The Marine Life Information Network

The characterizing species in these biotopes are widely distributed in the British Isles, Northeast Atlantic and beyond, from Norway to the Mediterranean, west and South Africa (Hayward & Ryland, 1995b). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007) In addition, Paramphinome jeffreysii seems to reach its southerly limit in UK waters suggesting a possible susceptibility to a long-term rise in summer water temperatures (Tillin & Tyler-Walters, 2014), although the sub-family to which it belongs seems to mainly occur in warm littoral waters (Rouse & Pleijel, 2001).Growth rates of Heteromastus filiformis have been shown to be very rapid in warmer environments, with no growth occurring during winter (Shaffer, 1983).

Kröncke et al. (2011) reported changes in abundance and distribution of various species with a southern distribution in the North Sea in 2000, and suggested the changes were largely associated with an increase in sea surface temperature, primary production and, thus, food supply. They suggested that the annual increase in average temperature of ~1.1 °C caused a decrease in Amphiura filiformis. In Galway Bay, long-term recordings of water temperature at a site of high-density aggregations of Amphiura filiformis showed the species is subject to annual variations in temperature of about 10 °C (O'Connor et al., 1983).

Increases in temperature may affect growth and fecundity. Muus (1981) showed that juvenile Amphiura filiformis are capable of much higher growth rates in experiments with temperatures between 12 and 17 °C.

Under climate change projections, Amphiura filiformis’ overall bioturbation potential has been predicted to increase by 42% by 2099 (Weinert et al., 2022). However, a mean bottom temperature increase of 5.4 °C over that period may also impose metabolic stress on individuals, potentially reducing their actual bioturbation activity in warmer regions. This physiological stress could lead to reduced sediment oxygenation and changes in benthic community structure. In addition, the amount of suitable habitat for this species is projected to decline by around 65% due to climate change (Weinert et al., 2016).

An increase in sea surface temperature (+1.5 to 1.8 °C from 1950 to 2015) at the Oysterground in the south-eastern North Sea, combined with a reduction of nutrient input into rivers, led to a decrease in phytoplankton primary productivity, which was strongly linked to the decline in abundance and biomass of Amphiura filiformis and Kurtiella bidentata, the latter of which was absent from 2010 onwards (Meyer et al., 2018). The resultant decline in the abundance of these two species also reduced bioturbation potential in the study area.

Ennucula tenuis is a cold-water species with a preference for benthic temperatures below 14 °C. Xu et al. (2024) used ensemble species distribution models to investigate the effects of global warming on Ennucula tenuis. They found that under the least extreme climate change scenario (global average sea surface temperature increase of 0.3 to 1.7 °C by the year 2100), Ennucula tenuis will experience a range contraction of up to 21.2% in the Yellow Sea. Under the most extreme scenario (warming of 2.6 to 4.8 °C), their range could contract by up to 43.5%.

Temperature not only limits the spatial distribution of bivalves, but is also a major controlling factor in many physiological rate processes like feeding and growth (Dame, 1996). For example, no spawning occurred in June in the wild, but Ennuculatenuis specimens (studied as Nucula tenuis) held in a laboratory at an elevated temperature of 23 °C were observed to spawn in July (Rachor, 1976, cited in Harvey & Gage, 1995).

Wilson (1981) investigated temperature tolerances of six bivalve species from Dublin Bay. They 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). Kurtiella bidentata (studied as Mysella bidentata) was recorded in Kinsale Harbour at temperatures ranging from 7.7 to 18.8 °C (O’Brien & Keegan, 2006), and Künitzer (1989) reported that the main factor affecting the growth rate of Kurtiella bidentata (studied as Mysella bidentata) was temperature.

According to OBIS (2025), Abra nitida has been recorded in sea surface temperatures ranging between 5 to 20 °C, with most observations in the 10 to 15 °C range. Its range extends from northern Norway to the mediterranean and the Black Sea. No evidence was found regarding the effects of higher temperatures on Abra nitida. However, evidence for the taxonomically similar Abra prismatica and Abra alba could be used for inference.

In a study modelling the effects of climate change on the distribution of benthic species in the Eastern Mediterranean, Moraitis et al. (2019) defined Abra prismatica as a tolerant species, but their model suggested the habitat suitability of the species was expected to decline as temperatures increase under predicted climate change (including scenario RCP 8.5) conditions by 2100. In a study on the changes in species occurrence as temperature increased in the English Channel from 1985 to 2012, Gaudin et al. (2018) observed extensions in the distribution range of Abra pristmatica and a more than 50% increase in its spatial occurrence in response to an increase in temperature over the time period. 

Bernard et al. (2016) experimentally tested how temperature and food availability affect Abra alba, a species which carries out bioturbation through its feeding and burrowing activity. They found that Abra alba showed much greater particle mixing at warmer ‘summer’ temperatures (around 18 to 22 °C), with more frequent and longer sediment movements and higher calculated biodiffusion rates. In contrast, mixing activity was very low at cooler ‘autumn’ temperatures (around 14 to 16 °C), suggesting that bioturbation by Abra alba is strongly temperature dependent and likely to decrease in colder conditions (Bernard et al., 2016).

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).

Heteromastus filiformis has been recorded in temperatures ranging between 3.21 °C to 29.27 °C (Zan et al., 2015). Between 1979-1993, Heteromastus filiformis and Thyasira equalis were dominant species in Byfjorden, Raunefjorden and Sørfjorden (Johansen et al., 2018). From the mid-1990s to 2016, Heteromastus filiformis increased 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.

Between 1986 and 2000, the North Sea experienced a mean sea bottom temperature increase of 0.31 °C and a mean sea surface temperature increase of 0.42 °C (Hiddink et al., 2015). Levinsenia gracilis have shifted their range north-westerly by ~6.8 km/year in this period, but Hiddink et al. (2015) show that this shift lags behind the shift of temperature zones, which could lead to local extinctions.

No evidence was found regarding Myrtea spinifera sensitivity to changes in temperature. This species has been recorded in temperatures between 5 to 25 °C, with most observations being in the 10 to 15 °C range (OBIS, 2025).

Sensitivity assessment: The characterizing species of the biotopes are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4 to 19°C (Huthnance, 2010). Elevated temperatures may affect metabolic activity, bioturbation, and growth of some of the characterizing species, but no mortality is expected at the benchmark level except for Thyasira spp. in the case of an acute increase in temperature. Resistance is therefore assessed as ‘Medium’ (loss <25%) for SS.SMu.CSaMu.ThyEten and SS.SMu.OMu.PjefThyAfil, and ‘High’ for the remaining biotopes. Resilience is likely to be ‘High’ so SS.SMu.CSaMu.ThyEten and SS.SMu.OMu.PjefThyAfil are considered to have ‘Low’ sensitivity to an increase in temperature at the pressure benchmark level, whereas the remaining biotopes are considered ‘Not Sensitive’.

The characterizing species in these biotopes are widely distributed in the British Isles, north-east Atlantic and beyond, from Norway to the Mediterranean and west and South Africa (Hayward & Ryland, 1995b). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). Additionally, Paramphinome jeffreysii seems to reach its southerly limit in UK waters (Tillin & Tyler-Walters, 2014), although the sub-family to which it belongs seems to mainly occur in warm littoral waters (Rouse & Pleijel, 2001). Heteromastus filiformis occur in the North Sea, English Channel, north-east Atlantic and Mediterranean (Hayward & Ryland, 1995b), and it has been suggested that no growth occurs during winter (Shaffer, 1983), suggesting intolerance to decreases in temperature.

Holme (1967) reported the absence of Amphiura filiformis from samples taken from Weymouth Bay and Poole Bay, England, after severe winter temperatures (4 and 5 °C, respectively, below the mean for about a month). In Galway Bay, long-term recordings of water temperature at a site of high-density aggregations of Amphiura filiformis showed the species is subject to annual variations in temperature of about 10°C (O'Connor et al., 1983). However, echinoderms, including Amphiura filiformis, in the North Sea, seem periodically affected by winter cold. A population at 27 m depth off the Danish coast was killed by the winter of 1962-63 (Muus, 1981) and a population at 35 to 50 m depth in the inner German Bight was killed in the winter of 1969-70. A new population was not re-established until 1974 (Gerdes, 1977). Ursin (1960, cited in Gerdes, 1977) suggests that Amphiura filiformis does not occur in areas with winter temperatures below 4 °C although in Helgoland waters it can tolerate temperatures as low as 3.5 °C.

Temperature not only limits the spatial distribution of bivalves, but also influences feeding and growth, with short-term acute periods of extreme cold and icing conditions considered likely to cause stress and some mortality in bivalve populations (Dame, 1996). For example, Kurtiella bidentata (studied as Mysella bidentata) was among the species that suffered high losses that could be related to low temperatures in the Wadden Sea area in 1979, where the temperature was 3 °C below average for 3 months (Beukema, 1979). During the 1978/79 winter, which was very cold with severe ice conditions, water temperature in the outer Weser estuary, Germany, remained below 0 °C on 45 successive days. Populations of the characteristic species of the benthos, including Abra spp. were considerably damaged (Buhr, 1981).

According to OBIS (2025), Abra nitida has been recorded in sea surface temperatures ranging between 5 to 20 °C, with most observations in the 10 to 15 °C range. Its range extends from northern Norway to the mediterranean and the Black Sea. No evidence was found regarding the effects of lower temperatures on Abra nitida. However, evidence for the taxonomically similar Abra alba could be used for inference.

Bernard et al. (2016) experimentally tested how temperature and food availability affect Abra alba, a species which carries out bioturbation through its feeding and burrowing activity. The authors found that Abra alba showed much greater particle mixing at warmer ‘summer’ temperatures (around 18 to 22 °C), with more frequent and longer sediment movements and higher calculated biodiffusion rates. In contrast, mixing activity was very low at cooler ‘autumn’ temperatures (around 14 to 16 °C), suggesting that bioturbation by Abra alba is strongly temperature dependent and likely to decrease in colder conditions (Bernard et al., 2016).

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 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.

Heteromastus filiformis has been recorded in temperatures ranging between 3.21 °C to 29.27 °C (Zan et al., 2015). In studies of the effects of cold winters on macrofauna communities in the North Sea, Kröncke et al. (2013) suggested that the overall trend was towards decreased abundance and biomass, including polychaetes, as a result of temperature anomalies of about 2 °C below normal. However, Holte et al. (2005) investigated the variations in soft bottom macrofauna from stratified Norwegian basins. Heteromastus filiformis occurred at the study sites, which experienced temperatures between 0.5 to 14°C.

No evidence was found regarding Myrtea spinifera sensitivity to changes in temperature. This species has been recorded in temperatures between 5 to 25 °C, with most observations being in the 10 to 15 °C range (OBIS, 2025).

Coyle et al. (2007) analysed temporal differences in benthic infaunal samples from the south-eastern Bering Sea shelf. Significant differences were observed for specific functional groups, namely carnivores, omnivores and surface detritivores, which suggested a mechanistic link between temperature changes and infaunal biomass, with exclusion of benthic predators on infaunal invertebrates by the cold bottom water on the shelf.

Sensitivity assessment: The characterizing species of these biotopes are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4 to 19 °C (Huthnance, 2010). Although it is likely that most of the characterizing species are able to resist a long-term decrease in temperature of 2 °C, Amphiura filiformis may suffer some mortality as a result of an acute decrease in temperature. Therefore, resistance for all biotopes under assessment is assessed as ‘Low’ (25-75% loss) and resilience is likely to be ‘Medium’, so the biotopes are considered to have ‘Medium’ sensitivity to a decrease in temperature at the pressure benchmark level.

The biotopes are found within fully marine subtidal locations (Connor et al., 2004). Therefore, it is highly unlikely that the biotopes would experience conditions of hyposalinity.

Amphiura filiformis are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Stickle & Diehl, 1987; Russell, 2013). However, Amphiura filiformis was recorded in hyposaline conditions in the Sado estuary in Portugal (Monteiro-Marques, 1982 cited in Russell, 2013) where the salinity was 25.5 ppt, and in the Black Sea where it tolerated 8.9 ppt (Russell, 2013). Pagett (1981) suggested that localised physiological adaption to reduced or variable salinities may occur in nearshore areas subject to freshwater runoffs. However, individuals in these biotopes are unlikely to experience variable salinities, and resident species are unlikely to be adapted to variation in salinity, as suggested by the results given by Pagett (1981). The full range of salinity in which Amphiura filiformis has been recorded is 10 to 40 PSU, with most observations recorded at 30 to 35 PSU (OBIS, 2025). Under climate change projections, it is predicted that salinity will decrease in some areas of the North Sea by up to 1.7 PSU by the year 2099 (Weinert et al., 2022). This is expected to have only a minor effect on the distribution and bioturbation potential of key bioturbator species including Amphiura filiformis.

Salinity may affect the structural and functional properties of bivalve organisms through changes in total osmotic concentration, relative proportion of solutes, coefficients of adsorption and saturation of dissolved gases and density and viscosity (Kinne, 1964, cited in Dame, 1996). There are records of Kurtiella bidentata (studied as Mysella bidentata) in Kinsale Harbour at salinities ranging from 19.3 to 35.0 (O’Brien & Keegan, 2006). However, Gogina et al. (2010a) reported that Kurtiella bidentata (studied as Mysella bidentata) showed a strong positive correlation with salinity varying from 8.30 to 27.10 PSU, which suggested that the species was affected adversely at the low end of the range.

Abra nitida occurs in salinities ranging from 10 to 40 PSU, with most records in the 30 to 35 PSU range (OBIS, 2025). No evidence was found regarding Abra nitida tolerance to salinities above this range. However, the taxonomically similar Abra alba has been identified as a dominant species in tidal channels in the Mediterranean where salinity can reach as high as 47 PSU (Fersi et al., 2023). This species has also been found in the Oualidia lagoon on the Moroccan Atlantic coast, exposed to salinities ranging from 10.1 to 39.5 ppt (El Asri et al., 2015, 2022).

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). 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 sulfur-oxidizing bacteria and hydrogen sulfide 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.

Levinsenia gracilis has been found in salinity ranges between 25 to 30 PSU (Onwuteaka, 2016) and 5 to 40 PSU by OBIS (2025). The salinity range with the most records of this species was 30 to 35 PSU (OBIS 2025).

In the Schelde estuary, Heteromastus filiformis was found in estimated salinity ranges from ~16 to ~24 PSU (Ysebaert et al. 2002). Yan et al. (2019) found that Heteromastus filiformis was positively correlated with proximity to the Yangtze (Changjiang) from which there was a significant amount of freshwater input.

The minimum and maximum recorded salinity ranges for the remaining characterizing species are 10 to 40 PSU for Ennucula tenuis, 5 to 40 PSU for Kurtiella bidentata, 15 to 40 PSU for Abra nitida, 20 to 40 PSU for Paramphinome jeffreysii, and 25 to 40 PSU for Myrtea spinifera, with most observations of these species occurring within the 30 to 35 PSU range (OBIS, 2025).

Sensitivity assessment: The evidence presented suggests that the characterizing species of these biotopes are likely to resist a decrease in salinity at the pressure benchmark level. Resistance is therefore assessed as ‘High’, resilience as ‘High’ by default, and sensitivity as ‘Not Sensitive’ for all biotopes under assessment.

The hydrographic regime, including flow rates, is an important structuring factor in sandy mud biotopes. Increased flow rate (above the pressure benchmark) could erode the substratum and could lead to loss of the biotope. This could also change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). Furthermore, increased water flow rate may prevent settlement of larvae and therefore reduce recruitment. Mature adults buried at depth are likely to be unaffected as muddy substrata are particularly cohesive. Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability.

Decreased water movement would result in increased deposition of suspended sediment (Hiscock, 1983). An increased rate of siltation may result in an increase in food availability for the characterizing species and therefore growth and reproduction may be enhanced, but only if food was previously limiting. Nevertheless, a decrease in water flow rates is unlikely to be relevant in the low energy environments where the biotopes occur.

Amphiura filiformis respond to currents by extending their arms into the water column to feed. Under laboratory conditions, they were shown to maintain this vertical position at currents of 0.3 m/s (Buchanan, 1964). Amphiura filiformis feed on suspended material in flowing water but change to deposit feeding in stagnant water or areas of very low water flow (Ockelmann & Muus, 1978). Food requirements probably set a lower limit on the current regime of areas able to support brittlestars. Amphiura filiformis has also been reported in the Northumberland coast, UK where tidal currents varied from surface speeds of 0.65 m/s at springs to 0.4 m/s at neaps, on a flood tide. Bottom residual currents were much weaker than near-surface, reaching a maximum of 0.07 m/s (Jones, 1979; cited in Birchenough & Frid, 2009).

No evidence was found for the effects of current speed on Abra nitida. However, the taxonomically similar Abra alba has been reported to be highly abundant and dominant in environments associated with strong tidal currents and high wave energy (Foulquier et al., 2020; El Asri et al., 2022). Dauvin et al. (2017) reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong hydrodynamic conditions (regular swell and tidal currents), river discharge, sediment changes and anthropogenic impacts from dredging and port construction. The study also found that Abra alba positively correlated with periods of low current speed, indicating a broad tolerance to changes in water flow.

Heteromastus filiformis has been recorded in areas where ebb-current velocity ranged between 0.01 and 1.64 m/s, and flood-current velocity ranged between 0.01 and 1.61 m/s (Ysebaert et al., 2002).

Sensitivity assessment: Sand particles are most likely to be eroded at about 0.2 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, silts and clays require greater critical erosion velocities because of their cohesiveness. These biotopes occur in stable areas of very weak (negligible) and weak (>0.5 m/s) tidal streams (Connor et al., 2004; JNCC, 2022). Although changes in water flow (above the benchmark) would be likely to change the sedimentary regime in the biotopes, the cohesive nature of the sandy muds that characterize the biotopes is likely to provide some protection to changes in water flow at the pressure benchmark. In addition, the characterizing species are likely to resist an increase in water flow at the benchmark level. Hence, resistance and resilience are assessed as ‘High’ and the biotopes are considered ‘Not Sensitive’ to a change in water flow at the pressure benchmark level.

Oxygen-deficient marine areas are characterized by a decline in the number and diversity of species. Cole et al. (1999) suggested possible adverse effects on marine species exposed to dissolved oxygen concentrations below 4 mg/l and probable adverse effects below 2 mg/l.

A number of animals have behavioural strategies to survive periodic events of reduced dissolved oxygen. These include shell closure and reduced metabolic rate in bivalve molluscs and either decreased burrowing depth or emergence from burrows for sediment dwelling crustaceans, molluscs and annelids. However, a decrease in oxygenation is likely to see the loss of the key species in the biotopes. During periods of hypoxia infaunal species migrate to the surface of the sediment (Diaz & Rosenberg, 1995).

Stachowitsch (1984) observed a mass mortality of benthic organisms in the Gulf of Trieste, northern Adriatic Sea, caused by the onset of severe hypoxia in the near-bottom water. A wide variety of organisms were affected, including burrowing invertebrates, sponges, and the brittlestar Ophiothrix quinquemaculata. However, Amphiura filiformis was reported as a species resistant to moderate hypoxia (Diaz & Rosenberg, 1995).

Mass mortality of Amphiura filiformis was observed during severely low oxygen events (<0.7 mg/l) (Nilsson, 1999). Mass mortality was observed following large increases in eutrophication and subsequent reductions in oxygen (Vistisen & Vismann, 1997). The regeneration rate of arms is significantly decreased at low oxygen concentrations (1.8 to 2.2 mg/l) (Nilsson, 1999), and growth rate is decreased in oxygen concentrations of <2.7 mg/l and spawning is restricted (Nilsson & Sköld, 1996).

Calder-Potts et al. (2015) and (2018) investigated the physiological and behavioural responses of Amphiura filiformis to short-term moderate hypoxia (3.59 mg/l over 14 days) using mesocosm experiments. Both studies revealed that hypoxia led to reduced aerobic metabolism and delayed recovery once normoxic conditions were restored. Reproductive development was also impaired, with females exhibiting smaller oocyte diameters and a higher proportion of pre-vitellogenic oocytes, suggesting a disruption in gametogenesis.

Although Amphiura filiformis density did not significantly affect physiological traits, it positively influenced bioturbation activity which was notably suppressed under hypoxic conditions. While the 2015 study found no significant effects of population density on metabolism or reproduction, the 2018 study did. Calder-Potts et al. (2018) also observed Amphiura filiformis individuals leaving their burrows under hypoxic conditions, possibly to find normoxic conditions. These findings highlight the sub-lethal vulnerability of Amphiura filiformis to episodic deoxygenation and suggest that repeated hypoxic events could have long-term consequences for population resilience, benthic community structure, and sediment processes in sandy mud biotopes.

Rosenberg et al. (1991) exposed benthic species from the NE Atlantic to gradually reduced oxygen concentrations. Hypoxic treatments were maintained at 0.5 to 1 ml/l (0.71–1.43 mg/l) for several weeks. Amphiura filiformis began to emerge from the sediment when oxygen levels reached of 0.85 ml/l (1.22 mg/l). Mortality became significant at 0.65 ml/l (0.93 mg/l) and lower, after which mortality progressively increased for the remainder of the study period. After 11 days of exposure to hypoxia levels of 0.86 ml/l (1.23 mg/l), there was a 60% mortality rate in Abra nitida compared to no mortality in the control which was kept at 6 to 7 ml/l (8.57 to 10 mg/l).

In a meta-analysis study of hypoxia, median sub-lethal oxygen concentrations were reported in experimental assessments. The thresholds of hypoxia for different benthic groups was LC50 1.42 mg/l for bivalves, and sub-lethal (SLC50) of 1.20 mg/l for annelids (Vaquer-Sunyer & Duarte, 2008). For Kurtiella bidentata (studied as Mysella bidentata), the median sub-lethal oxygen concentrations reported in experimental assessments was 1 mg/l, and for Abra spp. was 0.57 mg/l (Vaquer-Sunyer & Duarte, 2008).

At oxygen concentrations below ~0.4 mg/l, Kurtiella bidentata eventually emerged from the substratum (Ockelmann & Muus, 1978). Nilsson & Rosenberg (1994) investigated hypoxic responses of benthic communities and reported Kurtiella bidentata (studied as Mysella bidentata) leaving the sediment at oxygen concentrations of 1.7 mg/l. According to the authors, this behaviour that occurs at hypoxic oxygen concentrations that are slightly higher than those causing mortality, suggesting high levels of stress caused to the organisms.

Abra spp. are typically found in organically enriched sediments where it may be present in high densities (Dauvin & Gentil, 1989; Khedhri et al., 2016; Sciberras et al., 2017; Foulquier et al., 2020; Dilmi et al., 2024). Experimental examination of the interactions between eutrophication and oxygen deficiency (2.4 to 3.5 mg/l over a 93-day experimental period) revealed that Abra alba, closely related to Abra nitida, became inefficient in its use of the available organic matter under prolonged conditions of hypoxia (Hylland et al., 1996). Abra alba was also reported to be sensitive to lowered oxygen concentrations off the Swedish west coast (Rosenberg & Loo, 1988; Weigelt & Rumohr, 1986, both cited in Rees & Dare, 1993).

Benthic communities containing Abra alba, have been shown to influence chemical, physical and biological processes in marine sediments through bioturbation. Rius et al. (2018) showed evidence that muddy fine sand Abra alba communities in the English Channel persist and thrive in naturally oxygenated conditions, and drive high rates of sediment oxygen uptake, with oxygen saturation in the bottom waters between 90 to 100% (9.0 mg/l to 10.0 mg/l) (Rius et al., 2018). This suggests that Abra alba communities contribute to sediment oxygen consumption and are likely to be sensitive to declines in oxygen.

Dando & Spiro (1993) found that numbers of the congeners Thyasira equalis and Thyasira sarsi decreased rapidly following the de-oxygenation of bottom water in the deep basin of the Gullmar fjord in 1979-80. However, 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 individuals /m² at oxygen saturations of 23% (~2.3 mg/l).

Infaunal burrowers in the community live in close association with hypoxic and even anoxic muddy substrata, including the characterizing polychaetes. Heteromastus filiformis was recorded to occur in the Homa lagoon, eastern Aegean Sea, where high salinity was coupled with low oxygen concentrations (2.3 to 3.9 mg/l) with adverse negative effects on the abundance of the community (Can et al., 2012). However, the study focused on the effects of hypersalinity (>50 PSU) and the authors attributed mortality to increased salinity, leaving it unclear whether the hypoxic conditions also contributed to mortality of the population. This is consistent with Hiscock et al. (2005a), who described Heteromastus filiformis as a species resistant to severe hypoxia. According to Zan et al. (2015), dissolved oxygen levels suitable for Heteromastus filiformis ranges from 4.43 to 10.06 mg/l. However, they have been known to increase in abundance under hypoxic conditions. Between 1979-1993, Heteromastus filiformis and Thyasira equalis were dominant species in Byfjorden, Raunefjorden and Sørfjorden (Johansen et al., 2018). From the mid-1990s to 2016, Heteromastus filiformis increased 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: Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotopes will have varying responses to deoxygenation. Based on the evidence presented, the characterizing species are likely to only be affected by severe deoxygenation episodes. However, some mortality of Thyasira spp. might occur in near anoxic (0% oxygen) conditions. Resistance is therefore assessed as ‘Medium’ (loss <25%) for SS.SMu.CSaMu.AfilEten, SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.ThyEten and SS.SMu.OMu.PjefThyAfil, and ‘High’ for the remaining biotopes. Resilience is likely to be ‘High’ for all biotopes, so SS.SMu.CSaMu.AfilEten, SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.ThyEten and SS.SMu.OMu.PjefThyAfil are considered to have Low sensitivity to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week, whereas the remaining biotopes are considered ‘Not Sensitive’. 

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

Borja et al. (2000) and Gittenberger & Van Loon (2011) both assigned Amphiura filiformis to their Ecological Group II ‘species indifferent to enrichment, always present in low densities with non-significant variations with time (from initial state, to slight unbalance)’; Abra nitida and Thyasira flexuosa were assigned to Ecological Group III ‘species tolerant to excess organic matter enrichment); Kurtiella bidentata (referred to as Mysella bidnetata), Ennucula spp. and Myrtea spinifera were characterized as AMBI Group I – ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’. Heteromastus filiformis was considered in both cases as an opportunistic species, tolerant to excess organic matter enrichment, although assigned to different levels (III by Borja et al., 2000, and IV by Gittenberger & Van Loon, 2011).

Interface feeders (species which switch between suspension and deposit feeding depending on food availability) such as Amphiura filiformis have been reported to benefit from increased primary production (Pearson & Mannvik, 1998, cited in Schückel et al., 2010). Increases in primary production are a symptom of an increase in nutrients. Therefore, nutrient enrichment could indirectly affect Amphiura filiformis.

A reduction in nutrient input into rivers has led to the de-eutrophication of the Oysterground in the south-eastern North Sea. This reduction in nutrient input, combined with an increase in sea surface temperature (+1.5 to 1.8 °C from 1950 to 2015), has led to a decrease in phytoplankton primary productivity, which was strongly linked to the decline in abundance and biomass of Amphiura filiformis and Kurtiella bidentata, the latter of which was absent from 2010 onwards (Meyer et al., 2018). The resultant decline in the abundance of these two species also reduced bioturbation potential in the study area. This suggests that nutrient enrichment could be beneficial to these species to a certain extent.

In a sewage dumping region of the North Sea, a great increase in the abundance of Abra spp. occurred in much of the dumping area (Caspers, 1981). The Amoco Cadiz oil spill in March 1978 caused vast disturbance to the fine-sand communities of the Bay of Morlaix, France (Dauvin, 1982). Drastic changes in species abundance, diversity, and biomass were recorded after the spill. However, the Abra alba population persisted in the disturbed environment under eutrophic conditions and, as an 'opportunistic species' (Hily & Le Bris, 1984), rapidly adapting its reproductive strategy to three spawnings per year. Increased growth and abundance were attributable to increased food availability and vacant ecological niches (Dauvin & Gentil, 1989).

Abra alba is closely related to Abra nitida which characterizes SS.SMu.CSaMu.AfilKurAnit. Abra alba has been recorded as the most dominant species at a shellfish farm in the Mediterranean, amongst species tolerant of organic enrichment (Dilmi et al., 2024), and is dominant in organically enriched sediment within tidal channels in the Mediterranean where detritus accumulates (Fersi et al., 2023). This evidence is consistent with findings from Boughrara Lagoon, in the southwest Mediterranean, where the tolerant Abra alba increased in abundance despite improvements in measured nutrient concentrations following enlargement of the El-Kantra channel (Khedhri et al., 2016). The lagoon was described as an almost-closed, fine-sediment system impacted by increased pollution (such as aquaculture effluent, sewage outfalls, fishing ports and industrial discharges) and had high levels of nitrate, nitrite, phosphate and ammonium. These nutrient and organic levels significantly decreased after the expansion of the El-Kantra channel (e.g. nitrites 9.17 µg/l decreased to 0.20 µg/l; nitrates 55.53 µg/l to 2.46 µg/l; phosphates 61.07 µg/l to 0.44 µg/l; ammonia 14.23 µg/l to 4.07 µg/l), which allowed increased boat activities and increased water exchange into the lagoon (Khedhri et al., 2016). Khedhri et al. (2016) concluded that although physico-chemical conditions may have improved, the lagoon is still impacted by human pressures. The results found that species richness and abundance of macrofauna in the lagoon decreased, and tolerant species including Abra alba increased in abundance.

Enrichment from pulp mills is believed to have been the cause of the death of two populations of Thyasira gouldii in west Scotland sea lochs. However, Thyasira flexuosa has been recorded at densities of up to 4000 /m² in enriched areas (Dando & Southward, 1986).

Yan et al. (2019) found that Heteromastus filiformis was positively correlated with proximity to the Yangtze (Changjiang) river from which there was a significant input of dissolved inorganic nitrogen. A large increase in inorganic nitrogen and phosphorus in the lagoon of the Mar Menor (southeast Spain) from agricultural runoff led to a shift in ecological state in 2016 (Sandonnini et al., 2024). Heteromastus filiformis was one of the dominant polychaetes in the year following the eutrophication event, but then declined significantly by the 2018 sampling period due to competitive displacement by Hydroides. Heteromastus filiformis has also been found in areas with total nitrogen as high as 1900 mg/kg, total phosphorus as high as 580 mg/kg, and chlorophyll-α as high as 16,000 μg/kg (Ellis et al., 2015).

Sensitivity assessment: A decrease in nutrient availability may result in impaired growth and fecundity, although species diversity is not likely to be affected significantly. All characterizing species of these biotopes show ‘High’ resistance to nutrient enrichment except for interspecific differences within the Thyasira genus. Therefore, resistance and resilience for all biotopes are ‘High’, and all biotopes are considered to be ‘Not Sensitive’ with high confidence except for SS.SMu.CSaMu.ThyEten and SS.SMu.OMu.PjefThyAfil, for which confidence is medium.

Organic enrichment is likely to promote pelagic productivity and increase the amount of organic matter reaching the seabed, which may be beneficial to deposit feeders as a direct source of food. Nilsson (1999) investigated the effects of organic enrichment (control 0 g C /m², medium 27 g C /m² and high 55 g C /m²) on arm regeneration of Amphiura filiformis over a two-month period. Amphiura filiformis responded positively to increased organic enrichment. In the Skagerrak in the North Sea, a massive increase in abundance and biomass of the brittlestar between 1972 and 1988 was attributed to organic enrichment (Josefson, 1990; Hernroth et al., 2012). Rosenberg et al. (1997) also reported that Amphiura filiformis appeared to be more densely packed in the sediment when food occurred superabundantly compared to when food was less common. Sköld & Gunnarsson (1996) reported enhanced growth and gonad development in response to short-term enrichment of sediment cores containing Amphiura filiformis maintained in laboratory mesocosms. However, if increased organic input resulted in almost complete oxygen depletion, mortality of individuals was likely to occur (see de-oxygenation pressure). Mcleod et al. (2008) investigated the recovery of soft sediment benthic invertebrate community following removal of high levels of organic enrichment from fish farming in Tasmania. The authors observed that Amphiura species were associated with areas least impacted by organic enrichment.

Evidence from Gallmetzer et al. (2017) supports the sensitivity of these biotopes to organic enrichment. In the Gulf of Trieste, historical sediment cores revealed increasing concentrations of total organic carbon (TOC) and total nitrogen (TN) throughout the 20th century, driven by both natural inputs (e.g. riverine sedimentation) and anthropogenic sources. Kurtiella bidentata and Amphiura filiformis (with the latter inferred from ossicle records) were initially abundant, suggesting that moderate organic inputs may initially support bioturbators and deposit feeders. However, continued enrichment, combined with hypoxia and bottom trawling, led to declines in these sensitive species.

In contrast, Klunder et al. (2020) reported a positive correlation between total organic carbon, nitrogen, and the abundance of species from the Amphiuridae family. The highest level of sediment TOC found was ~0.566%. In addition, total nitrogen in the sediment was also positively correlated with Amphiuridae.

Abra alba is closely related to Abra nitida which characterizes SS.SMu.CSaMu.AfilKurAnit. It has been described as an indicator of increased organic matter (Dilmi et al., 2024) and has been frequently recorded in benthic assemblages, commonly associated with organic enrichment (Dauvin & Gentil, 1989; Khedhri et al., 2016; Sciberras et al., 2017; Foulquier et al., 2020; Dilmi et al., 2024). Multiple studies have reported Abra alba being highly abundant and/or a dominant species at sites with elevated sediment organic matter and in close proximity to organically enriched industrial discharge and sewage outfalls.

Abra alba has been recorded as the most dominant species at a shellfish farm in the Mediterranean, amongst species tolerant of organic enrichment (Dilmi et al., 2024), and is dominant in organically enriched sediment within tidal channels in the Mediterranean where detritus accumulates (Fersi et al., 2023). This evidence is consistent with findings from Boughrara Lagoon, in the Southwest Mediterranean, where the tolerant Abra alba increased in abundance despite improvements in measured nutrient concentrations following enlargement of the El-Kantra channel (Khedhri et al., 2016). The lagoon was described as an almost-closed, fine-sediment system impacted by increased pollution (such as aquaculture effluent, sewage outfalls, fishing ports and industrial discharges) and had high levels of nitrate, nitrite, phosphate and ammonium. These nutrient and organic levels significantly decreased after the expansion of the El-Kantra channel (e.g. nitrites 9.17 µg/l decreased to 0.20 µg/l; nitrates 55.53 µg/l to 2.46 µg/l; phosphates 61.07 µg/l to 0.44 µg/l; ammonia 14.23 µg/l to 4.07 µg/l), which allowed increased boat activities and increased water exchange into the lagoon (Khedhri et al., 2016). Khedhri et al. (2016) concluded that although physico-chemical conditions may have improved, the lagoon is still impacted by human pressures. The results found that species richness and abundance of macrofauna in the lagoon decreased, and tolerant species such as Abra alba increased in abundance.

Abra alba has been identified as one of the most abundant and dominant species in multiple benthic assemblages in the Oualidia lagoon, Moroccan Atlantic coast, exposed to both silty sediment with high organic content and coarser sand with low organic matter content and subject to strong tidal currents (El Asri et al., 2015; 2022). The Oualidia lagoon organic matter content ranged from 1.94 to 31.97% (El Asri et al., 2015). De Jong et al. (2015a) found that Abra alba dominated sites in the deepened shipping land and disposal sites in the Port of Rotterdam, which had the lowest mean bed shear stress, significantly higher sediment organic matter (2.1% mean sediment organic matter) and supported the highest biomass and species richness. In addition, Abra alba increased in abundance and became dominant following a summer flash flood in the Adour estuary coastal zone, off the French Basque coast, which caused a widespread fine sediment deposition (Foulquier et al., 2020). This site is a high energy and naturally physically stressed, and within four months, the site recovered rapidly. Abra alba density decreased, and the benthic assemblage shifted to a more stable community as fine particles decreased (Foulquier et al., 2020).

Dauvin et al. (2022) investigated the effects of dredged sediment disposal from the ports of Le Havre and Rouen on macrobenthic communities in the eastern Bay of Seine. The dumped sediment is largely composed of fine mud, sand and gravel with an elevated total organic carbon of around 1.2% on average, at impacted sites studied. Abra alba was among the ten dominant benthic species in the Bay of Seine. However, its abundance declined at some of the impacted disposal sites. Dauvin et al. (2022) found negative correlations between disposed volume and both taxonomic richness and abundance of the whole community, and noted seasonal recruitment with rapid community recovery after disturbance.

Sciberras et al. (2017) examined the chronic impacts of fishing frequency and organic matter enrichment on benthic communities and nitrogen cycling in sandy sediments from the Isle of Man. In laboratory conditions, sediment cores were enriched with a moderate dose of microalga, Isochrysis galbana, to simulate natural organic enrichment from an algal bloom. Results found that after one month, enrichment caused little difference in community composition, total density and species richness compared to non-enriched sediment. They also found that the macroinvertebrate community composition (including Abra prismatica) was dependent on the fishing history of the sediment collected rather than organic enrichment. Overall, enrichment acted as a food source for the sand-associated community, and nutrient cycling remained stable and efficient.

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). 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)”.

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., 2018). 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).

Birchenough & Frid (2009) analysed the succession of the macrobenthic community in the three years following cessation of sewage sludge disposal of 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 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). Similarly, Abra nitida occurs in organically enriched areas such as sediments beneath fish farms (Kutti et al., 2008).

Moreover, 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).

Organic input from deep-water fish farms can have severe effects on some of the characterising species of these biotopes. Paramphinome jeffreysii and 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). Paramphinome jeffreysii was a dominant species at reference sites, contributing up to 42% of the total faunal abundance at a site with low current speed and up to 31% at a site with moderate current speed. In the first sampling period under a low-current fish farm site, it was 12 times less abundant than in the low-current reference site, and was completely absent from samples taken 3 and 6 months later. At the moderate-current farm, Paramphinome jeffreysii was still present but its dominance was less than it was in the reference sites. 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.

Between 1979-1993, Heteromastus filiformis and Thyasira equalis were dominant species in Byfjorden, Raunefjorden and Sørfjorden (Johansen et al., 2018). From the mid-1990s to 2016, Heteromastus filiformis increased 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.

Heteromastus filiformis is a surface-deposit feeder, which may benefit from organic enrichment to an extent. Liao et al. (2019) investigated the effects of fish and oyster farms on microbenthic communities in Xiangshan Bay. Total organic carbon levels in the sediment in the fish cage culture area ranged from 0.66 to 0.89% compared to a range of 0.49 to 0.70% in the control area. Sediment nitrogen was overall higher (from 0.12 to 0.13%) than in the control area (0.09 to 0.10%). Suspended particulate matter in the fish cage culture area was as high as 243 mg/l due to particulate waste, compared to 61 to 142 mg/l in the control area. Heteromastus filiformis was found in all grab samples in the fish cage culture area and accounted for 17% of the total abundance of fauna, compared to being found in 88 to 100% of samples from the control area where it accounted for 11% of the faunal abundance. This suggests that Heteromastus filiformis is at least tolerant of slightly elevated levels of organic enrichment and may even benefit from such conditions. In Garolim Bay, South Korea, Heteromastus filiformis abundance has increased from 191 individuals/m² to 317.7 individuals/m² from 2006 to 2024 (Liang et al., 2024b). This increase has been interpreted as a bioindicator of ecosystem degradation linked to organic enrichment and heavy metal contamination from the 195 shellfish farms in the bay.

Liao et al. (2022) found that Heteromastus filiformis was more abundant at oyster farm sites than reference sites with no farming activity due to organic enrichment from oyster biodeposition. It was present in sediments with moderate total organic carbon levels (0.61 to 1.15%) but was negatively correlated with total organic carbon and ammonium, suggesting it prefers moderate enrichment over extreme organic enrichment. In addition, they found that it became significantly more abundant after the cessation of oyster farming activities and was among the top contributors to the shift in microbenthic community structure. An increase in abundance following the reduction of sediment organic matter content was also observed by Lee et al. (2025) at Lake Shihwa, an artificial lake in South Korea constructed in 1994 to supply agricultural water. This lake was heavily polluted from organic matter influx and nitrogen input, resulting in high chemical oxygen demand (the amount of oxygen required to chemically oxidise organic matter in water), seasonal hypoxia, a reduction in species richness and proliferation of pollution indicator species. In 2011, a tidal power plant was constructed between the lake and the outer sea to facilitate seawater mixing, after which Lake Shihwa communities and abiotic conditions became more similar to those of the outer sea within a year. In 2012. Heteromastus filiformis had increased in abundance more than any other species, suggesting that it is an opportunistic coloniser in recovering and organically enriched environments.

Borja et al. (2000) and Gittenberger & Van Loon (2011) both assigned Amphiura filiformis to their Ecological Group II ‘species indifferent to enrichment, always present in low densities with non-significant variations with time (from initial state, to slight unbalance)’; Abra nitida and Thyasira flexuosa were assigned to Ecological Group III ‘species tolerant to excess organic matter enrichment); Kurtiella bidentata (referred to as Mysella bidnetata), Ennucula spp. and Myrtea spinifera were characterized as AMBI Group I – ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’. Heteromastus filiformis was considered in both cases as an opportunistic species, tolerant to excess organic matter enrichment, although assigned to different levels (III by Borja et al., 2000, and IV by Gittenberger & Van Loon, 2011).

Sensitivity assessment: The above evidence suggests that most but not all of the important characteristic species are resistant of organic enrichment to varying degrees, with the Thyasira spp. being the most resistant and Kurtiella bidentata, Ennucula spp. and Myrtea spinifera the least resistant. Therefore, resistance is assessed as ‘Low’ (loss of 25 to 75%) for SS.SMu.CSaMu.AfilEten. SS.SMu.CSaMu.AfilKurAnit, SS.SMu.CSaMu.ThyEten, SS.SMu.OMu.PjefThyAfil and SS.SMu.OMu.MyrPo, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’. However, resistance is assessed ‘High’ and resilience ‘High’ for the opportunistic polychaete-dominated SS.SMu.OMu.LevHet, resulting in a sensitivity of ‘Not Sensitive’. Confidence in these assessments is low due to contrasting evidence for the responses of some characterizing species.

If the sediment that characterizes the biotopes was replaced with rock substrata, this would represent a fundamental change to the physical character of the biotopes. The characterizing species would no longer be supported and the biotopes would be lost and/or reclassified.

Jammar et al. (2025) reported that the installation of offshore wind farms (OWF) in soft sandy medium to coarse sediment habitats the Southern North Sea altered the seabed and shifted the microbenthic community structure. The OWF foundations provided new hard substrata, which increased the surfaces available for fouling organisms, and sediment near turbines became finer and organically enriched due to the increase in faecal pellets and detritus from fouling organisms. This resulted in a shift from a soft sediment Nepthys cirrosa community to a more diverse “intermediate community”, characterized by higher abundances of species associated with finer sediment, such as those typical of the Abra alba community (Jammar et al., 2025).

Sensitivity assessment: Resistance to the pressure is considered ‘None’, and resilience ‘Very Low’, given the permanent nature of this pressure. Sensitivity has been assessed as ‘High’. Although no specific evidence is described, confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Activities that disturb the surface could remove/damage infaunal species such as the characterizing species within the direct area of impact. Even shallow abrasion can damage feeding appendages which extend into the water column, temporarily reducing feeding efficiency and growth. More intense or repeated disturbance that penetrates the upper sediment layers may cause displacement or direct injury/mortality. Consequently, sensitivity to surface abrasion varies among taxa, with species such as Amphiura spp., burrowing bivalves, and polychaetes exhibiting differing levels of resistance and resilience depending on their morphology and sediment depth. The degree of impact therefore depends on the depth of abrasion (i.e. the gear used) and the vertical position of fauna within the sediment.

By extending their fragile arms from the sediment to feed, characterizing species Amphiura filiformis become vulnerable to damage by abrasion. Brittlestars can resist considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration (Sköld, 1998). Ramsay et al. (1998) suggested that Amphiura spp. may be less susceptible to beam trawl damage than other species like echinoids or tube dwelling amphipods and polychaetes. For example, Bergman & Hup (1992) found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Holtmann et al. (1996) reported a decrease in the abundance of the fragile burrowing heart urchins and Amphiura filiformis in areas of the southern North Sea between 1990 and 1995. These trends suggest that fishing activity may have been the main cause of these changes. However, Bradshaw et al. (2002) noted that the brittlestars Amphiura filiformis had increased in abundance in a long-term study of the effects of scallop dredging in the Irish Sea.

Rumohr & Kujawski (2000) compared qualitative historical benthos data (1902–1912) with data from 1986 to find long-term trends in epifauna species composition in the southern North Sea that may be attributed to fishery-induced changes. In general, the frequency of occurrence of bivalve species declined, whereas scavenger and predator species (crustaceans, gastropods, and sea stars) were observed more frequently in 1986. The authors suggested that these shifts could be attributed not only to the physical fishery impact but also to the additional potential food for scavenging and predator species provided by the large amounts of discards and moribund benthos. The brittlestar Amphiura filiformis occurred in 1986 on only 5% of the stations while it was present in most of the historical stations. Also, virtually all bivalve species originally present had decreased drastically, including Ennucula tenuis (also less than 5% of the sites by 1986). Despite the problems with the historical data set, the comparison presented was considered the best illustration achievable of the changes in the benthos from a near-pristine situation to the present conditions after long-term disturbance.

Amphiura filiformis was the dominant species in habitats sampled by Pommer et al. (2016) in the Kattegat, near Denmark. This species was significantly more abundant (~349 individuals/m²) in low-trawled sites (4.8 VMS points/year) compared to ~187 individuals/m² in high-trawled sites (63.8 VMS points/year). VMS points are hourly signals sent from fishing vessels to Danish and Swedish fishery management authorities whilst travelling at trawling speed.

Declines in species abundance from trawling events can continue to decline even after trawling has stopped. In an experiment conducted in the Frisian Front, North Sea, sediment cores were sampled before trawling and again 5.5, 29, and 75 hours after trawling. Amphiura filiformis showed a significant decline in abundance and was absent from the last sampling period (Tiano et al., 2020). They also found a 94% decrease in all epibenthos within trawled sediments and a 74% decrease in untrawled sediments of the same transect, suggesting that trawling can also significantly affect fauna in the surrounding sediments.

In habitats around the Isle of Man in 2003 to 2004, Amphiura filiformis was a dominant species accounting for 64.13% of the biomass (Sciberras et al., 2016). Trawling frequency between these surveys and repeated surveys in 2014 ranged from 2.95 to 8.51 sweeps per year. In the latter surveys, Amphiura filiformis was no longer present, and communities were instead dominated by burrowing shrimps.

Sköld et al. (2018) showed that Amphiura filiformis showed no significant response to bottom trawling intensity, while Amphiura chiajei abundance was shown to increase to an extent from trawling intensities of up to 5 trawls per year, beyond which its abundance declined. In contrast, Abra nitida decreased in abundance with increased trawling intensity.

In the five years following the establishment of a nearby MPA where trawling was prohibited, the abundance of Amphiura filiformis declined from ~50 individuals/m² to ~30 individuals/m², while Amphiura chiajei abundance declined from >20 individuals/m² to ~10 individuals/m² (Sköld et al., 2018). While this overall decline was statistically insignificant, the authors suggest it is still a meaningful decline due to the stability of the abundances of these two species in the trawled sights. It was presumed that this was due to reduced pressure on predatory fish and crustaceans which were target species of the fishery. This was later shown by Sköld et al. (2025) who reported significant declines in Amphiura abundance and biomass in the 12 years following the cessation of trawling in the Kattegat, with reductions in abundances estimated at 48% for Amphiura filiformis and 45% for Amphiura chiajei. Stomach content analyses confirmed that brittle stars were a staple prey item for benthivorus flatfish. Although no raw abundance values were reported, analyses of the data showed that Kurtiella bidentata, and Abra nitida also showed significant declines in abundance in the no-take zone following the cessation of trawling. This suggests that recovery from bottom-trawling is not only influenced by the removal of the pressure itself, but also by cascading ecological effects.

Direct mortality (percentage of initial density) from a single pass of a beam trawl was estimated from experimental studies on sandy and silty grounds as 9% for Amphiura spp., 20-65% for bivalves (including Kurtiella bidentata, studied as Mysella bidentata), and 5-40% for gastropods, starfish, small-medium sized crustaceans and annelid worms (Bergman & Van Santbrink, 2000). Some mortality was not caused directly by the passage of the trawl, but instead by disturbance, exposure and subsequent predation. Ball et al. (2000b) reported on the short-term effects of fishing on benthos from a mud patch in the northwestern part of the Irish Sea investigated in 1994 to 1996 by means of samples taken both before and shortly after (ca 24 hr) fishing activity. Kurtiella bidentata (previously studied as Mysella bidentata) was common at the inshore site, where estimates of mortality were calculated, but it was uncommon or entirely absent on the offshore fishing ground. Direct mortality from passage through an otter trawl was estimated at 70%.

Direct mortality (percentage of initial density) from a single pass of a beam trawl was estimated from experimental studies on sandy and silty grounds as 9% for Amphiura spp., 20-65% for bivalves (including Kurtiella bidentata, studied as Mysella bidentata), and 5-40% for gastropods, starfish, small-medium sized crustaceans and annelid worms (Bergman & Van Santbrink, 2000). Some mortality was not caused directly by the passage of the trawl, but instead by disturbance, exposure and subsequent predation. Ball et al. (2000b) reported on the short-term effects of fishing on benthos from a mud patch in the northwestern part of the Irish Sea investigated in 1994 to 1996 by means of samples taken both before and shortly after (~24 hr) fishing activity. Kurtiella bidentata (previously studied as Mysella bidentata) was common at the inshore site, where estimates of mortality were calculated, but it was uncommon or entirely absent on the offshore fishing ground. Direct mortality from passage through an otter trawl was estimated at 70%.

Abra spp. are shallow burrowers with a fragile shell (Tebble, 1976), and have been considered amongst the bivalve species most vulnerable to trawling by Bergmann & Van Santbrink (2000) who reported between <0.5% and 18% mortality of Abra alba due to trawling in the southern North Sea. However, the small size of Abra spp. relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993). Tiano et al. (2022) observed a significant decline in Abra alba from tickler trawling in the Netherlands. Three experimental plots (50 x 300 m) were trawled six times each, after which Abra alba density decreased from around 418 individuals/m² to 231 individuals/m². Due to the taxonomic similarities between Abra alba and Abra nitida, it is likely that the effects on the latter would be similar.

In laboratory conditions, Sciberras et al. (2017) examined sandy sediments collected from the Isle of Man, with different histories of bottom fishing disturbance, described as ‘Low’ and ‘High’ fishing frequency. Results found that the history of fishing activity influenced the community composition in the collected sediments. In sediments from sites which had previously experienced a low frequency of fishing, there was a higher abundance of deposit feeders (including Abra prismtica) than suspension feeders. However, in sediments from sites which had a high frequency of fishing, there was a higher abundance of disturbance-tolerant suspension feeders such as Abra alba.

Evidence from de Jong et al. (2015a, b) demonstrates that deep sand extraction caused a community shift, and two years after cessation of sand extraction, macrozoobenthic biomass significantly increased fivefold in the deepest areas, and Abra alba was the most abundant species in the burrow pit. Similarly, Pezy et al. (2017; 2018) reported that Abra alba recolonized dredged and disposal sites in the Bay of Seine, after the dumping of muddy fine sands.

Deep burrowing species are generally more resistant to bottom trawling, particularly at high trawling frequencies (Beauchard et al., 2023). Evidence found that trawling had most adverse effects on infaunal organisms (including Abra spp. and other bivalve species) positioned 0 to 5 cm in the seabed, but species positioned deeper in the sediment and species living on the seabed surface were less affected by trawling due to mobile swimmers or crawlers which may repopulate trawled grounds easily after disturbance (van Denderen et al., 2015). Tube-dwelling polychaetes were found associated with low trawling frequencies (Beauchard et al., 2023).

Thyasira spp., are small bivalves with thin fragile shells likely to be damaged and result in 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.

Levinsenia gracilis has shown resistance to iceberg grounding in the Antarctic (Paiva et al. (2015). Although the level of disturbance to the seabed and the abundance of Levinsenia gracilis was not quantified in this study, this species was one of the two dominant species in areas that were impacted by iceberg grounding, compared to non-impacted areas with much higher biodiversity.

The effects of trawling on infauna are greater in areas with low levels of natural disturbance compared to areas of high natural disturbance (e.g. Hiddink et al., 2006), and its cumulative impacts can lead to profound changes in benthic community composition, with far reaching implication for marine food webs (Hinz et al., 2009).

Furthermore, abrasion events are likely to cause turbulent re-suspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). Trawling can create suspended sediment plumes up to 10 m above the bottom (Churchill, 1989 cited in Clarke & Wilber, 2000). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures).

Trawling exposes older sediments that are depleted in organic matter. Paradis et al. (2019) found that trawling reduced concentrations of organic carbon, nitrogen and biopolymeric compounds by up to 60%. These conditions are less favourable for deposit feeders such as the species that characterize this biotope, which could therefore slow down the recovery of the biotope.

De Borger et al. (2021) demonstrated that the biogeochemistry of sediments is highly sensitive to bottom trawling disturbance. Their modelling showed that trawling intensities as low as two trawls per year can remove up to 96% of organic carbon and 99% of ammonium from the upper 10 cm of sediment. Furthermore, bioturbation activity decreased by up to 90% at trawling intensities as low as two trawls per year. Based on an estimated recovery rate of 0.04/yearr, it would take approximately 25 years without further disturbance for bioturbation to return to pre-trawling levels. These results indicate a substantial loss of bioturbating macrofauna and suggest that similar impacts are likely in biotopes dominated by Amphiura filiformis.

Tiano et al. (2019) found that sedimentary chlorophyll-α decreased following trawling, with a greater reduction after tickler chain use (83%) compared to PulseWing trawling (43%). PulseWing trawls use electrodes to instead of chains, and are lighter than conventional beam trawls , but apply electrical pulses to the sediment surface. The disturbance of surface sediments also led to notable declines in sediment oxygen consumption (41% for tickler chain and 33% for PulseWing trawled samples) and allowed oxygen to penetrate deeper into the sediment (3.78 mm for tickler chain, 3.17 mm for PulseWing) than in untrawled areas (2.27 mm). These results suggest that bottom trawling can cause immediate decreases in benthic community metabolism, with tickler chain trawling producing stronger effects on benthic biogeochemical processes than PulseWing trawling.

In a meta-analysis of the impacts of different fishing activities on the benthic biota of different habitats, muddy sands were found to be vulnerable to the impacts of fishing activities, with recovery times predicted to take years (Kaiser et al., 2006). The long recovery time for muddy sands is due to the fact that these habitats are mediated by a combination of physical, chemical and biological processes (compared to sand habitats which are dominated by physical processes and recovery time takes days-months).

Sensitivity assessment: Although burrowing life habits may provide some protection from damage by abrasion at the surface, a proportion of the population is likely to be damaged or removed. Significant impacts in population density would be expected if such physical disturbance were repeated at regular intervals. Furthermore, the nature of the soft sediment where the biotopes occur means that objects causing abrasion, such as fishing gears (including pots and creels) are likely to penetrate the surface and cause further damage to the characterizing species. Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’, so sensitivity is assessed as ‘Medium’.

The biotopes are found in weak and very weak tidal streams (Connor et al., 2004; JNCC, 2022). Clogging of feeding apparatus by suspended sediment is likely to be a major consideration for the characterizing species of the biotopes, which include a number of suspension feeders, such as brittlestar Amphiura filiformis, and bivalves Kurtiella bidentata, Abra spp., Ennucula nitida and Thyasira spp.. For example, according to Widdows et al. (1979) growth of filter-feeding bivalves may be impaired at suspended particulate matter (SPM) concentrations >250 mg/l. For instance, the abundance of Abra alba declined over two years within 1 km of an outfall pipe discharging fine-grained mineral waste from the china/clay industry at a rate of 450,000 tons per year to Mevagissey Bay, Cornwall. However, it was argued that persistent sediment instability was the more significant source of stress to the predominantly deposit-feeding community than the suspended sediment concentration (Probert, 1981).

Amphiura filiformis, Kurtiella bidentata and Abra spp. are able to switch between feeding methods (Hill & Wilson, 2008; Carter, 2008; Budd, 2007) and are likely to change to deposit feeding in stagnant waters or areas of very low water flow (Ockelmann & Muus, 1978). According to an analysis from Yan et al. (2019), Heteromastus filiformis was positively correlated with proximity to the Yangtze (Changjiang) river, the largest river in China, which significantly increased turbidity at their sampling sites, suggesting that they may benefit from higher levels of turbidity to an extent.

The characterizing polychaetes of the biotopes are thought to be predators or deposit feeders. For most benthic deposit feeders, food is suggested to be a limiting factor for body and gonad growth, at least between events of sedimentation of fresh organic matter (Hargrave, 1980; Tenore, 1988). Consequently, increased organic matter in suspension that is deposited may become incorporated into sediments via bioturbation and may enhance food supply. A decrease in the suspended sediment and hence siltation may reduce the flux of particulate material to the seabed. Since this includes organic matter the supply of food to the biotopes would probably also be reduced. While regenerating arms, the amount of food the brittlestars can feed on is decreased, meaning there is less energy to allocate to arm regeneration. If there is a change in the amount and quality of food available because of change in suspended solids in the biotopes, then this can have aggravated effects of the growth and development of brittlestars (Lawrence, 2010).

Where a change in suspended solids results in increases turbidity and change of light, the community is unlikely to be directly affected. The community is also unlikely to be directly affected by increased light penetration of the water column caused by a decrease in turbidity. Greater light penetration of the water column may improve primary production by phytoplankton in the water column and contribute to secondary productivity via the production of detritus from which the community may benefit.

Sensitivity assessment: An increase in the suspended matter settling out from the water column to the substratum may increase food availability. On the other hand, decreased siltation is unlikely to affect the mainly deposit feeding community that occur in the biotopes. Resistance of the biotopes is likely to be ‘High’, but with low confidence as no direct evidence was found. Resistance is likely to be ‘High’ (by default) and the biotopes are, therefore, assessed as ‘Not Sensitive’ to a change in suspended solids at the pressure benchmark level.

The biotopes are characterized by burrowing species that are likely to be able to burrow upwards and therefore unlikely to be adversely affected by smothering of 30 cm sediment.

Last et al. (2011) buried individuals of the brittlestar Ophiura ophiura under three different depths of sediment; shallow (2 cm), medium (5 cm) and deep (7 cm). The results indicated that Ophiura ophiura is highly tolerant of short-term (32 days) burial events, with less than 10% mortality of all buried specimens. This is largely a reflection of the ability of the species to re-emerge from all depths across all sediment fractions tested. Mortality after burial depended on the depth and duration of burials as well as the sediment type. Mortality was highest after 32 days of burial (18.5%), in the fine sediment (16.7%) and at 7 cm (22.2%) but was also highest in the largest individuals (6 to 9.7 cm across). However, overall mortality was low (~9.9%) due to its ability to emerge from burial. Emergence exceeded 60% in all durations and depths of burial, but was highest in medium sediment (94.4%) and low depths (94.4%) and lowest under coarse sediment (40.7%) decreasing with increasing depth (Last et al., 2011; Henrick et al., 2016). Survival of specimens that remained buried was low, with 100% mortality of individuals that remained buried after 32 days. The experiments utilized three different fractions of kiln dried, commercially obtained marine sediment: coarse (1.2 to 2.0 mm diameter), medium fine (0.25-0.95mm diameter) and fine (0.1-0.25mm diameter). Last et al. (2011) and Henrick et al. (2016) concluded that Ophiura ophiura was highly tolerant (resistant) of burial under 2 to 7 cm of fine to coarse sediment for up to 32 days.

The addition of mine tailings (waste materials from the extraction of valuable minerals) and dead sediment to the seabed can have significant consequences for benthic communities. Trannum et al. (2018) demonstrated that different materials can have distinct effects on benthic fauna, potentially altering community structure and ecosystem functioning. In their study, mesocosm communities collected from sediment cores were exposed to nominal burial layers of mine tailings ranging from 0.3 to 6 cm over a four-week period, simulating varying degrees of sedimentation. Carbonate- and silicate-rich fine-grained tailings (<63 µm) caused the strongest reductions in community-level remineralization of organic matter, increased nematode mortality at the thickest layers, and altered functional group composition. Coarser tailings (>63 µm) had less severe effects, with moderate changes in sediment mixing and vertical redistribution of infauna. Dead sediment (natural seabed sediment) had minimal impact, indicating that the observed effects are driven by tailing properties rather than mere sediment addition.

Trannum et al. (2020) placed experimental boxes with sediments capped with mine tailings on the seabed to investigate colonisation. They compared natural sediment (control), sediment capped with tailings and no chemicals, sediment capped with tailings and flocculation chemicals, and sediment capped with tailings with flotation chemicals. All sediments were successfully colonized within six months of deployment. The lowest colonisation occurred in the sediment capped with tailings and flotation chemicals. Heteromastus filiformis was among the species which colonized the sediment boxes.

Mevenkamp et al. (2017) found that even thin layers of mine tailings (≥0.1 cm) reduced the ability of benthic communities to process algal material, while deeper layers (up to 3 cm) led to partial vertical redistribution of some species, including higher densities of Levinsenia gracilis in deeper sediment layers, suggesting some tolerance to smothering. These experiments also tested different tailing types, showing that subtle differences in material properties can influence benthic responses.

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 all faunal groups studied—among them annelids (including Heteromastus filiformis), molluscs (including Ennucula tenuis [studied as Nuculoma tenuis] and Thyasira spp.), and echinoderms (including Amphiura filiformis)—declined significantly with increasing thickness of the water-based drill cuttings layer. At 2.4 cm of burial, abundances were ~0 and ~3 individuals /0.09 m² for annelida and mollusca respectively, while echinodermata abundance was 0 individuals /0.09 m² at ~1.9 cm of burial.

In mesocosm experiments, drill cuttings made up of crushed rock and drilling mud were shown to interfere with normal sediment-mixing processes carried out by the benthic organisms Amphiura filiformis and Abra segmentum (Trannum, 2017). A layer of 0.25 cm drill cuttings reduced their reworking activity, slowed their burrowing behavior, and limited how effectively they transported sediment particles downward. These changes could, in turn, affect how oxygen moves into the sediment and alter how organisms are distributed vertically within the seabed.

Characteristic suspension feeders may not persist in areas of excessive sedimentation. Material in suspension can affect the efficiency of filter and suspension feeding (Sherk & Cronin, 1970; Morton, 1976). Effects can include abrasion and clogging of gills, impaired respiration, clogging of filter mechanisms, and reduced feeding and pumping rates.

Hinchey et al. (2006) investigated the responses of estuarine benthic invertebrates to sediment burial and concluded that species-specific response to burial varied as a function of motility living position, and physiological tolerance of anoxic conditions while buried. Although the characterizing species of these biotopes were not included in the study, increased overburden stress did not significantly decrease survival and growth of the juvenile bivalve studied, Macoma balthica, but significantly caused a decline in juvenile Streblospio benedicti. The depth of sediment deposited varied between 0 to 24.6 cm and 0 to 8.4 cm, respectively. 

Furthermore, a study of the ecological effects of dumping dredged sediments by Essink (1999) reported that resistance of mobile macrobenthos varied greatly with species. For polychaetes, the author reported tolerances of up to 50 cm of mud for species such as Nepthys and Nereis, and up to 80 cm of sand. 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. Powilleit et al. (2009) studied responses to smothering for three bivalves; Arctica islandica, Macoma balthica and Mya arenaria. These successfully burrowed to the surface of a 32 to 41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water. These high escape potentials could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’. In comparison to a thick coverage, thin covering layers (i.e. 15 to 16 cm and 20 cm) increased the chance of the organisms to reach the sediment surface after burial. This suggests that characterizing species such as Kurtiella bidentata, Abra spp., Thyasira spp. and Ennucula tenuis are likely to be able to reburrow through similar overburdens, although sudden smothering with 5 cm of sediment would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Furthermore, Thyasira flexuosa have highly extensible feet (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

In the eastern Bay of Seine, Northern France, the Abra alba – Lagis koreni muddy fine sand community remained largely persistent in its species composition and densities of dominant species over a monitoring period (1988 to 2016), despite an increase in siltation from around 2006 (Bacouillard et al., 2020). It was suggested that the siltation increased due to changes in morpho-sedimentary dynamics and large inputs of dredged sediments from the extension of the Le Havre harbour (Bacouillard et al., 2020). Owenia fusiformis, Nephtys hombergii and Abra alba were consistently amongst the most abundant taxa recorded over the study period. Bacouillard et al. (2020) concluded that despite its exposure to multiple stressors, the Abra alba - Lagis koreni muddy fine sand community is dominated by highly resilient species capable of quickly rebuilding their populations (Bacouillard et al., 2020). Similar evidence was found by Dauvin et al. (2017) who reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong sediment changes, hydrodynamic conditions, river discharge, and anthropogenic impacts from dredging and port construction. This suggests that it could tolerate changes in sediment.

Being adapted for burrowing means these species are likely to resist additional fine sediment. However, it should be remembered that smothering by impermeable or viscous materials are likely to have some effect upon the animals, e.g. by causing deoxygenation.

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). For smothering by natural sediments, resistance is assessed as ‘High’, and resilience is ‘High’ (by default) so sensitivity of the biotopes is considered ‘Low’ to a ‘heavy’ deposition of up to 30 cm of fine material added to the seabed in a single, discrete event. For mine tailings and drill cuttings, abundances can decline at burial levels of 2.4 cm, far below the pressure benchmark. Therefore, it is highly likely that resistance to smothering by these materials is ‘Low’ for all biotopes under assessment. Resilience is assessed as ‘Medium’, and sensitivity as ‘Medium’.

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. No studies examining the effect of EMFs on macroalgae were found. 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. No studies investigating the effect of EMFs at the population or community level for benthic organisms were found.

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'.

Species in the biotopes may respond to vibrations from predators or excavation by retracting their palps or by burrowing deeper into the sediment. Solan et al. (2016) found no significant changes in tissue biochemistry and bioturbation behaviour in Amphiura filiformis that were exposed to noise ranging from ambient white noise to looped recordings of wind farm pile-driving at 60 m distance, which is ~150 dB re 1 μPa²/s).

 There is currently Insufficient evidence to assess the sensitivity of these biotopes to underwater noise changes.

Trawling for commercial species can disturb the seabed surface and penetrate below the surface, resulting in the removal or damage of non-targeted infaunal species such as the characterizing species within the direct area of impact. Even shallow abrasion can damage feeding appendages which extend into the water column, temporarily reducing feeding efficiency and growth. More intense or repeated disturbance that penetrates the upper sediment layers may cause displacement or direct injury/mortality. Consequently, sensitivity to surface abrasion varies among taxa, with species such as Amphiura spp., burrowing bivalves, and polychaetes exhibiting differing levels of resistance and resilience depending on their morphology and sediment depth. The degree of impact therefore depends on the depth of abrasion (i.e. the gear used) and the vertical position of fauna within the sediment.

By extending their fragile arms from the sediment to feed, characterizing species Amphiura filiformis become vulnerable to damage by abrasion. Brittlestars can resist considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration (Sköld, 1998). Ramsay et al. (1998) suggested that Amphiura spp. may be less susceptible to beam trawl damage than other species like echinoids or tube dwelling amphipods and polychaetes. For example, Bergman & Hup (1992) found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Holtmann et al. (1996) reported a decrease in the abundance of the fragile burrowing heart urchins and Amphiura filiformis in areas of the southern North Sea between 1990 and 1995. These trends suggest that fishing activity may have been the main cause of these changes. However, Bradshaw et al. (2002) noted that the brittlestars Amphiura filiformis had increased in abundance in a long-term study of the effects of scallop dredging in the Irish Sea.

Rumohr & Kujawski (2000) compared qualitative historical benthos data (1902–1912) with data from 1986 to find long-term trends in epifauna species composition in the southern North Sea that may be attributed to fishery-induced changes. In general, the frequency of occurrence of bivalve species declined, whereas scavenger and predator species (crustaceans, gastropods, and sea stars) were observed more frequently in 1986. The authors suggested that these shifts could be attributed not only to the physical fishery impact but also to the additional potential food for scavenging and predator species provided by the large amounts of discards and moribund benthos. The brittlestar Amphiura filiformis occurred in 1986 on only 5% of the stations while it was present in most of the historical stations. Also, virtually all bivalve species originally present had decreased drastically, including Ennucula tenuis (also less than 5% of the sites by 1986). Despite the problems with the historical data set, the comparison presented was considered the best illustration achievable of the changes in the benthos from a near-pristine situation to the present conditions after long-term disturbance.

Amphiura filiformis was the dominant species in habitats sampled by Pommer et al. (2016) in the Kattegat, near Denmark. This species was significantly more abundant (~349 individuals/m²) in low-trawled sites (4.8 VMS points/yr) compared to ~187 individuals/m² in high-trawled sites (63.8 VMS points/yr). VMS points are hourly signals sent from fishing vessels to Danish and Swedish fishery management authorities whilst travelling at trawling speed.

Declines in species abundance from trawling events can continue to decline even after trawling has stopped. In an experiment conducted in the Frisian Front, North Sea, sediment cores were sampled before trawling and again 5.5, 29, and 75 hours after trawling. Amphiura filiformis showed a significant decline in abundance and was absent from the last sampling period (Tiano et al., 2020). They also found a 94% decrease in all epibenthos within trawled sediments and a 74% decrease in untrawled sediments of the same transect, suggesting that trawling can also significantly affect fauna in the surrounding sediments.

In habitats around the Isle of Man in 2003 to 2004, Amphiura filiformis was a dominant species accounting for 64.13% of the biomass (Sciberras et al., 2016). Trawling frequency between these surveys and repeated surveys in 2014 ranged from 2.95 to 8.51 sweeps per year. In the latter surveys, Amphiura filiformis was no longer present, and communities were instead dominated by burrowing shrimps.

Sköld et al. (2018) showed that Amphiura filiformis showed no significant response to bottom trawling intensity, while Amphiura chiajei abundance was shown to increase to an extent from trawling intensities of up to 5 trawls per year, beyond which its abundance declined. In contrast, Abra nitida decreased in abundance with increased trawling intensity.

In the five years following the establishment of a nearby MPA where trawling was prohibited, the abundance of Amphiura filiformis declined from ~50 individuals/m² to ~30 individuals/m², while Amphiura chiajei abundance declined from >20 individuals/m² to ~10 individuals/m² (Sköld et al., 2018). While this overall decline was statistically insignificant, the authors suggest it is still a meaningful decline due to the stability of the abundances of these two species in the trawled sights. It was presumed that this was due to reduced pressure on predatory fish and crustaceans which were target species of the fishery. This was later proven to be true by Sköld et al. (2025) who reported significant declines in Amphiura abundance and biomass in the 12 years following the cessation of trawling in the Kattegat, with reductions in abundances estimated at 48% for Amphiura filiformis and 45% for Amphiura chiajei. Stomach content analyses confirmed that brittle stars were a staple prey item for benthivore flatfish. Although no raw abundance values were reported, analyses of the data showed that Kurtiella bidentata, and Abra nitida also showed significant declines in abundance in the no-take zone following the cessation of trawling. This suggests that recovery from bottom-trawling is not only influenced by the removal of the pressure itself, but also by cascading ecological effects.

Direct mortality (percentage of initial density) from a single pass of a beam trawl was estimated from experimental studies on sandy and silty grounds as 9% for Amphiura spp., 20 to 65% for bivalves (including Kurtiella bidentata, studied as Mysella bidentata), and 5 to 40% for gastropods, starfish, small-medium sized crustaceans and annelid worms (Bergman & Van Santbrink, 2000). Some mortality was not caused directly by the passage of the trawl, but instead by disturbance, exposure and subsequent predation. Ball et al. (2000b) reported on the short-term effects of fishing on benthos from a mud patch in the northwestern part of the Irish Sea investigated in 1994 to 1996 by means of samples taken both before and shortly after (~24 hr) fishing activity. Kurtiella bidentata (previously studied as Mysella bidentata) was common at the inshore site, where estimates of mortality were calculated, but it was uncommon or entirely absent on the offshore fishing ground. Direct mortality from passage through an otter trawl was estimated at 70%.

Abra spp. are shallow burrowers with a fragile shell (Tebble, 1976), and have been considered amongst the bivalve species most vulnerable to trawling by Bergmann & Van Santbrink (2000) who reported between <0.5% and 18% mortality of Abra alba due to trawling in the southern North Sea. However, the small size of Abra spp. relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993). Tiano et al. (2022) observed a significant decline in Abra alba from tickler trawling in the Netherlands. Three experimental plots (50 x 300 m) were trawled six times each, after which Abra alba density decreased from around 418 individuals/m² to 231 individuals/m². Due to the taxonomic similarities between Abra alba and Abra nitida, it is likely that the effects on the latter would be similar.

Thyasira spp., are small bivalves with thin fragile shells likely to be damaged and result in 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. 

Heteromastus filiformis was reported to occupy the top 15 cm of muddy sands and its limited mobility was considered to contribute to its vulnerability to dredging and to deposition of sediment mobilised by the dredging process by Shaffer (1983).

Heteromastus filiformis is an r-strategist, i.e. has a high reproductive potential which allows it to quickly recolonise disturbed habitats (Bettoso et al., 2020). It occupies the top 15 cm of muddy sands and its limited mobility was considered to contribute to its vulnerability to dredging and to deposition of sediment mobilised by the dredging process by Shaffer (1983). However, Bettoso et al. (2020) found no significant changes in microbenthic communities in the six months after 556,200 m³ of sediment was dredged in the Marano and Grado Lagoon in the northern Adriatic Sea. Heteromastus filiformis was a dominant species before and after the dredging period, and its abundance either remained stable or increased. In contrast, Loia et al. (2020) found that this species was the dominant polychaete in muddy habitats prior to a dredging event, but was almost completely absent 9 months later despite the recolonisation of other species.

The effects of trawling on infauna are greater in areas with low levels of natural disturbance compared to areas of high natural disturbance (e.g. Hiddink et al., 2006), and its cumulative impacts can lead to profound changes in benthic community composition, with far reaching implication for marine food webs (Hinz et al., 2009).

Furthermore, abrasion events are likely to cause turbulent re-suspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). Trawling can create suspended sediment plumes up to 10 m above the bottom (Churchill, 1989 cited in Clarke & Wilber, 2000). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures).

Trawling exposes older sediments which are depleted in organic matter. Paradis et al. (2019) found that trawling reduced concentrations of organic carbon, nitrogen and biopolymeric compounds by up to 60%. These conditions are less favourable for deposit feeders such as the species that characterize this biotope, which could therefore slow down the recovery of the biotope.

De Borger et al. (2021) demonstrated that the biogeochemistry of sediments is highly sensitive to bottom trawling disturbance. Their modelling showed that trawling intensities as low as 2 trawls per year can remove up to 96% of organic carbon and 99% of ammonium from the upper 10 cm of sediment. Furthermore, bioturbation activity decreased by up to 90% at trawling intensities as low as two trawls per year. Based on an estimated recovery rate of 0.04/yr, it would take approximately 25 years without further disturbance for bioturbation to return to pre-trawling levels. These results indicate a substantial loss of bioturbating macrofauna and suggest that similar impacts are likely in biotopes dominated by Amphiura filiformis.

Tiano et al. (2019) found that sedimentary chlorophyll-α decreased following trawling, with a greater reduction after tickler chain use (83%) compared to PulseWing trawling (43%). The disturbance of surface sediments also led to notable declines in sediment oxygen consumption (41% for tickler chain and 33% for PulseWing trawled samples) and allowed oxygen to penetrate deeper into the sediment (3.78 mm for tickler chain, 3.17 mm for PulseWing) than in untrawled areas (2.27 mm). These results suggest that bottom trawling can cause immediate decreases in benthic community metabolism, with tickler chain trawling producing stronger effects on benthic biogeochemical processes than PulseWing trawling.

In a meta-analysis of the impacts of different fishing activities on the benthic biota of different habitats, muddy sands were found to be vulnerable to the impacts of fishing activities, with recovery times predicted to take years (Kaiser et al., 2006). The long recovery time for muddy sands is due to the fact that these habitats are mediated by a combination of physical, chemical and biological processes (compared to sand habitats which are dominated by physical processes and recovery time takes days-months).

Sensitivity assessment: Although burrowing life habits may provide some protection from damage by abrasion at the surface, a proportion of the population is likely to be damaged or removed. Significant impacts in population density would be expected if such physical disturbance were repeated at regular intervals. Furthermore, the nature of the soft sediment where the biotopes occur means that objects causing abrasion, such as fishing gears (including pots and creels) are likely to penetrate the surface and cause further damage to the characterizing species. Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’, so sensitivity is assessed as ‘Medium’.

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).

Burrowing echinoderms are one of the functional groups that have been heavily reduced after red king crab invasions due to predation (Oug et al., 2018). Although Amphiura filiformis was not named, it is possible that it could be affected by red king crab invasion. Nuculid bivalves (which includes Ennucula tenuis although this species was not listed), Thyasira spp., Levinsenia gracilis, Heteromastus filiformis, and Paramphinome jeffreysii have been shown to increase in abundance (the latter of which increased most strongly) after red king crab invasion. 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 (Oug et al., 2018).

While some characteristic species of this group of biotopes 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, e.g. a shift from SS.SMu.CSaMu.AfilEten to SS.SMu.OMu.LevHet. However, there is currently Insufficient evidence for a sensitivity assessment for these biotopes to red king crab invasion.