope is likely to be unsuitable for the colonization of Didemnum vexillum due to the presence of mud and sandy mud, and the lack of hard substrata for colonization. The depth range of SS.SMu.OMu biotopes (50 to 200 m), which include SS.SMu.OMu.CapThy, is likely too deep for the colonization of Didemnum vexillum, which typically occurs from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020). Therefore, resistance is assessed as ‘High’, albeit with low confidence due to no direct evidence. Hence, resilience is assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’.

The Pacific oyster, Magallana gigas

Evidence

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

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

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

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020) because its larvae require hard substrata for successful settlement and development (McKinstry & Jensen, 2013; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020).

Magallana gigas has been reported from estuaries growing on intertidal mudflats and sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann et al., 2012, 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (McKinstry & Jensens, 2013). 

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

The majority of the evidence indicates that infralittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). However, in suitable situations (e.g. Oosterschelde) it may form beds down to 42 m.  

Sensitivity assessment: This biotope is likely to be unsuitable for the colonization of Magallana gigas due to the presence of mud and sandy mud, and the lack of any hard substrata suitable for colonization. The depth range of SS.SMu.OMu biotopes (50 to 200 m), which include SS.SMu.OMu.CapThy is likely too deep for the colonization of Magallana gigas, as the majority of evidence indicates that habitats that occur at depths of more than 10 m are unlikely to be suitable for colonization. Therefore, resistance is assessed as ‘High’, albeit with low confidence due to no direct evidence. Hence, resilience is assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’.

Wireweed, Sargassum muticum

Evidence

The depth range of SS.SMu.OMu biotopes (50 to 200 m), which include SS.SMu.OMu.CapThy. Therefore, the depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. 

Wakame, Undaria pinnatifida

Evidence

The depth range of SS.SMu.OMu biotopes (50 to 200 m), which include SS.SMu.OMu.CapThy. Therefore, the depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. 

Other INIS

Evidence

No other INIS were identified