The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

The bryozoan Conopeum reticulum has a wide distribution and is found from the North Sea and Atlantic coasts of Europe to North and South America, Africa, and Oceania; however, its original distribution is uncertain (López-Gappa & Pereyra, 2020; Yu et al., 2021). Little information was found on Hartlaubella gelatinosa and Bowerbankia imbricata. However, both have a broad geographic distribution, being found from the northeast and northwest Atlantic to the coasts of New Zealand (Waeschenbach et al., 2015; OBIS, 2025). Growth rates were reported to increase with temperature in several bryozoan species; however, zooid size decreased, which may be due to increased metabolic costs at higher temperatures (Menon, 1972; Ryland, 1976; Hunter & Hughes, 1994). Temperature is also a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). Most of the hydroid and bryozoan species within the biotope are recorded to the north or south of the British Isles and are unlikely to be adversely affected by long-term increases in temperature at the benchmark level, e.g. Hartlaubella gelatinosa has been recorded from southeast Sweden to the Mediterranean (Cornelius, 1995b). In Warnow estuary, northern Germany, Hartlaubella gelatinosa was documented at various depths and temperature ranges of a maximum of 21.1°C in July 2023 and a minimum of 0.8°C in January 2024 for surface waters, and a maximum of 19.5°C in September 2025 and a minimum of 2.8°C in January 2024 for bottom waters (Martin, 2024). Several members of the community may also survive acute temperature change, e.g. the upper lethal temperatures of Conopeum reticulum and Electra pilosa were 30 to 32°C and 25 to 29°C, respectively (Menon, 1972, 1974).

Sensitivity assessment. An increase in temperature at the benchmark level is likely to affect growth and reproduction in the encrusting bryozoan and hydroid species but otherwise has few adverse effects. Therefore, a resistance of 'High' has been recorded. Resilience is likely to be 'High', and, therefore, the biotope is probably 'Not sensitive' at the benchmark level. 

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

The bryozoan Conopeum reticulum has a wide distribution and is found from the North Sea and Atlantic coasts of Europe to North and South America, Africa, and Oceania; however, its original distribution is uncertain (López-Gappa & Pereyra, 2020; Yu et al., 2021). Little information was found on Hartlaubella gelatinosa and Bowerbankia imbricata. However, both have a broad geographic distribution, being found from the northeast and northwest Atlantic to the coasts of New Zealand (Waeschenbach et al., 2015; OBIS, 2025). Conopeum reticulum and Electra pilosa were reported to survive below freezing temperatures (Menon, 1972), although colonies are probably more tolerant of low temperatures in winter than summer (see species review for details). Low temperatures may trigger regression or dormancy in hydroids (e.g. Cordylophora caspia). Brault & Bourget (1985) noted that recruitment was delayed until spring on settlement plates deployed in winter. At lower temperatures, Conopeum reticulum has been recorded as having longer zooids, yet, to date, there is no study which investigates variability in bryozoan zooid body size across a natural depth-related thermal gradient (Stępień et al., 2017). In Warnow estuary, northern Germany, Hartlaubella gelatinosa was documented at various depths and temperature ranges of a maximum of 21.1°C in July 2023 and a minimum of 0.8°C in January 2024 for surface waters, and a maximum of 19.5°C in September 2025 and a minimum of 2.8°C in January 2024 for bottom waters (Martin, 2024). However, all the dominant species within the biotope are boreal or recorded from north of the British Isles.

Sensitivity assessment. Although growth and reproduction may be reduced, they are unlikely to be adversely affected by reductions in temperature in British waters.  Therefore, resistance and resilience are likely to be 'High’, and the biotope is probably 'Not sensitive' at the benchmark level. 

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

The hydroids and bryozoans present in the biotope are characteristic of brackish waters. The biotope itself is only recorded from 'low' or 'reduced' salinity environments. The dominance of hydroid and bryozoan species in this biotope is probably due to the exclusion of predators and competitors (con-specifics and ascidians) by the reduced and variable salinity. Conopeum reticulum has been recorded as having a salinity preference of 30 to 32 psu, and Conopeum species seem well adapted to waters with low salinity and salinity variations; it is likely that salinity is an important factor in determining their distribution (Yu et al., 2021). In Warnow estuary, northern Germany, Hartlaubella gelatinosa was documented at very low salinities, 0.4 psu in January 2024, which coincided with the lowest recorded temperature of 0.8 °C (Martin, 2024). Despite some inconsistencies, salinity increased with rising water temperature, and the highest salinity was recorded in October 2023, with 13.1 psu and a temperature of 15.7 °C (Martin, 2024). While the salinity of surface waters varied with temperature, bottom waters had a consistent salinity ranging from 10.5 psu in June 2023 to 14.8 psu in December 2023 (Martin, 2024). An increase in salinity at the benchmark level (i.e. from 'reduced' to 'variable' or 'full') is likely to allow more marine species to colonize the biotope and potentially out-compete the hydroid and bryozoan members of the community. A year-long increase in salinity would probably result in loss of the community at the marine limit of its range. It may be able to colonize new space at the upper estuarine limit of its range, providing suitable hard substrata are present.

Sensitivity assessment. A resistance of 'Low’ has been recorded to represent a loss of the extent of the biotope and/or the change in abundance of characteristic species. Resilience is probably 'High'. Therefore, an overall sensitivity of 'Low' is recorded. 

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

The hydroids and bryozoans present in the biotope are characteristic of brackish waters. Lower limits of tolerance include, for example, 6.2 psu for Hartlaubella gelatinosa, 1 psu for Obelia geniculata, 12 psu for Obelia dichotoma, 21.5 psu for Conopeum reticulum and 13.7 psu in Einhornia crustulenta (as Electra crustulenta) (Ryland, 1970; Cornelius, 1995b; Hayward & Ryland, 1998). [Please note:Conopeum seurati is considered a truly brackish water species surviving down to 1 psu, and often confused with Conopeum reticulum (Ryland, 1970; Hayward & Ryland, 1998)]. Therefore, they are tolerant of the low and variable salinities experienced within the habitat and probably not sensitive to short-term changes in the salinity for a week. However, a long-term change in salinity from, e.g. low to <5 psu for a year is likely to adversely affect the community. In the Tamar estuary, Devon, this biotope (IR.LIR.IFaVS.HarCon) is replaced by the IR.LIR.IFaVS.CcasEin at the riverine/estuarine transition where the salinity is always below 20 psu and can drop to 0 psu (Hiscock & Moore, 1986).

Conopeum reticulum has been recorded as having a salinity preference of 30 to 32 psu, and Conopeum species seem well adapted to waters with low salinity and salinity variations; it is likely that salinity is an important factor in determining their distribution (Yu et al., 2021). In Warnow estuary, northern Germany, Hartlaubella gelatinosa was documented at very low salinities, 0.4 psu in January 2024, which coincided with the lowest recorded temperature of 0.8 °C (Martin, 2024). Despite some inconsistencies, salinity increased with rising water temperature, and the highest salinity was recorded in October 2023, with 13.1 psu and a temperature of 15.7 °C (Martin, 2024). While the salinity of surface waters varied with temperature, bottom waters had a consistent salinity ranging from 10.5 psu in June 2023 to 14.8 psu in December 2023 (Martin, 2024).

Sensitivity assessment. A long-term decrease in salinity is likely to result in loss of the biotope in the uppermost parts of the estuary, although it may extend its range into reduced salinity waters further down the estuary. Therefore, a resistance of 'None' is suggested. However, resilience is probably 'High', so that sensitivity is 'Medium'. 

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Water movement is essential for hydroids and other suspension feeders such as encrusting bryozoans and barnacles, to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Most hydroids have a narrow range of water flow rates for effective feeding, and feeding efficiency decreasing a high water flow rates (Gili & Hughes, 1995). Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). Okamura (1985) noted that encrusting bryozoans live and feed in the boundary layer, so that the water flow rates they experience are much lower than that of the surrounding environment.  In the laboratory, feeding efficiency of Conopeum reticulum colonies was higher at slow water flow (0.01-0.02 m/s) that fast (0.1-0.2 m/s), depending on colony size, with large colonies experiencing less effect (Okamura, 1985). However, Conopeum reticulum has been reported in strong to weak tidal streams (JNCC, 1999). Therefore, it is probably tolerant of a wide range of water flow rates. Similarly, Hartlaubella gelatinosa has been recorded in very strong, moderately strong and weak tidal streams (JNCC, 1999).  Therefore, the characteristic species in the biotope are probably tolerant of changes in the water flow. This biotope is recorded from moderately strong tidal flow (0.5-1.5 m/s), so a change of 0.1-0.2 m/s is probably not significant. Therefore, a resistance of 'High' is recorded to represent potential changes in feeding efficiency, while resilience is probably 'High', and the biotope is probably 'Not sensitive' at the 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. Further detail

Evidence

This biotope is subtidal occurring from 0-5 m. However, its upper extent may be exposed during periods of low water level. Intertidal populations of hydroids, Conopeum reticulum, and Electra sp. are restricted to damp habitats such as underboulders, overhangs or the interstices between macroalgae. The branched growth form of hydroids is likely to retain water on emersion (e.g. see Cordylophora caspia).  The biotope is found on a mixture of hard substrata, included cobbles and small boulder, so that damp areas probably exist to protect parts of the population form dessication.  A decrease in emergence may allow the biotope to extent its range if suitable habitat exists. 

Nevertheless, hydroids and bryozoans are likely to be adversely affected by desiccation as a result of increase in emergence at the benchmark level and the upper limit of the population may be removed.  Therefore a resistance of 'Low'  has been recorded to represent the potential loss of a proportion of the biotope. Resilience is probably 'High' so that the biotope has a 'Low' sensitivity to this pressure. 

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

The biotope occurs in very wave sheltered situations, although storms may create significant oscillatory water movement in shallow depths. The oscillatory water flow caused by wave action is potentially more damaging to delicate marine organisms than unidirectional flow. The encrusting bryozoans are tolerant of a wide range of wave exposures (e.g. see Electra pilosa review). Wave exposed conditions tend to favour small, less branched species of hydroid than are found in this biotope (Boero, 1984; Gili & Hughes, 1995). Hartlaubella gelatinosa has only been recorded from wave sheltered conditions. The most likely adverse effect of wave action is the displacement of hard substrata (e.g. small rocks, cobbles or pebbles) and attached organisms. The resultant movement of the substratum and sediment scour may remove attached hydrorhizae and even resting stages of hydroids but many are likely to survive. Therefore, it is likely than an small increase in wave exposure at the benchmark level  is likely to result in loss or damage of the hydroid and bryozoan colonies. Therefore, a resistance of 'Medium' is suggested, so that with a resilience of 'High', sensitivity to this pressure is probably 'Low'. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001). Bryozoans are common members of the fouling community and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints, and bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1977; Holt et al., 1995). Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l. Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that several species of epifauna may be at least inhibited by the presence of TBT. However, Hartlaubella gelatinosa has been recorded in the Crouch estuary where TBT were very high, and was recorded under boat moorings at Cargreen in the river Tamar, and may be relatively tolerant of TBT (Keith Hiscock pers. comm.). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100 ng/l TBT.

Howethis pressure is Not assessed.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Although subtidal, this biotope is relatively shallow and may be exposed to oils and hydrocarbons adsorbed onto particulates and ingested or through the water soluble fractions of oils and hydrocarbons. Species of the encrusting bryozoan Membranipora and the erect bryozoan Bugula were reported to be lost or excluded from areas subject to oil spills (Mohammad, 1974; Soule & Soule, 1979). Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoans (no species given) at oiled sites after the Exxon Valdez oil spill. Encrusting bryozoans are also probably intolerant of the smothering effects of oil pollution, resulting in suffocation of colonies. The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995). However, hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Calder (1976) suggested that hydroids found in the low salinity areas of south Carolina, such as Cordylophora caspia, were also present in relatively polluted waters, such as Charleston Harbour. Loss of the dominant, abundant species Conopeum reticulum and Electra spp. from the biotope would result in significant change in the community and perhaps loss of the biotope as described.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Chemical contaminants are likely to affect different species inthe biotope to varying degrees, depending on the nature of the contaminant and its concentration.

• Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth.
• Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l.
• Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that several species of epifauna may be at least inhibited by the presence of TBT.
• However, Hartlaubella gelatinosa has been recorded in the Crouch estuary where TBT were very high, and was recorded under boat moorings at Cargreen in the river Tamar, and may be relatively tolerant of TBT (Keith Hiscock pers. comm.).
• Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1977; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100 ng/l TBT.
• Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey.

The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Bryozoans are probably intolerant of chemical pollution.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

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). Further detail

Evidence

Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans Conopeum tenuissimum and Membranipora tenuis, and the hydroid Obelia bicuspidata, were unaffected by periods of moderate hypoxia (ca 0.35 to 1.4 ml/l; 0.49 to 1.96 mg/l) and short periods of hypoxia (<0.35 ml/l; 0.49 mg/l) in the York River, Chesapeake Bay. Although the exact species examined differ, their study suggests that estuarine epifaunal communities are relatively tolerant of hypoxia. In Warnow estuary, northern Germany, Hartlaubella gelatinosa was documented in waters where oxygen saturation ranged from its maximum in September 2023, with 12.32 mg/l and decreased to its minimum in October 2023 at 6.31 mg/l; however, only surface waters were tested (Martin, 2024).

Sensitivity assessment. The above evidence suggests that estuarine epifaunal communities are probably resistant to hypoxia. Hence, Resistance is assessed as 'High', resilience as 'High', and sensitivity as 'Not sensitive' at the benchmark level, but with ‘Low’ confidence due to the lack of direct evidence concerning characteristic species.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Estuarine habitats are generally higher in nutrient levels than coastal waters. A moderate increase in nutrients may increase food availability for suspension feeders, in the form of organic particulates. Eutrophication may result in local hypoxic conditions (see below) and /or blooms of ephemeral algae. Yet, in this turbid environment, ephemeral algae are likely to be limited to the very shallow water near the top of the shore, and unlikely to adversely affect the biotope. However, there is ‘Insufficient evidence’ on which to base an assessment at present.  

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Little direct evidence was found.  Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  Nevertheless, Conopeum reticulum was not recorded.  Therefore, it suggests that a proportion of the community in the biotope is resistant of organic enrichment and siltation, while other members of the community are probalby adversely affected. Therefore, a resistance of 'Medium' is suggested. However, as resilience is likely to be 'High', sensitivity is, therefore, 'Low'.  

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

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. Further detail

Evidence

All members of this biotope require hard substratum for attachment, ranging from bedrock and cobbles, to shell, artificial substrata and plants. Therefore a permanent change from a hard rock (or similar substratum) to soft (sedimentary) substratum would result in loss of the biotope.  Members of the biotope may survive on scattered shell and stones on a sedimentary substratum but the biotope would not remain.  Therefore, a resistance of 'None' is recorded. As the change in permanent, the resilience is 'Very low' (by definition) and sensitivity is 'High'. 

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

All members of this biotope require hard substratum for attachment, ranging from bedrock and cobbles, to shell, artificial substrata and plants. Where the biotope occurs on mixed substrata, the characterizing species occur on stones, shell and debris. Therefore, a change in one Folk class from mixed to sand or gravel dominated sediment that resulted in removal of stones or other suitable hard substrata, would result in loss of the biotope.  A change to fine sediment may also increase localised scour resutling in loss of the part of the biotope.  Therefore, a resistance of 'Low' is recorded. As the change in permanent, the resilience is 'Very low' (by definition) and sensitivity is '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). Further detail

Evidence

Where the biotope occured on bedrock or other hard substrata, extraction is unlikely, and the pressure is not relevant. However, where the biotopes occurs on stones, shell and other hard substrata on mixed sediment then extraction (to 30 cm) would probably remove the biotope completely within the affected area. The biotope would be lost and resistance is 'None'.  Resilience is probably 'High' so that sensitivity is '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). Further detail

Evidence

Abrasion by an anchor or fishing gear is likely to remove relatively delicate uprights of hydroids, and damage bryozoan colonies, and erect epifauna are considered particularly sensitive to physical abrasion (López-Gappa & Pereyra, 2020). For example, drop-down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011). The study showed that damage was incremental, with damage increasing with frequency of trawls rather than a blanket effect occurring on the pass of the first trawls. The level of impact may be mediated by the rugosity of the attachment, with surfaces with greater damage occurring over smooth terrains, where the fishing gear could move unimpeded across a flat surface. Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Erect epifauna can be directly removed and brought to the surface in trawl hauls. De Groot (1984), for example, found that beam trawls with or without tickler chains removed the hydrozoan Tubularia spp. (mostly Tubularia indivisa). He suggested that nearly all individuals in the path of a beam trawl would be destroyed. This study was based on observations of species caught as by-catch and did not assess in-situ damage rates. Re-sampling of grounds that were historically studied (from the 1930s) indicates that some upright species have increased in areas subject to scallop fishing (Bradshaw et al., 2002). This study also found increases in the tough-stemmed hydroids, including Nemertesia spp., whose morphology may prevent excessive damage. Bradshaw et al. (2002) suggested that, as well as having high resistance to abrasion pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult. Other population-level effects have also been recorded. The scallop fishery has been implicated for altering genetic diversity within Sertularia cupressina populations on commercial scallop grounds in Atlantic Canada, where increased damage rates have increased clonality from injury-induced fragmentation (Henry & Kenchington 2004). This meant that genetic diversity in fished areas was lower than in unfished areas.

No specific information was available to assess the resistance of the characteristic species within this biotope. But erect epifauna exposed to abrasion could displace, damage and remove individuals (De Groot 1984; Veale et al., 2001; Boulcott & Howell, 2011; López-Gappa & Pereyra, 2020). Colonies of hydroids and bryozoans attached to mobile substrata may suffer damage during the displacement of stones and small boulders, but are likely to survive. However, removal of a bryozoan or hydroid colony from its substratum would probably be fatal, and encrusting bryozoans are not known to be able to reattach. Fragmentation is thought to be a possible mode of asexual reproduction in hydroids (Gili & Hughes, 1995), and it is possible that a proportion of displaced hydroid fragments may attach to new substrata, enhancing recovery. However, the encrusting bryozoans, hydroids and barnacles are likely to be lost.

Sensitivity assessment. Although trawling activities may be unlikely where this biotope occurs, anchoring and dredging activities may have similar effects. Overall, the biotope is likely to have a 'Low' resistance to this pressure. However, resilience is likely to be 'High', so that sensitivity is probably 'Low’. 

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Subsurface penetrative activities are not relevant where this biotope occurs on bedrock and other hard substratum.  Where the biotope occurs on small boulders, cobbles and stones on mixed substrata, then sub-surface penetrative activities may physically displace or remove the biotope (see abrasion above). However, where suitable substrata remain, the biotope is highly resilient. Therefore, a resistance of 'Low' is suggested, with 'High' resilience and hence 'Low' sensitivity. 

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. Further detail

Evidence

The biotope occurs in high-turbidity estuarine waters. Estuarine waters can exhibit high turbidities, measured in grammes per litre rather than milligrammes per litre and probably exceed the lower limit of 'very turbid' (>300 mg/l) on the UKTAG scale (see benchmark). Therefore, an increase in turbidity is unlikely. If a decrease in turbidity were to occur, then conditions may favour macroalgae, which would compete for space with the dominant epifauna and potentially grow over the epifauna. However, records of the biotope suggest that it contains bare areas, and the hydroid and bryozoan epifauna can probably tolerate being grown over, so that the effects are limited. If conditions return to the prior norm after a year, then recovery will be rapid. In Gomso Bay, South Korea, Conopeum reticulum was found close to a tidal mud flat that experienced high turbidity (max 500 mg/l) due to nearby riverine inflow and strong tidal currents in this macrotidal setting (Yu et al., 2021). In contrast, in the ports on the west coast of Korea, where another bryozoan, Conopeum seurati, was collected, the waters were less turbid than in Gomso Bay (20 to 50, max 100 mg/L), suggesting that this species is less tolerant to turbidity (Yu et al., 2021).

Sensitivity assessment. A resistance of 'Medium' is suggested to represent changes in the abundance of epifauna, but with a resilience of 'High', sensitivity is 'Low'. 

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

No direct evidence was found to assess the impact of this pressure at the pressure benchmark. The characteristic species are attached to the substratum and are usually shorter than 30 cm (e.g. Obelia dicotoma can form large, erect and loosely fan-shaped or elongate up to 35 cm in height, or short and either bushy or unbranched colonies up to 5 cm in height in British waters (see review), and Hartlaubella gelatinosa can grow up to 20 cm in height (see review). Encrusting bryozoans are easily buried.  Siltation by fine sediments would also prevent larval settlement by the characteristic species which require hard substratum (Berghahn & Offermann,  1999). The intensity and duration of siltation will be mediated by site-specific hydrodynamic conditions, such as water-flow and wave action that determine the dispersal of deposits.

In general it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea has been linked to environmental changes including siltation. Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions. Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. However, Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  

Smothering by 5 cm of sediment (see benchmark) is likely to prevent feeding and hence reduce growth and reproduction in encrusting bryozoans. The hydroid hydranths are relatively large and some parts of the colony are likely to protrude above 5 cm of sediment. However reduced feeding, together with local hypoxic conditions under the sediment layer will probably reduce growth and reproduction rates. In addition, associated sediment abrasion may remove the bryozoan colonies.  Therefore, a biotope resistance of ' Medium' has been recorded, as a proportion of the community may be adversely affected. The resilience is probably 'High'  so that the sensitivity is likely to be 'Low'. 

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

No direct evidence was found to assess the impact of this pressure at the pressure benchmark. The characteristic species are attached to the substratum and are usually shorter than 30 cm (e.g. Obelia dicotoma can form large, erect and loosely fan-shaped or elongate up to 35 cm in height, or short and either bushy or unbranched colonies up to 5 cm in height in British waters (see review), and Hartlaubella gelatinosa can grow up to 20 cm in height (see review). Encrusting bryozoans are easily buried.  Siltation by fine sediments would also prevent larval settlement by the characteristic species which require hard substratum (Berghahn & Offermann,  1999). The intensity and duration of siltation will be mediated by site-specific hydrodynamic conditions, such as water-flow and wave action that determine the dispersal of deposits.

In general it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea has been linked to environmental changes including siltation. Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions. Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. However, Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  

Smothering by 30 cm of sediment (see benchmark) is likely smother encrusting bryozoans. The hydroid hydranths are relatively large and a limited number may protrude above even 30 cm of sediment. However reduced feeding, together with local hypoxic conditions under the sediment layer will probably reduce growth and reproduction rates. In the moderately strong tidal  flow the sediment may remain for some time, so that the hydroid uprights may die back. In addition, associated sediment abrasion may remove the bryozoan colonies. Therefore, while Hartlaubella gelatinosa and Obelia sp.may be tolerant, other members of the community are likely to be removed, so a resistance of 'Low' is suggested. As resilience is probably 'High, sensitivity is likely to be 'Low'. 

Litter

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

Evidence

Not assessed

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

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

Underwater noise changes

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

Evidence

Not relevant to the charaterizing species of this biotope. 

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

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

Sensitivity assessment. Given the rapid expansion of the evidence base but the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Barrier to water flow may restrict larval recruitment for this biotope.   The hydroid or colonies are probably not dependent on external recruitment for it continued survival, except if severely damaged. In addition, members of this biotope are typical of fouling communities and noted for high rates of recovery and recruitment. Therefore, only complete cessation of water flow and water transport is likely to adversely affect recruitment of the dominant characteristic species. Therefore, this pressure is 'Not relevant' to this biotope. 

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. Further detail

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. Further detail

Evidence

None of the charateristic species within this biotope have any know visual acuity, and while they may react to shading of the polyps themselves, this presure is 'Not relevant'. 

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

None of the characteristic species are known to be 'translocated', subject to breeding programmes, or liable to hybridize with other species. Therefore this pressure is 'Not relevant' to this biotope. 

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). Further detail

Evidence

No evidence found

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

It is extremely unlikely that any of the species indicative of sensitivity would be targeted and we have no evidence for the indirect effects of removal of other species on this biotope. Therefore, this pressure is 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. Further detail

Evidence

It is extremely unlikely that any of the species indicative of sensitivity would be targeted and we have no evidence for the indirect effects of removal of other species on this biotope. Therefore, this pressure is not relevant. 

The American slipper limpet, Crepidula fornicata

Evidence

This biotope is classed as mixed infralittoral and, therefore, can support seaweed communities, however, the characterizing species in this biotope have a preference for lower salinities, whereas most seaweeds have a preference for full salinity conditions. Crepidula fornicata 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 from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1 to 0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders, so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space, and many are fast-growing and opportunistic, so they may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment. The mixed infralittoral rock characterizing this biotope could be suitable for the colonization by Crepidula fornicate, however, the lower salinity of estuaries and the mix of conditions they experience would make colonization difficult, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011b). In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the infralittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required.

The carpet sea squirt, Didemnum vexillum

Evidence

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

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

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

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

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

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

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

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

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

Didemnum vexillum has not been reported to colonize hydroid and bryozoan communities. Nevertheless, Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope occurs on hard mixed substratum, which could provide a suitable hard substratum for colonization by Didemnum sp. 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 a moderately strong water flow (0.5 to 1.5 m/s) but very sheltered to extremely sheltered wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of hydroids and bryozoans. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'.

The Pacific oyster, Magallana gigas

Evidence

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

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

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

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020) because its larvae require hard substrata for successful settlement and development (McKinstry & Jensen, 2013; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020). Invasive populations of Magallana gigas have been found on wave-exposed rocky shores to wave-sheltered soft sediment environments, and it has been described as a habitat generalist (Troost, 2010; Kochmann et al., 2012, 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). They are unlikely to occur under boulders as they require access to the water column (Tillin et al., 2020). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).  

The Pacific oyster can withstand a wide range of salinities (from 11 to 34 psu), but no oysters were observed in areas which had salinities less than 20 psu, and most abundant populations occur in salinities above 20 psu on the Swedish west coastline (Wrange et al., 2010; Kochmann, 2012; Chu et al., 1996 cited in Tillin et al., 2020). Bergstrom et al. (2021) noted that in the Skagerrak, Sweden native and Pacific oyster densities increased with rising salinity above 15 to 21 psu up to the full range measured (27 psu). Larvae can survive salinities between 19 to 35 psu (Troost, 2010; Tillin et al., 2020). Kochmann (2012) reported 11 to 35 psu as the optimal salinity range for Magallana gigas (cited in Wood et al., 2021). Growth of Pacific oysters can occur between 10 to 30 psu (Troost, 2010).

Sensitivity assessment. The hard substrata in this biotope may provide attachment for Magallana. However, it prefers salinities of 20 psu or above, which would probably mitigate its colonization to a few individuals. Therefore, resistance is assessed as ‘High’, resilience and ‘High’, and the biotope is probably ‘Not sensitive’, albeit with ‘Low’ confidence.

Wireweed, Sargassum muticum

Evidence

Sargassum muticum can survive in estuarine conditions but has a preference for full salinity ranges, 30 to 34 psu. Therefore, the salinity and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum.   The biotope is assessed as ‘Not sensitive’

Wakame, Undaria pinnatifida

Evidence

Undaria pinnatifida can survive in estuarine conditions but has a preference for full salinity ranges, 27 to 33 psu. Therefore, the salinity and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria and the biotope is assessed as ‘Not sensitive’

Other INIS

Evidence

This epifaunal-dominated biotope could be threatened by other more aggressive epifauna, e.g. Perophora japonica. However, such species have not been reported from the upper and mid Tamar estuary, where the biotope is found. No evidence of impact was found, although the evidence may need to be revisited in the future.