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

Serpula vermicularis is found throughout the British Isles (NBN, 2016), and its range extends through the North East Atlantic and the Mediterranean (Holt et al., 1998).  This range suggests it is tolerant of some temperature change.  In Loch Creran in Scotland, the temperature regime within the main basin is similar to that in the adjacent sea with a low of about 6°C in February/March and a high of 13 -15°C in August/September (Gage, 1972).  The peak in larval recruitment of Serpula vermicularis in Loch Creran coincides with this peak in temperature (Chapman et al., 2007).  Additionally, Hughes et al. (2005) found the species to be tolerant of changes in temperature “with upper lethal limits exceeding any value that they could conceivably experience in the field”.  

Sensitivity assessment.  At the pressure benchmark, the characterizing species Serpula vermicularis is not thought to be sensitive.  Some of the other species in the biotope may be less tolerant of an increase in temperature, although overall species diversity is not expected to be significantly affected.  Therefore, resistance and resilience have been assessed as ‘High’ so that sensitivity is assessed as ‘Not sensitive’ at the benchmark level. 

Temperature decrease (local)

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

Evidence

Serpula vermicularis is found throughout the British Isles (NBN, 2016), and its range extends through the North East Atlantic and the Mediterranean (Holt et al., 1998).  It would be expected that in shallow enclosed areas, the temperature will fall during periods of cold winter weather so decreases in temperature are probably tolerated by reefs.  Hughes et al. (2005) found the species to be tolerant of a wide range of temperatures.

Sensitivity assessment.  At the pressure benchmark, the characterizing species Serpula vermicularis is not thought to be sensitive.  Some of the other species in the biotope may be less tolerant of an increase in temperature, although overall species diversity is not expected to be significantly affected.  Therefore, resistance and resilience have been assessed as ‘High’ so that sensitivity is assessed as ‘Not sensitive’ at the benchmark level. 

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

This biotope occurs in variable and full marine salinity regimes (Connor et al., 2004).  Serpula vermicularis is not found in areas where hypersaline conditions occur, such as rock pools or lagoons, so it seems likely that the species would be intolerant of increases in salinity.  An increase in salinity at the benchmark level would result in a salinity of >40 psu and, as hypersaline water is likely to sink to the seabed, the biotope may be affected by hypersaline effluents. Ruso et al. (2007) reported that changes in the community structure of soft sediment communities due to desalinisation plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b). 

Sensitivity assessment.  A long-term increase to hypersaline conditions would probably result in loss of reefs and the loss of many of the other species that colonize the reefs. Therefore, resistance is assessed as ‘None’, as a long-term change in the salinity regime at the benchmark would most likely cause the severe mortality of the characterizing species and a large number of the other species found within the biotope.  Resilience is assessed as ‘Very low’ due to the very unlikely recovery of the biotope so that sensitivity is assessed as ‘High’.

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

This biotope is recorded from both full marine and variable salinity regimes (Connor et al., 2004). Bosence (1979b) suggested that the layer of low salinity water in the upper layers of the water in Ardbear Loch in Galway, Eire is partly responsible for the lack of Serpula vermicularis individuals above a depth of 2 m.  However, Serpula vermicularis is known to tolerate reduced salinities (Hartmann-Schröder, 1971; Mastrangelo & Passeri, 1975; cited in Moore et al., 1998b; Cook, 2016) and in Loch Creran in Scotland, individual specimens of Serpula vermicularis were commonly observed in shallow waters where salinities could fall to around 23 psu at 4 m (Moore et al., 1998b).  Small enclosed lochs such as Loch Sween & Ardbear Loch are often subject to extremely variable salinity so the species seems to be tolerant of shorter-term changes.  Serpula vermicularis reefs were also observed in intertidal areas of Loch Creran during the 19th century, where salinity is likely to vary.  Therefore, it seems likely that the species can tolerate some decreases in salinity.  However, when reduced salinity interacts with variation in temperature, larval mortality occurs (Gray, 1976). Cook (2016) also noted that Loch Creran experienced an extreme rainfall event in January 2014 that resulted in a reduction in salinity of at least 3.5 ppt at 6 m at all of his study sites.

Sensitivity assessment.  A long-term (one year) reduction in salinity to reduced (18-30) regime will remove both Serpula vermicularis and the other species within this biotope from their optimal habitat conditions.  It is possible that some of the Serpula vermicularis would die because of this change in salinity, although their tubes and the structure of the reef would remain.  Hence, resistance is assessed as ‘Medium’. Therefore, resilience is assessed as ‘Low’ so that sensitivity is 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 and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

The tidal flows within this biotope are recorded as weak (<1 knot) and very weak (negligible) (Connor et al., 2004).  All of the Serpula vermicularis reefs recorded in Ireland and Scotland are found in sheltered sea lochs/loughs with restricted entrances.  This has led to the suggestion that sheltered conditions with a limited turnover of water are required for the development of Serpula vermicularis reefs (Bosence, 1979b; Dodd et al., 2009).  In Loch Creran, no reefs were recorded in the outer section of the loch beyond the narrows at Sgeir Calliach, despite the presence of suitable depths and substrata.  This is a well-flushed section of the loch, where the larvae of Serpula vermicularis will presumably be at lower concentrations than further up the loch (Moore, 1998b). Moore et al. (1998b) also noted that reefs were absent from parts of the upper basin exposed to strong currents even though suitable substratum was present. Therefore, the biotope is likely to be intolerant of an increase in water flow rate as larvae are likely to be taken away from the reefs, old worms will die and, without a supply of new individuals, the reef will die.  With the collapse of dead Serpula vermicularis reefs, species diversity will decline significantly because the open structure of the reefs provides substratum and crevices for many other organisms.

Sensitivity assessment.  Although water flows seem to be an important environmental factor for the growth of Serpula vermicularis reefs, this pressure is unlikely to have a negative impact on the biotope at the level of the benchmark (a change of 0.1-0.2 m/s).  Therefore, resistance and resilience are assessed as ‘High’ and sensitivity 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. (Emergence regime change pressure definition).

Evidence

This biotope does not occur in the intertidal and a change in emergence is not relevant to this biotope.

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

This biotope occurs in sea lochs where wave exposure is recorded as sheltered to extremely sheltered (Connor et al., 2004).  The Serpula vermicularis reefs are open and quite fragile structures and are not likely to be resistant of wave exposure.  No reefs are reported at depths of zero metres, which may be the effect of turbulence on larval recruitment (Champan et al., 2007).  Tulbure (2015) found that in Loch Creran the more wave exposed areas of the loch contained lower total coverage of Serpula vermicularis.

Sensitivity assessment.  An increase in wave exposure may damage existing reefs, while a decrease might increase the available space for colonization. A single storm may cause significant damage to a reef (D. Harries, pers. comm.). However, an increase in the pressure at the benchmark (an increase of 3-5% in significant wave height) is unlikely to affect the biotope.  Therefore, resistance and resilience are assessed as ‘High’ and sensitivity 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 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.

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.

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.

Radionuclide contamination

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

Evidence

No evidence.

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 no information regarding the tolerance of Serpula vermicularis to deoxygenation. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Bosence (1979b) observed that the lower limit of larval settlement in Ardbear Lough, Eire coincided with mud-rich and possibly oxygen poor water.  Gage (1972) found the dissolved oxygen concentration in the lower basin of Loch Creran in Scotland, where Serpula vermicularis reefs form, did not fall below 87% saturation.  Therefore, the species, and the larvae, in particular, may be intolerant of deoxygenated water.  Other species in the biotope may also be intolerant of changes in oxygen availability resulting in a possible loss of species diversity. 

Sensitivity assessment.  The resistance of this biotope to de-oxygenation at the level of the benchmark is assessed as ‘Low’ but with 'Low' confidence. Therefore,  resilience is assessed as ‘Very low’ and the overall sensitivity of the biotope to this pressure is assessed as ‘High’.

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

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) decreases in dissolved oxygen and uncharacteristic microalgal blooms (Bricker et al., 1999, 2008).

The hypersaline Mar Menor lagoon, Spain, became a severely eutrophic system in 2016 after decades of nutrient and organic matter input. In 2019, further anthropogenic inputs, as well as a major flood event, caused water-column stratification that produced anoxic, hydrogen sulphide rich (euxinic) conditions, resulting in extensive mortality of benthic fauna (Sandonnini et al., 2021a ). Once oxygen levels returned to normal, serpulid species began to recolonize rapidly forming reefs (Sandonnini et al., 2021a). Although Serpula vermicularis occurred at lower abundances compared with the opportunistic Hydroides species, it was among the four serpulids that successfully established reefs in the lagoon within weeks of conditions returning to normal (Sandonnini et al., 2021a). While it did not reach the extremely high densities characteristic of Hydroides elegans or Hydroides dianthus, Serpula vermicularis was recorded forming part of the recovering assemblage across a range of substrata (Sandonnini et al., 2021a). The low density of Serpula vermicularis may partly reflect the 2016 eutrophication event, which caused a marked drop in lagoon pH and likely reduced calcification capacity in Serpula spp., particularly when compared with more opportunistic or tolerant serpulid species (Sandonnini et al., 2021b).

Sensitivity assessment. Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for all the suspension feeders within the biotope. However, long-term or high levels of organic enrichment may result in eutrophication and have indirect adverse effects, such as increased turbidity, increased suspended sediment, increased risk of deoxygenation and the risk of algal blooms. D. Harries (pers comm.) noted that increased algal growth on reefs was suggested as a possible cause of the deterioration in Loch Teacuis and could certainly hamper serpulid feeding and cause increased drag. However, evidence from the eutrophic Mar Menor lagoon suggests that Serpula vermicularis can persist in nutrient enriched conditions (Sandonnini et al., 2021a). The indirect effects of nutrient enrichment (e.g. turbidity, deoxygenation, or algal blooms) may be detrimental, while nutrient enrichment alone may be tolerated. Nevertheless, no direct evidence on the effects on reefs was found, and there is ‘Insufficient evidence’ to form the basis of an assessment.

Organic enrichment

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

Evidence

Organic enrichment leads to organisms no longer being limited by the availability of organic carbon. The consequent changes in ecosystem function can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) and decreases in dissolved oxygen and uncharacteristic microalgal blooms (Bricker et al., 1999, 2008). Indirect adverse effects associated with organic enrichment include increased turbidity, increased suspended sediment and the increased risk of deoxygenation. Organic effluent from an alginate factory in Loch Creran, Scotland, appeared to be responsible for the exclusion of Serpula vermicularis reefs along a 1 km stretch of the coast centred on the discharge and may have reduced reef development at greater distances (Moore et al., 1998b). The affected area was covered in a bacterial mat of Beggiatoa spp. and no reefs were present even though suitable substrata was present. However, this level of organic pollution was extreme and does not give any indication of the intolerance of Serpula vermicularis reefs to an increase in organic enrichment at the benchmark level. Species diversity may decline but the overall impact on reefs is unknown.

The hypersaline Mar Menor lagoon, Spain, became a severely eutrophic system in 2016 after decades of nutrient and organic matter input. In 2019, further anthropogenic inputs, as well as a major flood event, caused water-column stratification that produced anoxic, hydrogen sulfide rich (euxinic) conditions, resulting in extensive mortality of benthic fauna (Sandonnini et al., 2021a and references therein). Once oxygen levels returned to normal, serpulid polychaetes began to recolonise rapidly (Sandonnini et al., 2021a). Although Serpula vermicularis occurred at lower abundances compared with the opportunistic Hydroides species, it was among the four serpulids that successfully re-established in the lagoon within weeks of conditions returning to normal (Sandonnini et al., 2021a). While it did not reach the extremely high densities characteristic of Hydroides elegans or Hydroides dianthus, Serpula vermicularis was recorded forming part of the recovering assemblage across a range of substrata (Sandonnini et al., 2021a). The low density of Serpula vermicularis may partly reflect the 2016 eutrophication event, which caused a marked drop in lagoon pH and likely reduced calcification capacity in Serpula spp., particularly when compared with more opportunistic or tolerant serpulid species (Sandonnini et al., 2021b).

Sensitivity assessment. Little empirical evidence was found to support an assessment of this biotope at this benchmark. However, evidence from the eutrophic Mar Menor lagoon suggests that Serpula vermicularis can persist in organically enriched conditions (Sandonnini et al., 2021a). The lack of direct evidence for the characterizing species has resulted in this pressure being assessed as ‘Insufficient evidence’.

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

Serpula vermicularis requires a hard substratum on which to construct its tube. The most common substratum for settlement is bivalve shells. In Loch Creran, it was particularly common on shells of Pecten, Aequipecten and Modiolus. Reefs form predominantly in areas where there is suitable substratum scattered throughout a muddy sand bottom (Moore et al., 1998b, 2009; Chapman et al., 2007). In Loch Creran, reefs were found growing in bedrock, boulders, stones, shells and man-made substrata (Moore et al., 1998b). Large reefs were only rarely found growing on rock (Moore et al., 1998b, 2009). 

Sensitivity assessment.  If the sediment was replaced with rock, 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. Therefore, resistance to the pressure is assessed as ‘None’, resilience as ‘Very low’ and sensitivity 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

In Loch Teacuis, Serpula vermicularis reefs were recorded to grow on rocks and amongst the Laminaria saccharina holdfasts (Dodd et al., 2009).  In Loch Creran, Serpula vermicularis grows mainly on bivalve shells on muddy sand (Moore et al., 1998b, 2009; Chapman et al., 2007;  Dodd et al., 2009).  Serpula vermicularis typically grow up from a shell, cobble or boulder substratum on muddy sand to produce patch reefs (Chapman et al., 2007, 2012).  The species needs to be able to attach to a hard substratum, and can’t settle on to soft sediment alone. In Loch Creran, Moore et al. (1998b, 2009) noted that reefs were mainly restricted to muddy sands with 'low representation' in muddy areas even when suitable substratum was present. Moore et al. (2009) suggested that the distribution of fine muds in Loch Creran defined the lower limit of the reefs and explained their absence from the head of the loch but the cause (siltation, suspended sediment etc.) was unknown. Bosence (1979b) suggested that raised levels of suspended mud defined the lower limit of the reefs in Ardbear Lough, Ireland. Chapman et al. (2007) suggested that the lower reef boundary in Loch Creran resulted from a depth-correlated settlement response rather than lack of suitable substratum or depth-correlated mortality and that light or another factor was more important in determining the depth distribution of settlement than siltation. 

Sensitivity assessment.  A change in sediment type from muddy sand to 'mud and sandy muds' may reduce the abundance or extent of the serpulid reef, based on their habitat preferences in Loch Creran and Ardbear Logh. The effect of a change from muddy sand to coarse, gravel dominated sediments is unclear as no records of reefs on this habitat was found, presumably as their preference for sheltered environments favours finer sediments. Therefore, resistance is assessed as 'Low' based on their possible response to a change to muddy sediments. Hence, resistance is assessed as 'Very low' and sensitivity as 'High'. 

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Serpula vermicularis reefs are always attached to a hard substratum as the larvae have to be able to settle on a stable anchor point.  However, these solid anchor points can be bivalve shells, pebbles, cobbles, and boulders, all of which could be extracted under this pressure.  The removal of this substratum would remove all of the characterizing species, Serpula vermicularis, as well as a large majority of the other species found in this biotope.  Therefore, the resistance of this biotope to this pressure is assessed as ‘None’, resilience as ‘Very low’, and sensitivity is assessed as ‘High’. 

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

Serpula vermicularis aggregations and reefs are fragile and can be easily damaged.  For example, in Loch Creran severe damage, although on a local scale, was caused by the movement of mooring blocks and chains (Moore, 1996; Moore et al., 2009, Figure 8).  Although the effects were localized, mooring had reduced colonies to rubble within a radius of 10 m in one instance, and extensive damage was reported within 50 m of salmon cages (Holt et al., 1998).  Although individual worms survived and were seen to continue feeding, the reefs were broken up so that the value of the habitat was greatly diminished.  Moore et al. (2009, Figure 7) also detected single and double tracks through the reef at Rubha Mor in Loch Creran using side-scan sonar. The single tracks were ca 3 m wide. Diver survey revealed the tracks consisted of scattered reef rubble through the dense area of reef, and estimated the tracks had removed ca 11% of the reef in Rubha Mor (Moore et al., 2009). 

Sensitivity assessment.  Serpula vermicularis aggregations and reefs are considered to be extremely fragile (Holt et al.,1998; Moore et al., 1998b, 2009; Chapman et al., 2017, 2012). The evidence suggests that physical disturbance from mooring chains and bottom gear would remove the majority of the reef within the affected area. While some individual worms and fragments may remain, the aggregates, reefs, and the biodiversity they host would be lost within the affected area.  Therefore, the resistance of this biotope to this pressure is assessed as ‘None’, resilience as ‘Very low’, and sensitivity is assessed as ‘High’.

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

Due to the epifaunal nature of the characterizing species within this biotope, the physical effect of penetration will be extremely similar to the effects of abrasion and disturbance. Therefore, the resistance of this biotope to this pressure is assessed as ‘None’, resilience as ‘Very low’, and sensitivity is assessed as ‘High’.

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

Siltation can have a negative impact on site selection by serpulid larvae (Rodriguez et al., 1993).  Bosence (1979b) suggested from observations and transplant experiments, that the lower depth limit of Serpula vermicularis was probably determined by suspended sediment and deoxygenation. In contrast, Moore et al. (1998b) found no horizontal layers of suspended mud in Loch Creran, and although the authors do not rule out the possibility that storm-generated, suspended mud may inhibit reef development, the lower limit of reefs could also be due to inadequate current velocities for suspension feeding.  Chapman et al. (2007) suggested that the lower reef boundary resulted from a depth-correlated settlement response rather than lack of suitable substratum or depth-correlated mortality and that light or another factor was more important in determining the depth distribution of settlement than siltation.   A supply of suspended sediment may be important to Serpula vermicularis because the species requires a supply of particulate matter for suspension feeding. 

Sensitivity assessment.  There is a lack of empirical evidence to suggest how the pressure at the benchmark might affect this biotope.  An increase in suspended sediment may affect the ability of Serpula vermicularis larvae to settle.  In addition, an increase in suspended sediment may change the rate at which Serpula vermicularis has to clean its branchial plume.  A decrease in the level of suspended sediment could reduce the amount of particulate food in the water column and consequently reduce food availability.  However, due to the lack of information a sensitivity assessment of ‘No evidence’ is given. 

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

Siltation can have a negative impact on site selection by serpulid larvae (Rodriguez et al., 1993).  Bosence (1979b) suggested from observations and transplant experiments, that the lower depth limit of Serpula vermicularis was probably determined by suspended sediment and deoxygenation. In contrast, Moore et al. (1998b) found no horizontal layers of suspended mud in Loch Creran, and although the authors do not rule out the possibility that storm-generated, suspended mud may inhibit reef development, the lower limit of reefs could also be due to inadequate current velocities for suspension feeding.  Chapman et al. (2007) suggested that the lower reef boundary resulted from a depth-correlated settlement response rather than lack of suitable substratum or depth-correlated mortality and that light or another factor was more important in determining the depth distribution of settlement than siltation.   A supply of suspended sediment may be important to Serpula vermicularis because the species requires a supply of particulate matter for suspension feeding. 

Sensitivity assessment.  There is a lack of empirical evidence to suggest how the pressure at the benchmark might affect this biotope.  An increase in suspended sediment may affect the ability of Serpula vermicularis larvae to settle.  In addition, an increase in suspended sediment may change the rate at which Serpula vermicularis has to clean its branchial plume.  A decrease in the level of suspended sediment could reduce the amount of particulate food in the water column and consequently reduce food availability.  However, due to the lack of information a sensitivity assessment of ‘No evidence’ is given. 

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

The mean height of Serpula vermicularis reefs in Loch Teacuis was 26 ± 9 cm (Dodd et al., 2009).  In other reefs, such as those in Loch Creran, reefs have been recorded to have a maximum height often exceeding 50 cm (Moore et al., 2003).  This difference in reef height will mean that the effect of this pressure will vary depending on the reef which is going to be affected.  Therefore, each reef should be assessed on a case by case basis. Taking into consideration the reef in Loch Teacuis, the pressure at the benchmark would smother a vast majority of the reef, causing the worms to asphyxiate as gaseous exchange would be inhibited and the worms would also not be able to feed.

Sensitivity assessment.  A large proportion of the biotope could be smothered by a 30 cm deposit of fine material.  Therefore, resistance is assessed as ‘None’, resilience as assessed as ‘Very low’, and the sensitivity of this biotope is assessed as ‘High’.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020a), 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 Serpula vermicularis. However, one study was performed on another 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.

Sensitivity assessment. No studies have examined the effect of EMFs on Serpula vermicularis, therefore, 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

Species characterizing this habitat do not have hearing perception but vibrations may cause an impact, however, no studies exist to support an assessment.

Introduction of light or shading

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

Evidence

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

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

Barrier to species movement

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

Evidence

Not relevant – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

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 – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

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

No evidence for the effect of this pressure on the characterizing species within this biotope was found.

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

No information on diseases of Serpula vermicularis was found.  The species is known to be parasitized by the protozoan Haplosporidium parisi (Ormieres, 1980) but the effects of this infestation are unknown.  There are no reports of loss of the biotope from the disease.  There is insufficient evidence to assess the effect of the pressure on this biotope, therefore, an assessment of ‘No evidence’ has been given. 

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

The characterizing species (Serpula vermicularis) are not known to be targeted by commercial fisheries.

Removal of non-target species

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

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  The characterizing species within this biotope could easily be incidentally removed from this biotope as by-catch when other species are being targeted.  The loss of these species and other associated species would decrease species richness and negatively impact on the ecosystem function.

Sensitivity assessment. Removal of a large percentage of the characterizing species would alter the character of the biotope. The resistance to removal is ‘Low’ due to the easy accessibility of the biotopes location and the inability of these species to evade the effects of collection or harvesting. Resilience is assessed as ‘Very low’, with recovery only being able to begin when the harvesting pressure is removed altogether. Therefore, sensitivity is assessed as ‘High’.

The American slipper limpet, Crepidula fornicata

Evidence

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

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

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

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

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogenous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density.

Sensitivity assessment. The above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of pebbles, shells, cobbles, or any other hard substrata that can be used for larvae settlement (Tillin et al., 2020). This habitat is sheltered from wave action, which is also suitable for Crepidula colonization. Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020). Serpula vermicularis requires hard substrata for settlement, with a preference for shells. Hence, it may be able to settle on Crepidula beds, especially dead Crepidula. However, the Crepidula bed is likely to ingest larvae, including those of Serpula vermicularis which is also very slow-growing, so Serpula may not be able to compete with Crepidula for space and access to suitable substratum. Existing reefs may persist but degrade without a fresh supply of larvae (removed by Crepidula) or suitable substratum for larval settlement. Therefore, resistance is assessed as 'Low' and resilience is assessed as 'Very low' as Crepidula would need to be removed by artificial means, so the biotope sensitivity is assessed as 'High'. Crepidula has not yet been reported to occur in this biotope and there is a lack of direct evidence on the interaction between Crepidula and Serpula vermicularis, so confidence in the assessment is 'Low' and 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; Michin & 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-meditated 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).

The seasonal growth cycle of Didemnum vexillum 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 to 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., 2007 a&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).

Some species have been shown to tolerate overgrowth by Didemnum vexillum. Such as anemones (did not specify species name) that were observed in high densities of 10 to 339 individuals in transects with a high percentage cover of Didemnum vexillum (Lengyel et al., 2009). In the Netherlands, the sea anemone Sagartia elegans and Sabella pavonia tubes were not overgrown by Didemnum sp. (Gittenberger, 2007). Botrylloides violaceus can overgrow Didemnum sp. (Gittenberger, 2007) although it was noted to be overgrown in other studies (Valentine et al., 2007a). In addition, Styela clava and Ascidiella aspera survived overgrowth by Didemnum vexillum as long as their siphons remained free (Gittenberger, 2007). However, Gittenberger (2007) stated that the boring sponge Clione celata, the sea anemone Diadumene cincta, Mytilus edulis, Magallana (syn. Crassostrea) gigas, Ostrea edulis, a variety of hydroids, the colonial ascidians Aplidium (Fig. 4) and Diplosoma listerianum and the solitary ascidians Ciona intestinalis start to die on contact with Didemnum sp.

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

However, some studies on Georges Bank, USA and Sandwich, Massachusetts observed that colonies survived partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow sandy bottom (cited Gittenberger, 2007). In the Netherlands, the coastal zone is composed of mud and sand, with only shells as hard substrata. Didemnum sp. remained rare until 1996 when populations quickly expanded and it became a dominant invasive species because of an increase in available hard substrata for colonization after a cold winter between 1995 and 1996 caused a decrease in the abundance of many marine animals (Gittenberger, 2007). Thus, Didemnum vexillum colonized and established mud and sand habitats where hard substrata was present.

In contrast to Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated gravel, cobbles and pebbles. 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). 

The Sandwich tide pools (USA) were subject to air exposure at low tide, and daily changes in water depth and temperatures (Valentine et al., 2007a). Didemnum vexillum colonies survived exposure to air at low tides for a short time (not exceeding two hours) during rapid colony growth in the summer months of July to September (Valentine et al., 2007a). However, parts of the large established colonies, which were artificially exposed to air for two to three hours in October, were observed desiccated or predated on by grazing periwinkles 30 days later, in the winter month of November (Valentine et al., 2007a). They suggested that the invasive tunicates’ ability to tolerate exposure to air varies with the seasonal growth cycle. Didemnum vexillum also tolerated emersion in Kent, as colonies on the mid-shore at Reculver flourish and survive in air exposure for up to three hours per cycle during springs (Hitchin, 2012). Hitchin (2012) suggested the porous nature of the sandstone boulders the species colonized retained water. The Kent shore was sheltered but held water due to its shallow slope and flats, which may allow Didemnum sp. to survive in the low to mid-shore. Didemnum vexillum died when exposed to air for more than six hours (Laing et al., 2010).

Sensitivity assessment. Didemnum vexillum requires hard substrata for successful colonization, therefore, it could colonize the pebbles, shells and gravel typical of this biotope. Didemnum vexillum may be able to survive close to the sandy mud substratum as long as sedimentation is not too high and the risk of burial is low. Also, Didemnum vexillum has a preference for wave-sheltered conditions typical of this biotope. There is no evidence that Didemnum vexillum can overgrow Serpula vermincularis but evidence has recorded Didemnum overgrowing and displacing other calcareous tube worms. The Serpula vermicularis reefs in Loch Creran were monitored for Didemnum vexillum which was confirmed at an intertidal oyster farm in the loch but was not yet found on the reefs (D. Harries, pers. comm., 2018; Cottier-Cook et al., 2019). The long-term survival of Surpula vermincularis is dependent on the presence of hard substrata (e.g. cobbles, pebbles, gravel). Didemnum vexillum may compete with this species and other epifauna, potentially contributing to Serpula vermincularis population decline. Hence, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution but with 'Low' confidence. Resilience is likely to be 'Very low' as Didemnum vexillum would need to be physically removed to allow recovery. Hence, sensitivity to invasion by Didemnum is assessed as 'Medium'. 

The Pacific oyster, Magallana gigas

Evidence

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

Since introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSS, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergstrom et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024). Shipping activity has also been associated with the introduction of Magallana gigas in the north-eastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019) and possibly in south-west England from France possibly via fouling on ships (GBNNSS, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Larval mortality can be as large as 99%, as larvae are sensitive to environmental conditions (Alves et al., 2021), but adults are long-lived so populations can survive with infrequent recruitment (Padilla, 2010). Larval dispersal and mass spawning events have facilitated the settlement and establishment of Pacific oysters, as seen in the Oosterschelde estuary, Netherlands (Hansen et al., 2023). It has been suggested that the spread of the Pacific oyster in Scandinavia is due to northward larval drift on tidal and wind-driven currents (Hansen et al., 2023). Wood et al. (2021) suggested that larval dispersal of the Pacific oyster from populations within and outside the UK was possible via unaided (passive) transport by currents, but that aquaculture and offshore structures (e.g. windfarms) increased the risk of the invasive species spreading and the geographical extent of spread.

Pacific oyster reefs, in the Wadden Sea and Brittany, on littoral muddy and sandy habitats formed predominantly at lower tidal levels from Mean Low Water levels to the shallow subtidal (Herbert et al., 2012, 2016). Pacific oyster spatfall was recorded in the estuarine intertidal zone on areas with hard substrata of stone and shell, particularly between the low water of spring tides and high water of neap tides, such as in the Menai Strait (Spencer et al., 1994). In Lim Bay, Adriatic Sea, Magallana gigas is only found in the intertidal and on the sublittoral edge (at a depth of 1 m) and not at 3 m or 6 m depth (Stagličić et al., 2020; Tillin et al., 2020). It coexists here with Ostrea edulis which is abundant in the subtidal (Stagličić et al., 2020). Bergstrom et al. (2021) found that depth was one of the most important predictors of the occurrence of Magallana gigas in the Skagerrak and suggested the optimal depth of the species was 0.5 m in the shallow subtidal, although it occurred down to 5 m. On littoral rock in Brittany, the Pacific oyster colonizes all intertidal levels from Mean High Water to Mean Low Water on sheltered (low energy), moderately exposed (moderate energy) and high energy rock shores (Herbert et al., 2012). 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). However, in suitable situations (e.g. Oosterschelde) it may form beds down to 42 m. Magallana gigas has not been found at extreme low water levels or subtidally beneath rocky habitats, as it has been in soft sediment areas (Herbert et al., 2012).

Sensitivity assessment. Most of the evidence suggests that Magallana gigas is limited to 10 m but can form beds down to 42 m where conditions allow (Smaal et al., 2009; Herbert et al., 2012, 2016; Tillin et al., 2020). In addition, Magallana gigas is known to prefer wave-sheltered conditions. Magallana gigas can settle on hard surfaces such as shells, pebbles, and biogenic structures created by reef-forming organisms (Tillin et al., 2020), which are found within the muddy sediment of this biotope. Therefore, this biotope may be suitable for Magallana gigas colonization down to 10 m. However, Magallana gigas’ preference for the shallow intertidal would probably exclude it from the deeper examples of this biotope, which occur down to 30 m. Therefore, because Magallana gigas has not yet (2026) been reported co-occurring with Serpula vermicularis reefs, a resistance of ‘High’ is suggested. Hence, resilience is assessed as 'High' and sensitivity as ‘Not sensitive’, but with 'Low' confidence due to a lack of evidence.

Wireweed, Sargassum muticum

Evidence

Wireweed, Sargassum muticum. Sargassum muticum is known to grow in the shallow subtidal around the UK, usually in areas sheltered from wave action. Its distribution is limited by the availability of hard substratum (e.g. stones >10 cm) and light (Staehr et al., 2000; Strong & Dring 2011; Engelen et al., 2015). It is most abundant between 1 and 3 m below mean water, is a poor competitor under low light and only develops dense canopies in shallow areas (Engelen et al., 2015). Where this biotope occurs within shallower water (0 to 5 m), the sheltered conditions may be suitable for Sargassum, however, a lack of large enough hard attachment substrata within the muddy sediment may prevent colonization. Serpula vermicularis reefs found at 5 to 20 m are unlikely to be colonized by Sargassum muticum due to their depth. Resistance and resilience are therefore 'High' and the biotope 'Not sensitive' to Sargassum muticum.

Wakame, Undaria pinnatifida

Evidence

Wakame, Undaria pinnatifida. Undaria pinnatifida is known to grow in the shallow subtidal around the UK but is usually found in areas sheltered from wave action, with a depth range of -1 to 4 m. Where this biotope occurs within shallower water (0 to 5 m), the sheltered conditions may be suitable for Undaria, however, a lack of hard attachment substrata within the muddy sediment may prevent colonization. Serpula vermicularis reefs found at 5 to 30 m are unlikely to be colonized by Undaria pinnatifida due to their depth. Resistance and resilience are therefore 'High' and the biotope 'Not sensitive' to Undaria pinnatifida.

Other INIS

Evidence

Although several species of serpulid polychaetes have been introduced into British waters none are reported to compete with Serpula vermicularis (Eno et al., 1997). The red seaweed Heterosiphonia japonica was also present in the loch but no evidence of impacts on the reefs was observed (D. Harries, pers. comm., 2018). 

Orange striped anemone, Diadumene lineata. This species tends to be found in brackish waters, particularly in bays, estuaries, and marinas where its only requirement is hard substrata on which to attach. As such, it is often associated with biogenic reefs (Tillin et al., 2020). It can tolerate a large salinity range, from 0.5 to 35 ppt, and is found in shallow waters to depths of a few hundred meters (Cohen, 2011), preferring sheltered areas with low wave exposure (Fofonoff et al., 2003). This species has not been shown to cause negative impacts on the habitats that it colonizes (Fofonoff et al., 2003). This biotope can be found within suitable depths and salinities, in sheltered conditions, and contains suitable hard attachment substrata, making it potentially suitable habitat for Diadumene lineata (Tillin et al., 2020). However, there has been no evidence of this species colonizing this biotope around the UK and Ireland.

Asian rapa whelk, Rapana venosa. This species colonizes subtidal habitats up to 90 m deep and in salinities of 16 to 35 ppt (Tillin et al., 2020). It can live on a variety of substrata including rock and mixed sediment. Its preferred wave exposure and tidal currents are not documented. Therefore, it is not known whether Rapana venosa would succeed on Serpula vermicularis reefs. However, sublittoral biogenic reefs have been suggested as potentially suitable habitat for Rapana venosa given their occurrence on sediment and suitable depths (Tillin et al., 2020). There have been no documented occurrences of Rapana venosa on this biotope around the UK and Ireland from which to assess sensitivity.

Red ripple bryozoan, Watersipora subatra. This species colonizes a variety of substrata and is often found in the intertidal and shallow subtidal, though has been recorded deeper than 10 m, and in salinities between 18 and 49 psu as well as a wide range of wave exposures from sheltered to exposed (Tillin et al., 2020). The availability of suitable hard attachment substrata, salinity range, and sheltered conditions makes this biotope potentially suitable habitat for Watersipora subatra where it occurs in the lower littoral and shallow subtidal (Tillin et al., 2020). However, there have been no records of Watersipora subatra colonizing Serpula vermicularis reefs around the UK and Ireland.

Japanese skeleton shrimp, Caprella mutica. This species has been found on a range of different substrata between 18 to 35 ppt salinity, intertidally and subtidally up to 20 m deep and in sheltered areas to those with high tidal flow regimes (Tillin et al., 2020). While this species does not usually associate directly with hard surfaces, it favours filamentous structures like hydroids and turf algae that it can hold onto, which can be found growing on Serpula vermicularis reefs. At depths less than 13 m, this biotope has been considered as potentially suitable habitat for Caprella mutica due to its associated epifaunal community and suitable salinity and hydrodynamic regimes (Tillin et al., 2020). However, there have been no reports of Caprella mutica colonizing Serpula vermicularis reefs around the UK and Ireland.

Sensitivity assessment. While this biotope may provide potentially suitable habitat for these species, there has been no direct evidence of their occurrence on Serpula vermicularis reefs around the UK and Ireland. Therefore, there is ‘Insufficient evidence’ from which to assess the sensitivity of this biotope to these species.