The Marine Life Information Network

Temperature increase (local)

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

Evidence

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the Mediterranean Sea and Indian Ocean (Farke, 1979). Therefore, the species should be capable of tolerating higher temperatures than it experiences in Northern Europe. For example, Covazzi Harriague et al. (2007) reported Aphelochate marioni occurring in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at 24°C. Also, Aphelochate marioni occurring in Richards Bay Harbour, South Africa, is found in temperature ranges from 17.2 to 27.1°C at a depth range of 0.2 to 16.1 meters (Izegaegbe, Vivier & Mzimela, 2020). In addition, Aphelochate marioni numbers in the harbour were high during spring (11,653), winter (13,008) and summer (6,356), but low during autumn (5,932). Analysis of environmental data suggested that a number of variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment (Izegaegbe, Vivier & Mzimela, 2020). Furthermore, Aphelochaeta marioni lives infaunally, and so is likely to be insulated from rapid temperature change. An increase in temperature would be expected to cause some physiological stress but no mortality.

Murina (1997) categorized Polydora ciliata as a eurythermal species because of its ability to spawn in temperatures ranging from 10.6 to 19.9°C. This is consistent with a wide distribution in north-west Europe, which extends into the warmer waters of Portugal and Italy (Pardal et al., 1993; Sordino et al., 1989; Bertasi, 2016). In the western Baltic Sea, Gulliksen (1977) recorded high abundances of Polydora ciliata in temperatures of 7.5 to 11.5°C and in Whitstable in Kent, where sea temperatures varied between 0.5 and 17°C (Dorsett, 1961). Growth rates may increase if the temperature rises. For example, at Whitstable in Kent, Dorsett (1961) found that a rapid increase in growth of Polydora ciliata coincided with the rising temperature of the seawater during March. Bertasi (2016) observed Polydora cornuta in north Adriatic lagoons and stated that populations of the species would likely increase quickly owing to high larval output and reproduction even in winter. However, Bertasi (2016) also stated that other studies showed that lower temperatures slowed development time from hatching to metamorphosis in Polydora cornuta by 1.4 to 3.1 times slower at 12°C than at 18°C, and that development time for egg capsules, from deposition of eggs to release of three-chaetiger larvae, is around four to five days at 20°C.

Experimental studies on Polydora hoplura populations from oysters in South Africa found that increased water temperatures decreased brooding time but also resulted in a combination of both larger-sized larvae at hatching and lower survivorship rates when compared to the control treatment (David, 2021). Polydora hoplura produces a combination of planktotrophic and adelphophagic (the consumption of nurse eggs or sibling larvae) larvae, and increases in temperature (3 to 5°C) have been shown to result in the faster development of adelphophagic larvae and increase competition for nurse eggs; in extreme cases, sibling cannibalism of smaller planktotrophs was found to be more frequent at higher temperatures (David, 2021). One long-term consequence of shorter Planktonic Larval Duration (PLD) times would be the higher probability of local recruitment to the natal site and the subsequent contraction of the species’ distributional range (David, 2021).

As ocean temperatures rise, faster developmental rates and shorter PLD times of polydorid larvae, coupled with increases in asexually reproducing propagules (see above), mean that shellfish infestations of shell-boring polychaetes will potentially increase, as seen before on farmed abalone (Haliotis midae) in South Africa (David, 2021). However, high temperatures may remove parasitic species like Polydora entirely from ecosystems. In warming models predicting the future interactions of the barnacle parasite Loxothylacus panopaei (not a polydorid species) on the marine crab Eurypanopeus depressus, a warming of only 2°C caused the extinction of the parasite in the southern United States (David, 2021). These examples re-emphasize the plastic nature of invertebrate symbioses; but, more importantly, they show that subtle changes in life-history traits and behaviour could have wider adaptive consequences for whole species and communities (David, 2021). 

Most organisms in the biotope are distributed to the north and south of Britain and Ireland and are unlikely to be affected adversely by long-term temperature change. In addition, subtidal and especially infaunal species are likely to be protected from acute temperature change. Nevertheless, an increase in temperature may indirectly affect some species as microbial activity within the sediments will be stimulated, increasing oxygen consumption and promoting hypoxia (see de-oxygenation pressure).

Sensitivity assessment

Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). No information was found on the maximum temperature tolerated by the important characteristic species Aphelochaeta marioni and Polydora ciliata. However, it is likely that the species can resist a long-term increase in temperature of 2°C and may resist a short-term increase of 5°C. Therefore, resistance and resilience are assessed as ‘High’, and the biotope is assessed as ‘Not Sensitive’.

Temperature decrease (local)

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

Evidence

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the western Baltic Sea, South Atlantic Ocean, and North Sea (Farke, 1979). Therefore, the species should be capable of tolerating low temperatures in the UK. Aphelochaeta marioni lives buried in sediment and is therefore well insulated from decreases in temperature. In the Wadden Sea, the population was apparently unaffected by a short period of severe frost in 1973 (Farke, 1979). Kędra et al. (2010) reported Aphelochaete marioni occurring in the Svalbard Archipelago where temperatures below zero may be experienced in the winter. A decrease in temperature would be likely to cause some physiological stress but no mortality. Another similar bristleworm species, Aphelochaeta palmeri, is found in the Antarctic Ocean and is adapted to freezing conditions. The temperatures Aphelochaeta palmeri were found in ranged from −0.29° to −1.09°C at 25 meters deep (in Rod Bay and Adelie Cove, respectively), from −1.23° to −1.85°C at 70 meters deep (in Adelie Cove and Central Bay, respectively), and from −1.89° to −1.95°C at 140 meters deep (in Adelie Cove and Central Bay, respectively) (Buschi et al., 2024).

In Richards Bay Harbour, South Africa, Aphelochate marioni is found in temperature ranges from 17.2 to 27.1°C at a depth range of 0.2 to 16.1 meters (Izegaegbe, Vivier & Mzimela, 2020). In addition, Aphelochate marioni numbers in the harbour were high during spring (11,653), winter (13,008), and summer (6,356), but low during autumn (5,932). Analysis of environmental data suggested that a number of variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment (Izegaegbe, Vivier & Mzimela, 2020).

Murina (1997) categorized Polydora ciliata as a eurythermal species because of its ability to spawn in temperatures ranging from 10.6 to 19.9°C. This is consistent with a wide distribution in north-west Europe. In the western Baltic Sea, Gulliksen (1977) recorded high abundances of Polydora ciliata in temperatures of 7.5 to 11.5°C, and in Whitstable in Kent, abundance was high when winter water temperatures dropped to 0.5°C (Dorsett, 1961). During the extremely cold winter of 1962 to 1963, Polydora ciliata was apparently unaffected when temperature anomalies of between 2.5 and 5.8°C were observed (Crisp, 1964). Bertasi (2016) observed Polydora cornuta in north Adriatic lagoons and stated that populations of the species would likely increase quickly owing to high larval output and reproduction even in winter. However, Bertasi (2016) also stated that other studies showed that lower temperatures slowed development time from hatching to metamorphosis in Polydora cornuta by 1.4 to 3.1 times slower at 12°C than at 18°C. The development time for egg capsules, from deposition of eggs to release of three-chaetiger larvae, is around 4 to 5 days at 20°C (Bertasi, 2016).

Sensitivity assessment

Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). Aphelochaeta marioni and Polydora ciliata are likely to be able to resist a long-term decrease in temperature of 2°C and may resist a short-term decrease of 5°C. Temperature may act as a spawning cue, and an acute or chronic decrease may result in some delay in spawning, however, this is not considered to impact the adult population and may be compensated by later spawning events. Resistance and resilience are therefore assessed as 'High', and the biotope assessed as 'Not Sensitive'.

Salinity increase (local)

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

Evidence

Populations of Aphelochaeta marioni inhabit the open coast where seawater is at full salinity. Covazzi-Harriague et al. (2007) reported Aphelochate marioni in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at salinities above 38 psu. In Richards Bay Harbour, South Africa, Aphelochate marioni is found in salinity ranges from 18.9 to 34.8 at a depth range of 0.2 to 16.1 meters (Izegaegbe, Vivier & Mzimela, 2020). Although limited freshwater input reaches the harbour due to its isolation from any major river drainage, some salinity fluctuations were observed due to inflow from surrounding wetlands, particularly via the Bhizolo (from the Mhlathuze Papyrus swamps) and Mzingazi (overflow from Lake Mzingazi) canals. However, the strong tidal influence resulted in salinities remaining at close to marine levels throughout most of the year (Izegaegbe, Vivier & Mzimela, 2020). The analysis of environmental data by Izegaegbe, Vivier & Mzimela (2020) suggested that several variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment.

Farke (1979) studied the effects of changing salinity on Aphelochaeta marioni (studied as Tharyx marioni) in the laboratory. Over several weeks, the salinity in a microsystem was increased from 25 to 40 psu and no adverse reaction was noted. However, when individuals were removed from the sediment and displaced to a new habitat, they only dug into their new substratum if the salinities in the two habitats were similar. If the salinities differed by 3 to 5 psu, the worms carried out random digging movements, failed to penetrate the sediment, and died at the substratum surface after a few hours. This suggests that Aphelochaeta marioni can tolerate salinity changes when living infaunally but may not resist an increase in salinity when removed from its habitat. Furthermore, in the Severn Estuary, Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated, poorly sorted mud with relatively high interstitial salinity (Broom et al., 1991).

Another similar bristleworm species, Aphelochaeta palmeri, is found in the Antarctic Ocean where salinity ranged from 33.74 to 34.52 at 25 meters (in Rod Bay and Adelie Cove, respectively) and remained stable at around 34.69 at both 70- and 140- meters deep in each area, except for Adelie Cove at 70 metres where it decreased to 34.32 (Buschi et al., 2024).

Polydora ciliata is a euryhaline species inhabiting fully marine and estuarine habitats. However, there are no records of the species or the biotope occurring in hypersaline waters (>40 psu).

Sensitivity assessment

In SS.SMx.SMxVS.AphPol, an increase in salinity at the pressure benchmark (one MNCR salinity category above the usual range of the biotope) would represent an increase to full salinity. Based on the evidence presented, both characterizing species occur in environments of full salinity and are likely to resist an increase in salinity at the pressure benchmark level. However, as salinity increases SS.SMx.SMxVS.AphPol may grade into SS.SMu.ISaMu.MelMagThy, with those species characteristic of the latter increasing in abundance (Connor et al., 2004; JNCC, 2022), so the biotope may be gradually lost and eventually reclassified at the pressure benchmark level that assumes a change in salinity for one year. Resistance is therefore assessed as ‘Low’ (loss 25-75%) and resilience is likely to be ‘High’, so the biotope is considered to have ‘Low’ sensitivity to an increase in salinity at the pressure benchmark level.

Salinity decrease (local)

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

Evidence

Aphelochaeta marioni thrives in estuaries and is therefore likely to be tolerant of decreases in salinity. It has been recorded from brackish inland waters in the Southern Netherlands with a salinity of 16 psu, but not in areas permanently exposed to lower salinities (Wolff, 1973). However, it also occurs in areas exposed to salinities as low as 4 psu for short periods at low tide when freshwater discharge from rivers is high (Farke, 1979).

Polydora ciliata is a euryhaline species inhabiting fully marine and estuarine habitats (Bertasi, 2016). In an area of the western Baltic Sea, where bottom salinity was between 11.1 and 15.0 psu, Polydora ciliata was the second most abundant species with over 1000 individuals per m² (Gulliksen, 1977). OBIS (2025) recorded Polydora ciliata in salinities ranging from 0 to 40 PSU, with most records at 5 to 20 and 30 to 35 psu.

Overall, the important characterizing species are likely to tolerate a short-term change in salinity from e.g. variable to low salinity and a long-term change from variable to reduced salinity. The species richness of the biotope may decline, but the biotope will probably not be adversely affected.

Sensitivity assessment

Records indicate SS.SMx.SMxVS.AphPol occurs in areas of reduced (18 to 30 ppt), variable (18 to 35 ppt) and low (<18 ppt) salinity (Connor et al., 2004; JNCC, 2022). The characterizing species Aphelochaeta marioni and Polydora ciliata are therefore likely to resist a decrease in salinity at the pressure benchmark level. Resistance is therefore assessed as ‘High’ and resilience as ‘High’ (by default), and the biotope is considered ‘Not Sensitive’ to a decrease in salinity at the pressure benchmark level.

Water flow (tidal current) changes (local)

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

Evidence

Aphelochaeta marioni has been recorded in the Wadden Sea with flow rates up to 0.45 m/s (Compton et al., 2013), and in the Westerscheld estuary (SW Netherlands), where spring current velocities vary between 0.1 and 0.58 m/s (Van Colen et al., 2010a). Polydora ciliata colonized test panels in Helgoland in three areas, two exposed to strong tidal currents and one site sheltered from currents (Harms & Anger, 1983). Very strong water flows may sweep away Polydora colonies, where these are present as a thick layer of mud on a hard substratum.

The hydrographic regime is an important structuring factor in sedimentary habitats. An increase in water flow rate is not likely to affect Aphelochaeta marioni directly as it lives infaunally. The most damaging effect of increased flow rate would be the erosion of the substratum, as this could eventually lead to loss of the habitat. Orvain et al. (2007) investigated the spatio-temporal variations in intertidal mudflat erodibility in Western France and suggested a potential link between Polychaeta and bed erodibility given the high polychaete abundances observed in the study.

Increased water flow rates are likely to change the sediment characteristics in which the species live, primarily by re-suspending and preventing deposition of finer particles (Hiscock, 1983). The characterizing species prefer habitats with silty/muddy substrata, which would not occur in very strong tidal streams. Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability. Decreased water movement would result in increased deposition of suspended sediment (Hiscock, 1983). An increased rate of siltation resulting from a decrease in water flow may result in an increase in food availability for the characterizing species, and therefore growth and reproduction may be enhanced, but only if food was previously limiting.

Sensitivity assessment

Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on the Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, clays and silts require greater critical erosion velocities because of their cohesiveness. SS.SMx.SMxVS.AphPol is recorded in moderately strong (0.5-1 m/s) and weak (>0.5 m/s) tidal streams (Connor et al., 2004). A change in water flow rate at the pressure benchmark level of 0.1-0.2 m/s is considered to fall within the range of flow speeds experienced by populations in the middle of their range. Resistance and resilience are, therefore, assessed as 'High', and the biotope is considered 'Not Sensitive' to a change in water flow at the pressure benchmark level.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year, or 2) an increase in relative sea level or decrease in high water level for ≥1 year. (Emergence regime change pressure definition).

Evidence

SS.SMx.SMxVS.AphPol occurs in the infralittoral, so only the upper extent of shallow examples of the biotope is likely to be emersed at extreme low tides. Aphelochaeta marioni lives in the intertidal zone in significant numbers (Gibbs, 1969; Farke, 1979), and Polydora ciliata occurs in the mid to low intertidal. Both characterizing species would probably survive an increase in emergence. However, the species can only feed when immersed and is therefore likely to experience reduced feeding opportunities. Over the course of a year, the resultant energetic cost is likely to cause some mortality. In addition, increased emergence is likely to increase the vulnerability to predation from shore birds. A decrease in emergence is likely to allow the biotope to extend its upper limit, where suitable substrata exist.

Sensitivity assessment

Some mortality of the characterizing species is likely to occur because of emergence regime changes. Resistance is therefore assessed as 'Medium' and resilience is likely to be 'High', so the biotope is considered to have 'Low' sensitivity to emergence regime changes at the pressure benchmark level.

Wave exposure changes (local)

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

Evidence

The biotope is found in sheltered, very sheltered and extremely sheltered sites (Connor et al., 2004; JNCC, 2022). Strong wave action is likely to cause damage or withdrawal of delicate feeding and respiration structures, resulting in loss of feeding opportunities and compromised growth of Aphelochaeta marioni and Polydora ciliata, and changes in wave exposure may also influence the supply of particulate matter for tube-building Polydora. Decreases in wave exposure may influence the supply of particulate matter because wave action may have an important role in re-suspending the sediment that is required by the species to build its tubes. Furthermore, Aphelochaeta marioni characteristically inhabits soft sediments in sheltered areas (Broom et al., 1991), so individuals may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action.

Sensitivity assessment

Hydrographic regimes are an important structuring factor in sedimentary habitats, and an increase in wave exposure could result in fine sediments being eroded (Hiscock, 1983), resulting in the likely reduction of the habitat and a decrease in food availability. Some erosion will occur naturally, and storm events may be more significant in loss and damage of the substratum than changes in wave height at the pressure benchmark, but these events will be short-lived. SS.SMx.SMxVS.AphPol occurs in sheltered, very sheltered and extremely sheltered areas (Connor et al., 2004; JNCC, 2022), and a change at the benchmark level, by a single wave exposure category for one year, is likely to fall within the range experienced by the mid-range examples of this biotope.

However, in the worst-case scenario, examples of biotope that occur in ‘sheltered’ conditions would experience moderately wave-exposed conditions at the benchmark level. Moderate wave exposure may winnow away the sediment surface, but most importantly, damage or remove the mud layer created by Polydora. However, a change from sheltered to moderate for one year would be considered the worst-case scenario, but due to the nature of the biotope and its quick recoverability, this is likely not to result in any significant changes. Therefore, the resistance is assessed as ‘Low’. Nevertheless, resilience is assessed as ‘High’ (within two years), and sensitivity as ‘Low’ at the pressure benchmark. The assessment is based on the biotope classification and, hence, expert judgement. While unlikely, a prolonged increase in wave exposure would probably result in loss of the biotope and modification of the underlying sediment.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition).

Evidence

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

Evidence suggests that polychaetes are fairly resistant to the effects of heavy metals (Bryan, 1984). Aphelochaeta marioni is tolerant of heavy metal contamination occurring in the heavily polluted Restronguet Creek (Bryan & Gibbs, 1983), and it is also an accumulator of arsenic (Gibbs et al., 1983). Polydora ciliata occurs in an area of the southern North Sea polluted by heavy metals but was absent from sediments with very high heavy metal levels (Diaz-Castaneda et al., 1989).

Taking into account the variable salinity conditions that affect this biotope (in general, for estuarine animals, heavy metal toxicity increases as salinity decreases and temperature increases: McLusky et al., 1986), it seems possible that some polychaete species, at least in the biotope, might be adversely affected by high contamination by heavy metals.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

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

Cirratulids seem to be mostly immune to oil spills, probably because their feeding tentacles are protected by a heavy secretion of mucus (Suchanek, 1993). This is supported by observations of Aphelochaeta marioni following the Amoco Cadiz oil spill in March, 1978 (Dauvin, 1982, 2000). Prior to the spill, Aphelochaeta marioni (studied as Tharyx marioni) was present in very low numbers in the Bay of Morlaix, western English Channel. Following the spill, the level of hydrocarbons in the sediment increased from 10 mg/kg dry sediment to 1443 mg/kg dry sediment 6 months afterwards. In the same period, Aphelochaeta marioni increased in abundance to a mean of 76 individuals/m², which placed it among the top five dominant species in the faunal assemblage. It was suggested that the population explosion occurred due to the increased food availability because of the accumulation of organic matter resulting from high mortality of browsers. Six years later, the abundance of Aphelochaeta marioni began to fall away again, accompanied by gradual decontamination of the sediments.

In analysis of kelp holdfast fauna following the Sea Empress oil spill in Milford Haven, the fauna present, including Polydora ciliata, showed a strong negative correlation between numbers of species and distance from the spill (SEEEC, 1998). After the extensive oil spill in West Falmouth, Massachusetts, Grassle & Grassle (1974) followed the settlement of polychaetes in the disturbed area. Species with the most opportunistic life histories, including Polydora ligni, were able to settle in the area. This species has some brood protection, which enables larvae to settle almost immediately in the nearby area (Reish, 1979).

Overall, hydrocarbon contamination is likely to adversely affect some members of the community, resulting in more tolerant or opportunistic species increasing in abundance, and a consequent reduction in species richness.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

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

There is little evidence directly relating to the effects of synthetic chemicals on Aphelochaeta marioni. Waldock et al. (1999) reported that the species diversity of polychaete infauna, including Aphelochaeta marioni, in the Crouch estuary increased in the three years after the use of TBT was banned within the estuary, suggesting that TBT had suppressed their abundance previously.

Polydora ciliata was abundant at polluted sites close to acidified, halogenated effluent discharge from a bromide-extraction plant in Amlwch, Anglesey (Hoare & Hiscock, 1974). Spionid polychaetes were found by McLusky (1982) to be relatively resistant to distilling and petrochemical industrial waste in Scotland.

Furthermore, Beaumont et al. (1989) concluded that TBT had a detrimental effect on the larval and/or juvenile stages of infaunal polychaetes.

Radionuclide contamination

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

Evidence

No Evidence is available on which to assess this pressure.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status) (deoxygenation pressure definition).

Evidence

Connor et al. (1997a) described sediments in which Aphelochaeta marioni is commonly found as usually having a "black anoxic layer close to the sediment surface". In Richards Bay Harbour, South Africa, Aphelochate marioni is found in dissolved oxygen ranges from 3.9 to 8.8 mg/l at a depth range of 0.2 to 16.1 meters, including one site with hypoxic conditions, where a very low dissolved oxygen of 0.5 mg/l was recorded (Izegaegbe, Vivier & Mzimela, 2020). Broom et al. (1991) recorded that Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated mud in the Severn Estuary. They found Aphelochaeta marioni to be dominant where the redox potential at 4 cm sediment depth was 56 mV and, therefore, concluded that the species was tolerant of very low oxygen tensions. It is likely that feeding, growth, and reproduction would be impaired under sustained low oxygen conditions.

Polydora ciliata is frequently found at localities with oxygen deficiency (Pearson & Rosenberg, 1978; Vedenin et al., 2025). For example, in polluted waters in Los Angeles and Long Beach harbours, Polydora ciliata was present in the oxygen range of 0.0 to 3.9 mg/l, and the species was abundant in hypoxic fjord habitats (Rosenberg, 1977). Furthermore, in a study investigating a polychaete community in the northwest Black Sea, Polydora ciliata was observed in all four study sites, including those severely affected by eutrophication and hypoxia due to discharges of wastewater (Vorobyova et al., 2008). However, Polydora ciliata is unlikely to be able to resist anoxic conditions. Hansen et al. (2002) reported near-total extinction of all metazoa in the Mariager Fjord (Denmark), including Polydora spp., after a severe hypoxia event that resulted in complete anoxia in the water column for two weeks. In the southwestern Baltic Sea, Polydora ciliata are typically recorded in areas that experience oxygen deficiency, yet do not have such tolerance to anoxic conditions (Vedenin et al., 2025). Vedenin et al. (2025) explained this as due to the ability of Polydora ciliata to develop rapidly, with full metamorphosis from hatching to maturity lasting 9 to 11 days; they can form dense populations within less than a month and bounce back quickly from anoxic events. In addition, Como & Magni (2009) investigated seasonal variations in benthic communities known to be affected by episodic events of hypoxia. The authors observed that the abundance of Polydora ciliata varied seasonally, decreasing during the summer months, and suggested it could be explained by the occurrence of hypoxic/anoxic conditions and sulphidic sediments during the summer. No details of the levels of dissolved oxygen leading to these community responses were provided.

Sensitivity assessment

The characterizing species are likely to only be affected by severe de-oxygenation episodes. Resistance to de-oxygenation at the pressure benchmark level is likely to be ‘High’. Opportunistic Polydora spp. have also repeatedly been reported amongst the first to recover from hypoxia events (Hansen et al., 2002; Van Colen et al., 2010). Resilience of the biotope is likely to also be ‘High’, and the biotope is therefore considered ‘Not Sensitive’ to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

Nutrient enrichment

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

Evidence

Raman & Ganapati (1983) studied the distribution of Aphelochaeta marioni (studied as Tharyx marioni) in relation to a sewage outfall in Visakhapatnam Harbour, Bay of Bengal. Increased nutrients often derive from sewage inputs, and the presence of species such as Aphelochaeta marioni in such situations (for instance, Broom et al., 1991) may reflect tolerance to high nutrients or to de-oxygenated conditions or both. Aphelochaeta marioni was found to be dominant in the 'semi-healthy zone' characterized by low nutrients (nitrate 0.02 mg/l, phosphate 0.88 mg/l). Aphelochaeta marioni was not found in high numbers in the polluted zone close to the sewage outfall, characterized by high nutrients (nitrate 0.042 to 0.105 mg/l, phosphate 2.35 to 3.76 mg/l) (Rayment, 2007a). This would suggest that Aphelochaeta marioni is intolerant of eutrophication. However, it would be expected that an increase in organic nutrients would lead to increased food availability for the deposit-feeding Aphelochaeta marioni. Furthermore, Dauvin (1982, 2000) recorded an increase in abundance of Aphelochaeta marioni following an oil spill, which resulted in an explosion of plant growth due to high mortality of grazers. Therefore, the available evidence on the resistance of Aphelochaeta marioni to nutrient changes does not allow consistent conclusions to be drawn.

Polydora ciliata is often found in environments subject to high levels of nutrients. For example, the species was abundant in areas of the Firth of Forth, Scotland, exposed to high levels of sewage pollution (Smyth, 1968), in nutrient-rich sediments in the Mondego estuary, Portugal (Pardal et al., 1993), and the coastal lagoon Lago Fusaro in Naples (Sordino et al., 1989). The extensive growth of Polydora ciliata in mat formations were recorded at West Ganton, in the Firth of Forth, prior to the introduction of the Sewage Scheme (Read et al., 1983). The abundance of the species was probably associated with their ability to use the increased availability of nutrients as a food source and silt for tube building.

Sensitivity assessment

Nutrient enrichment may reduce the abundance of Aphelochaeta marioni, although there is conflicting evidence, while Polydora is probably resistant. However, for both species, increased availability of nutrients may be used as a source of food (Hiscock et al., 2005a). Therefore, the evidence is ‘insufficient’ to form the basis of an assessment.

Organic enrichment

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

Evidence

Kędra et al. (2010) reported Aphelochaeta mariori that occurred in the Hornsundfjord, Svalbard, where primary production has been recorded as 120 gC/m²/yr. Covazzi-Harriague et al. (2007) reported Aphelochaeta mariori at sites with organic matter sedimentation as high as 359 mg/m²/hr in the Ligurian Sea, Italy. Furthermore, Markert et al. (2010) compared macrofaunal communities in the Wadden Sea in reefs dominated by Mytilus edulis and Cassostrea gigas and found Aphelochaeta mariori as a dominant species throughout the study site, which suggested the species was unlikely to be affected by organic enrichment.

In colonization experiments in an organically polluted fjord receiving effluent discharge from Oslo, Polydora ciliata settled in large numbers within the first month (Green, 1983; Pardal et al., 1993). However, Callier et al. (2007) investigated the spatial distribution of macrobenthos under a suspended mussel culture in eastern Canada, where the sedimentation of organic matter to the bottom was approx. 1-3 gC/m²/d. Polydora ciliata was recorded as absent in the sites under the suspended mussel farm after one year and as dominant in reference areas of the study. It should be noted that the organic matter input from the mussel farm exceeds the pressure benchmark. Como & Magni (2009) investigated seasonal variations in benthic communities known to be affected by episodic events of sediment over-enrichment. The authors observed that the abundance of Polydora ciliata varied seasonally, and suggested this could be a result of major accumulation of organic carbon-binding fine sediments in the study site. Studies by Almeda et al. (2009) and Pedersen et al. (2010) investigated larval energetic requirements for Polydora ciliata, and suggested maximum growth rates were reached at food concentrations ranging from 2.5 to 1.4 μg C/ml depending on larval size, and energetic carbon requirements of 0.09 to 3.15 μg C l/d, respectively. On the other hand, Polydora ciliata can also occur in organically poor areas (Pearson & Rosenberg, 1978).

Borja et al. (2000) and Gittenberger & Van Loon (2011) both assigned Aphelochaeta marioni and Polydora ciliata to their AMBI Ecological Group IV ‘Second-order opportunistic species present in slight to pronounced unbalanced situations’.

Sensitivity assessment

The evidence presented suggests the characterizing species may not be affected by organic enrichment at the benchmark level. Resistance and resilience are therefore assessed as 'High', and the biotope is considered 'Not Sensitive' to organic enrichment (deposit of 100 gC/m²/yr).

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

Evidence

All marine habitats and benthic species are considered to have a resistance of None to this pressure and to be unable to recover from a permanent loss of habitat (Resilience is Very Low). Sensitivity within the direct spatial footprint of this pressure is therefore High. Although no specific evidence is described, confidence in this assessment is ‘High’ due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata, or vice versa (Physical change in subtratum type pressure definition).

Evidence

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

Sensitivity assessment

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

Physical change (to another sediment type)

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

Evidence

Aphelochaeta marioni has been recorded from a variety of different sediment types. In the intertidal area of the Wadden Sea, it achieved the highest abundance where the sediment fraction smaller than 0.04 mm in diameter was greater than 10% of the total sediment (Farke, 1979).

In Richards Bay Harbour, South Africa, a study was conducted to examine the status of the macrobenthic community structure. Sediment and benthic samples were collected quarterly (summer, autumn, winter, and spring) using a Van Veen grab (area size of 0.0236 m²) between 2016 and 2017 from seven sites in the harbour to represent different habitat types, with one site being a reference site (site 7). The majority of habitat types had sediment compositions ranging from very fine sand to mud, and the implication of this difference in sediment substrata was profound in the benthic abundance and distribution (Izegaegbe, Vivier & Mzimela, 2020). Analyses of the benthic fauna identified 98 taxa, of which the tanaid Halmyrapseudes digitalis, the polychaetes Mediomastus capensis, Aphelochaeta marioni and Prionospio sexoculata, and the brachyuran crab Paratylodiplax blephariskios were numerically abundant, accounting for 79% of the organisms recorded (Izegaegbe, Vivier & Mzimela, 2020). Aphelochaeta marioni made up 17.81% of the organisms recorded (Izegaegbe, Vivier & Mzimela, 2020). In detail, site 2 (8,814 ind.), 3 (10,678 ind.), and 7 (7,331 ind.) had the most Aphelochaeta marioni, which was the intertidal mudflat (Izegaegbe, Vivier & Mzimela, 2020). The other sites being the deep (15 m) muddy channel (made from a mix of mud and sand) (site 1 and 6; 3,771 and 2,712 ind. respectively) and the mid channel (muddy) (site 4; 3,136 ind.), and the sandflat (characterized by fine sand and low organic content, largely due to the influence of wave action and currents) (site 5; 508 ind.) (Izegaegbe, Vivier & Mzimela, 2020).

The importance of sediment character (grain size and organic content) in structuring estuarine benthic communities, coupled with salinity, has been widely reported (Teske and Wooldridge, 2003, cited in Izegaegbe, Vivier & Mzimela, 2020). In large permanently open estuaries, sediment type was more important than salinity in structuring benthic assemblages (Teske and Wooldridge, 2004, cited in Izegaegbe, Vivier & Mzimela, 2020). This corresponds to results from Izegaegbe, Vivier & Mzimela (2020), who concluded that several variables, including sediment characteristics, depth, salinity, and temperature, were most important and responsible for the variability in the abiotic environment.

Where a change in sediment type results in a decrease in the mud fraction in the sediment, tube building for the characterizing species Polydora ciliata could be compromised.

Sensitivity assessment

This biotope (SS.SMx.SMxVS.AphPol) is characterized by mixed sediment (sandy, gravelly mud) (Connor et al., 2004; JNCC, 2022). Therefore, a change in one Folk would represent a change to either coarse sediments (sand- and gravel-dominated) or to sandy muds and muds (e.g. SMuVS.PolCvol or SMuVS.AphTubi). While the characteristic species would probably survive, their abundance would change, and the biotope would require re-classification and be lost. Therefore, resistance to the pressure is assessed as ‘None’. Resilience is ‘Very Low’ given the permanent nature of the pressure, so sensitivity has been assessed as ‘High’.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Aphelochaeta marioni lives buried in soft sediments, with most individuals found in the upper 4 cm of the sediment (Rayment, 2007a). The tubes built by Polydora sometimes agglomerate to form layers of mud up to an average of 20 cm thick (Hill, 2007).

Removal of the substratum to 30 cm would result in the loss of the characterizing species. Resistance to the pressure is considered ‘None’, and resilience ‘Medium’. Sensitivity has been assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Aphelochaeta marioni is a soft-bodied organism which exposes its palps and cirri at the surface while feeding (Rayment, 2007a). The species lives infaunally in soft sediment, usually within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would be likely to penetrate the upper few centimetres of the sediment and cause physical damage to Aphelochaeta marioni. De Biasi & Pacciardi (2008) compared macrobenthic communities in a commercial fishing ground exploited by otter trawling with an area closed to fishing for over 10 years in the Adriatic Sea. The authors found that polychaetes, including Aphelochaeta spp., were among the species dominating the disturbed areas, which is likely to result from the ability of the species to recolonize disturbed areas rapidly, rather than indicate that the polychaetes are resistant to disturbance of the seabed surface.

In Poole Harbour, UK, a similar result was observed. Local fishermen utilise a unique ‘pump-scoop’ dredge (which can penetrate shallow soft sediments; approx. 3 to 5 cm) to harvest the introduced Manila clam Ruditapes philippinarum. A Before-After-Control-Impact sampling design was used to assess the impacts of pump-scoop dredging on benthic physical characteristics and community structure in an area where there was no dredging, an area newly opened to dredging and an area subject to high levels of historic dredging (Clarke et al., 2018). A sampling grid was used in each area to best capture any fishing effort in the newly opened area, and core samples were taken to a depth of 30 cm within intertidal mudflats. A significant loss of fine sediments was observed in the site subject to high-intensity dredging, and a significant change in community structure occurred in both dredged sites throughout the study period (Clarke et al., 2018). In the newly opened site, this was characterized by a relative increase in species richness, including increased abundance of annelid worms, notably Hediste diversicolor and Aphelochaeta marioni and a decline in the abundance of the bivalve mollusc Abra tenuis (Clarke et al., 2018). Clarke et al. (2018) concluded that these changes, albeit relatively small, are attributed to physical disturbance as a direct result of pump-scoop dredging, and despite the significant change in community structure at the newly opened site, no difference in the classification of the biotope or ecological quality of either of the dredged sites was observed.

The tubes of characterizing species Polydora spp. are also likely to be removed by abrasion, as these project above the surface and are not physically robust. The thick Polydora mud may crumble away and be swept away by water currents, so it is probably vulnerable to abrasion.

Sensitivity assessment

The characterizing community in this biotope is considered likely to be damaged and removed by abrasion. As soft-bodied species, both Aphelochaeta marioni and Polydora ciliata are likely to be crushed and killed by an abrasive force or physical blow. Resistance to abrasion is considered ‘Low’. However, the community is likely to be able to re-establish rapidly, so resilience of the biotope is assessed as ‘High’, with the biotope considered to have ‘Low’ sensitivity to abrasion or disturbance of the surface of the seabed.

Penetration or disturbance of the substratum subsurface

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

Evidence

Activities that penetrate below the surface would remove a significant proportion of the characterizing species within the direct area of impact (see evidence under ‘abrasion’ above). However, it is expected that the community is likely to be able to re-establish rapidly.

For example, in Poole Harbour, UK, local fishermen utilise a unique ‘pump-scoop’ dredge (which can penetrate shallow soft sediments; approx. 3 to 4 cm) to harvest the introduced Manila clam Ruditapes philippinarum. A Before-After-Control-Impact sampling design was used to assess the impacts of pump-scoop dredging on benthic physical characteristics and community structure in an area where there was no dredging, an area newly opened to dredging and an area subject to high levels of historic dredging (Clarke et al., 2018). A sampling grid was used in each area to best capture any fishing effort in the newly opened area, and core samples were taken to a depth of 30 cm within intertidal mudflats. A significant loss of fine sediments was observed in the site subject to high-intensity dredging, and a significant change in community structure occurred in both dredged sites throughout the study period (Clarke et al., 2018). In the newly opened site, this was characterized by a relative increase in species richness, including increased abundance of annelid worms, notably Hediste diversicolor and Aphelochaeta marioni and a decline in the abundance of the bivalve mollusc Abra tenuis (Clarke et al., 2018). Clarke et al. (2018) concluded that these changes, albeit relatively small, are attributed to physical disturbance as a direct result of pump-scoop dredging, and despite the significant change in community structure at the newly opened site, no difference in the classification of the biotope or ecological quality of either of the dredged sites was observed. 

Sensitivity assessment

The characterizing community in this biotope is considered likely to be damaged and removed by penetration. As soft-bodied species, both Aphelochaeta marioni and Polydora ciliata are likely to be crushed and killed by an abrasive force or physical blow. However, the community is likely to be able to re-establish rapidly. Biotope resistance is therefore assessed as ‘Low’, and recovery is assessed as ‘High’ based on the assumption that the suitable substratum to support the community of the characterizing species would not be lost. Sensitivity is therefore assessed as ‘Low’.

Changes in suspended solids (water clarity)

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

Evidence

This biotope is probably exposed to the high levels of suspended sediment characteristic of estuarine conditions. Therefore, the resident species are probably adapted to high suspended sediment levels. Aphelochaeta marioni lives infaunally and is a surface deposit feeder (Rayment, 2007a), therefore relying on a supply of nutrients at the sediment surface. An increased rate of siltation may result in an increase in food availability and therefore growth and reproduction of Aphelochaeta marioni. However, food availability would only increase if the additional suspended sediment contained a significant proportion of organic matter and the population would only be enhanced if food was previously limiting. A decrease in the suspended sediment would result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for Aphelochaeta marioni. This would be likely to impair growth and reproduction.

In the Firth of Forth, Polydora ciliata formed extensive mats in areas that had an average of 68 mg/l suspended solids and a maximum of approximately 680 mg/l, indicating the species is able to tolerate different levels of suspended solids (Read et al., 1982; Read et al., 1983). Occasionally, in certain places, siltation is sped up when Polydora ciliata is present because the species actually produces a 'mud' as it perforates soft rock and chalk habitats, and larvae settle preferentially on substrates covered with mud (Lagadeuc, 1991). Suspended sediment and siltation of particles is important for tube building in Polydora ciliata so a decrease in suspended solids may reduce tube building or the thickness of the mud surrounding the 'colonies'. Daro & Polk (1973) reported that the success of Polydora is directly related to the quantities of muds of any origin carried along by rivers or coastal currents.

An increase in turbidity, reducing light availability, may reduce primary production by phytoplankton in the water column. A reduction in primary production in the water column and by the microphytobenthos on the sediment surface may result indirectly in reduced food supply to the characterizing species, which in turn may affect growth rates and fecundity.

Sensitivity assessment

An increase in suspended solids at the pressure benchmark level is unlikely to affect the characterizing species of this biotope. However, a decrease in suspended matter in the biotope could result in limitation of material for tube building of Polydora and in the substratum being no longer suitable for colonization by new recruits. Resistance of the biotope is therefore assessed as 'Low' (loss of 25-75%) and resilience is 'High' (following a return to normal conditions), so the biotope is considered to have 'Low' sensitivity to a decrease in suspended solids at the pressure benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

This biotope is probably exposed to the high levels of suspended sediment characteristic of estuarine conditions. Therefore, the resident species are probably adapted to high suspended sediment levels. Aphelochaeta marioni lives infaunally and is a surface deposit feeder (Rayment, 2007a), therefore relying on a supply of nutrients at the sediment surface. An increased rate of siltation may result in an increase in food availability and therefore growth and reproduction of Aphelochaeta marioni. However, food availability would only increase if the additional suspended sediment contained a significant proportion of organic matter and the population would only be enhanced if food was previously limiting. A decrease in the suspended sediment would result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for Aphelochaeta marioni. This would be likely to impair growth and reproduction.

In the Firth of Forth, Polydora ciliata formed extensive mats in areas that had an average of 68 mg/l suspended solids and a maximum of approximately 680 mg/l, indicating the species is able to tolerate different levels of suspended solids (Read et al., 1982; Read et al., 1983). Occasionally, in certain places, siltation is sped up when Polydora ciliata is present because the species actually produces a 'mud' as it perforates soft rock and chalk habitats, and larvae settle preferentially on substrates covered with mud (Lagadeuc, 1991). Suspended sediment and siltation of particles is important for tube building in Polydora ciliata, so a decrease in suspended solids may reduce tube building or the thickness of the mud surrounding the 'colonies'. Daro & Polk (1973) reported that the success of Polydora is directly related to the quantities of muds of any origin carried along by rivers or coastal currents.

An increase in turbidity, reducing light availability, may reduce primary production by phytoplankton in the water column. A reduction in primary production in the water column and by the microphytobenthos on the sediment surface may result indirectly in reduced food supply to the characterizing species, which in turn may affect growth rates and fecundity.

Sensitivity assessment

An increase in suspended solids at the pressure benchmark level is unlikely to affect the characterizing species of this biotope. However, a decrease in suspended matter in the biotope could result in limitation of material for tube building of Polydora and in the substratum being no longer suitable for colonization by new recruits. Resistance of the biotope is therefore assessed as 'Low' (loss of 25-75%) and resilience is 'High' (following a return to normal conditions), so the biotope is considered to have 'Low' sensitivity to a decrease in suspended solids at the pressure benchmark level.

Smothering and siltation rate changes (heavy)

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

Evidence

Aphelochaeta marioni lives infaunally in soft sediments and moves by burrowing. It deposit feeds at the surface, extending contractile palps from its burrow. An additional layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. Kędra et al. (2010) reported Aphelochaeta mariori to occur in the Hornsundfjord, Svalbard, where sedimentation rates can vary between 0.1-35 cm/yr. Furthermore, Do et al. (2012) studied the macrobenthos recovery in the Arcachon Bay (France) following a deposition of sediment up to 10 cm thick that resulted from dredging activities. The authors reported Aphelochaeta marioni as considerably reduced or absent from impacted areas characterized mainly by mud substrata.

Adults of Polydora ciliata produce a 'mud' resulting from the perforation of soft rock substrates (Lagadeuc, 1991). A Polydora mud can be up to 50 cm thick, but the animals themselves occupy only the first few centimetres. They either elongate their tubes or leave them to rebuild close to the surface. Munari & Mistri (2014) investigated the spatio-temporal variation pattern of a benthic community following deposition of dredged material, at a maximum thickness of 30–40 cm. Polydora ciliata was amongst the first colonizers of the newly deposited sediments. The authors suggested that it was possible that the individuals migrated vertically through the deep layer of dredged sand. This was based on the results of Roberts et al. (1998), who suggested 15 cm as the maximum depth of overburden through which benthic infauna can successfully migrate. After one year, no adverse impact of sand disposal on the benthic fauna was detected on the study site.

Sensitivity assessment

Polychaete species have been reported to migrate through depositions of sediment greater than the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as 'Low' and resilience as 'High', and the biotope is considered to have 'Low' sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

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

No studies have examined the effect of EMFs on Polydora sp. or Aphelochaeta sp. However, one study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of a static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity indicative of a stress response were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

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

Evidence

None of the species in the biotope are likely to be sensitive to noise or vibration at the benchmark level, and no information was found concerning the intolerance of Aphelochaeta marioni to noise. Polydora ciliata may respond to vibrations from predators or bait diggers by retracting their palps into their tubes. However, the characterizing species are unlikely to be affected by noise pollution, and so the biotope is assessed as Not Sensitive.

Introduction of light or shading

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

Evidence

Changes in light availability impact primary production by phytoplankton in the water column and by the microphytobenthos on the sediment surface, which in turn may affect food availability for the characterizing species. However, SS.SMx.SMxVS.AphPol is not directly dependent on sunlight, so the biotope is likely unaffected by changes in light.

Since 2016, research on artificial light at night (ALAN) has expanded considerably in the marine and coastal environment. Light was previously assumed to be of low ecological significance in subtidal and intertidal habitats, but there is now evidence that ALAN is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organisation. Documented responses include disruption of diel and circalunar rhythms, changes in activity and foraging, altered predator–prey interactions, shifts in community composition, and impacts on algal growth and phenology (Davies et al., 2014, 2015; Gaston et al., 2017; Tidau et al., 2021; Lynn et al., 2022; Marangoni et al., 2022; Miller & Rice 2023; Ferretti et al., 2025). Evidence for benthic habitats and assemblages specifically is beginning to emerge (e.g. Trethewy et al., 2023; Schaefer et al., 2025), but remains limited and fragmented, often focusing on single taxa or short-term experiments. Mortality thresholds, long-term consequences, and responses at the biotope scale are rarely addressed, and there are major gaps around indirect effects such as trophic cascades or habitat modification.

Sensitivity assessment

Although both Aphelochaeta marioni and Polydora ciliata can perceive light, no evidence on their response to artificial light was found. Given the rapid expansion of the evidence base, but the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion (Barrier to species movement pressure definition).

Evidence

Not Relevant to biotopes restricted to open waters.

Death or injury by collision

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

Evidence

Not Relevant to seabed habitats. NB. Collision by grounding vessels is addressed under surface abrasion

Visual disturbance

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

Evidence

Most species will respond to the shading caused by the approach of a predator, however, their visual acuity is probably very low. Farke (1979) noted that Aphelochaeta marioni is intolerant of visual disturbance in a microsystem in the laboratory, possibly due to its nocturnal life habits (Farke, 1979). In order to observe feeding and breeding in the microsystem, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Polydora ciliata exhibits shadow responses withdrawing its palps into its burrow, believed to be a defence against predation. However, since the withdrawal of the palps interrupts feeding and possibly respiration the species also shows habituation of the response (Kinne, 1970). 

Sensitivity assessment

Nevertheless, the characterizing species are unlikely to have the visual acuity to respond to visual disturbance and are defined by pressure. Resistance and resilience are therefore assessed as 'High', and the biotope judged as 'Not Sensitive' to visual disturbance.

Genetic modification & translocation of indigenous species

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

Evidence

The important characterizing species in the biotope are not cultivated or likely to be translocated. This pressure is therefore considered Not Relevant.

Introduction of microbial pathogens

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

Evidence

Introduced organisms (especially parasites or pathogens) are a potential threat in all coastal ecosystems. Little information was found regarding microbial infection of polychaetes, although Gibbs (1971) recorded that nearly all of the population of Aphelochaeta marioni in Stonehouse Pool, Plymouth Sound, was infected with a sporozoan parasite belonging to the acephaline gregarine genus Gonospora, which inhabits the coelom of the host. No evidence was found to suggest that gametogenesis was affected by Gonospora infection, and there was no apparent reduction in fecundity. No information was found on microbial pathogens affecting Polydora ciliata.

Sensitivity assessment

The biotope is judged to have 'High' resistance to this pressure. By default, resilience is assessed as 'High', and the biotope is classed as 'Not Sensitive'.

Removal of target species

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

Evidence

SS.SMx.SMxVS.AphPol is currently not targeted by commercial fisheries and hence is not directly affected by this pressure. This pressure is therefore considered Not Relevant.

Removal of non-target species

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

Evidence

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in this biotope, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Hall & Harding (1997) demonstrated that commercial cockle harvesting by suction dredging had significant effects on soft-sediment infaunal communities. Following dredging, species numbers were reduced by up to 30% and abundances by up to 50%.

Sensitivity assessment

Removal of the characterizing species would result in the biotope being lost or re-classified. Therefore, the biotope is considered to have a resistance of 'Low' to this pressure and to have 'High' resilience, resulting in the sensitivity being judged as 'Low'.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and on the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40), although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel or shells of other Crepidula or other species, e.g., oysters and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020; Fitzgerald, 2007; Blanchard, 2009; Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and, once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015; Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupies spaces between or within shells, resulting in a homogeneous Crepidula-dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance, and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330 to 1300 ind./m²), but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula, with only moderate or low abundances (11 ind./m²), and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm), but while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting that the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action, which may be less suitable (Tillin et al., 2020). 

Sensitivity assessment

The above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of gravel, shells, cobbles, or any other hard substrata that can be used for larval settlement (Tillin et al., 2020). Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. However, the low proportion of gravel recorded in this biotope (<5%, Connor et al., 2004) may mitigate colonization. This is a sheltered to extremely sheltered habitat and is suitable for Crepidula. However, the reduced to low salinity levels in the biotope might mitigate or prevent colonization at high densities. Most of the evidence records Crepidula in salinities from 25 to 35 psu (OBIS, 2023). There is evidence of Crepidula occurring in low densities within this biotope, but it is unclear whether the population will dominate. Crepidula occurs in higher abundance in the SMuVS.CreMed biotope, which occurs in variable salinity (18 to 30) and has a higher proportion of gravel (ca 24%). SMuVS.CreMed is recorded adjacent to SMuVS.AphTubi and SMxVS.AphPol. While Crepidula individuals (or their shells) could spill over into this biotope (AphPol), Crepidula is probably prevented from dominating (and hence modifying) the habitats by the reduced or low salinity regime as well as the sediment characteristics. However, those examples of the AphPol biotope that occur in variable salinity may be vulnerable. Hence, resistance is assessed as 'Medium' and resilience as 'Very low’, so the biotope is assessed as 'Medium'. Further evidence is required on the presence of Crepidula within this biotope, so the confidence in the assessment is 'Low'.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). A lack of published descriptions and an incomplete historical record have led to the widespread misidentification of Didemnum vexillum, and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; McKenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan, with its native range possibly extending into continental Asia and the north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; McKenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; McKenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024). Zhang et al. (2020) suggested that in the current climate conditions (based on depth, current, temperature, and salinity), Didemnum vexillum had not yet occupied their predicted suitable habitats, and predicted that the Northern Atlantic coast is susceptible to invasion by Didemnum vexillum and that climate change could cause a poleward expansion of Didemnum vexillum.

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

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

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

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

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

Didemnum vexillum requires suitable hard substrata for successful settlement and establishment. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). The extensive mats formed by the invasive species over cobble-pebble substrata can bind or ‘glue’ small pebbles and cobbles together by filling spaces between the sediment particles, which alters the habitat complexity of the seafloor, turning it into a more homogeneous two-dimensional habitat rather than a heterogeneous three-dimensional one (Griffith et al., 2009; Mercer et al., 2009; Lengyel et al., 2009).

In addition, Didemnum vexillum is commonly found associated with artificial hard substrata, being mostly found in harbours and marinas where it covers a variety of maritime structures such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ships' hulls and at aquaculture facilities (Valentine et al., 2007 a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012). Didemnum vexillum was abundant in the marinas at Terschelling, Texel, and Vlieland, in the Wadden Sea (Gittenberger et al., 2015). In the UK, Didemnum vexillum was initially recorded in marinas and adjacent shallow man-made structures (Tillin et al., 2020). In Wales, it was first recorded in Holyhead Marina, then subsequently reported in Plymouth Marina and other marinas around the UK (Griffith et al., 2009; Minchin & Nunn, 2013; Bishop et al., 2015).

Didemnum sp. can colonize both horizontal and vertical surfaces of fouling and benthic communities, commonly occurring on upper horizontal surfaces in benthic habitats (Dijkstra et al., 2007; Tillin et al., 2020). It has been recorded on overhangs or the underside of boulders (Hitchin, 2012) or on the underside of docks, boat hulls, and pontoons (Griffiths et al., 2009; Minchin & Nunn, 2013). In sheltered areas, colonies are lobed and beard-like, forming long tendrils that drop down from the underside of docks or other artificial substrata to establish new colonies if there are suitable substrata available (Valentine et al., 2007a). In areas of stronger current, colonies are low, undulating mats (Valentine et al., 2007a,b).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015). Some species have been shown to tolerate overgrowth by Didemnum vexillum. Such as anemones (species not specified), which were observed in high densities of 10 to 339 individuals in transects with high percentage cover of Didemnum vexillum (Lengyel et al., 2009). In the Netherlands, the sea anemone Sagartia elegans and Sabella pavonia tubes were not overgrown by Didemnum sp. (Gittenberger, 2007). Botrylloides violaceus can overgrow Didemnum sp. (Gittenberger, 2007), although it was noted to be overgrown in other studies (Valentine et al., 2007a). In addition, Styela clava and Ascidiella aspera survived overgrowth by Didemnum vexillum as long as their siphons remained free (Gittenberger, 2007). However, Gittenberger (2007) stated that the boring sponge Clione celata, the sea anemone Diadumene cincta, Mytilus edulis, Magallana (syn. Crassostrea) gigas, Ostrea edulis, a variety of hydroids, the colonial ascidians Aplidium (Fig. 4) and Diplosoma listerianum and the solitary ascidians Ciona intestinalis start to die on contact with Didemnum sp. Didemnum vexillum can overgrow bivalve species, such as oysters, scallops, and mussels, as the hard shells can provide suitable hard substrata for settlement. It has been described as a ‘shellfish pest’ by the aquaculture industry because it is likely to completely encapsulate bivalves and smother them resulting in death or partially encapsulate and partially smother them resulting in reduced bivalve growth (Auker, 2010; Bullard et al., 2007; Coutts & Forrest, 2007, Valentine et al., 2007a; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020). Didemnum vexillum has been recorded overgrowing mussels in Strangford Lough, Northern Ireland (Minchin & Nunn, 2013) and recorded forming large mats over Blue Mussel beds in the Gulf of Maine, completely covering individuals (Auker et al., 2014).

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

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

Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020). It is abundant across various shore heights, thriving in both nearshore and offshore sites, particularly in subtidal areas. For example, colonies of Didemnum vexillum were dominant at depths between 45 and 60 m, occupying 50 to 90% of available space in two gravelly areas (more than 230 km²) composed of immobile pebble and cobble pavement on Georges Bank fishing ground, USA (Bullard et al., 2007; Valentine et al., 2007b; Lengyel et al., 2009). In addition, patchy mats have been observed covering approximately 1 to 1.5 km² of the pebble-cobble seabed, which is interspersed with large boulders and 30 m deep in Long Island Sound, USA (Mercer et al., 2009). In an offshore scallop dredge survey, Didemnum sp. was found attached to cobbles and boulders at 10 to 34 m (Vercaemer et al., 2015). An experiment in the Thames River estuary, Connecticut, found a significant difference between Didemnum vexillum growth rates at different depths, with faster growth rates seen in shallow water (1.0 m) compared to deeper depths (4.0 m) (Bullard & Whitlatch, 2009). It was also found that although Didemnum vexillum grew faster in shallow depths during the experiment, it grew well at all depths examined (1.0 m, 2.5 m, and 4.0 m), and there was no significant difference in survival between the depths.

Didemnum vexillum tolerates a wide range of environmental conditions, including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 ppt, is commonly found in marine waters around 33 pp,t but is unable to survive in salinities below 20 ppt (Bullard & Whitlatch, 2009; Groner et al., 2011; Tillin et al., 2020). It has been recorded in estuarine conditions and tidal lagoons (Dijkstra et al., 2007; Tillin et al., 2020). In the Lagoon of Venice, Didemnum vexillum is found in waters at 30 PSU. It was absent in low salinity, such as the estuary and around the salt marshes, but well established in the euhaline and tidally well-flushed zones of the Lagoon of Venice (Tagliapietra et al., 2012). Similar results were found in Connecticut and Rhode Island, where Didemnum vexillum was not found in environments with salinity less than 20 ppt (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 and 25.97 ppt (Gittenberger, 2007; Gittenberger et al., 2015). Salinity can influence the growth rates of Didemnum vexillum. For example, in an experiment in the Thames River estuary, Connecticut, Bullard & Whitlatch (2009) found growth rates were significantly higher in high salinity areas (26 to 30 ppt) and although survival at different salinities was not significantly different, the Didemnum vexillum colonies in low (10 to 26 ppt) and medium (15 to 28 ppt) salinities were bloated, discoloured and appeared to be dying. In unpublished data from Bullard & Whitlatch (2009), similar results were found in the laboratory, as most colonies appeared to be dying after one week in 20 ppt and healthy in 30 ppt.

A study on Didemnum vexillum colonies from Holyhead Marina, Isle of Anglesey, found colony growth within a week was significantly impaired and reduced by two-thirds at lower salinities (27 PSU and 20 PSU), while in ambient Holyhead Marina salinity (34 PSU), the growth increased and surface area doubled (Groner et al., 2011). Mortality was described as negligible in colonies of Didemnum vexillum in ambient salinity (34 PSU) after two weeks. However, mortality increased as salinity decreased. At the end of the two-week experiment, 72% of invasive colonies survived in 27 PSU and 55% of colonies survived in 20 ppt (Groner et al., 2011). When exposed to severe low salinity of 10 PSU for two hours, Didemnum vexillum showed no mortality, which suggested the duration of exposure influences mortality, not the stress intensity (Groner et al., 2011). Colonies of Didemnum vexillum collected from Anglesey, Wales, experienced more mortality under severe hypo-salinity (20 PSU, 38% colonies survived) compared to moderate hypo-salinity (27 PSU, 82% colonies survived) after two weeks, showing severe hypo-salinity creates more stressful conditions for Didemnum vexillum (Lenz et al., 2011). Therefore, Didemnum vexillum can tolerate a short-term severe decline in salinity, but prolonged exposure over two weeks caused chronic stress and increases in mortality.

Didemnum vexillum is a temperate species that can survive a broad temperature range of -2 to 24°C, with an upper survival limit suggested to be 25°C (Bullard et al., 2007; Valentine et al., 2007a; Herborg et al., 2009; Kleeman, 2009; McKenzie et al., 2017; Holt, 2024). It thrives best at 14 to 20 °C, with optimal growth temperature between 14 and 18°C during summer months (May, June, September, October) (Gittenberger, 2007; Kleeman, 2009; McKenzie et al., 2017). Didemnum vexillum has been recorded surviving in 4 to 15°C in Georges Bank and 5 to 22°C in Holyhead (Bullard et al., 2007; Valentine et al., 2007b; Griffith et al., 2009). In New England, colonies tolerate temperatures as low as -2°C (Bullard et al., 2007), but reports from the Netherlands show colonies “die off” when temperatures drop below 5°C during winter months from November to April (Gittenberger, 2007; Herborg et al., 2009). Cold winter months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). Temperature changes are an important factor influencing the seasonal growth cycle and reproduction of Didemnum vexillum (Valentine et al., 2007a).

Once established, Didemnum vexillum can expand rapidly, taking over most available hard substrata. Gittenberger (2007) stated that in the Oosterschelde, Netherlands, Didemnum sp. could cover around 95% of hard substrata, leaving little space for recruitment and growth of other species. On Georges Bank, USA, Didemnum vexillum has altered the benthic community (Lengyel et al., 2009; Tillin et al., 2020). The pebble gravel substrata on Georges Bank are important to the success and survival of haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua), and the settlement of sea scallop larvae (Placopecten magellanicus). Therefore, the invasion of Didemnum vexillum and its ability to change the habitat complexity of the seafloor may, in turn, negatively impact the benthic community (Lengyel et al., 2009). In Georges Bank, Lengyel et al. (2009) analysed photographs of the seabed and suggested that Didemnum vexillum outcompeted other epifaunal and macrofaunal species. Changes were seen in hydroids, the second most abundant epifaunal species at the location, which were overgrown by the invasive tunicate and negatively correlated with the percentage cover of Didemnum vexillum (Lengyel et al., 2009). The number of non-colonial macrofauna was also negatively related to the percentage cover of Didemnum vexillum (Lengyel et al., 2009). Dredge samples revealed clear differences in benthic species composition and revealed a significant difference in the species abundance before and after the colonization of Didemnum vexillum (Lengyel et al., 2009). Invasion of Didemnum vexillum also provided a new habitat for species not normally present, such as two polychaete species, Nereis zonata and Harmothoe extenuata, changing the species composition. The increase in abundance of polychaetes Nereis zonata and Harmothoe extenuata was also seen in dredge samples collected from Georges Bank (Valentine et al., 2007b).

In contrast, some studies have suggested that potentially the overgrowth of Didemnum vexillum has little impact on benthic communities. In Long Island Sound, USA, Mercer et al. (2009) found that the total abundance and richness of native epifaunal and infaunal species were either not different or significantly higher in samples taken inside Didemnum vexillum mats compared with samples collected outside the mats. While the mats did lead to subtle changes in community structure and shifts in species dominance, the authors suggested that benthic species may use Didemnum vexillum mats as a novel habitat and species living beneath the mats may use it for shelter and protection from epibenthic predators (Mercer et al., 2009). In addition, dredge samples taken from Georges Bank found 15 polychaete species and seven bivalve species living beneath the Didemnum vexillum mat (Valentine et al., 2007b). The comparisons of 85 benthic megafauna collected from dredge samples before and after Didemnum sp. became abundant in the Georges Bank fishing ground showed slight changes in abundance, but changes to the invertebrate species composition were statistically marginally insignificant (Valentine et al., 2007b).

Sensitivity Assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope occurs on infralittoral sandy, gravelly, muddy mixed sediment, which could provide a suitable hard substratum for colonization by Didemnum sp., as Didemnum vexillum has been recorded on clay boulders (Hitchin, 2012). Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope experiences weak to moderately strong water flow (<1 to 3 knots; <0.5 to 1.5 m/s) but extremely sheltered to sheltered wave exposure. The community is dominated by infauna, and it is unclear if Didemnum could colonize the biotope. If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of Polydora spp.. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'.

The Pacific oyster, Magallana gigas

Evidence

The majority of the evidence indicates that infralittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). In addition, the sandy, gravelly, muddy mixed sediment would not provide a suitable attachment for oysters. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The infralittoral sandy, gravelly, muddy mixed sediment, combined with the low levels of light within this biotope, is considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The infralittoral sandy, gravelly, muddy mixed sediment, combined with the low levels of light within this biotope, is considered to inhibit colonization of invasive algal species. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

The friable nature of the substratum, which is subject to ongoing erosion, means this biotope supports only a sparse epifauna and flora. This biotope is therefore unlikely to be invaded by sessile invasive non-indigenous species. As the biotope occurs in the infralittoral, and turbidity levels are often high, this biotope is likely to be unsuitable for invasive non-indigenous algae. 

The American piddock, Petricolaria pholadiformis, is a non-native, boring piddock that was unintentionally introduced from America with the American oyster, Crassostrea virginica, not later than 1890 (Naylor, 1957). Rosenthal (1980) suggested that from the British Isles, the species has colonized several northern European countries by means of its pelagic larva and may also spread via driftwood, although it usually bores into clay, peat, or soft rock shores. In Belgium and the Netherlands, Petricolaria pholadiformis almost completely displaced the native piddock, Barnea candida (ICES, 1972). However, this has not been observed elsewhere, and later studies have found that Barnea candida is now more common than Petricolaria pholadiformis in Belgium (Wouters, 1993), and there is no documentary evidence to suggest that Barnea candida has been displaced in the British Isles (J. Light & I. Kileen pers. comm. to Eno et al., 1997).

Sensitivity assessment

No evidence of impacts from ‘other ISNIS’ was found. Therefore, the evidence is presently ‘Insufficient’ to support an assessment.