The Marine Life Information Network

Temperature increase (local)

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

Evidence

Alcyonium digitatum is described as a northern species by Hiscock et al. (2004) but is distributed from northern Norway (70°N) to Portugal (41°N) (Hartnoll, 1975; Budd, 2008). Temperature at the seafloor is an important predictor of Alcyonium digitatum, yet there are no published studies that explore the thermal minima or maxima for the species, so the only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Alcyonium digitatum is commonly found in inshore and offshore areas of northwest and northern France, the Channel Islands, most of the British Isles, including the Shetland Islands, parts of the southern North Sea, parts of southern Norway, and along the Atlantic slope (Jenkins & Stevens, 2022). Alcyonium digitatum can be found alongside Eunicella verrucosa, and in Britain and Ireland, Eunicella verrucosa has been observed in waters where 9.2°C was the lowest average seafloor temperature, the median temperature was 10.5°C, and the highest temperature was 11.4°C (Jenkins & Stevens, 2022). While trying to understand the impact of climate change on Alcyonium digitatum distribution, Jenkins & Stevens (2022) noted that there will likely be a shift north as more suitable habitat becomes available in higher latitudes. However, suitability predictions in the southern portion of their study area decreased.

Securiflustra securifrons is recorded from Kongsfjorden, Svalbard (Gontar et al., 2001), to the Iberian Peninsula in both Spain and Portugal (Ramos, 2010). Across this latitudinal gradient, species are likely to experience a range of temperatures from approximately 5 to 18°C (Seatemperature, 2015). Flustra foliacea is recorded in the German North Sea (Helgoländer Steingrund), which experiences seasonal water temperatures from 2 to 19°C (Becker et al., 2020).

Spirobranchus triqueter is described as a temperate species by Kupriyanova & Badyaev (1998). Spirobranchus triqueter is recorded as abundant in sub-tidal habitats of Trondheimsfjord (63°N) (Kukliński & Barnes, 2008); no survey reports could be found further north. The most southerly records are from the Iberian Peninsula, Spain (Ramos, 2010), as well as from the Alexandria coast of Egypt, Mediterranean Sea (Dorgham et al., 2013). Across this latitudinal gradient, Spirobranchus triqueter is likely to experience a range of temperatures from approximately 5 to 28°C (Seatemperature, 2015).

Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Ursin (1960) reported that Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11 to 19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998). In addition, maximum spawning occurs in spring, although individuals may spawn over a protracted period throughout the year. Gonad weight is at its maximum in February/March in the English Channel (Comely & Ansell, 1988; Hamed et al., 2024), but decreases during spawning in spring and then increases again through summer and winter until the next spawning season.

Caryophyllia smithii is found across the British Isles (NBN, 2015; Coolen et al., 2015) and has been recorded in Greece (Koukouras, 2010). In the Mediterranean, Caryophyllia smithii has been recorded in seawater between 13 and 14°C (Rodolfo‐Metalpa et al., 2015). It is therefore unlikely to be significantly affected by an increase at the benchmark level. However, Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns. Holt (pers. comm.) also suggested that long-term increases in temperature due to climate change may allow the parasitic barnacle Adna anglica to extend its range northwards and overlap the range of this biotope. Adna anglica is a southern species, limited to the southwest of Britain, where it parasitizes Caryophillia and has probably contributed to the decrease in abundance of Leptopsammia (Holt pers. comm.). It may impact the abundance of Caryophyllia if climate change allows it to extend its range northwards (Holt pers. comm.).

CR.MCR.EcCr.FaAlCr.Adig, CR.MCR.EcCr.FaAlCr.Sp, and CR.MCR.EcCr.FaAlCr.Sec are mostly restricted to the north of the British Isles, with some records from Wales (Pembrokeshire and Holyhead) and across the south coast of England. Sea surface temperature across this distribution ranges from northern to southern Sea Surface Temperature (SST) of 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013).

Sensitivity assessment

Spirobranchus triqueter records from the Alexandria coast of Egypt, Mediterranean Sea (Dorgham et al., 2013) indicate the species is unlikely to be affected at the benchmark level. An increase in sea surface temperature of 2°C for a period of one year, combined with high temperatures, may approach the upper temperature threshold of Alcyonium digitatum, Echinus esculentus, and/or Securiflustra securifrons, and may, therefore, cause minor declines in abundance. There is insufficient evidence to assess the effect of a short-term increase in temperature of 5°C on Alcyonium digitatum, however, it may disrupt Echinus esculentus spawning in southern examples of this biotope. Resistance has been assessed as ‘Medium’, and resilience has been assessed as ‘High’. Sensitivity has been assessed as ‘Low’.

Temperature decrease (local)

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

Evidence

Alcyonium digitatum is described as a northern species by Hiscock et al. (2004) but is distributed from Northern Norway (70°N) to Portugal (41°N) (Hartnoll, 1975; Budd, 2008). Across this latitudinal gradient, both species are likely to experience a range of temperatures from approximately 5 to 18°C. Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962 to 1963, when air temperature reached -5.8°C (Crisp, 1964). Temperature at the seafloor is an important predictor of Alcyonium digitatum, yet there are no published studies that explore the thermal minima or maxima for the species, so the only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Currently, Alcyonium digitatum is commonly found in inshore and offshore areas of northwest and northern France, the Channel Islands, most of the British Isles, including the Shetland Islands, parts of the southern North Sea, parts of southern Norway, and along the Atlantic slope (Jenkins & Stevens, 2022). Alcyonium digitatum can be found alongside Eunicella verrucosa, and in Britain and Ireland, Eunicella verrucosa has been observed in waters where 9.2°C was the lowest average seafloor temperature, the median temperature was 10.5°C, and the highest temperature was 11.4°C (Jenkins & Stevens, 2022).

Securiflustra securifrons is recorded from Kongsfjorden, Svalbard (Gontar et al., 2001) to the Iberian Peninsula in both Spain and Portugal (Ramos, 2010).

Ursin (1960) reported that Echinus esculentus occurred at temperatures between zero and 18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11 to 19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998). Echinus esculentus has been recorded from the Murmansk Coast, Russia. Due to the high latitude at which Echinus esculentus can occur, it is unlikely to be affected by a decrease in temperature at the pressure benchmark. In addition, maximum spawning occurs in spring, although individuals may spawn over a protracted period throughout the year. Gonad weight is at its maximum in February/March in the English Channel (Comely & Ansell, 1988; Hamed et al., 2024) but decreases during spawning in spring and then increases again through summer and winter until the next spawning season.

Spirobranchus triqueter is described as a temperate species by Kupriyanova & Badyaev (1998). Spirobranchus triqueter is recorded as abundant in sub-tidal habitats of Trondheimsfjord (63°N) (Kukliński & Barnes, 2008); no survey reports could be found further north. Averaged across several years, the lowest winter temperature within Trondheimsfjord is 4.9°C (Seatemperature, 2015). Below 7°C, Spirobranchus triqueter is unable to build calcareous tubes (Thomas, 1940). Mature adults may survive a decrease at the pressure benchmark; however, larvae may not be able to attach to the substrate (Riley & Ballerstedt, 2005) if a temperature decrease co-occurred with cold winter temperatures in the UK. However, settlement is reportedly low during winter (See resilience section), and therefore, the effects on recruitment are likely to be minor.

Caryophyllia smithii is a southern species (Fish & Fish, 1996) with a northern range limit in the Shetland Isles (NBN, 2015). It is therefore likely to be close to its northerly range limit and is likely to be negatively affected by a decrease in temperature at the benchmark level. 

CR.MCR.EcCr.FaAlCr.Adig, CR.MCR.EcCr.FaAlCr.Sp, and CR.MCR.EcCr.FaAlCr.Sec are mostly restricted to the north of the British Isles, with some records from Wales (Pembrokeshire and Holyhead) and across the south coast of England. Sea surface temperature across this distribution ranges from northern to southern Sea Surface Temperature (SST) ranges of 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013).

Sensitivity assessment

Alcyonium digitatum, Echinus esculentus & Securiflustra securifrons have northern/boreal distributions and are unlikely to be affected at the benchmark level. Spirobranchus triqueter is unable to build calcareous tubes at low temperatures, however, during winter, this is unlikely to have any significant effects on recruitment. Resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has been assessed as ‘Not sensitive’.

Salinity increase (local)

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

Evidence

Lyster (1965) tested the tolerance of Spirobranchus triqueter larvae to various hyper- and hypo-salinity treatments. Larvae were placed in cultures ranging from 0 to 90‰, and notes were made on the time taken for larvae to die or begin displaying abnormal behaviour. Spirobranchus triqueter larvae were tolerant of salinities ranging from 20 to 50‰; salinities above 50‰ caused high mortality. Spirobranchus triqueter is, therefore, unlikely to be affected at the pressure benchmark.

Echinoderms are generally stenohaline and possess no osmoregulatory organ (Boolootian, 1966). Therefore, an increase in salinity may cause Echinus esculentus mortality. Alcyonium digitatum’s distribution and the depth at which it occurs also suggest it would not likely experience regular salinity fluctuations and therefore tolerate significant increases in salinity. Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experiences salinities ranging from 30 to 33 psu, which is influenced by the fluctuating estuarine water inflow from the river Elbe (Becker et al., 2020).

Sensitivity assessment

The CR.MCR.EcCr.FaAlCr biotope and sub-biotopes are restricted to full salinity (Connor et al., 2004; JNCC, 2022), hence, an increase in salinity at the benchmark level would result in hypersaline conditions.  Therefore, seems likely that an increase in salinity to >40‰ may cause a decline in the abundance of Alcyonium digitatum, Echinus esculentus, Flustra folicacea, Securiflustra securifrons, and other component species. Hence, resistance has been assessed as ‘Low’ as a precaution. Resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’. Due to the lack of information regarding salinity effects on Alcyonium digitatum, Echinus esculentus & Securiflustra securifrons, confidence in this assessment has been assessed as low.

Salinity decrease (local)

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

Evidence

Alcyonium digitatum does inhabit situations such as the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where salinity may vary occasionally. Furthermore, Alcyonium digitatum is found within a number of variable salinity biotopes, e.g. MCR.BYH.Flu.Hocu. However, its distribution and the depth at which it occurs suggest that Alcyonium digitatum would not likely often experience salinity fluctuations and therefore is unlikely to survive significant reductions in salinity (Budd, 2008).

Echinoderms are generally unable to tolerate low salinity (stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). At low salinity, urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. However, within Echinus esculentus, there is some evidence to suggest intracellular regulation of osmotic pressure due to increased amino acid concentrations. Barrett et al. (2024) studied the effect of low salinity on Echinus esculentus. Experiments were conducted on populations of Echinus esculentus originally collected from Loch Linnhe, Scotland, and were exposed to low salinity over the short term (11‰, 16, 21, 26 and 31‰ for 24 hours) and long term (21, 26 and 31‰ for 25 days). Over the short term, oxygen consumption, activity coefficient and coelomic fluid osmolality were directly correlated with reduced salinity, with 100% survival at ≥21‰ and 0% at ≤16‰ (Barrett et al., 2024). Over the long term at 21‰ (25 days), oxygen consumption was significantly higher, feeding was significantly reduced, and activity coefficient values were significantly lower than at control salinity (31‰) (Barrett et al., 2024). Furthermore, beneficial functional resistance (righting ability and metabolic capacity) to acute low salinity was observed at 26‰. Barrett et al. (2024) concluded that Echinus esculentus demonstrates phenotypic plasticity that enabled acclimation to reduced salinity around 26‰; however, 21‰ represented a lower acclimation threshold, potentially limiting its distribution in coastal areas prone to high freshwater input. In addition, Echinus esculentus is found within a number of variable and reduced salinity biotopes, e.g. IR.LIR.KVS.SlatPsaVS (Connor et al., 2004).

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata (the class of Bryozoans which includes Securiflustra securifrons and Flustra foliacea) were fairly stenohaline and restricted to full salinity (35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna. Similarly, Dyrynda (1994) noted that Flustra foliacea were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experiences salinities ranging from 30 to 33 psu, which are influenced by the fluctuating estuarine water inflow from the river Elbe (Becker et al., 2020). Although protected from extreme changes in salinity due to their subtidal habitat, the introduction of freshwater or hyposaline effluents may adversely affect Flustra foliacea colonies.

Lyster (1965) tested the tolerance of Spirobranchus triqueter larvae to various hyper- and hypo-salinity treatments. Larvae were placed in cultures ranging from 0 to 90‰, and notes were made on the time taken for larvae to die or begin displaying abnormal behaviour. Spirobranchus triqueter larvae can survive very well in salinities down to 20‰ and can tolerate salinities down to 10‰. Adults are tolerant of salinities as low as 3‰ and can be found in areas where salinity ranges from 18 to 23‰ (Alexander et al., 1935).

Sensitivity assessment

The CR.MCR.EcCr.FaAlCr biotope and sub-biotopes are restricted to full salinity (Connor et al., 2004; JNCC, 2022), hence, a decrease in salinity at the benchmark level would result in reduced (18 to 30) conditions. The lack of records within “Reduced” salinity (18 to 30‰) suggests the community would not persist/be recognisable if salinity was reduced. Securiflustra securifrons is unlikely to tolerate low salinity environments. Spirobranchus triqueter is likely to be able to tolerate reduced salinity. Records from the MNCR suggest Alcyonium digitatum & Echinus esculentus can occur in reduced salinity habitats, however, the general evidence suggests that these species would decrease in abundance. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Medium’, and sensitivity as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

CR.MCR.EcCr.FaAlCr.Adig is recorded from weak-strong tidal streams (0.5 to 3 m/s), CR.MCR.EcCr.FaAlCr.Car is recorded from weak to very-weak (>1 m/s to negligible), and the other associated biotopes are recorded from very weak to moderately strong tidal streams (negligible to 3 m/s) (Connor et al., 2004).

Alcyonium digitatum, Caryophyllia smithii, and Spirobranchus triqueter are suspension feeders, relying on water currents to supply food (Hiscock, 1983; O’Reilly et al., 2022). These taxa, therefore, thrive in conditions of vigorous water flow, e.g. around Orkney and St Abbs, Scotland, where Alcyonium digitatum-dominated biotopes may experience tidal currents of 3 and 4 knots (approximately 1.5 m/s) during spring tides (De Kluijver, 1993; Coolen et al., 2015).

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to the high mass transport of water, such as the Menai Strait and tidal rapids, generally support large numbers of bryozoan species (Moore, 1977). Although active suspension feeders, their feeding currents are probably localised, and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability, and an increased risk of siltation (Tyler-Walters, 2005c).

Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams (Stebbing, 1971; Eggleston, 1972; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substratum due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current-swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. Flustra foliacea found in the German North Sea (Helgoländer Steingrund between 9 and 18 m deep) experiences tidal currents with a mean velocity of 0.6 kt, which is strong enough to saturate bottom waters with 8 to 10 mg/l of oxygen (Kühne and Rachor, 1996 cited in Becker et al., 2020).

Spirobranchus triqueter has been recorded in areas with very sheltered to exposed water flow rates (Price et al., 1980). Wood (1988) observed Spirobranchus sp. in strong tidal streams, and Hiscock (1983) found that in strong tidal streams or strong wave action where abrasion occurs, fast-growing species such as Spirobranchus triqueter occur.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Urchins are removed from the stipe of kelps by wave and current action. Echinus esculentus are also displaced by storm action. After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Therefore, increased water flow may remove the population from the affected area, probably to deeper water, although individuals would probably not be killed in the process and could recolonize the area quickly.

In flume experiments, Hiscock (1983) noted that the tentacles of Caryophyllia smithii were displaced by currents over ca 0.5 m/s but withdrawn at 0.75 m/s and took several hours to re-emerge after cessation of strong flow. Hiscock (1983) noted that Caryophyllia smithii was most abundant in semi-exposed and sheltered habitats. Other cup corals (Balanophyllia regia, Hoplangia durotrix, and Leptopsammia pruvoti) were recorded from weak and very weak tidal streams in cave and overhang biotopes (Connor et al., 2004). 

The life cycle of Caryophyllia smithii includes a larval planktotrophic stage with a duration of 8 to 10 weeks, and during this time, the released larvae float freely in the water column and are transported in the direction of net water movement, which is driven by tidal currents and wind. These residual currents in the North Sea, UK, range between 0.02 and 0.08 cm/s (Coolen et al., 2015). Caryophyllia smithii is described as favouring sites with a high tidal flow (Bell & Turner, 2000; Wood, 2005; Coolen et al., 2015). Caryophyllia smithii has been recorded in biotopes from negligible to strong water flow (0 to 6 knots; 0 to >3 m/s) (Connor et al., 2004). Rodolfo‐Metalpa et al. (2015) noted that Caryophyllia smithii was recorded in waters with a tidal current of 24 cm/s (±15) off the coast of Italy. This biotope consists mainly of species firmly attached to the substratum, which would be unlikely to be displaced by an increase in the strength of tidal streams at the benchmark level.

Sensitivity assessment

The CR.MCR.EcCr.FaAlCr biotope and sub-biotopes are recorded from moderately strong (0.5 to 1.5 m/sec) to very weak tidal streams and wave-exposed to moderately wave-exposed conditions. Echinus esculentus may become dislodged, but are unlikely to be killed, and may recolonize quickly. Bryozoan communities rely on the movement of water for feeding, and a severe reduction in water flow over an extended period could cause mortality. Most of the sub-biotopes occur in moderately strong to weak flow, but most records of the FaAlCr.Car sub-biotopes occur in very weak flow. However, in weak tidal flow, water movement is probably dominated by wave action. Therefore, a decrease in tidal velocity of 0.1 to 0.2 m/s for one year (the benchmark) is not likely to have a significant effect on the biological community within these biotopes. Hence, resistance has been assessed as ‘High’, and resilience has been assessed as ‘High’. Sensitivity has been assessed as ‘Not sensitive’.

Emergence regime changes

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

Evidence

Changes in emergence are 'Not relevant' to CR.MCR.EcCr.FaAlCr.Adig, CR.MCR.EcCr.FaAlCr.Spi, and CR.MCR.EcCr.FaAlCr.Sec, which are restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

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

Evidence

Alcyonium digitatum, Securiflustra securifrons, and Spirobranchus triqueter are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow. Jenkins & Stevens (2022) noted how seabed slope, temperature at the seafloor, and wave orbital velocity were important predictors of distribution in Alcyonium digitatum, and that, specifically, wave orbital velocity is more important than tidal velocity for bringing in fresh nutrients and oxygen, both for polyps to feed on and for exporting waste products.

Caryophyllia smithii has been recorded in very sheltered to extremely exposed biotopes (Connor et al., 2004; JNCC, 2015). Bell (2002) reported that Caryophyllia smithii near Lough Hyne (Ireland), exposed to strong wave action on open coasts, were relatively small, possibly due to juvenile morphological variability, as Caryophyllia smithii found deeper and in sediment were thinner and taller.

Flustra foliacea occurs from very wave-exposed to sheltered waters, although probably limited to deeper waters in very wave-exposed conditions (Tyler-Walters & Ballerstedt, 2007). The oscillatory water flow generated by wave action may be more damaging than constant strong currents, e.g. strong wave action may generate an oscillatory flow of 2 m/sec at 20 m (Hiscock, 1983, 1985). Flustra foliacea is a common member of the flotsam, having been removed from its substratum by storms. Whilst the biotope is circalittoral, a severe increase in wave exposure (e.g. storms) could affect bryozoan colonies, especially on mobile substrata such as cobbles and pebbles. Cocito et al. (1998) described how a severe winter storm in 1993 had devastating effects on the same Flustra foliacea population, sweeping away most of the colonies down to 11 m.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Urchins are removed from the stipe of kelps by wave and current action. Echinus esculentus are also displaced by storm action. After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Keith Hiscock (pers. comm.) reported that Echinus esculentus occurred in significant numbers as shallow as 15 m below low water at the extremely wave-exposed site of Rockall, Scotland. Therefore, localised increases in wave height may remove the population from the affected area, probably to deeper water, although individuals would probably not be killed in the process and could recolonize the area quickly.

Sensitivity assessment

Wave action is a fundamental environmental variable controlling the biological community of sub-littoral biotopes. The CR.MCR.EcCr.FaAlCr biotope and sub-biotopes are recorded from moderately strong (0.5 to 1.5 m/sec) to very weak tidal streams and exposed to moderately wave-exposed conditions. These biotopes are structured by grazing, probably primarily due to the urchin Echinus esculentus, but the differences in dominant species between sub-biotopes are probably due to variation in grazing pressure, wave action, tidal streams, depth, substratum type, and slope.

FaAlCr.Adig is recorded from extremely wave-exposed to moderately wave-exposed sites, FaAlCr.Spi, FaAlCr.Sec, and FaAlCr.Flu are recorded from exposed to moderately exposed sites, and FaAlCr.Car is recorded from exposed to sheltered wave exposure (Connor et al., 2004; JNCC, 2022). The depth of these biotopes also affects wave action, as wave energy is attenuated with depth. FaAlCr.Adig, FaAlCr.Flu, FaAlCr.Spi occur from 5 to 50 m, FaAlCr.Sec and FaAlCr.Car from 10 to 30 m.

An increase in wave exposure by one MNCR category (the benchmark) is unlikely to affect FaAlCr.Adig, which experiences the highest energy of the group, but a decrease may expose moderately exposed examples to wave-sheltered conditions and cause the biotope to transition to FaAlCr.Sec, unless the tidal streams are strong enough to dominate. Most of the characteristic species are long-lived, so unlikely to be lost during one year (at the benchmark duration), but opportunistic species may be able to increase in abundance. Hence, resistance is assessed as ‘Medium’ to reflect some changes in the biotope. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’.

An increase in wave exposure from exposed to very exposed may reduce grazing pressure in some examples of FaAlCr.Spi, FaAlCr.Sec, and FaAlCr.Flu, while a decrease to sheltered conditions may allow other opportunists to colonize while grazing may intensify. As above, the biotopes are unlikely to be lost within one year of a change in wave exposure, so resistance is assessed as Medium to reflect some changes in the biotope. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’.

FaAlCr.Car is the most sheltered and low-energy sub-biotope in this group. An increase in wave exposure (e.g. from exposed to very exposed) might favour transition to other FaAlCr biotopes, while a decrease (e.g. from sheltered to very sheltered) might favour transition to low-energy circalittoral biotopes, e.g. CR.LCR.BrAs.AmenCio. As above, the biotopes are unlikely to be lost within one year of change in wave exposure, so resistance is assessed as ‘Medium’ to reflect some changes in the biotope. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’.

Transition elements & organo-metal contamination

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

Evidence

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

No information on the direct biological effects of heavy metal contamination on Alcyonium digitatum. Possible sub-lethal effects of exposure to heavy metals may result in a change in morphology, growth rate or disruption of the reproductive cycle. The vulnerability of this species to concentrations of pollutants may also depend on variations in other factors, e.g. temperature and salinity conditions outside the normal range. Based on the available evidence for several species, Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals.

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, 1979; Holt et al., 1995). Bryozoans were shown to bioaccumulate heavy metals to a certain extent (Holt et al., 1995). For example, Bowerbankia gracialis and Nolella pusilla accumulated Cd, exhibiting sublethal effects (reduced sexual reproduction and inhibited resting spore formation) between 10-100 µg Cd /l and fatality above 500 µg Cd/l (Kayser, 1990).

Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were sensitive to heavy metal contamination, for example, Migliaccio et al. (2014) reported that exposure of Paracentrotus lividis larvae to increased levels of cadmium and manganese caused abnormal larval development and skeletal malformations. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu).

Hydrocarbon & PAH contamination

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

Evidence

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

CR.MCR.EcCr.FaAlCr, CR.MCR.EcCr.FaAlCr.Spi & CR.MCR.EcCr.FaAlCr.Sec are sub-tidal biotopes (Connor et al., 2004). Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sublittoral habitats (Castège et al., 2014). Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16 m in the locality of Sennen Cove, Cornwall, which was likely a result of toxic detergents sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (Budd, 2008).

At the time of writing, little information on the effects of hydrocarbons on bryozoans was found. Ryland & Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. in Milford Haven or St. Catherine's Island, south Pembrokeshire, although it did alter the breeding period.

Large numbers of dead polychaetes and other fauna were washed up at Rulosquet marsh near Isle de Grand following the Amoco Cadiz oil spill in 1978 (Cross et al., 1978). However, no information was found relating to Spirobranchus triqueter in particular.

Echinus esculentus is subtidal and unlikely to be directly exposed to oil spills. However, as with the ‘Prestige’ oil spill, rough seas can cause mixing with the oil and the seawater, and therefore, sub-tidal habitats can be affected by the oil spill. Castège et al. (2014) recorded the recovery of rocky shore communities following the Prestige oil spill, which impacted the French Atlantic coast. Rough weather at the time of the spill increased mixing between the oil and seawater, causing subtidal communities/habitats to be affected. The urchin Echinus esculentus was reported absent after the oil spill, however, it returned after 2-5 years. Large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen Cove, presumably due to a combination of wave exposure and heavy spraying of dispersants following the ‘Torrey Canyon’ oil spill (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in its echinopluteus larvae. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez, 1999).

Synthetic compound contamination

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

Evidence

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

Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16 m in the locality of Sennen Cove, Cornwall, resulting from the offshore spread and toxic effect of detergents (a mixture of a surfactant and an organic solvent), e.g. BP 1002 sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill. Possible sub-lethal effects of exposure to synthetic chemicals may result in a change in morphology, growth rate or disruption of the reproductive cycle. The vulnerability of this species to concentrations of pollutants may also depend on variations in other factors, e.g. temperature and salinity conditions outside the normal range (Budd, 2008).

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, 1979; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoa with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100 ng/l TBT. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the 5 years since TBT was banned from use on small vessels. This last report suggests that bryozoans may be at least inhibited by the presence of TBT. Hoare & Hiscock (1974) suggested that polyzoa (bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur less than 165 m from the effluent source. The evidence, therefore, suggests that Securiflustra securifrons would be sensitive to synthetic compounds.

Large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area following the Torrey Canyon oil spill (Smith, 1968). Smith (1968) also demonstrated that 0.5 to 1 ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez, 1999).

Radionuclide contamination

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

Evidence

No Evidence was found.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

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

Evidence

There is anecdotal evidence to suggest that Alcyonium digitatum is sensitive to hypoxic events. However, the degree of deoxygenation was not quantified, and the evidence cannot be compared to the pressure benchmark. There was insufficient evidence to assess the sensitivity of Securiflustra securifrons or Spirobranchus triqueter.

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l, or even less (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995).

Alcyonium digitatum mainly inhabits environments in which the oxygen concentration usually exceeds 5 ml/l, and respiration is aerobic (Budd, 2008). In August 1978, a dense bloom of a dinoflagellate, Gyrodinium aureolum, occurred surrounding Geer Reef in Penzance Bay, Cornwall and persisted until September that year. Observations by local divers indicated a decrease in underwater visibility (<1 m) from below 8 m Below Sea Level. It was also noted that many of the faunal species appeared to be affected, e.g. no live Echinus esculentus were observed, whereas on surveys prior to August they were abundant; Alcyonium sp. and bryozoans were also in an impoverished state. During follow-up surveys conducted in early September, Alcyonium sp. was noted to be much healthier and feeding. It was suggested that the decay of Gyrodinium aureolum either reduced oxygen levels or physically clogged faunal feeding mechanisms. Adjacent reefs were also surveyed during the same period, and the effects of the Gyrodinium aureolum bloom were less apparent. It was suggested that higher water agitation in shallow water on reefs more exposed to wave action were less affected by the phytoplankton bloom (Griffiths et al., 1979).

Bell (2002) reported that an oxycline at Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, (<5% surface concentration; ca 0.5 mg/l) limited vertical colonization by Caryophillia smithii. Micaroni et al. (2025) conducted further studies at Lough Hyne (2018 to 2021), mainly focusing on an unknown disturbance event (possibly relating to deoxygenation) which caused a large decline of sponges between 2010 and 2015. However, Caryophyllia smithii was mentioned as one of the most stable cnidarians in the studied communities, with low temporal fluctuations, likely due to their slow growth, low recruitment, and low mortality rates (Micaroni et al., 2025).

Little information on the effects of oxygenation on bryozoans was found. Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans were unaffected by periods of moderate hypoxia (ca 0.35 to 1.4 ml/l which corresponds to ca 0.5 to 2 mg/l) and short periods of hypoxia (<0.35 ml/l which corresponds to <0.5 mg/l) in the York River, Chesapeake Bay, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. 

Hiscock & Hoare (1975) reported an oxycline forming in the summer months (June to September) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation below ca 10 m. During these summer events, no echinoderms were recorded at depths below 10 to 11 m. 

Sensitivity assessment

The CR.MCR.EcCr.FaAlCr biotope and sub-biotopes are recorded in moderate energy conditions from moderately strong (0.5 to 1.5 m/sec) to very weak tidal streams and wave-exposed to moderately wave-exposed conditions (Connor et al., 2004; JNCC, 2022). The high-water movement, which is indicative of these biotopes, is likely to increase mixing with surrounding oxygenated water (Griffiths et al., 1979) and may, therefore, decrease the effects of deoxygenation. In addition, whilst the majority of assessed species are sessile, Echinus esculentus is mobile and may escape the hypoxic event (depending on the extent and conditions). However, the evidence from Griffiths et al. (1979) suggests that grazing echinoderms such as Echinus may be affected. Therefore, a resistance of ‘Medium’ is suggested. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’.

Nutrient enrichment

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

Evidence

Alcyonium digitatum, Securiflustra securifrons & Spirobranchus triqueter are suspension feeders of phytoplankton and zooplankton. Nutrient enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to Alcyonium digitatum, Securiflustra securifrons & Spirobranchus triqueter in terms of an increased food supply, but the effects are uncertain (Hartnoll, 1998). The survival of Alcyonium digitatum, Securiflustra securifrons & Spirobranchus triqueter may be influenced indirectly. High primary productivity in the water column combined with high summer temperature and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters, which faunal species are likely to be highly intolerant of (see de-oxygenation pressure).

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified in all habitats exposed to the contaminant types.

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Nutrient enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton, and microalgae, although individuals died at an early age. 

Sensitivity assessment

Limited evidence on the effects of nutrient enrichment on the characteristic species was found, but it suggests that the characteristic species vary in their response to nutrients. However, the evidence is ‘Insufficient’ to form the basis of an assessment.

Organic enrichment

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

Evidence

Alcyonium digitatum and Spirobranchus triqueter are suspension feeders of phytoplankton and zooplankton. Organic enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to Alcyonium digitatum and Spirobranchus triqueter in terms of an increased food supply, but the effects are uncertain (Hartnoll, 1998). The survival of Alcyonium digitatum and Spirobranchus triqueter may be influenced indirectly. High primary productivity in the water column combined with high summer temperature and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters (see de-oxygenation pressure).

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness was identified in all habitats exposed to the contaminant types.

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Organic enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.

Sensitivity assessment

Organic enrichment is not likely to directly affect the characterizing species within this biotope, however, chronic organic enrichment may cause secondary effects such as hypoxia. Resistance has been assessed as ‘Medium’, resilience as ‘High ’, and sensitivity has been assessed as ‘Low’.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

If rock, boulders, or cobbles were replaced with sediment, this would represent a fundamental change to the physical character of the parent and child biotopes, and the species would be unlikely to recover. The biotope would be lost. The characterizing species (except Echinus esculentus) each require a hard substratum to attach to, such as rock, steel, and other coralligenous or hard formations (Langhamer, 2016; Chava & Mokievsky, 2022; Jenkins & Stevens, 2022). However, Spirobranchus triqueter is also known to colonize floating marine debris, mostly favouring plastics (Gündogdu, Çevik & Karaca, 2017; Abelouah et al., 2024). In addition, Spirobranchus triqueter can colonize mobile hard substrata. In Northern Norway, Spirobranchus triqueter had a coverage range of 20 to 73% on Buccinum sp. and Littorina sp. shells utilized by Pagurus pubescens (Balazy & Kuklinski, 2018). Settling on mobile species or floating marine debris may aid Spirobranchus dispersal and recovery.

Alcyonium digitatum is also capable of settling on other substrata, including shells, cobble and other (unstable) coarse substrates (Jenkins & Stevens, 2022). High terrain ruggedness index (TRI) values are often associated with hard substrates, which may explain the positive relationship between the density of records and TRI. Areas with a high TRI would indicate a more complex seabed with local topographic highs, which coral species have been found to prefer (Langton, Stirling & Boulcott, 2023). A study by Becker et al. (2020) focusing on the German North Sea (Helgoländer Steingrund between 9 and 18 m deep) noted how Alcyonium digitatum had a significant positive correlation with hard ground greater than 22%. Becker et al. (2020) also stated that Flustra foliacea dominated the surface structure of bottom waters (meaning coverage of nearly 100%), being significantly positively correlated with hard ground (25%) and shell <2 mm more than 0.05 weight% (Becker et al., 2020); in the same region, Michaelis et al. (2019) noted how Flustra foliacea largely colonized boulder-sized stones. In addition, the probability of occurrence of Echinus esculentus was highest in areas with hard ground greater than 19% and shell >2 mm, more than 0.04 weight% (Becker et al., 2020). While observing an undersea pipeline in the North Sea, Rouse et al. (2019) recorded Alcyonium digitatum at its highest densities on pipelines located on mud, while Echinus esculentus were more common on pipelines in sand.

Furthermore, Flustra foliacea growing on ‘complex’ artificial reefs (in the Loch Linnhe Artificial Reef complex on the west coast of Scotland) experienced productivity rates that were 2.4 times higher than those growing on ‘simple’ reefs, which had a smaller surface area (Rouse, Porter & Wilding, 2020). Productivity rates were highest on external areas of reefs and decreased by 1.56% per cm distance into the reef on complex reefs and 2.93% per cm into the reef on simple block reefs (Rouse, Porter & Wilding, 2020). The differences in productivity rates between reefs constructed from simple and complex blocks were assumed to reflect different current regimes and food supply between the external and internal reef areas, according to reef type (Rouse, Porter & Wilding, 2020). More complex habitats could encourage the growth and recovery of Flustra foliacea.

Sensitivity assessment

Nevertheless, loss of hard substrata would significantly alter the character of the biotope and its reclassification. Hence, resistance to the pressure is considered ‘None’, resilience is ‘Very low’ (permanent change), and sensitivity is assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

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) (Removal of substratum pressure definition). 

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) (Surface abrasion/disturbance pressure definition).

Evidence

Alcyonium digitatum, Echinus esculentus, Securiflustra securifrons & Spirobranchus triqueter are sedentary or slow-moving species that might be expected to suffer from the effects of dredging. Boulcott & Howell (2011) conducted experimental Newhaven scallop dredging over a circalittoral rock habitat in the Sound of Jura, Scotland and recorded the damage to the resident community. The results indicated that the sponge Pachymatisma johnstoni was highly damaged by the experimental trawl. However, only 13% of photographic samples showed visible damage to Alcyonium digitatum. Where Alcyonium digitatum damage was evident, it tended to be small colonies that were ripped off the rock. The authors highlight that physical damage to faunal turfs (erect bryozoans and hydroids) was difficult to quantify in the study. However, the faunal turf communities did not show large signs of damage and were only damaged by the scallop dredge teeth, which was often limited in extent (approximately 2 cm wide tracts). The authors indicated that species such as Alcyonium digitatum and faunal turf communities were not as vulnerable to damage through trawling as sedimentary fauna, and whilst damage to circalittoral rock fauna did occur, it was of an incremental nature, with the loss of species such as Alcyonium digitatum and faunal turf communities increasing with repeated trawls. Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the Newhaven dredges. A fully loaded Newhaven dredge may cause higher damage to the community, as indicated in their study.

Alcyonium digitatum has been documented as locally depleted (reductions in colony numbers and size) in some areas due to benthic trawling, such as Lyme Bay, before the trawling ban (Holland, Jenkins, & Stevens, 2017). Magorrian & Service (1998) reported that trawling for queen scallops resulted in the removal of emergent epifauna and damage to horse mussel beds in Strangford Lough. They suggested that the emergent epifauna, such as Alcyonium digitatum, were more intolerant than the horse mussels themselves and reflected early signs of damage (Service & Magorrian, 1997; Magorrian & Service, 1998; Service, 1998). Veale et al. (2000) reported that the abundance, biomass, and production of epifaunal assemblages, including Alcyonium digitatum, decreased with increasing fishing effort. However, the fact that Alcyonium digitatum was more abundant on high fishing effort grounds suggested that this seemingly fragile species was more resistant to abrasive disturbance than might be assumed (Bradshaw et al., 2000), presumably owing to the ability for the replacement of senescent cells and regeneration of damaged tissue, in addition to the early larval colonization of available substrata.

Picton & Goodwin (2007) noted that an area of boulders with a rich fauna of sponges and hydroids on the east coast of Rathlin Island, Northern Ireland, was significantly altered since the 1980s. Scallop dredging had begun in 1989, and boulders were observed to have been turned and the gravel harrowed. In addition, many of the boulders had disappeared, and rare hydroid communities were greatly reduced (Picton & Goodwin, 2007). Prior records also indicated the presence of large sponges, mainly Axinella infundibuliformis (Picton & Goodwin, 2007).

Staniforth et al. (2023) studied the decadal-scale temporal change in epibenthic megafaunal assemblages on scallop fishing grounds in the Bay of Fundy. Previous data from 1967 to 1997 showed that the epibenthic assemblage in the Digby scallop ground had become more homogeneous over the 30-year interval, with attached, fragile, epifaunal taxa declining, whereas scavengers and robust burrowing filter-feeders increased in frequency of occurrence (Staniforth et al., 2023). Staniforth et al. (2023) resampled the Digby ground along with the commercial scallop grounds around Grand Manan Island and on Lurcher Shoal in 2007 and 2008 to determine whether the changes off Digby had continued and if they were also occurring in other parts of the Bay. They observed significant spatial differences among the three scallop grounds, including the leafy bryozoan, Flustra foliacea, which was expanding its local range through time and was the leading contributor to the spatial dissimilarity in the Bay of Fundy (Staniforth et al., 2023). For example, Flustra foliacea was the greatest contributor to the dissimilarity in assemblage composition, which was found at 97% of the stations off Digby but not at all around Grand Manan, nor on Lurcher Shoal in 1997, although it was present at a minority of the stations there ten years later, such as making up to 14% of the observed taxa in 2007 and 2008 at Lurcher Shoal (Staniforth et al., 2023). In addition, significantly more taxa were recorded per station in 1997 than in 2007 and 2008, while assemblage compositions were significantly different between the two periods (Staniforth et al., 2023). Staniforth et al. (2023) concluded that decadal-scale declines in the frequencies of occurrence of attached, epibenthic filter-feeders were observed, continuing the trend previously reported and consistent with ongoing impacts of bottom-contact fishing gears, despite decades of trawling and dredging. Therefore, although fishing in the Bay of Fundy has negatively impacted some epibenthic taxa, Flustra foliacea is one such species that seems to be benefiting from the regular abrasion occurring in the area.

Species with fragile tests, such as Echinus esculentus, were reported to suffer badly because of scallop or queen scallop dredging (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000). Kaiser et al. (2000) reported that Echinus esculentus were less abundant in areas subject to high trawling disturbance in the Irish Sea. Jenkins et al. (2001) conducted experimental scallop trawling in the North Irish Sea and recorded the damage caused to several conspicuous megafauna species, both when caught as by-catch and when left on the seabed. The authors reported that 16.4% of Echinus esculentus were crushed/dead, 29.3% would have >50% spine loss/minor cracks, 1.1% would have <50% spine loss, and the remaining 53.3% would be in good condition. Sea urchins can rapidly regenerate spines, e.g. Psammechinus miliaris were found to re-grow all spines within a period of two months (Hobson, 1930). The trawling examples mentioned above were conducted on sedimentary habitats, and thus the evidence is not directly relevant to the rock-based CR.MCR.EcCr.FaAlCr biotopes, but does indicate the likely effects of abrasion on Echinus esculentus.

Sensitivity assessment

The FaAlCr sub-biotopes are biotopes are dominated by epifauna, so damage and mortality following a physical disturbance are likely to be significant. However, some studies have brought into question the extent of damage to the faunal turf, so resistance will vary between sub-biotopes. The physiology of the bryozoans affords some protection in the event of abrasion events, and recovery is likely to be rapid if stolons remain undamaged. Where present, hydroids have rapid growth rates, potentially high recruitment, and can recover quickly from fragments or dormant resting stages. The above evidence suggests that Alcyonium dominated sub-biotopes FaAlCr.Adig and FaAlCr.Ser would probably lose a proportion of the Alcyonium population and probably has a ‘Low’ resistance, ‘High’ resistance, and ‘Low’ sensitivity to abrasion. The evidence suggests that Flustra and other faunal turfs were little damaged by fishing gear over rocky habitats (Boulcott & Howell, 2011), so the resistance of FaAlCr.Spi, and FaAlCr.Car is also probably a ‘Low’ resistance, ‘High’ resistance, and ‘Low’ sensitivity to abrasion. However, examples of FaAlCr.Flu that occur on mixed substrata (cobbles and pebbles) may be less resistant due to mobilization of the substratum by passing gears.

Nevertheless, abrasion due to scallop dredging was reported to cause damage or mortality to nearly 50% of Echinus esculentus (Jenkins et al., 2001), and fragile species such as Echinus or Echinocardium were reported to suffer badly from scallop dredging. Therefore, the resistance of Echinus is probably ‘Low’, with a resistance of ‘Medium’, and a sensitivity of ‘Medium’ to abrasion. Most importantly, the FaAlCr biotope and its sub-biotopes are structured by grazing, primarily by Echinus, so its loss from the biotope could result in significant changes to the biotopes. Therefore, the overall FaAlCr biotope and sub-biotopes resistance is assessed as ‘Low’, with resilience of ‘Medium’ determined by worst-case recovery of the Echinus population, resulting in a sensitivity of ‘Medium’ to abrasion.

Penetration or disturbance of the substratum subsurface

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

Evidence

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. This pressure is not thought to be relevant to hard rock biotopes.

Changes in suspended solids (water clarity)

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

Evidence

The characteristic epifauna (faunal turfs) (e.g. Alcyonium digitatum, Flustra foliacea, Securiflustra securifrons, Caryophyllia smithii & Spirobranchus triqueter) are not thought to be highly susceptible to changes in water clarity because they are suspension-feeding organisms and are not directly dependent on sunlight for nutrition.

Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucus. Alcyonium digitatum is also known to inhabit the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where water clarity is likely to be highly variable.

Bell & Turner (2000) studied populations of Caryophyllia smithii at three sites of differing sedimentation regimes in Lough Hyne, Ireland. Calyx size was largest at the site of least sedimentation and smallest at the site of most sedimentation. In contrast, the height of individuals was greatest at the site of most sedimentation and smallest at the site of least sedimentation. The height of individuals correlated with the level of surrounding sediment. High density was correlated with high sedimentation and depth (Bell & Turner, 2000).

Moore (1977) suggested that Echinus esculentus was unaffected by turbid conditions. Echinus esculentus is an important grazer of red macro-algae within the CR.MCR.EcCr habitat complex. Increased turbidity and resultant reduced light penetration is likely to negatively affect algal growth. However, Echinus esculentus feeds on a variety of prey including epifauna, detritus or dissolved organic material (Lawrence, 1975; Comely & Ansell, 1988).

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. However, Tyler-Walters & Ballerstedt (2007) reported Flustra foliacea as tolerant to increased suspended sediment based on its occurrence in areas of high suspended sediment, e.g. abundant in turbid, fast-flowing waters of the Menai Straits (Moore 1977). Also, communities dominated by Flustra foliacea were described on tide-swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel (Holme & Wilson, 1985). Flustra foliacea is also characteristic of sediment-scoured, silty rock communities CR.HCR.XFa.FluCoAs and CR.MCR.EcCr.UrtScr (Connor et al., 2004), and is also found at greater depths, where organic particulates (detritus) are probably more important.

According to Bacescu (1972), sabellids are accustomed to turbidity and silt. Spirobranchus triqueter has also recently been recorded by De Kluijver (1993) from Scotland in the aphotic zone, indicating that the species would not be sensitive to an increase in turbidity.

Sensitivity assessment

Resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’. 

Smothering and siltation rate changes (light)

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

Evidence

Smothering by 5 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c). While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Alcyonium digitatum, Flustra foliacea, Securiflustra securifrons & Spirobranchus triqueter are sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Some Alcyonium digitatum colonies can attain a height of up to 20 cm (Edwards, 2008), and Securiflustra securifrons colonies can attain a height of 10 cm (Porter, 2012), so they would still be able to feed in the event of sediment deposition. However, Spirobranchus triqueter are an encrusting species and would thus likely be smothered, and depending on sediment retention, could block larval settlement.

Holme & Wilson (1985) examined the bottom fauna in a tide-swept region of the central English Channel. Flustra foliacea-dominated communities were reported to form in areas subject to sediment transport (mainly sand) and periodic, temporary submergence by thin layers of sand (ca <5 cm).

Caryophyllia smithii is a small (approx. <3 cm height from the seabed) species and would therefore likely be inundated in a “light” sedimentation event. Coolen et al. (2015) noted how a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. For example, in the Skomer Island, UK, Marine Conservation Zone, higher numbers of Caryophyllia smithii were observed on vertical walls, likely due to less surface sediment accumulating there (Lock et al., 2025). However, Bell & Turner (2000) reported Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7 mm ± 0.5 mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 5 cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope. Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover. Bell (2002) reported that juvenile Caryophyllia smithii are morphologically variable and initially undergo rapid growth with tall and thin forms in deeper, sheltered, relatively sedimented conditions near Lough Hyne, Ireland. It was concluded that this was to escape the thin layer of sediment present. Hiscock (1983) reported that Caryophyllia smithii was able to clear the surface of the polyp of silt using ciliary cleaning mechanisms. It was also able to survive for five days smothered by fine sediment (mud) (Hiscock, 1983).

Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2008). Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be killed by smothering, however, smaller specimens and juveniles may be less resistant. A layer of sediment may interfere with larval settlement. If retained within the host biotope for extended periods, a layer of 5 cm of sediment may negatively affect successive recruitment events.

Sensitivity assessment

The FaAlCr biotopes are recorded from vertical and upper faces of circalittoral rock. Those examples of the biotope that occur on vertical faces are unlikely to be susceptible to smothering due to deposition of fine sediments. Examples of biotopes that occur on upper faces, or that occur only on upper faces (FaAlCr.Flu and FaAlCr.Spi) may be more vulnerable. However, FaAlCr.Flu is associated with sediment scour and is probably resistant to smothering. In addition, all the FaAlCr biotopes are associated with moderately strong water movement, so deposited sediment is unlikely to be retained for more than a few tidal cycles. Hence, resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has therefore been assessed as ‘Not Sensitive’.

Smothering and siltation rate changes (heavy)

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

Evidence

Smothering by 30 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c). While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Alcyonium digitatum, Securiflustra securifrons & Spirobranchus triqueter are sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Alcyonium digitatum colonies can attain a height of up to 20 cm (Edwards, 2008), Securiflustra securifrons colonies can attain a height of 10 cm (Porter, 2012), and Spirobranchus triqueter are encrusting species. Echinus esculentus are large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2008). Therefore, it is likely that all characterizing species that occur on upper faces within the FaAlCr biotopes would be totally inundated.

Holme & Wilson (1985) examined the bottom fauna in a tide-swept region of the central English Channel. Flustra foliacea-dominated communities were reported to form in areas subject to sediment transport (mainly sand) and periodic, temporary submergence by thin layers of sand (ca <5 cm). If inundated by 30cm of sediment, respiration and larval settlement are likely to be blocked until the deposited sediment is removed.

Caryophyllia smithii is a small (approx. <3 cm height from the seabed) species and would therefore likely be inundated in a “light” sedimentation event. Coolen et al. (2015) noted how a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. For example, in the Skomer Island, UK, Marine Conservation Zone, higher numbers of Caryophyllia smithii were observed on vertical walls, likely due to less surface sediment accumulating there (Lock et al., 2025). However, Bell & Turner (2000) reported Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7 mm ± 0.5 mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 5 cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope. Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover. Bell (2002) reported that juvenile Caryophyllia smithii are morphologically variable and initially undergo rapid growth with tall and thin forms in deeper, sheltered, relatively sedimented conditions near Lough Hyne, Ireland. It was concluded that this was to escape the thin layer of sediment present. Whilst most of the characterizing species are likely to be buried in 30 cm of sediment deposition, the biotope occurs on vertical rock in high-energy conditions, and burial is unlikely.

Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2008). Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be killed by smothering, however, smaller specimens and juveniles may be less resistant. A layer of sediment could interfere with larval settlement. If sediment is retained within the host biotope for extended periods, a layer of sediment may negatively affect successive recruitment events.

Sensitivity assessment

The FaAlCr biotopes are recorded from vertical and upper faces of circalittoral rock. Those examples of the biotope that occur on vertical faces are unlikely to be susceptible to smothering due to deposition of fine sediments. Examples of biotope that occur on upper faces, or that occur only on upper faces (FaAlCr.Flu and FaAlCr.Spi) may be more vulnerable. However, FaAlCr.Flu is associated with sediment scour and is probably resistant to smothering. In addition, all the FaAlCr biotopes are associated with moderately strong water movement, so 30 cm of deposited sediment is likely to be removed within several tidal cycles, depending on the local conditions. Hence, resistance has been assessed as ‘Medium’ as a precaution, and resilience as ‘High’. Sensitivity has therefore been assessed as ‘Low’.

Litter

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

Evidence

All characterizing species for this biotope are sessile epifauna or slow-moving, being either encrusting, branching or cup-like, or having a fragile test. Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increased when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018). Both Sheehan et al. (2017) and Giusti et al. (2019) highlight how, in addition to the direct damage from fishing, ghost fishing may also be responsible for some coral mortality, Eunicella verrucosa in this case, either through direct damage or by making them more vulnerable to removal from their anchorage to the sea floor, particularly during storms.

In addition, Spirobranchus triqueter is also known to colonize floating marine debris, mostly favouring plastics (Gündogdu, Çevik & Karaca, 2017; Abelouah et al., 2024). Settling on floating marine debris may aid Spirobranchus dispersal and recovery.

There are no records of ghost fishing affecting the characterizing species for this biotope. However, epifaunal communities are vulnerable to damage from fishing gear and are likely to be dislodged or damaged through lost fishing gear, and possibly certain types of marine litter. At present, there is 'Insufficient evidence' to complete a sensitivity assessment on the effect of litter on this biotope.

Electromagnetic changes

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

Evidence

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

One study exists on the effect of EMFs on Echinus esculentus. Chapman et al. (2023) simulated an EMF of 500 μT, as modelled for an export cable over a rocky shore, where the industry standard cable burial would not be possible. However, no significant differences were found in either behavioural or physiological responses on Echinus esculentus. A similar study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of a static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity, indicative of a stress response, were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

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

Evidence

Alcyonium digitatum, Echinus esculentus, Securiflustra securifrons, and Spirobranchus triqueter have no hearing perception, but vibrations may cause an impact, however, no studies exist to support an assessment (where relevant).

Introduction of light or shading

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

Evidence

Although no evidence was found for the effect of light on the biotopes characterizing species, it is well understood that light influences the spawning of tropical corals, along with other environmental cues such as solar insolation, day length, and temperature (Davies et al., 2023; Egger et al., 2025). However, for temperate and intermediate-water species, some of these cues may be absent or differ significantly. Many cold-water corals live beyond the reach of moonlight, but the species in this biotope, Caryophyllia smithii, can be found within the first 200 m of depth where light, both natural and artificial, would reach (Holland, Jenkins, & Stevens, 2017; Egger et al., 2025). In addition, shading of light or the introduction of light within the first 50 m could influence marine organisms, such as triggering early coral spawning or affecting the opening and reproduction rhythm of bivalves (Charifi et al., 2023; Davies et al., 2023; Smyth et al.,2021). Below 200 m, it is unlikely that these species would be impacted, as the light level that reaches beyond this point is very low and unsuitable for photosynthesis.

Although there is some evidence that the basiepithelial nerve plexus below the entire outer skin of echinoderms is sensitive to light (Hill, 2008), there is no evidence to suggest the impact of light on echinoderms.

Sensitivity assessment

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

Barrier to species movement

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

Evidence

Not relevant: barriers and changes in tidal excursion are not relevant to biotopes restricted to open waters.

Death or injury by collision

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

Evidence

Not relevant to seabed habitats. NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

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

Evidence

Not relevant

Genetic modification & translocation of indigenous species

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

Evidence

Alcyonium digitatum or Spirobranchus triqueter are not cultivated or translocated. Echinus esculentus was identified by Kelly & Pantazis (2001) as a species suitable for culture for the urchin roe industry. However,no evidence was found to suggest that significant Echinus esculentus mariculture was present in the UK. If industrially cultivated it is feasible that Echinus esculentus individuals could be translocated.

Translocation also has the potential to transport pathogens to uninfected areas (see pressure ‘introduction of microbial pathogens’). The sensitivity of the ‘donor’ population to harvesting to supply stock for translocation is assessed for the pressure ‘removal of target species’.

Introduction of microbial pathogens

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

Evidence

There was 'no evidence' to suggest that any of the characterizing species within CR.MCR.EcCr.FaAlCr biotopes are sensitive to current/known microbial pathogens.

Alcyonium digitatum acts as the host for the endoparasitic species Enalcyonium forbesiand and Enalcyonium rubicundum (Stock, 1988). Parasitisation may reduce the viability of a colony but not to the extent of killing them but no further evidence was found to substantiate this suggestion. Thomas (1940) recorded parasites of Spirobranchus triqueter. Trichodina pediculus (a ciliate) was observed in high numbers moving over the branchial crown. However, this relationship is symbiotic, not parasitic. Parasites found in the worm include gregarines & ciliated protozoa and parasites that had the appearance of sporozoan cysts. However, no information was found about the effects of microbial pathogens on Spirobranchus triqueter.

Echinus esculentus is susceptible to 'Bald-sea-urchin disease', which causes lesions, loss of spines, tube feet, pedicellariae, destruction of the upper layer of skeletal tissue and death. It is thought to be caused by the bacteria Vibrio anguillarum and Aeromonas salmonicida. Bald sea-urchin disease was recorded from Echinus esculentus on the Brittany Coast. Although associated with mass mortalities of Strongylocentrotus franciscanus in California and Paracentrotus lividus in the French Mediterranean it is not known if the disease induces mass mortality (Bower, 1996).

Removal of target species

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

Evidence

None of the characterizing species within CR.MCR.EcCr.FaAlCr or CR.MCR.EcCr.FaAlCr.Spi are commercially exploited. This pressure is considered ‘Not Relevant’.

Removal of non-target species

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

Evidence

Alcyonium digitatum and faunal turf communities (which include bryozoans such as Securiflustra securifrons) are probably resistant to abrasion through bottom fishing (see abrasion pressure). Alcyonium digitatum goes through an annual cycle, From February to July all Alcyonium digitatum colonies are feeding, from July to November an increasing number of colonies stop feeding. During this period a large number of polyps can retract and a variety of filamentous algae, hydroids and amphipods can colonize the surface of colonies epiphytically. From December-February the epiphytic community is however sloughed off (Hartnoll, 1975). If Alcyonium digitatum were removed the epiphytic species would likely colonize rock surfaces and are therefore not dependant on Alcyonium digitatum.

Sensitivity assessment.  The sessile fauna probably compete for space, however, there isn’t any evidence to suggest other interspecific relationships or dependencies between these species. Therefore, removal of one or a number of these species would provide colonization space and most likely benefit the species with rapid colonization rates (e.g. Spirobranchus triqueter). Echinus esculentus is an important red algal grazer (Connor et al., 2004), without which the abundance of red algae may increase and possibly displace some of the faunal turf species.

Sensitivity assessment. If Alcyonium digitatum, Spirobranchus triqueter or Echinus were removed (e.g. as incidental by-catch) the character of the biotope would probably change. Therefore, resistance has been assessed as ‘Low’, resilience  as ’High’ and sensitivity has been assessed as ‘Low’.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the 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 at 60 metres 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 a gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders. 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 (0.1-0.9/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 wave-exposed circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata. Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). It has been recorded from the lower intertidal to ca 160 m in depth, but it is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). Therefore, colonization of Crepidula would be limited to low densities in deeper examples of the biotope. However, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral 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 the water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests 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, 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 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and 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 are enough to cause dislodgement (Reinhardt et al., 2012). 

Sensitivity assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope, and child biotopes, occurs on bedrock, boulders, or cobbles, 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, and child biotopes, experiences very weak to strong water flow (1 to 6 m/s) and sheltered to extremely exposed 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). The FaAlCr biotopes are characterized by grazing, primarily due to Echinus esculentus, a general forager and grazer of faunal and floral turfs. Switzer et al. (2011) reported that green (Strongylocentrotus droebachiensis) and red (Strongylocentrotus franciscanus) sea urchins consumed this tunicate, but concluded that Strongylocentrotus droebachiensis was an ineffective biological control for the tunicate in oyster culture. No evidence was found to suggest that Echinus could not consume Didemnum, but the sea squirt’s ability to produce acidic secretions suggests it could be unpalatable. Hence, if Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of epifauna. 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 majority of the evidence indicates that infralittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

This biotope is classified as circalittoral and therefore no algal species have been considered. Several invasive bryozoans are of concern, including Bugula neritina (Porter et al., 2017), Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b), however, evidence of potential effects is sparse. Limited evidence exists for Bugula neritina in the UK; however, it was found in Northern Ireland for the first time in 2006, then seen again in both 2008 and 2013 in abundance and attached to buoys and kelp stipes throughout Carrickfergus Marina (Porter et al., 2017). Tricellaria inopinata has been reported to colonize the byssal threads of the mussel Mytilus galloprovincialis, Hymeniacidon perleve and the ascidian Styela plicata (Dyrynda et al., 2000). Tricellaria inopinata dominated the fouling community in the Lagoon of Venice, within seven years of being introduced (Ambrogi, 2000). At present, there is 'Insufficient evidence' to suggest that the circalittoral biotopes are sensitive to colonization by algal or other invasive species; further evidence is required.