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

Cordylophora caspia can survive as resistant dormant stages (menonts) at -10°C and at 35°C. Colonies tolerate 5 to 35°C and reproduce between 10 and 28°C (Kinne, 1971; reviewed by Arndt, 1986, 1989). Arndt (1989) reported that colonies died back in autumn when the temperature fell to about 10°C, only to germinate in spring when the temperature exceeded 5°C. Arndt (1989) concluded that Cordylophora caspia was thermophilic but that low temperature had an important influence on growth and reproduction. Cordylophora caspia growth rates have also been found to increase non-linearly with temperature, with a positive growth rate occurring at temperatures above 14°C and a peak growth rate occurring above 19°C (Pucherelli et al., 2016). In another study observing the effect of multiple factors on Cordylophora caspia colonies, the highest densities were observed at temperatures above 20.3°C (Obolewski, Jarosiewicz & Ozgo, 2015), and the species' optimal range seems to be between 18 and 26°C, but polyps survive temperatures ranging from 8 to 30°C (Seyer et al., 2017). However, Suutari et al. (2017) observed Cordylophora caspia in temperature ranges of −0.2 to 24.2°C in the northern Baltic Sea. In addition, the distribution of Cordylophora caspia extends into subtropical habitats (Arndt, 1986, 1989). In laboratory experiments, colonies degenerated but recovered after 1 to 8 hours of exposure to 35°C and 36.1°C, but died (and did not regenerate within seven days) after exposure to 1 to 2 hr at 37.7°C and 40.5°C. However, it was unknown if they would have regenerated if examined for longer than seven days (Folino-Rorem & Indelicato, 2005). Therefore, this species is unlikely to be adversely affected by chronic or acute temperature change at the benchmark level in British waters. 

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). The recorded distribution of Einhornia crustulenta is limited from southern England to Orkney but may be more widespread (Hayward & Ryland, 1998; JNCC, 1999). Balanus crenatus, however, is a boreal species and may be lost due to long-term increases in temperature at the benchmark level, causing a minor decline in species richness.

Sensitivity assessment. Overall, the biotope is unlikely to be adversely affected at the benchmark level. Therefore, a resistance of 'High', resilience of 'High' and hence sensitivity of 'Not sensitive' is recorded 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

Cordylophora caspia can survive as resistant dormant stages (menonts) at -10°C and at 35°C. Colonies tolerate 5 to 35°C and reproduce between 10 and 28°C (Kinne, 1971; reviewed by Arndt, 1986, 1989). Arndt (1989) reported that colonies died back in autumn when the temperature fell to about 10°C, only to germinate in spring when the temperature exceeded 5°C. Arndt (1989) concluded that Cordylophora caspia was thermophilic but that low temperature had an important influence on growth and reproduction. In another study observing the effect of multiple factors on Cordylophora caspia colonies, the highest densities were observed at temperatures above 20.3°C (Obolewski, Jarosiewicz & Ozgo, 2015), and the species' optimal range seems to be between 18 and 26°C, but polyps survive temperatures ranging from 8 to 30°C (Seyer et al., 2017). However, Suutari et al. (2017) observed Cordylophora caspia in temperature ranges of −0.2 to 24.2°C in the northern Baltic Sea.

Electra pilosa was reported to survive below freezing temperatures (Menon, 1972), although colonies are probably more tolerant of low temperatures in winter than summer (see review for details). Einhornia crustulenta may exhibit a similar response. Brault & Bourget (1985) noted that recruitment was delayed until spring on settlement plates deployed in winter. However, all the dominant species within the biotope are boreal or recorded from north of the British Isles. Therefore, although growth and reproduction may be reduced, they are unlikely to be adversely affected by reductions in temperature in British waters. 

Sensitivity assessment. While low temperatures may trigger premature dieback or regression, colonies are likely to survive changes in temperature at the benchmark level. A resistance of 'High' is suggested, with a resilience of 'High' and a sensitivity of 'Not sensitive'. 

Salinity increase (local)

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

Evidence

Cordylophora caspia are observed in their highest densities predominantly in brackish waters but also freshwater lakes, can survive 0 to 40 psu as resistant stages, grow between 0.2 to 30 psu, reproduce between 0.2 to 20 psu and possess the ability to ionic regulate (Kinne, 1971; reviewed by Arndt, 1986, 1989; Obolewski, Jarosiewicz & Ozgo, 2015; Pucherelli et al., 2016; Seyer et al., 2017). Cordylophora caspia are noted as having an optimal growth at a salinity between 15 and 17 psu (Obolewski, Jarosiewicz & Ozgo, 2015; Seyer et al., 2017). Despite this tolerance, the occurrence of Cordylophora caspia can vary. Seyer et al. (2017) did not observe any at points in an estuary (the Guadiana River, Portugal) where salinity was 34 and 0.1 psu, respectively. In nature, well-developed colonies are usually found in water of 2 to 12 psu, where tidal influence is considerable, or between 2 to 6 psu, where conditions are constant (Arndt, 1989; Suutari et al., 2017; Seyer et al., 2017), but it may also occur at full salinities. Kinne (1971) noted that high salinities (24 or 30 psu) occasionally resulted in developmental abnormalities in older colonies in the laboratory. Arndt (1989) suggested that its marine distribution was probably limited by food availability, competition from Clava spp. or Laomedea spp. and predation, e.g. from the nudibranch Tenellia adspersa (as Embletonia pallida).

Einhornia crustulenta are a typical brackish water bryozoan. In the Baltic Sea, populations have been noted to decrease when salinity exceeds 15 psu, parallel to the appearance of stenohaline bryozoan species (Piwoni-Piórewicz et al., 2022). However, Einhornia crustulenta are found in waters with a salinity range of 9.6 to 15.1 psu, with other bryozoa having a higher salinity tolerance, such as Cribrilina cryptooecium (21.5), Cryptosula pallasiana (21.5 to 26.8), Electra pilosa (18.5), and Escharella immersa (27.2) (Piwoni-Piórewicz et al., 2022). For Electra Pilosa, in a laboratory setting, decreasing activity of colonies was observed with a decrease in salinity (Dethlefs & Bathelt, 2025). Colonies of Electra Pilosa showed reduced activity of individuals in salinities of 25.5‰ to 17.1‰ after one hour but adapted to these conditions within two hours (Dethlefs & Bathelt, 2025). Irreversible damage to some colonies was observed in solutions with 14.3‰ salinity, and all colonies died in solutions of 8.6‰ (Dethlefs & Bathelt, 2025).

Sensitivity assessment. A change in full salinity to hypersaline (>40 units) may result in loss of a proportion of the population due to dieback or competitive exclusion, and predation. Hence, a resistance is assessed as 'Low'. Survival of resting stages is likely to result in rapid recovery, so resilience is 'High', resulting in a sensitivity of 'Low'. 

Salinity decrease (local)

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

Evidence

Cordylophora caspia are observed in their highest densities predominantly in brackish waters but also freshwater lakes, can survive 0 to 40 psu as resistant stages, grow between 0.2 to 30 psu, reproduce between 0.2 to 20psu and possess the ability to ionic regulate (Kinne, 1971; reviewed by Arndt, 1986, 1989; Obolewski, Jarosiewicz & Ozgo, 2015; Pucherelli et al., 2016; Seyer et al., 2017). Cordylophora caspia are noted as having an optimal growth at a salinity between 15 and 17 psu (Obolewski, Jarosiewicz & Ozgo, 2015; Seyer et al., 2017). Despite this tolerance, the occurrence of Cordylophora caspia can vary, with Seyer et al. (2017) not observing any at points in an estuary (the Guadiana River, Portugal) where salinity was 34 and 0.1 psu, respectively. In nature, well-developed colonies are usually found in water of 2 to 12 psu, where tidal influence is considerable, or between 2 to 6 psu, where conditions are constant (Arndt, 1989; Suutari et al., 2017; Seyer et al., 2017). It may also occur at full salinities, and fast-flowing, well-oxygenated freshwater containing Ca, Mg, Na, Cl and K ions (Fulton, 1962; Arndt, 1989). It has been reported from estuaries that receive significant seasonal freshwater input, and tolerates variable salinities (Arndt, 1986; 1989). Kinne (1971) noted that high salinities (24 or 30 psu) occasionally resulted in developmental abnormalities in older colonies in the laboratory. Arndt (1989) suggested that its marine distribution was probably limited by food availability, competition from Clava spp. or Laomedea spp. and predation, e.g. from the nudibranch Tenellia adspersa (as Embletonia pallida). Therefore, it is probably relatively tolerant of a change in salinity at the benchmark level. A reduction from full to reduced salinity may be beneficial and allow Cordylophora caspia to colonize new habitats. In addition, this biotope was recorded at the riverine/estuarine transition, where the salinity was reported to be always below 20 psu but could drop to zero (Hiscock & Moore, 1986; Moore et al., 1999). 

Einhornia crustulenta are a typical brackish water bryozoan, and in the Baltic Sea, populations have been noted to decrease when salinity exceeds 15 psu, parallel to the appearance of stenohaline bryozoan species (Piwoni-Piórewicz et al., 2022). However, Einhornia crustulenta are found in waters with a salinity range of 9.6 to 15.1 psu, with other bryozoa having a higher salinity tolerance, such as Cribrilina cryptooecium (21.5), Cryptosula pallasiana (21.5 to 26.8), Electra pilosa (18.5), and Escharella immersa (27.2) (Piwoni-Piórewicz et al., 2022). For Electra iilosa, in a laboratory setting, decreasing activity of colonies was observed with a decrease in salinity (Dethlefs & Bathelt, 2025). Colonies of Electra pilosa showed reduced activity of individuals in salinities of 25.5‰ to 17.1‰ after one hour but adapted to these conditions within two hours (Dethlefs & Bathelt, 2025). Irreversible damage to some colonies was observed in solutions with 14.3‰ salinity, and all colonies died in solutions of 8.6‰ (Dethlefs & Bathelt, 2025).

Sensitivity assessment. A long-term change in salinity (see benchmark) from full to reduced, reduced to low, or low to freshwater, is unlikely to adversely affect the dominant species (Cordylophora caspia) and may allow it to extend its range. Therefore, the biotope is probably 'Not sensitive' at the benchmark level (resistance and resilience are 'High'). 

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 suspension-feeding species to supply adequate food, remove metabolic waste products, prevent the accumulation of sediment and disperse larvae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). In Cordylophora caspia, the flexibility of the otherwise rigid perisarc of hydroids is provided by annulations at the base of branches. In addition, in athecates, the neck of the polyp is flexible enough to allow the polyp to adopt an efficient 'lee-side' feeding posture in water flow. However, most hydroids have a narrow range of water flow rates for effective feeding. For example, in the athecate Tubularia indivisa, food capture rate increased up to 20 cm/s, but decreased as water flow rates increased (Hiscock, 1983). In Cordylophora inkermania, food capture rates were higher in fluctuating flows than in unidirectional flows (Gili & Hughes, 1995), presumably because more polyps were brought into play in fluctuating flow than in unidirectional flow, where upstream branches 'shaded' downstream branches. Loomis (in Fulton, 1961) noted that Cordylophora caspia did not grow in still water cultures, presumably because of the build-up of CO₂ from respiration. 

Electra pilosa (and by inference Einhornia crustulenta) was able to grow in strong water flows (e.g. Menai Strait and Lough Ine rapids) (Ryland, 1970; Hermansen et al., 2001). Balanus crenatus is found in a wide range of water flow rates and is often dominant in very strong tidal streams. However, Yakovis & Artemieva (2015) noted that, at a depth of 12 m, the disturbance level is low enough for epibenthic patches of Balanus crenatus on cockle shells sized about 40 cm² to persist for years.

Sensitivity assessment. This biotope occurs in moderately strong to strong tidal streams (i.e. between 0.5 and 3 m/s). Therefore, a change of 0.1 to 0.2 m/s is unlikely to adversely affect the biotope, although a reduction may decrease feeding and hence growth rates. Therefore, a resistance of 'High' is recorded, with a resilience of 'High' and a sensitivity of '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 was recorded from steeply broken shale bedrock at +1 to 3m deep, dominated by a 100% cover of Cordylophora caspia (as lacustris), and from deeper sloping horizontal bedrock with scattered Cordylophora caspia, frequent Einhornia crustulenta and rare Balanus spp. (Hiscock & Moore, 1986; Moore et al., 1999). Intertidal populations of Cordylophora caspia are restricted to damp habitats such as underboulders and overhangs. The branched growth form of this species is likely to retain water on emersion (see image). However, an increase in desiccation (associated with increased emergence) is likely to result in drying and death of the uprights. Increased desiccation may result in the formation of resistant, dormant stages, however, no information on their desiccation tolerance was found. 

Sensitivity assessment. An increase in emergence is likely to result in death of a proportion of the population and reduction in its upper shore extent.  Therefore, a resistance of Low is recorded. However, recovery from resting stages and recolonization is likely to be rapid, so that resilience is probably 'High and sensitivity, therefore, 'Low'. 

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 very sheltered situations in which this biotope occurs are unlikely to experience an increase in wave exposure at the benchmark level due to natural causes, except perhaps extreme storm conditions. However, an increase in large or fast boat traffic and the resultant wash may have a similar effect to an increase in wave exposure. 

Cordylophora caspia and Einhornia crustulenta have only been recorded from very or extremely wave-sheltered habitats, and this biotope has only been recorded in very to extremely wave-sheltered conditions. Therefore, it is likely that an increase in wave exposure at the benchmark level is likely to result in loss of or damage to their colonies. Populations occupying small rocks, cobbles or pebbles are likely to be more sensitive, and the resultant movement of the substratum and sediment scour may also remove attached hydrorhizae, the resting stages of the hydroid, and encrusting bryozoan colonies. However, Balanus crenatus is tolerant of a wide range of wave exposures, and in low-exposure environments, can settle on many types of hard surfaces. For example, Yakovis & Artemieva (2015) noted how at a depth of 12 m, the disturbance level is low enough for epibenthic patches of Balanus crenatus on cockle shells sized about 40 cm² to persist for years.

Sensitivity assessment. A resistance of 'Low’ has been recorded. Recovery of the biotope will depend on recruitment of Cordylophora caspia from other areas. However, any resting stages and fragments of colonies remaining may contribute to the recovery. Therefore, resilience is probably 'High' and sensitivity 'Low', albeit with ‘Low’ confidence.

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

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

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant 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. 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). The athecate Cordylophora caspia may show similar sublethal effects assuming similar physiology. 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. Littoral populations of encrusting bryozoans and hydroids are also probably intolerant of the smothering effects of oil pollution, resulting in suffocation of colonies. Littoral barnacles generally have a high tolerance to oil (Holt et al., 1995) and were little impacted by the Torrey Canyon oil spill (Smith, 1968) so Balanus crenatus is probably fairly resistant to oil.

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.

Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (asCampanularia) 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. 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 100ng/l TBT. However, Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey. Hoare & Hiscock (1974) found that Balanus crenatus survived near to an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. However, barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved and Holt et al. (1995) concluded that barnacles were fairly intolerant of chemical pollution.

Therefore, hydroids are probably intolerant of TBT contamination (which may be highest in estuarine environments) and bryozoans and barnacles are probably intolerant of chemical pollution. Cordylophora caspia was also a dominant species on settlement plates placed on a floating shipyard dock in Warnock river (Sandrock et al., 1991). Floating docks are likely to result in local contamination with heavy metals and antifouling agents from ship paints, as well as oils and other chemicals used in ship maintenance. 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).

Radionuclide contamination

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

Evidence

No evidence was found.

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

Fulton (1962) found that some polyps of Cordylophora caspia fell off or were reabsorbed after seven days in the complete absence of oxygen, but remaining polyps began feeding shortly after the reintroduction of oxygen. Fulton (1962) concluded that Cordylophora caspia had a low oxygen requirement for growth and was able to grow at a reduced rate at 1% oxygen (ca 0.32 mg/l) and achieved maximal growth at 4% oxygen (ca 1.3 mg/l). Similarly, the hydroid Melicertum octocostatum annually over-summers as stolons in anoxic conditions in Abereiddy Quarry, growing back in autumn (Hiscock & Hoare, 1975).

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) and short periods of hypoxia (<0.35 ml/l) in the York River, Chesapeake Bay. Their study suggests that estuarine epifaunal communities are relatively tolerant of hypoxia. However, Balanus crenatus was reported to survive an average of 3.2 days in the absence of oxygen (Barnes et al., 1963), and it is probable that a proportion of the Balanus crenatus population would be lost, resulting in a loss of species richness.

Sensitivity assessment. Overall, Cordylophora caspia is probably resistant to low oxygen levels (Fulton, 1962); resistance of 'High' is recorded. Therefore, resilience is 'High’ and the biotope is probably 'Not sensitive' at the benchmark level. 

Nutrient enrichment

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

Evidence

Cordylophora caspia became one of the dominant species to colonize settlement plates placed beneath a floating dock in the Warnock River (Sandrock et al., 1991). This station was characterized by low salinities and higher organic and mineral nutrient loads (ca 20 to 100 µmol NO₃/l) than their other experimental station. Cordylophora caspia in the southern Baltic have been observed in their highest densities at chloride concentration above 1.18 g Cl^-/l; however, chloride concentration did not exceed 5 g Cl⁻/l and thus should be considered low for a species which reaches optimal growth at 15 psu (Obolewski, Jarosiewicz & Ozgo, 2015). In terms of nitrogen and phosphorus, Cordylophora caspia in the northern Baltic Sea was observed growing in concentrations of ~150-400 tot-N μg/l and ~14-40 tot-P μg/l, respectively, and invertebrates made up more than 94% of the average total biomass collected from the substrata a year after deployment (Suutari et al., 2017). Therefore, Cordylophora caspia seems able to tolerate high N values in coastal sites (with additional nutrient inputs from nearby fish farm pens, 2 to 150 m away), whereas these values are expected in transitional waters where Cordylophora caspia was also observed. Arndt (1986, 1989) suggested that food intake in Cordylophora caspia was high in comparison to other hydroids, so that growth and reproduction rates required for the survival of the species could only occur in eutrophic or hypertrophic waters where food is plentiful. Therefore, Cordylophora caspia is likely to tolerate relatively high nutrient levels and may benefit from moderate increases in nutrients levels and may benefit from moderate increases in nutrients.

Sensitivity assessment. The evidence suggests that Cordylophora caspia tolerates or prefers high nutrient loads. No evidence of the effects of nutrient enrichment on the other characteristic species was found. Hence, resistance is assessed as ‘High’, albeit with ‘Low’ confidence. Hence, resilience is ‘High’, and the biotope is probably ‘Not sensitive’.

Organic enrichment

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

Evidence

Cordylophora caspia became one of the dominant species to colonize settlement plates placed beneath a floating dock in the Warnock river (Sandrock et al., 1991). This station was characterized by low salinities, and higher organic and mineral nutrient loads (ca 20-100 µmol NO3/l) than their other experimental station. Arndt (1986, 1989) suggested that food intake in Cordylophora caspia was high in comparison to other hydroids so that growth and reproduction rates required for the survival of the species could only occur in eutrophic or hypertrophic waters where food is plentiful. Therefore, Cordylophora caspia is may tolerate relatively high nutrient levels, and may benefit from moderate increases in organic enrichment, although no direct evidence was found. Therefore, a resistance of High is suggested, with a resilience of 'High', resulting in a sensitivity rank of 'Not sensitive'. 

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

This biotope is recorded from hard substrata, that is bedrock, small to large boulders, and cobbles. In addition, the key species, Cordylophora caspia, Einhornia crustulenta, and Balanus crenatus also grow on artificial hard substrata, ships hulls, plants, wood, and debris (Dauvin et al., 2021; see species review). These species can also be considered epibionts, settling on other species such as crabs and mussels (Yakovis & Artemieva, 2015; Hildebrandt, Wiesenthal & Müller, 2018; Dvoretsky & Dvoretsky, 2024). However, a permanent change to soft substrata (muds, sands, etc.) would result in exclusion of the species from the habitat. They may colonize areas where boulders remain, but removal or loss of the available substratum (the benchmark) would result in loss of all encrusting species and hence loss of the biotope.

Sensitivity assessment. Resistance is 'None', and resilience is 'Very low' (it is a permanent change), and hence 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

This biotope is recorded from hard substrata, that is, bedrock, small to large boulders and cobbles.  Therefore a change in sediment type is not relevant. 

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

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

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 a passing anchor or fishing gear, or by trampling on the lower shore is likely to remove relatively delicate uprights of hydroids, damage bryozoan colonies and crush barnacles. However, in hydroids the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of hydroid colonies may be able to develop into new colonies. Populations on small hard substrata (e.g. cobbles) may be removed by fishing gear, constituting substratum loss. Overall, a proportion of the hydroid and bryozoan colonies or barnacles are likely to be destroyed and an resistance of 'Low' has been recorded. However, recovery from surviving hydrorhizae and occasional fragments is likely to be rapid so that resilience is probably 'High', resulting in a sensitivity of '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

The species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure.

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

Cordylophora caspia and Einhornia crustulenta are found in estuarine and sheltered lagoonal habitats, which are characterized by relatively high suspended sediment loads. This biotope probably experiences marked changes in suspended sediment loads between winter and summer, due to winter storms and rainfall. Cordylophora caspia was also reported in saltmarsh pools (JNCC, 1999) and salt marshes are a depositional environment characterized by siltation. Therefore, Cordylophora caspia and Einhornia crustulenta are probably not sensitive to increases in suspended sediment loads at the benchmark level. Balanus crenatus is found a wide variety of habitats including estuaries and on the back of crustaceans in sedimentary habitats, although increased sediment loads may reduce growth rates.

A reduction in suspended sediment is unlikely to directly affect the biotope. A decrease in suspended sediment may reduce the availability of organic particulates and hence reduce food availability. Arndt (1986, 1989) suggested that Cordylophora caspia had a high food requirement for growth and reproduction  and that the species could only occur in eutrophic or hypertrophic waters where food is plentiful. It is therefore, likely to be intolerant of any reduction in food availability. Overall, a reduction in suspended sediment may reduce food availability and hence growth and reproduction in all the species in the biotope. 

Estuarine environments are typically very turbid in comparison to coastal waters, therefore a change in turbidity of one rank in the UKTAG scale (the benchmark) may not be significant. In addition the biotope probably experiences marked changes in sedimentary loads between the winter and summer months.  Therefore, the resistance is probably 'High',  resilience is  'High', and the biotope 'Not sensitive' at the 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. Further detail

Evidence

At low salinities Cordylophora caspia forms short, un-branched colonies and smothering by 5 cm of sediment is likely to cover a large proportion of the colony, preventing feeding and hence reducing growth and reproduction in the hydroid, while local hypoxic conditions are also likely to inhibit growth (Fulton, 1961, 1963).  Smothering will also prevent feeding and growth in both Einhornia crustulenta and Balanus crenatus. The encrusting bryozoan grows rapidly and may be adversely affected, while Balanus crenatus was considered to be resistant (see review). The hydroid colony is likely to survive or become dormant, and recover rapidly once the sediment is removed. Examples of the biotope on overhangs and other vertical or near vertical surfaces are unlikely to be affected, while examples on small boulders and cobbles that retain the sediment will probably be the most affected. In estuarine conditions, tidal flow is likely to remove 5 cm of deposited sediment within a number of tidal cycles, depending on site, so the duration of smothering is probably fairly short. Therefore, a resistance of 'Moderate' is suggested, with a 'High' resilience',  so that sensitivity is probably '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

At low salinities Cordylophora caspia forms short, un-branched colonies and smothering by 5 cm of sediment is likely to cover a large proportion of the colony, preventing feeding and hence reducing growth and reproduction in the hydroid, while local hypoxic conditions are also likely to inhibit growth (Fulton, 1961, 1963).  Smothering will also prevent feeding and growth in both Einhornia crustulenta and Balanus crenatus. The encrusting bryozoan grows rapidly and may be adversely affected, while Balanus crenatus was considered to be resistant (see review). The hydroid colony is likely to survive or become dormant, and recover rapidly once the sediment is removed. Examples of the biotope on overhangs and other vertical or near vertical surfaces are unlikely to be affected, while examples on small boulders and cobbles that retain the sediment will probably be the most affected. In estuarine conditions, tidal flow is likely to remove 30 cm of deposited sediment within several months, depending on site, so the duration of smothering is probably variable. Therefore, a resistance of 'Low' is suggested, with a 'High' resilience', so that sensitivity is probably '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 Cordylophora caspia, Einhornia crustulenta, 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

The characterizing epifauna have no known ability to perceive noise. although they can perceive localised vibration, changes in noise levels are probably 'Not relevant' to 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. However, Cordylophora caspia is a potentially invasive non-indigenous species,thought to have been transported in ballast water and on ship hulls, as larvae, fragments or dormant stages. Therefore, only complete cessation of water flow and water transport is likely to adversely affect recruitment of the dominant characteristic species.  In addition, the hydroid colony (or colonies) is probably not dependent on external recruitment for it continued survival, except if severely damaged. 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

The characteristic epifaunal species have no known visual perception. 

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

Cordylophora caspia is considered to be a invasive non-indigenous species in Europe, and both South and North America, probably transported by shipping (Fofonoff et al., 2015).  Its 'translocation' has probably extended its geographic range and can be regarded as beneficial to the species.  Therefore, this biotope is probably 'Not sensitive'  to this pressure. 

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 (i.e. Cordylophora caspia) would be targeted by a fishery or other commercial activity.  Therefore, this pressure (as defined in the benchmark) is 'Not relevant' to this biotope.  However, in areas where the Cordylophora caspia is regarded as an invasive or biofouling species, physical removal might be adopted as a control measure but his is not considered here. 

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

This biotope is unlikely to be exposed to commercial fisheries or shell fisheries, angling or harvesting activities  (as defined in the benchmark) . Therefore, this pressure is 'Not relevant'.  Any potential abrasion to the habitat via canoe or boat landing points, recreational access etc., is considered under the abrasion pressure above. 

The American slipper limpet, Crepidula fornicata

Evidence

This biotope is classed as 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 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 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 moderately strong to strong water flow (0.5 to 3 m/s) but sheltered to very 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. 

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. 

Other INIS

Evidence

Cordylophora caspia is regarded as an invasive non-indigenous species (INIS) in parts of Europe and both North and South America. However, no evidence on the effects of other INIS on Cordylophora caspia was found.