The Marine Life Information Network

Temperature increase (local)

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

Evidence

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.).

Mitchell et al. (1983) suggested that the Scottish and Irish populations of Swiftia pallida were at the southern limit of the species range. It should be noted that there are reports of Swiftia pallida in deep waters (518 to 766 m depth) in the Mediterranean (Mastrototaro et al., 2010); however, its distribution in the British Isles appears to be limited to the Atlantic coasts of Scotland and Ireland (NBN, 2015; Langton, Stirling & Boulcott, 2023). Hiscock et al. (2001) predicted the loss of all populations occurring in the Inner Hebrides and mainland western Scotland with a 2°C increase in summer surface temperatures over a 20-year period. 

Alcyonium glomeratum has been recorded from Scotland to the Bay of Biscay (Hayward & Ryland, 1995b) and would probably tolerate an increase at the benchmark level. Hiscock et al. (2004) predicted that Alcyonium glomeratum would spread northwards with warming seas. Other species present in the biotope are widespread across the British Isles or are not important to the classification of this biotope.

Sensitivity assessment. The CR.MCR.EcCr.CarSwi biotope generally has a northern distribution within the British Isles, with the characterizing Swiftia pallida being intolerant of warmer conditions. Therefore, resistance is likely to be ‘Low,’ resistance is probably ‘Medium’ and sensitivity is assessed as ‘Medium’.

Temperature decrease (local)

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

Evidence

Caryophyllia smithii is a southern species (Fish & Fish, 1992) with a northern range limit in the Shetland Isles (NBN, 2015).  It is therefore likely to be close to its northerly range limit and therefore likely to be negatively affected by a decrease in temperature at the benchmark level. Swiftia pallida is classed as a northerly species and is recorded in Scotland, south-west Ireland (e.g. Kenmare Bay) on the west coasts of Norway and Sweden and in deep water from the Bay of Biscay and the Mediterranean (Wilding & Wilson, 2009; Langton, Stirling & Boulcott, 2023). Alcyonium glomeratum has been recorded from Scotland to Biscay (Hayward & Ryland, 1995b) and, being close to its northerly distribution limit, is likely to experience a significant decline.

Sensitivity assessment. Caryophyllia smithii and Alcyonium glomeratum are already close to their northern range limit, and a decrease would significantly affect the northern populations of the species and hence the biotope. Resistance is assessed as ‘Low,’ resilience as ‘Medium’, and sensitivity as ‘Medium’.

Salinity increase (local)

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

Evidence

CR.MCR.EcCr.CarSwi is a circalittoral biotope and an increase in salinity at the benchmark would result in a change from 'full' to hypersalinity.  No records of the characterizing Caryophyllia smithii or Swiftia pallida in hypersaline conditions was found. Hence, no assessment was made due to the lack of evidence.

Salinity decrease (local)

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

Evidence

This biotope occurs in full salinity.  Caryophyllia smithii has been recorded in biotopes from Full to Low salinity (Connor et al., 2004) and would probably tolerate a change at the benchmark level. Swiftia pallida has only been recorded in full salinity biotopes (Connor et al., 2004) and is likely to be intolerant of a decrease in salinity. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Medium’ and sensitivity has been assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

Alcyonium digitatum, Caryophyllia smithii, Spirobranchus triqueter and sponges 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/sec) during spring tides (De Kluijver, 1993; Coolen et al., 2015). O’Reilly et al. (2022) studied environmental factors in relation to cold water corals in the northeast Atlantic and found that current speeds determined living and dead (i.e. rubble) distributions, whereby slower current speeds (average 25.4 and 9.4 cm/s) favour living corals, and higher dead coral ratios are concurrent with elevated average current speeds (31.3 cm/s) which are subjected to periodic intensified pulses (max 114 cm/s). Overall, where live coral framework dominated, current speeds fail to exceed 66 cm/s. 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, in particular, 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.

Swiftia pallida is a filter feeder and therefore relies on a supply of organic matter suspended in the water column. Topographic highs elevate colonies into sections of the water column with higher currents, and a complex seabed stimulates the localized resuspension of organic matter, both of which enhance the feeding rates of benthic filter feeders (Langton, Stirling & Boulcott, 2023). Swiftia pallida prefer locations with gravel content (around 10%), as high gravel content is associated with strong currents, yet there may be an upper tolerance limit to strong currents that is reflected in the seabed type (Langton, Stirling & Boulcott, 2023). Sea fans are found in strong tidal streams but retract their polyps when current velocity gets too high for the polyps to retain food. Tidal streams exert a steady pull on the colonies and are therefore likely to detach only very weakly attached colonies. Colonies rely on high water flow rates to bring food and to remove silt (Hiscock, 2007). No evidence for Swiftia pallida was found; however, Bunker (1986) reported that the sea fan Eunicella verrucosa was present in areas subject to at least moderate tidal stream but was most abundant in strong tidal streams. There is a tendency for Eunicella verrucosa to grow aligned across the direction of the prevailing current (Bunker, 1986).

Little information was available for Alcyonium glomeratum, yet in Lough Hyne, UK, the species was recorded in areas of high current flow (Trowbridge et al., 2016). Alcyonium glomeratum is similar to Alcyonium digitatum, and colonies of these species also occur in subtidal areas with high water motion (currents and/or waves), and in Lough Hyne, UK, are abundant on Whirlpool Cliff and other subtidal cliffs which experience rapid current flow and/or high turbulence (Trowbridge et al., 2016).

Sensitivity assessment. The CR.MCR.EcCr.CarSwi biotope is found from negligible to moderately strong water flow (0 to 3 knots) but can be found from extremely exposed to sheltered wave exposure. It is likely that the biotope exists in moderate energy, with either water flow or wave action prevailing. The characterizing species (including gorgonians, soft corals, and Caryophyllia smithii) are generally associated with moderate to high energy environments. Change in water flow is therefore probably only relevant to wave-sheltered examples. However, a change at the benchmark level is unlikely to be significant. Resistance is, therefore, assessed as ‘High,’ resilience as ‘High,’ and the biotope is assessed as ‘Not Sensitive’ at the benchmark level.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Changes in emergence are Not Relevant to this biotope as it is 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. Further detail

Evidence

Natural events, such as storms, can lead to temporary reductions in the abundance of reef species (Langton, Stirling & Boulcott, 2023). No direct evidence on Swiftia pallida was found, however, Eunicella verrucosa can be used as a proxy species. Dead sea fans (Eunicella verrucosa) have been recorded washed up along Chesil Beach (UK) following winter storms (Hatcher & Trewhella, 2006). However, Bunker (1986) reported that Eunicella verrucosa was most abundant in moderately exposed locations. Jenkins & Stevens (2022) noted how seabed slope, temperature at the seafloor, and wave orbital velocity were important predictors of distribution in Eunicella verrucosa, 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 down to juvenile morphological variability, as Caryophyllia smithii found deeper and in sediment were thinner and taller.

Sensitivity assessment. The CR.MCR.EcCr.CarSwi biotope is recorded from extremely exposed to sheltered wave exposure and but can be found in negligible to moderately strong water flow (0 to 3 knots). It is likely that the biotope exists in moderate to high energy, with either water flow or wave action prevailing. Change in wave exposure is therefore probably only relevant to habitats that experience weak water flow. Nevertheless, the characterizing species (including gorgonians and Caryophyllia smithii) are generally associated with moderate to high energy environments. Hence, a change at the benchmark level is unlikely to be significant. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is assessed as ‘Not Sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

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

Evidence

Chan et al. (2012) studied the response of the gorgonian Subergorgia suberosa to heavy metal-contaminated seawater from a former coastal mining site in Taiwan. Cu, Zn, and Cd each showed characteristic bioaccumulation. Metallic Zn accumulated but rapidly dissipated. In contrast, Cu easily accumulated but was slow to dissipate, and Cd was only slowly absorbed and dissipated. Associated polyp necrosis, mucus secretion, tissue expansion, and increased mortality were reported in Subergorgia suberosa exposed to water polluted with heavy metals. However, this pressure is Not assessed

Hydrocarbon & PAH contamination

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

Evidence

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

CR.MCR.EcCr.CarSwi is a sub-tidal biotope complex (Connor et al., 2004). Oil pollution is mainly a surface phenomenon and its impact on 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 could potentially negatively affect sub-littoral habitats (Castège et al., 2014).

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones which experience high exposure and show correspondingly high mortality, as are bottom-dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981). White et al. (2012) reported on deep-water gorgonian communities, including Swiftia pallida six months after the Deep Water Horizon oil spill. Stress in the gorgonians was observed including excessive mucous production, retracted polyps and smothering of brown flocculent material (floc) which contained oil from the Macondo well. Hsing et al. (2013) reported that, following smothering by floc associated with the Deep Water Horizon spill, recovery of corals and gorgonians was inversely correlated with floc presence.

Synthetic compound contamination

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

Evidence

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

Radionuclide contamination

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

Evidence

'No evidence'.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

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

Evidence

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 (ca 2.66 mg/l) (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l.

Little information on the effects of oxygenation on bryozoans was found.  No evidence was found concerning the effects of hypoxia for Swiftia pallida. However, as a species that lives in fully oxygenated waters in conditions of flowing waters, it is expected that it would be intolerant to decreased oxygen levels. Bell (2002) reported that an oxycline at Lough Hyne (<5 % surface concentration) limited vertical colonization by Caryophillia smithii.

Sensitivity assessment

Despite limited evidence, Swiftia pallida and Caryophyllia smithii are unlikely to tolerate hypoxic events given their preference for moderate water movement. Resistance is ‘Low’, resilience is ‘Medium’ and sensitivity is ‘Medium’.  It should be noted that, as these biotopes occur in high energy habitats and low oxygen events are likely to be short-lived.

Nutrient enrichment

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

Evidence

Echavarri-Erasun et al. (2007) described the effects of deepwater sewage outfall discharges on the relative abundance of rocky reef communities. Species typical of hard substrata (including Caryophyllia smithii and bryozoans) increased in total richness and abundance near the outfall.

Whilst Swifita pallida could be at risk of competition from algae in shallow waters due to nutrient enrichment, this biotope occurs in the circalittoral below the depth suitable for most macroalgae. If nutrient enrichment resulted in algal blooms, then their subsequent death could result in deposition of dead algae on the seabed and resultant localised hypoxia (see above).

However, there is 'Insufficient evidence' for a nutrient enrichment sensitivity assessment of this biotope.

Organic enrichment

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

Evidence

Echavarri-Erasun et al. (2007) described the effects of deepwater sewage outfall discharges on the relative abundance of rocky reef communities.  Species typical of hard substrata (including Caryophyllia smithii and bryozoans) increased in total richness and abundance near the outfall.

Sensitivity assessment. Evidence for some of the characterizing species suggests some tolerance or even increased abundance when exposed to organic enrichment in the circalittoral.  However, ‘No evidence’ for the important characterizing Swiftia pallida could be found.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

If the rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope, and the species would be unlikely to recover. The biotope would be lost. Caryophillia smithii, Swiftia pallida, and Alcyonium glomeratum each require a hard substratum to attach to, such as rock, steel, and other coralligenous formations (Trowbridge et al., 2016; Fabri et al., 2022; Jenkins & Stevens, 2022; Langton, Stirling & Boulcott, 2023). 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).

Sensitivity assessment. Resistance to the pressure is considered ‘None,’ and resilience is ‘Very low’. Sensitivity has been assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

Not relevant’ to biotopes occurring on bedrock.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

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

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Caryophillia smithii, Swiftia pallida, and Alcyonium glomeratum are likely to be affected by physical disturbances. 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 increase when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018; Long et al., 2021; Langton, Stirling & Boulcott, 2023). Also, heavy mobile gears could also result in movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998). Bottom-contacting fishing activity is the main pressure-causing activity to adversely affect Swiftia pallida. However, the ability of Swiftia pallida to recover after the removal or reduction of fishing pressure is unknown (Langton, Stirling & Boulcott, 2023). Yet Langton, Stirling & Boulcott (2023) predicted Swiftia pallida habitat to be found in areas of complex seabed, on or near to exposed rock, as it was more likely to have experienced no benthic trawling or dredging (because of the risk of gear being damaged) for the last 10  years, and was more likely to be within MPAs or within areas with MPA management measures. While the patchy records of Swiftia pallida throughout the waters of west Scotland and Northern Ireland do not constitute a detailed baseline dataset, they are currently the only occupied areas of suitable habitat predicted by the model presented by Langton, Stirling & Boulcott (2023) to have experienced a release from fishing pressures and could be a good candidate for repeated monitoring to study the impact of implementing management measures if appropriate control sites are identified.

Little evidence for Swiftia pallida was found. However, reviews have considered the sea fan Eunicella verrucosa to be sensitive to abrasion (MacDonald et al., 1996; Hall et al., 2008; Tillin et al., 2010; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Egger et al., 2025). For example, strandings of Eunicella verrucosa have been observed on the southwest coast of England, where the coral was entangled in lost fishing gear, as well as in domestic marine litter (Sheehan et al., 2017; Chimienti, Nisio, & Lanzolla, 2020). Swiftia pallida has also been observed tangled in lobster pot netting and detached in the vicinity of lobster pots (Holt, pers comm.). In addition, physical contact with fishing gear scrapes has been noted to favour and increase the development of epibionts on gorgonian corals (Canessa et al., 2022). Epibionts substantially modify host–environment interactions (e.g., transference of energy or matter), eventually reducing their fitness, and large masses of epibionts lead to a burdening of the colonies and greater mechanical stress (Canessa et al., 2022). The response of Eunicella verrucosa colonies to physical stress and epibionts was studied in the Catalan Sea, Spain. Canessa et al. (2022) observed that Eunicella verrucosa in unprotected areas, which experienced fishing damage, experienced epibiosis at least four times higher than colonies in protected areas; 10 to 30% compared to 4 to 10%, respectively.

Fishing gear such as bottom trawling, bottom longlines, trammel nets, and gillnets have all been observed to adversely affect Eunicella verrucosa (Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020). For example, Eunicella verrucosa was the most common coral bycatch species (32%) over two fishing seasons (summer to autumn and spring) in southern Portugal (Dias et al., 2020). In Lyme Bay, UK, before the exclusion of bottom trawling, Eunicella verrucosa would occur as bycatch during fishing operations. The populations of Eunicella verrucosa and other benthic taxa in Lyme Bay have benefited since the trawling ban in 2008 (Kaiser et al., 2018; Chimienti, Nisio, & Lanzolla, 2020). Although not recovered, Sheehan et al. (2013) noted that within three years of closing an area in Lyme Bay, UK, to fishing, some recovery of Eunicella verrucosa had occurred, with a marked increase compared to areas that were still fished. However, recovery of slow-growing, long-lived, sessile epifauna is expected to take decades (Kaiser et al., 2018; Long et al., 2021). Kaiser et al. (2018) specifically studied the recovery of sessile epifauna following the exclusion of towed mobile fishing gear in Lyme Bay, UK. Their estimates suggest that no recovery occurred within the timescale of the study, and that some biogenic habitats (particularly sponges and soft corals) could require up to, or more than, 20 to 30 years before signs of recolonization and recovery may occur. The maximum recovery time modelled was 51 years for yellow branched sponges, while Eunicella verrucosa and Pentapora foliacea increased in abundance, but had not fully recovered, with their projected recovery time being 17 to 20 years (Kaiser et al., 2018). Therefore, recovery rates of biota depend on life-history factors and habitat-specific requirements, with the longer-lived species that require specific habitats and have low dispersal potential taking longer to recover (Kaiser et al., 2018).

A 15-year review of the Lyme Bay trawling ban by Renn et al. (2024) highlighted definitive evidence of recovery, in terms of increased species richness, with key sessile taxa (Pentapora foliacea and Phallusia mammillata) showing signs of early recovery between 2008 and 2013. In terms of exploited species, between 2008 and 2019, fish experienced a 430% increase in taxon richness and a 370% increase in total abundance inside the Marine Protected Area (MPA), but invertebrates (crab, lobster, cuttlefish, and whelk) exhibited no signs of recovery (Renn et al., 2024). Renn et al. (2024) concluded that the evidence of recovery recorded in Lyme Bay broadly aligned with the wider literature by detecting early stages of recovery within the first few years of MPA establishment. However, full recovery is thought to occur over decadal timescales, and measuring full recovery rates in-situ remains a priority for future research in Lyme Bay. Coma et al. (2006) reported ongoing recovery in Eunicella singularis populations in the Mediterranean four years following a mass-mortality event. 

Other studies suggest that Eunicella verrucosa may be more resistant to abrasion pressures. Eno et al. (2001) conducted experimental potting on areas containing fragile epifaunal species in Lyme Bay, south-west England. Divers observed that pink sea fans flexed and bent before returning to an upright position under the weight of pots. Although relatively resistant to a single event, long-term deterioration or the effects of repeated exposure were not clear (Eno et al., 2001). Observation of pots suggested that they were dragged along the bottom when wind and tidal streams were strong. However, little damage to epifauna was observed. Eunicella verrucosa were patchily distributed in areas subject to potting damage, but the study could not determine whether this was due to damage from potting (Eno et al., 2001). A further four-year study on potting in the Lundy Marine Protected Area detected no significant differences in Eunicella verrucosa between areas subject to commercial potting and those where this activity was excluded (Sheehan et al., 2013). However, Tinsley (2006) observed flattened sea fans that had continued growing, with new growth being aligned perpendicular to the current, so even colonies of Eunicella verrucosa that are damaged can survive. 

Healthy Eunicella verrucosa can recover from minor damage and scratches to the coenenchyme (Tinsley, 2006), and the coenenchyme covering the axial skeleton will re-grow over scrapes on one side of the skeleton in about one week (Hiscock, pers. comm, cited in Hiscock, 2007). While Hinz et al. (2011a) reported that abundance and average body size of Eunicella verrucosa were not significantly affected by scallop dredging intensity, there is evidence of Eunicella verrucosa detached by mobile gear (Hiscock, pers. comm.). In addition, Eunicella verrucosa that has been accidentally collected in fishing activities has been proven to survive if returned quickly to the sea when it settles on cobbles and is less damaged when removed (Enrichetti et al., 2019).

A study by Boulcott & Howell (2011) on the effects of scallop dredging in rocky substrata suggested that associated epifaunal communities, such as bryozoans, hydroids, soft corals, and sponges, were removed by a passing scallop dredge. However, on hard, uneven rock, damage to more resistant epifauna, whilst in evidence, was restricted. The study also recorded that mobile substrata present were likely to be moved and turned by the passing dredge, leading to further damage to the epifaunal communities. 

This biotope is also characterized by the bryozoan Porella compressa. Porella compressa is a delicate, 'stony' and brittle, erect bryozoan that is likely to be damaged by abrasion (Holt pers. comm.). However, its loss from the biotope would not result in loss of the biotope.

Sensitivity assessment. Swiftia pallida is sessile and epifaunal, and based on evidence for Eunicella verrucosa, is likely to be severely damaged by heavy gears, such as scallop dredging (MacDonald et al., 1996; Hiscock, pers. comm.; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Langton, Stirling & Boulcott, 2023; Egger et al., 2025). However, some studies suggest the sea fan Eunicella verrucosa may be more resistant, particularly to low-intensity, lighter abrasion pressures, such as pots and associated anchor damage (Eno et al. 2001; Sheehan et al., 2013), and this could be the case for Swiftia pallida. Caryophyllia smithii and Alcyonium glomeratum is also likely to be severely damaged by towed fishing gear. Taking all the evidence into account, resistance is ‘Low’, resilience is ‘Low’, and sensitivity is ‘High.’

Penetration or disturbance of the substratum subsurface

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

Evidence

The species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure. This pressure is thought ‘Not 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. Further detail

Evidence

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 correlated with high sedimentation and depth (Bell & Turner, 2000). 

While siltation may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007), and sea fans are probably tolerant of increases in suspended sediment (Hiscock et al., 2004). Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but did occur at sites up to ‘moderately silted’.

Cold-water corals are documented in areas with high concentrations of resuspended particulate organic matter, 1,330 to 3,965 μg l⁻¹, which acts as an abundant food source for benthic communities (O’Reilly et al., 2022). Bilan et al. (2023) studied the vulnerability of cold-water corals to sediment resuspension from bottom trawling in the Mediterranean and found that cup coral and octocoral did not exhibit symptoms of distress, whereas colonial scleractinian corals and black coral experienced substantial polyp mortality in enhanced suspended sediment concentration treatments.

Sensitivity assessment. CR.MCR.EcCr.CarSwi occurs on bedrock in the circalittoral and is unlikely to experience highly turbid conditions. From the evidence presented above, the characterizing species tolerate some siltation, and a change at the benchmark level is unlikely to cause mortality. Resistance is recorded as ‘High,’ resilience as ‘High,’ and the biotope is ‘Not sensitive’ at the benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

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 low abundance of Caryophyllia smithii is 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.  

Swiftia pallida generally grows to a height of about 7 to 10 cm (Wilson, 2007). It is found on rocks covered with a fine layer of silt (Mitchell et al., 1983). While siltation may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007). It is, however, thought that smothering causes mortality (Hiscock et al., 2004). Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but did occur at sites up to ‘moderately silted’. However, Eunicella verrucosa seems to tolerate heavy silting, as Canessa et al. (2022) noted how Eunicella verrucosa colonies of northeast Sardinia, Italy, occurred mainly on sub-horizontal rocks characterized by heavy silting between 30 and 215 m.

Sensitivity assessment. Smothering by 5 cm would cover the majority of Caryophyllia smithii and the smallest examples of the other characterizing species and could result in limited mortality. Caryophyllia smithii has been reported as quite tolerant of temporary burial, and the biotope occurs in moderate water flow, and the sediment would likely be removed rapidly.  Resistance was assessed as ‘High,’ resilience as ‘High,’ and the biotope is ‘Not sensitive’ at the benchmark level.

Smothering and siltation rate changes (heavy)

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

Evidence

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 low abundance of Caryophyllia smithii is 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.  

Swiftia pallida generally grows to a height of about 7 to 10 cm (Wilson, 2007). It is found on rocks covered with a fine layer of silt (Mitchell et al., 1983). While siltation may inhibit feeding, colonies of the sea fan Eunicella verrucosa produce mucus to clear themselves of silt (Hiscock, 2007). It is, however, thought that smothering causes mortality (Hiscock et al., 2004). Bunker (1986) reported that Eunicella verrucosa were mostly observed on bedrock or boulders, but did occur at sites up to ‘moderately silted’. However, Eunicella verrucosa seems to tolerate heavy silting, as Canessa et al. (2022) noted how Eunicella verrucosa colonies of northeast Sardinia, Italy, occurred mainly on sub-horizontal rocks characterized by heavy silting between 30 and 215 m.

Sensitivity assessment. Smothering by 30 cm of sediment would likely bury most characterizing species, with only those individuals on boulders and vertical surfaces escaping burial. The biotope occurs in moderate water flow, and it is likely that the sediment would probably be removed rapidly. Resistance was assessed as ‘Medium’, resilience as ‘Medium’, and sensitivity as ‘Medium’.

Litter

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

Evidence

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). Since little evidence for Swiftia pallida was found, reviews have considered the sea fan Eunicella verrucosa to be sensitive to abrasion. Sensitivity of Eunicella verrucosa to abrasion events has been assessed as High in previous reviews, largely due to its slow growth rate and fragility to physical damage (MacDonald, 1996; Hall et al., 2008; Tillin et al., 2010; Kaiser et al., 2018; Chimienti, 2020; Chimienti, Nisio, & Lanzolla, 2020; Canessa et al., 2022; Egger et al., 2025). 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 Eunicella verrucosa mortality, either through direct damage or making them more vulnerable to removal from their anchorage to the sea floor, particularly during storms.

For example, during 2015, strandings of Eunicella verrucosa were observed on the southwest coast of England, where the coral appeared to be entangled in lost fishing gears, as well as in domestic marine litter, and almost all of the tangled bundles of marine debris contained a central dead, black or brownish skeletal remains of Eunicella verrucosa (Sheehan et al., 2017; Chimienti, Nisio, & Lanzolla, 2020). Divers have often encountered plastic fishing gear, fishing line, and other marine debris, such as plastic bags, amongst living coral gardens on rocky reefs off the coasts of southwest England and have become snagged and subsequently overgrown by Eunicella verrucosa (Sheehan et al., 2017). When colonies are broken, such as through being severed by fishing line, the corals then lie flat on the seafloor and eventually die; the pink or white outer coenenchyme rots, leaving the black internal skeleton visible (Sheehan et al., 2017; Giusti et al., 2019). Alternatively, Eunicella verrucosa entangled with marine debris could have formed after gorgonians were detached from the seabed, for example, due to damage from gill nets, and then picked up debris as they travel along the seabed with the currents (Sheehan et al., 2017).

In addition, physical contact with fishing gear scrapes (lost lines entangled in colonies) has been noted to favour and increase the development of epibionts on gorgonian corals (Canessa et al., 2022). Epibionts modify host–environment interactions (e.g., transference of energy or matter), eventually reducing their fitness, and large masses of epibionts lead to a burdening of the colonies and greater mechanical stress (Canessa et al., 2022). The response of Eunicella verrucosa colonies to physical stress and epibionts was studied in the Catalan Sea, Spain. Canessa et al. (2022) observed that Eunicella verrucosa in unprotected areas, which experienced fishing damage, experienced epibiosis at least four times higher than colonies in protected areas, 10 to 30% compared to 4 to 10%, respectively.

There are no records of ghost fishing affecting Caryophyllia smithii or Alcyonium glomeratum. However, epifaunal communities are vulnerable to damage from fishing gear, and are likely vulnerable to being dislodged or damaged through lost fishing gear, and possibly certain types of marine litter.

Sensitivity assessment. Ghost fishing by discarded fishing gear, lines and pots could cause severe damage to the community, especially the tall erect epifauna such as Swiftia, where discarded lines may catch the upright epifauna and increase drag, especially in stormy weather (Sheehan et al., 2017; Giusti et al., 2019). Fishing lines can cause lesions to the gorgonian coenenchyme, leading to greater aggregates of epibionts, which can eventually cause the branch to rupture (Bo et al., 2014; Canessa et al., 2022). Taking all the evidence from ghost fishing and discarded lines into account, resistance is ‘Low.’ Resilience is ‘Low’, and sensitivity is ‘High.’

Electromagnetic changes

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

Evidence

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

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

Underwater noise changes

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

Evidence

Whilst no evidence could be found on the effects of noise or vibrations on the characterizing species, it is unlikely that these species would be adversely affected by noise.  This pressure ‘Not relevant’.

Introduction of light or shading

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

Evidence

Although no evidence was found for the effect of light on Caryophyllia smithii, Swiftia pallida, or Alcyonium glomeratum, 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 can be found within the first 200 m of depth where light, both natural and artificial, would reach (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.

For comparison, Egger et al. (2025) is the first study that provides the first description of the spawning and early life ecology of Eunicella verrucosa, and through lab-based studies with artificial moonlight, found that the spawning of Eunicella verrucosa was less pronounced, occurring over 2 to 3 consecutive days and 6 to 7 days after the full moon. Egger et al. (2025) concluded that Eunicella verrucosa spawning may be influenced by the lunar patterns as observed in other temperate gorgonians, like Paramuricea clavata, but likely relies on a combination of additional cues to regulate the exact timing of spawning (Egger et al., 2025).

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

Barrier to species movement

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

Evidence

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. Further detail

Evidence

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

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

‘Not relevant’.

Genetic modification & translocation of indigenous species

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

Evidence

No evidence of translocation or genetic modification of populations of the characterizing species was found. Therefore, there is currently ‘No evidence’ on which to assess this pressure.

Introduction of microbial pathogens

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

Evidence

Whilst no evidence of disease in Swiftia pallida could be found, the first recorded incidence of cold-water coral disease was noted in the sea fan Eunicella verrucosa, in south-west England in 2002 (Hall-Spencer et al., 2007). Video surveys of 634 separate colonies at 13 sites revealed that disease outbreaks were widespread in south-west England from 2003 to 2006. Coenenchyme became necrotic in diseased specimens, leading to tissue sloughing and exposing skeletal gorgonin to settlement by fouling organisms. Sites where necrosis was found had significantly higher incidences of fouling. No fungi were isolated from diseased or healthy tissue, but significantly higher concentrations of bacteria occurred in diseased specimens. Vibrios isolated from Eunicella verrucosa did not induce disease at 15°C, but at 20°C, controls remained healthy, and test gorgonians became diseased. Bacteria associated with diseased tissue produced proteolytic and cytolytic enzymes that damaged Eunicella verrucosa tissue and may be responsible for the necrosis observed. Monitoring at the site where the disease was first noted showed new gorgonian recruitment from 2003 to 2006; some individuals had died and become completely overgrown, whereas others had continued to grow around a dead central area (Hall-Spencer et al., 2007). In addition, corals are more susceptible to diseases when stressed. Eunicella species were affected by mass mortality events linked to positive thermal anomalies, and evidence of a disease affecting Eunicella verrucosa was correlated to high concentrations of Vibrio bacteria, most likely due to the elevated seawater temperature (Chimienti, 2020). Furthermore, damaged corals, such as those with injury from tissue abrasion via fishing gear, can lead to infection and disease, particularly in tropical corals, where a four-fold higher level of coral disease was observed outside of a marine no-take reserve (Sheehan et al., 2017). No evidence of disease in the Caryophyllia smithii or Alcyonium glomeratum was found.

Sensitivity assessment. Based on reports of mortality linked to disease in the sea fan Eunicella verrucosa, a disease may result in mortality of Swiftia pallida. It should be noted that the colder temperatures in which Swiftia pallida occurs may confer some resistance. Resistance is assessed as ‘Medium’, resilience as ‘Medium’, and sensitivity as ‘Medium’. 

Removal of target species

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

Evidence

Eunicella verrucosa was collected historically as a curio by divers and was collected until recently in the British Isles (Wells et al., 1983; Bunker, 1986). It is now protected under schedule 5 of the Wildlife and Countryside Act 1981, no evidence of harvesting of the sea fan Swifita pallida or any of the other characterizing species was found. 

As there is historical evidence of harvesting of other sea fans, the sessile, epifaunal Swifita pallida would have no resistance to harvesting by divers.  Resistance has been assessed as ‘None’, resilience as ‘Low’ and sensitivity is, therefore ‘High’.

Removal of non-target species

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

Evidence

The characteristic species probably compete for space within the biotope, so that loss of one species would probably have little if any effect on the other members of the community. However, removal of the characteristic epifauna due to by-catch is likely to remove a proportion of the biotope and change the biological character of the biotope.  As sessile epifauna, the characterizing species and likely to be severely damaged by heavy gears, such as scallop dredging (MacDonald et al., 1996).  However,  some studies suggest that sea fans may be more resistant, particularly to low intensity, lighter abrasion pressures, such as pots and associated anchor damage (Eno et al. 1996; Sheehan et al., 2013)  Taking all the evidence into account, a resistance of ‘Low’ is recorded, albeit with a low confidence value owing to the lack of consensus in the literature. Resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

This biotope is classed as circalittoral and therefore no algal species have been considered. Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, or shells of other Crepidula, or other species, e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on a gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’, it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1 to 0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders, so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space, and many are fast-growing and opportunistic, so they may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment. The circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata due to the extremely wave-exposed to moderately wave-exposed conditions, in which wave action and storms may mitigate or prevent the colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011b). In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the 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 water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

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

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

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

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

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

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope occurs on bedrock, which could provide a suitable hard substratum for colonization by Didemnum sp. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope experiences very weak to moderately strong water flow (0 to 1.5 m/s) but moderate to extreme wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of corals, bryozoans, and sponges. 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 INIS is probably 'Not relevant' in this biotope.

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 INIS is probably 'Not relevant' in this biotope. 

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 INIS is probably 'Not relevant' in this biotope. 

Other INIS

Evidence

Hesperibalanus (syn. Solidobalanus) fallax is an invasive southern species of barnacle only recently recorded in southwest England (Southward et al., 2004). It has been observed fouling (primarily damaged or diseased) gorgonians (Hall-Spencer et al., 2007). However, it has not yet been recorded in Scottish waters (NBN, 2023). Therefore, resistance to fouling by this barnacle is assessed as ‘Medium’ as a precaution based on its potential to foul sea fans, but with 'Low' confidence. Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium.’ Due to the constant risk of new invasive species, the literature on this pressure should be revisited, and the confidence for this assessment is 'Low'.