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

Sabellaria alveolata are a southern species reaching their northern limit in Britain and Ireland and whose global distribution extends south to Morocco (Gruet, 1982). Studies at Hinkley Point, Somerset, found that growth of the tubes in the winter was considerably greater in the cooling water outfall, where the water temperature was raised by around 8-10 °C, than at a control site, although the size of the individual worms themselves seemed to be unaffected (Bamber & Irving, 1997). Dubois et al. (2007) observed that in autumn where water temperatures are 8 °C higher than in spring, a shorter period was required for larvae to metamorphose. 

 Differences between spawning regimes which may be due  to different water temperatures have been observed, where conditions for more northern population are less favourable and lead to single annual spawning events of shorter duration (Culloty et al., 2010). Intertidal populations of Sabellaria alveolata are susceptible to low temperatures in winter.

Sensitivity assessment. Based on distribution and temperature enhancement of duration and frequency of spawning, metamorphosis and growth rates, Sabellaria alveolata is considered to be ‘Not sensitive’ to an increase in temperature at the pressure benchmark (resistance and resilience are therefore both considered to be 'High').

Temperature decrease (local)

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

Evidence

Sabellaria alveolata are a southern species reaching their northern limit in Britain and Ireland. Studies at Hinkley Point, Somerset, found that growth of the tubes in the winter was considerably greater in the cooling water outfall, where the water temperature was raised by around 8-10 °C, than at a control site, although the size of the individual worms themselves seemed to be unaffected (Bamber & Irving, 1997). 

Dubois et al. (2007) observed that in autumn where water temperatures are 8 °C higher than in spring, a shorter period was required for larvae to metamorphose. Differences between spawning regimes which may be due  to different water temperatures have been observed, where conditions for more northern population are less favourable and lead to single annual spawning events of shorter duration (Culloty et al., 2010).

Sensitivity assessment. Based on distribution and  reported temperature effects on duration and frequency of spawning, metamorphosis and growth rates. The effects of acute decreases in temperature at the benchmark will depend on the seasonality of occurrence. Decreases in winter are likely to stress populations more than decreases in summer (although there may be effects on larval supply).  At the centre of their UK range, adult Sabellaria alveolata are considered to have 'High' resistance to both an acute and chronic change at the pressure benchmark as subtidal populations will be buffered from large changes in temperature and protected from frost. 

Salinity increase (local)

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

Evidence

No empirical evidence was found to assess the impact of increases in salinity on subtidal, reef forming populations. This biotope occurs in areas of variable salinity from 18-35 ppt (Connor et al., 2004). Based on the tolerance of intertidal populations for full salinity is is likely that this biotope would not be sensitive to a change to full salinity. However, it should be noted that reefs could be sensitive to hyper saline conditions above this benchmark. Quintino et al. (2008) examined through laboratory experiments the sub-lethal endpoints of brine exposure on Sabellaria alveolata larvae. Natural seawater where salinities had been increased using commercial salts used to prepare artificial seawater were used as the control. At a salinity of 36 (natural seawater artificially concentrated) 20% of Sabellaria alveolata developed abnormally, at a salinity of 40 this increased to about about 70% of the larvae developed abnormally, clearly indicating the effect of increasing salinity on larvae. Although not directly relevant to the pressure benchmark the experiments do suggest that increasing salinity would lead to sub-lethal effects on larvae. It is not clear how these supply effects would ramify at the population level. Recruitment success varies between years (see resilience information) and a shortfall in one year may be compensated in another year when salinity returns to normal, providing the source population is unaffected.

Sensitivity assessment. This biotope has only been recorded from areas of variable salinity but intertidal reefs are present in areas of full salinity (Connor et al., 2004), it is therefore considered that subtidal reefs would have a similar tolerance, resistance is therefore assessed as 'High' and resilience as 'High' (by default). This biotope is therefore considered to be 'Not sensitive'. 

Salinity decrease (local)

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

Evidence

It is likely that Sabellaria alveolata can tolerate small declines in salinity as it occurs intertidally where freshwater inputs may lower salinity, either on a semi-permanent basis where rivers discharge into estuaries and bays, or where rainfall and land run-off cause an acute lowering of salinity. In the Bay of Mont-Saint-Michel , for example, where large reefs are found salinities are lower (at <34.8) than in the open sea. (Dubois et al., 2007). This biotope is reported to occur in areas experiencing variable salinity (Connor et al., 2004). Lancaster (1993, cited from Holt et al., 1998) also found extensive, healthy hummocks of Sabellaria at Drigg, Cumbria, where there is a large freshwater input from the Drigg BNFL plant. However, based on a lack of records from habitats experiencing reduced (18-30ppt) or Low (18ppt) salinity regimes this biotope is considered likely to be sensitive to reduced salinity at the benchmark level.

Sensitivity assessment.  Based on distribution with only occasional records within estuaries, this biotope is considered likely to be sensitive at the lower limits of the pressure benchmark (a change to reduced or low salinity; 18-35ppt or <18 ppt). Resistance is therefore assessed as ‘Low', as a reduction in salinity at the pressure benchmark is considered to result in the loss of most of the reef. Resilience (following habitat recovery) is assessed as ‘Medium’. Sensitivity is therefore ‘Medium’. The observed distribution of this biotope may be based on other factors than salinity, such as availability of suitable sediments, and confidence in this assessment is low. 

Water flow (tidal current) changes (local)

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

Evidence

 Water flow will be a key driver of habitat suitability for subtidal Sabellaria alveolata, due to the requirement for suspended sand for tube building and the supply or organic particles for food. Tests on the mechanical strength and properties of Sabellaria alveolata tubes were performed by Le Cam et al. (2011).  These found that the biomineralised cement the worms produce to cement sand grains to form tubes confer wave resistance. Although thresholds of resistance are not known, the visco-elastic behaviour of the cement enables tubes to dissipate the mechanical energy of breaking waves and presumably also confers resistance to increased water flow rates (Le Cam et al. 2011).

In general sediment re-suspension and transport models indicate that sands are suspended by currents around 0.20-0.25 m/s and will stay in suspension until flow drops below 0.15-0.18 m/s (ref). Sabellaria alveolata may be relatively insensitive to changes above these flow rates (although the upper tolerance limit is not clear). In sheltered habitats where the water flow rates are approaching the lower limits of water flow tolerance a further reduction at the pressure benchmark may have negative impacts. Desroy et al., (2011) suggested that modifications to hydrodynamics (where current speed decreased downstream of new mussel farming infrastructure installations facing the reef) indirectly impacted sedimentary patterns and led to increased silt deposition resulting in the deterioration of Sabellaria alveolata reefs in the Bay of Mont-Saint Michel, France.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of Sabellaria alveolata changes over environmental gradients within the Severn Estuary.  Model predicted response surfaces were derived for each biotope for each of the selected habitat variables, using logistic regression.  From these response surfaces the optimum habitat range for each biotope could be defined based on the range of each environmental variable where the probability of occurrence, divided by the maximum probability of occurrence, is 0.75 or higher.  These results identify the range for each significant variable where the habitat is most likely to occur.  The modelled ranges should be interpreted with caution and apply to the Severn Estuary alone (which experiences large tidal ranges, high currents and extremely high suspended sediment loads and is therefore distinct from many other estuarine systems).  However, these ranges do provide some useful information on environmental tolerances.  The models indicate that for subtidal Sabellaria alveolata the maximum optimal current speed (the range in which it is most likely to occur) ranges from 1.26-2.46 m/s and the optimal mean current speed ranges from 0.5-1.22 m/s.  Although the results should be interpreted with caution, the modelled habitat suitability for Sabellaria alveolata indicates that the range of water flow tolerances is relatively broad.

Changes in water flow potentially also have implications for larval transport and recruitment. Sabellaria alveolata is generally absent from very exposed peninsulas such as the Lleyn, Pembrokeshire and the extreme south west of Cornwall, which probably relates to the effect of water movement on recruitment (Cunningham et al., 1984, cited from Holt et al. 1998). However, behavioural responses by larvae to different flow rates may result in some control over movement.  Dubois et al. (2007) observed vertical migration of Sabellaria alveolata larvae during the tidal cycle, where larvae migrate upwards in the water column to faster near-surface currents and migrate down the water column on the ebb flow to where currents are weaker. This migration enhances landward transport of larvae to more suitable habitats and prevents seaward loss.

Sensitivity assessment. A long-term decrease in water flow may reduce the viability of populations by limiting growth and tube building. No evidence was found for threshold levels relating to impacts although Tillin (2010) modelled optimal flow speeds of 0.5-1.22 m/s.  The worms may retract into tubes to withstand periods of high flows at spring tides and some non-lethal reduction in feeding efficiency and growth rate may occur at the edge of the optimal range.  Similarly a reduction in flow may reduce supply of tube-building materials and food but again, given the range of reported tolerances a change at the pressure benchmark, mid-range is not considered to result in mortality.  Resistance is therefore assessed as ‘High’ and resilience as ‘High’ (no impact to recover from).  All the biotopes within this biotope group are therefore considered to be ‘Not sensitive’.

Emergence regime changes

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

Evidence

Changes in emergence are not relevant to this biotope  which is restricted to fully subtidal habitats  The shoreward fringes are unlikely to be sensitive to increased emergence as reefs of Sabellaria alveolata are found in the intertidal.

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

As the reefs are subtidal they are protected from breaking waves, although wave action may lead to oscillatory water movements at the reef surface. Connor et al. (2004) indicate that this biotope is found in locations that vary from wave exposed to sheltered indicating a broad tolerance to a range of wave heights (as wave height is broadly correlated with the degree of wave exposure). Tests on the mechanical strength and properties of Sabellaria alveolata tubes found that the biomineralised cement the worms produce to cement sand grains to form tubes confer wave resistance preventing the reef from breaking (Le Cam et al. 2011). 

Sensitivity assessment.  At the pressure benchmark, Sabellaria alveolata are considered to be able to mechanically withstand an increase in wave exposure and to be unaffected by a decrease. The biotope group is therefore considered to be ‘Not Sensitive’ at the pressure benchmark (resistance and resilience are assessed as ‘High’ by default).

Transition elements & organo-metal contamination

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

Evidence

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

Hydrocarbon & PAH contamination

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

Evidence

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

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

Mauchline et al. (1964) examined concentration of radioactive isotopes by organisms on Windscale beach. Sabellaria alveolata built reefs with the smaller particles on the beach which adsorb the greatest amount of radioactivity per weight (due to surface-area effects). Thus Sabellaria reefs could concentrate radioactivity. However, the study by Mauchline et al. (1964) did not look for or identify any potential negative effects on the worms such as changes in reproductive success or mortality rates.

Sensitivity assessment. 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

No direct evidence was found to assess this pressure. 

Nutrient enrichment

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

Evidence

No evidence was found to assess this pressure, however  the benchmark is relatively protective and the worms may benefit from localised increases in phytoplankton supported by enrichment and are unlikley to be affected by opprtunistic algae due to depth and suspended sediment concentrations. Resistance is therefore assessed as 'High' and resilience as 'HIgh' (by default). The biotope is therefore considered to be 'Not sensitive'.

Organic enrichment

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

Evidence

No evidence was found to support this sensitivity assessment.  Habitat preferences for areas of high water movement suggest that organic matter would not accumulate on reefs, limiting exposure to this pressure.  Sabellaria alveolata would be able to consume re-suspended particulate organic matter.  This conclusion is supported by the enhanced growth rates observed in the congener Sabellaria spinulosa that have been recorded on the vicinity of sewage disposal areas (Walker & Rees, 1980).  Resistance is therefore assessed as ‘High’ to this pressure and recovery is assessed as ‘High’ (no impact to recover from), resulting in a sensitivity of 'Not sensitive'. 

Physical loss (to land or freshwater habitat)

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

Evidence

All marine habitats and benthic species are considered to have ‘No Resistance’ to this pressure and to be unable to recover from a permanent loss of habitat.  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in the resistance assessment is ‘High’, due to the incontrovertible nature of this pressure.  Adjacent habitats and species populations may be indirectly affected where meta-population dynamics and trophic networks are disrupted and where the flow of resources e.g. sediments, prey items, loss of nursery habitat etc. is altered. No recovery is predicted to occur and the rate and confidence in resilience are not assessed.

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

The introduction of artificial hard substratum is considered at the pressure benchmark level and it is noted that Sabellaria spinulosa can colonize bedrock and artificial structures in the intertidal. An increase in the availability of hard substratum may therefore be beneficial in areas where sedimentary habitats were previously unsuitable for colonization.  

Sensitivity assessment. Based on reported habitat preferences the species (rather than the biotope) is considered to be ‘Not Sensitive’ as the resulting habitat is suitable for the development of reefs. However these would be classified as a different biotope type.  Resistance of the biotope is therefore assessed as None (loss of >75% of extent), resilience is Very low (the pressure is a permanent change) and sensitivity is assessed as High. The more precautionary assessment for the biotope, rather than the species, is presented in the table as it is considered that any change to a sedimentary habitat from a rock reef habitat would alter the biotope classification and hence the more sensitive assessment is appropriate.

Physical change (to another sediment type)

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

Evidence

Sabellaria alveolata biotopes that occur on mixed sediments are not considered to be affected by a change in sediment type of 1 Folk class that leads to a change to ‘coarse sediments’ characterized as gravel, sandy gravel or gravelly sand (based on the Long (2006) simplified Folk classification) or a change to an intertidal Sabellaria alveolata reefs are found on this species is found on sands (George & Warwick 1985).   Larsonneur et al. (1984), working in the Bay of St Michel in Normandy, noted that the sand mason Lanice conchilega can stabilize sand well enough to allow subsequent colonization by Sabellaria alveolata. Settlement is also enhanced by the presence of existing colonies or their dead remains (Holt et al. 1998).

This biotope is, however, considered to be negatively impacted by a change to the finest sediment class e.g. a change in the sediment classification to ‘mud and sandy mud’ (based on the Long, (2006) classification).  This assessment is based on the lack of records of reefs occurring on these sediment types and is likely due to the mobility of the sediment, the lack of sand for tube-building and possibly the re-suspension of fine sediments clogging feeding structures and gills, however this is assumed rather than based on direct evidence.

Sensitivity assessment.  Based on reported habitat preferences and evidence from Foster-Smith (2001), where a change in one Folk class results in increased coarseness (e.g. a change to a coarse sediment of gravel, sandy gravel or gravelly sand) then the biotope is considered to be ‘Not Sensitive’ as the resulting habitat is suitable for this species, although the biotope character would alter.  A change in sediments would alter the biotope and an increase in fine sediments to the degree that sediments are re-classified as mud or sandy mud would severely reduce habitat suitability for the species.  Biotope resistance to a change in fine sediments or sediment type is, therefore, assessed as ‘None’ (loss of >75% of extent), resilience as Very low (the pressure is a permanent change), 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). Further detail

Evidence

The removal of substratum down to 30 cm depth is likely to remove the whole Sabellaria alveolata reef within the extraction footprint.  At an expert workshop convened to assess the sensitivity of marine features to support MCZ planning, Sabellaria alveolata reefs were assessed as having no resistance to extraction of the feature (benchmark was the removal of feature/substratum to 50 cm depth) (Tillin et al. 2010).

Sensitivity assessment. As Sabellaria alveolata reefs are surface features they will be directly removed by extraction of the reef to 30 cm depth.  Resistance to this pressure is, therefore, assessed as ‘None’.  Resilience is considered to be ‘Medium’ to allow for the establishment of reef structure and the potential for variable recruitment and this biotope is, therefore, considered to have ‘Medium’ sensitivity to this pressure. Confidence in this assessment is assessed as 'High' due to the incontrovertible nature of the pressure. 

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

Impacts of surface abrasion from fishing trawls and trampling have been investigated on shallow subtidal and intertidal reefs and the evidence is considered applicable to the subtidal biotopes.  To address concerns regarding damage from fishing activities in the Wadden Sea, Vorberg (2000) used video cameras to study the effect of shrimp fisheries on Sabellaria alveolata reefs.  The imagery showed that the 3 m beam trawl easily ran over a reef that rose to 30 to 40 cm, although the beam was occasionally caught and misshaped on the higher sections of the reef.  At low tide there were no signs of the reef being destroyed although the trawl had left impressions and all traces had disappeared four to five days later due to the rapid rebuilding of tubes by the worms.  The daily growth rate of the worms during the restoration phase was significantly higher than undisturbed growth (undisturbed: 0.7 mm, after removal of 2 cm of surface: 4.4 mm) and indicates that as long as the reef is not completely destroyed recovery can occur rapidly.  These recovery rates are as a result of short-term effects following once-only disturbance.  Cunningham et al. (1984) examined the effects of trampling on Sabellaria alveolata reefs. The reef recovered within 23 days from the effects of trampling, (i.e. treading, walking or stamping on the reef structures) repairing minor damage to the worm tube porches. However, severe damage, estimated by kicking and jumping on the reef structure, resulted in large cracks between the tubes, and removal of sections (ca 15x15x10 cm) of the structure (Cunningham et al., 1984). Subsequent wave action enlarged the holes or cracks. However, after 23 days, at one site, one side of the hole had begun to repair, and tubes had begun to extend into the eroded area. At another site, a smaller section (10x10x10 cm) was lost but after 23 days the space was already smaller due to rapid growth.

Sensitivity assessment. Based on the evidence above resistance to abrasion was assessed as ‘Medium’ as the tubes are able to withstand some damage and be rebuilt, recovery to a single event was considered to take place through tube repair by adults so recovery was assessed as ‘High’ and sensitivity was categorised as ‘Low’. The scale and intensity of impacts would influence the level of resistance and the mechanism of recovery. Where reefs suffer extensive spatial damage requiring larval settlement to return to pre-impact conditions then recovery would be prolonged (years).

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

This pressure will result in the surface disturbance effects outlined above but effects will be compounded by the penetration and sub-surface damage aspect of this pressure. No empirical evidence was found to assess impacts however it is considered that the deeper and more significant the damage, the higher the risk of removing complete tubes and limiting recovery of the reefs.

Sensitivity assessment. Based on the evidence cited above for abrasion, resistance was assessed as ’Low’ (taking into account deeper penetration of the disturbance), recovery was assessed as ‘Medium’ (2-10 years) to take into account that larval recruitment may be necessary for the reef structure to recover although small, localised areas of repair would take place within months.  Sensitivity is therefore assessed as ‘Medium’. 

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

Sabellaria alveolata do not rely on light penetration for photosynthesis and their visual perception is believed to be limited.  Changes in light penetration or attenuation associated with this pressure are, therefore, not relevant to the Sabellaria alveolata reef biotope. However alterations in the availability of suspended organic matter that can be used as food and the availability of suspended sediment for tube building  could either increase or decrease habitat suitability for Sabellaria alveolata reefs.

The effect of increased seston concentration on Sabellaria alveolata clearance rates was investigated by Dubois et al. (2009). The range of experimental suspended particulate matter (SPM) concentrations (65-153.8 mg/l) correspond to clear to medium turbidity at the pressure benchmark scale. The number of polychaetes actively feeding increased between SPM 6.5-12.3 mg/l and no change was observed  between SPM 12.3 and 55.5 mg/l.  At higher levels of SPM clearance rates were reduced, the decline in filter feeding efficiency (measured as a clearance rate) declined at around SPM 45 mg/l and thereafter remained relatively stable.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of Sabellaria alveolata changes over environmental gradients within the Severn Estuary.  The model predicted response surfaces were derived for each biotope for each of the selected habitat variables, using logistic regression.  From these response surfaces the optimum habitat range for each biotope could be defined based on the range of each environmental variable where the probability of occurrence, divided by the maximum probability of occurrence, is 0.75 or higher.  These results identify the range for each significant variable where the habitat is most likely to occur.  The modelled ranges should be interpreted with caution and apply to the Severn Estuary alone (which experiences large tidal ranges, high currents and extremely high suspended sediment loads and is therefore distinct from many other estuarine systems).  However, these ranges do provide some useful information on environmental tolerances.  The models indicate that for subtidal Sabellaria alveolata the optimal mean neap sediment concentrations range from 515.7-906 mg/l and optimal mean spring sediment concentrations range from 855.3-1631 mg/l.  The upper levels of these modelled optima broadly correspond with observations by Cayocca et al. (2008, cited in Dubois et al. 2009) who recorded SPM peaks ranging between 200 and 1000 mg/l depending on the flow and ebb conditions, in the vicinity of the largest Sabellaria alveolata reef in the Bay of Mont-Saint-Michel. Outside of these peaks the SPM remained around 50 mg/l the level at which Dubois et al. (2009) recorded changes in clearance rate.

Sensitivity assessment. Sabellaria alveolata is adapted to turbid systems and can maintain its filtering activity under high seston loads (Dubois et al., 2009).   A supply of suspended sediment is a requirement for the development of reefs (Cunningham et al. 1984). Based on Cayocca et al. (2008, cited in Dubois et al., 2009) the normal range of SPM in which Sabellaria alveolata reefs occur is probably in the intermediate range (based on UKTAG, 2014 ranks).  It is therefore considered that Sabellaria alveolata reef biotoes are ‘Not sensitive’ to increases in peak suspended sediment concentration to the medium turbidity level (100-300 mg/l)  at the pressure benchmark . However, if the increase was constant then reductions in filtration efficiency may negatively affect a proportion of the population , resistance was therefore assessed as ‘Medium’ and recovery as ‘High’ following habitat recovery. Sensitivity is therefore considered to be ‘Low’.  But, a reduction from intermediate levels to clear (<10 mg/l) where the reduction is due to a reduced supply of organic matter and particulate matter suitable for tube building and food may restrict reef development and reduce the food supply to this species. Resistance was assessed as ‘Low’ and recovery as ‘Medium’ so that overall sensitivity is considered to be ‘Medium’. 

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

Sabellaria alveolata was reported to survive short-term burial for days and even weeks in the south west of England as a result of storms that altered sand levels up to two meters.  They were, however killed by longer-term burial (Earll & Erwin 1983). In Brittany intensive mussel cultivation on ropes wound around intertidal oak stakes affected nearby Sabellaria alveolata reefs by smothering with faeces and pseudofaeces, though it was not clear if this resulted in any harm (cited from Holt et al. 1998, no reference given). It should be noted that if siltation is associated with altered water flows to allow accumulation, then long-term habitat suitability for this species would be unfavourably altered .

Sensitivity assessment.  Where siltation does occur, currents are likely to rapidly remove silty deposits. As reefs have some resistance to periodic smothering and burial, resistance to siltation is assessed as ‘High’ and recovery as ‘High’, so that this biotope is considered to be ‘Not Sensitive’.

Smothering and siltation rate changes (heavy)

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

Evidence

Sabellaria alveolata was reported to survive short-term burial for days and even weeks in the south west of England as a result of storms that altered sand levels up to two meters. they were, however killed by longer-term burial (Earll & Erwin 1983). Sabellaria alveolata has been identified as sensitive to changes in sediment regime in the Mediterranean Gulf of Valencia, Spain, where Sabellaria alveolata populations were lost as a result of sand level rise resulting from the construction of seawalls, marinas/harbours, and beach nourishment projects (Porras et al., 1996). It is likely that the length of survival, while dependent on length of burial, may be influenced by temperatures and oxygen levels so that seasonality and the depth and character of overburden partially determine sensitivity.

Sensitivity assessment.  Natural events such as storms may lead to episodic burial by coarse sediments with subsequent removal by water action and the degree of mortality will depend on a number of factors including the length of burial. As fine sediments may be relatively cohesive and as water and air penetration is limited the addition of an overburden of 30 cm is considered to potentially lead to some mortality if large areas are impacted. Resistance is therefore assessed as ‘Low’ and recovery is assessed as ‘Medium’, and sensitivity to this pressure is categorised as ‘Medium’.

Litter

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

Evidence

Not assessed.. 

Electromagnetic changes

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

Evidence

No evidence.

Underwater noise changes

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

Evidence

Not relevant.

Introduction of light or shading

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

Evidence

No 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 that reduce the degree of tidal excursion may reduce the supply of Sabellaria alveolata larvae moving landwards to suitable habitats from source populations. However the presence of barriers may enhance local population supply by preventing the seaward loss of larvae. The residual tidal currents in Bay of Mont-Saint-Saint Michel (France) naturally prevent the loss of larvae from the bay and are believed to enhance settlement locally (Dubois et al., 2007). This species is therefore potentially sensitive to barriers that restrict water movements, whether this will lead to beneficial or negative effects will depend on whether enclosed populations are sources of larvae or are ‘sink’ populations that depend on outside supply of larvae to sustain the local population.

Sensitivity assessment. As this habitat is potentially sensitive to changes in tidal excursion and exchange, resistance is assessed as ‘Medium’ and resilience as ‘High’, sensitivity is therefore ‘Low’.

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

Sabellaria alveolata is not farmed or translocated, therefore this pressure is 'Not relevant'.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

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 a 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).

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 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).

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).  

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 northeastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019). It was also suggested that some Magallana gigas populations were established in southwest England from France possibly via fouling on ships (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 that 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.

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 ind./m² little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilize the sediment surface locally (Troost, 2010). When such reefs are formed or, particularly when the species colonizes soft sediments such as mud or sand, it can change and affect local communities, by creating hard substrata for mobile species, which might not otherwise be present before the invasion (Padilla, 2010). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; Mckinstry & Jensen, 2013; Tillin et al., 2020). 

Magallana gigas also colonizes littoral intertidal biogenic reefs formed by the blue mussel Mytilus edulis or honeycomb worm Sabellaria alveolata (GBNNSS, 2011, 2012; Kochmann, 2012; Kochmann et al., 2013; Herbert et al., 2016; Tillin et al., 2020). The colonization and overgrowth of Magallana gigas may have impacts on Sabellaria alveolata and its habitat formation (Herbert et al., 2012, 2016). Pacific oysters may smother Sabellaria alveolata because it grows over tube ends and could outcompete it for space (Dubois et al., 2006; Desroy et al., 2011). The colonization of the Pacific oyster has been linked to the degradation and deterioration of Sabellaria reef health (Desroy et al., 2011). Desroy et al. (2011) reported several contributing factors, including an increase in silt deposits and fine particles in the sediment from pseudo-faeces produced by the oysters, which can cause increased sedimentation and nutrient enrichment (Green & Crowe, 2013). It has been suggested the increased sediment from oysters might explain why some species normally found in muddy-sand environments were present, further creating new species associations (Dubois et al., 2006).

Dubois et al. (2006) found that Magallana gigas had invaded some Sabellaria alveolata reefs in the Bay of Mont-Saint Michel, France, resulting in densities of more than 100 oysters /m² on some of them. In this area, Sabellaria alveolata reefs were the only available hard substratum for settlement of the Pacific oyster. The study found that an intermediate covering of the Pacific oyster introduced greater species richness and heterogeneity of diversity on the Sabellaria reefs by creating hard substrata habitats and refuges for sessile or mobile species not usually present (Dubois et al., 2006). Green & Crowe (2013) found less percentage cover of Sabellaria alveolata on boulders colonized by Magallana gigas in Ireland. 

The presence of other filter feeders such as Magallana gigas increases trophic competition (Desroy et al., 2011, Green & Crowe, 2013). However, high densities of filter-feeding species alter the settlement of particulate matter and larvae as turbulence in the water column is increased (Green & Crowe, 2013). The physical structure of the oyster beds changes the hydrography and provides refuge from predators for oyster larvae, which increases their recruitment (Soniat et al., 2004). Pacific oysters might improve the recruitment of Sabellaria alveolata by increasing the probability of Sabellaria larvae swimming or sinking down the water column (Tillin et al., 2020). However, Dubois et al. (2006) reported that the abundance of smaller class sizes of Sabellaria alveolata was reduced in Sabellaria reefs with epibionts (Pacific oyster or Ulva spp.) indicating negative impacts of Pacific oysters on recruitment, although not as marked as in the presence of algae (Padilla, 2010; Tillin et al., 2020). Secondary impacts have also been reported, including increased recreational harvesting of the oysters on Sabellaria reefs, which led to reef trampling, physical damage and fragmentation (Dubois et al., 2006; Desroy et al., 2011). In the northern part of Bourgneuf Bay, France Magallana gigas was observed in rocky areas usually occupied by Sabellaria alveolata (Cognie et al., 2006; Herbert et al., 2012, 2016). Cognie et al. (2006) suggested that Magallana gigas could compete with Sabellaria alveolata for food and space, leading Herbert et al. (2012) to suggest that the Pacific oyster may prevent new colonization by Sabellaria alveolata.

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 slipper limpet Crepidula fornicata was recorded on intertidal Sabellaria alveolata reefs in Champeaux, west Cotentin coast, northern France, at a low density of ca 0.75+/-2 /m² (Schlung et al., 2016) Powell-Jennings & Calloway (2018) reported that Crepidula had a preference for hard grounds colonized by Sabellaria alveolata in Swansea Bay, south Wales, with over 80% of the records of Crepidula associated with the Sabellaria alveolata reef. However, no evidence of their relationship was available and they may be in completion or facilitate each other's presence (Powell-Jennings & Calloway, 2018). Crepidula has not yet been recorded from sublittoral Sabellaria alveolata reefs.

Sensitivity assessment. The evidence above suggests that both Crepidula and Magallana have the potential to colonize intertidal Sabellaria alveolata reefs. 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). However, this sublittoral biotope is exposed or moderately exposed to, or sheltered from wave action. Hence, colonization by Crepidula may be prevented or limited to only low densities in areas subject to wave action and especially winter storms. However, Crepidula may colonize sheltered examples of the biotope 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), as occurs in maerl gravels (Grall & Hall-Spencer, 2003), resulting in the loss of the biotope. There is no evidence to suggest that Crepidula has a detrimental effect on the reefs in the intertidal. However, Crepidula is reported to colonize similar sedimentary habitats depending on wave exposure. Therefore, resistance is assessed as 'Medium' in examples of the biotope exposed to wave action but 'Low' is wave sheltered examples. Resilience is assessed as 'Very low' as a bed of Crepidula would need to be removed (by human intervention) before recovery could begin. Therefore, sensitivity is assessed as 'High' based on the worst-case scenario but with 'Low' confidence due to the lack of evidence for the occurrence of Crepidula in this biotope. 

The above evidence also suggests that mixed sediment habitats, typical of this biotope, could be suitable for the colonization of Magallana gigas due to the presence of gravel, shells, or other hard substrata required for successful settlement and establishment (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). Sabellaria alveolata binds the sediment and may provide additional hard substratum for settlement by Magallana. The presence of Magallana gigas at low densities can increase the species richness and diversity on Sabellaria alveolata reefs, but higher densities of the Pacific oyster may smother Sabellaria alveolata, and outcompete it for space and food (Cognie et al., 2006; Dubois et al., 2006; Desroy et al., 2011). This may prevent the colonization of Sabellaria alveolata in some areas (Herbert et al., 2012). However, Magallana gigas populations may be limited to low densities in wave exposed to moderately wave exposed conditions (Teschke et al., 2020) typical of most (ca 70%) records of this biotope.  Therefore, resistance to colonization by Magallana is assessed as ‘Medium’. Resilience is likely to be 'Very low' as the Magallana gigas population would need to be removed for recovery to occur. Therefore, sensitivity is assessed as ‘Medium'.

Didemnum vexillum has not been recorded colonizing Sabellaria reefs or associating with Sabellaria alveolata. No evidence was found on the potential effects of Didemnum sp. on Sabellaria. The presence of hard substrata,  such as pebbles and cobbles, in this Sabellaria biotope could provide suitable hard substrata for the successful colonization of Didemnum vexillum, which may otherwise not colonize the sandy sediment. Didemnum vexillum has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). Therefore, Didemnum vexillum could colonize examples of these biotopes that occur in tide-swept conditions. Didemnum vexillum has been recorded in the lower intertidal but in the mid-shore examples of the biotope, the abundance and extent of colonies may be limited due to emersion. Didemnum vexillum colonies can survive exposure to air at low tides for a short time (not exceeding two hours) (Valentine et al., 2007a). It is also unclear if Sabellaria could out-compete Didemnum sp. for space, or vice-versa. If Didemnum sp. was able to gain a 'foothold' it could overgrow, smother or cause mortality on the Sabellaria reefs. 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 is able to 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'. 

Introduction of microbial pathogens

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

Evidence

No evidence found for pathogens or diseases impacting Sabellaria alveolata.

Removal of target species

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

Evidence

Sabellaria alveolata biotopes may be removed or damaged through contact with static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. No evidence was found for trophic or other ecological interactions between commercially targeted species and Sabellaria alveolata.

Sensitivity assessment. As Sabellaria alveolata is not commercially targeted the habitat is assessed as ‘Not Sensitive’. Resistance is therefore assessed as ‘High’, resilience as ‘High’ and sensitivity as 'Not sensitive'.

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

Sabellaria alveolata biotopes may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures.  Sabellaria alveolata creates the biogenic reefs that characterise this biotope, removal of this species as by-catch would therefore remove the biotope.  No evidence was found for key trophic or other ecological interactions between other species within the biotope and Sabellaria alveolata.

Sensitivity assessment. Removal of the worms and tubes as by-catch would remove the biotope and hence this group is considered to have ‘None’ resistance to this pressure and to have ‘Medium’ recovery. Sensitivity is therefore ‘Medium’.