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

No empirical evidence was found for the temperature tolerance of Sabellaria spinulosa; nevertheless, its widespread distribution suggests that it is tolerant to temperature variations (Gibb et al., 2014). Sabellaria spinulosa has the greatest geographical range of all the sabellariids, according to current records, encompassing Iceland, the Skagerrak and the Kattegat, the North Sea, the English Channel, the northeast Atlantic, the Mediterranean, the Wadden Sea and the Indian Ocean, (Achari, 1974; Riesen & Reise, 1982; Reise & Schubert,1987; Hayward & Ryland, 1998; Foster-Smith, 2000; Collins, 2005).

Temperature decrease (local)

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

Evidence

Sabellaria spinulosa occurs north to the Arctic and is therefore considered tolerant of decrease in temperature at the pressure benchmark. This conclusion is supported by observations made on oyster grounds in the River Crouch throughout the severe winter of 1962–1963 that Sabellaria spinulosa appeared unaffected by the cold. The mean daily temperature was recorded at a depth of 1 fathom (1.8 m) below low water (equinoctial spring tide) and the lowest temperature recorded was -1.8°C (Crisp, 1964). At Penmon in Bangor, Sabellaria spinulosa also appeared not to suffer from the low temperatures and live individuals were readily found (Crisp, 1964). 

Sensitivity assessment. Given the widespread distribution of Sabellaria spinulosa it is unlikely that this species is sensitive to temperature variations at the pressure benchmark. Resistance is therefore assessed as ‘High’ and resilience is assessed as ‘High’ (no impact to recover from), so that all the Sabellaria biotopes within this group are assessed as ‘Not Sensitive’.

Salinity increase (local)

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

Evidence

No evidence for the range of physiological tolerances to salinity changes were found for Sabellaria spinulosa by Gibb et al. (2014).  As reefs are largely subtidal they are less exposed to hypersaline conditions resulting from coastal brine discharge and natural evaporation (lagoons).  There is therefore no direct or indirect evidence for sensitivity to an increase in salinity and this element of the pressure is not assessed. 

Sensitivity assessment. No evidence was found for tolerance of hypersaline conditions and sensitivity to this benchmark is not assessed based on lack of evidence.

Salinity decrease (local)

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

Evidence

No evidence for the range of physiological tolerances to decreases in salinity was found for Sabellaria spinulosa by Gibb et al. (2014).  The sensitivity assessment made in that report was therefore based on recorded habitat preferences, as described below. 

Sabellaria spinulosa does not seem to occur in very low salinity areas (Holt et al.,1998) but has been recorded from estuaries including the Crouch, Mersey (Killeen & Light, 2000) and the Thames (Limpenny, 2010).  Buhs & Reise (1997) surveyed 12 channel systems in the Wadden Sea and found that Sabellaria spinulosa reefs occurred in the northern tidal inlets which experienced salinity levels ranging from 28 to 30 psu.  There is some speculation (Foster-Smith & Hendrick, 2003) that Mcintosh (1922) misidentified samples of Sabellaria spinulosa as the congener Sabellaria alveolata from the Humber estuarine population (Holt et al., 1998).  These records indicate that reduced and variable salinities can be tolerated to some extent but the paucity of records suggests that areas of reduced salinity do not provide optimal habitat.

Sensitivity assessment. As the salinity tolerances of Sabellaria spinulosa are unclear the potential impact of salinity change, at the pressure benchmark, is uncertain.  The reported distribution of Sabellaria spinulosa from fully marine to estuarine habitats does suggest some tolerance of changes in salinity although a decrease in salinity at the extreme of the pressure benchmark (reduction in 10 psu) may not be tolerated.  Resistance is therefore assessed as ‘Low’ (loss of 25-75% of extent).  Reef resilience (following habitat recovery) is considered to be “Medium’ (2-10 years).  Sensitivity, based on combined resistance and resilience, is therefore assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

Sabellaria spinulosa tend to occur in areas of high water movement where larvae, tube building materials and food particles are suspended and transported (Jones et al., 2000).  The relative importance of tidal versus wave induced movements to support reefs is, however, unclear (Holt et al., 1998). There is currently limited in-situ data on the specific water flow tolerances of Sabellaria spinulosa, although colonies have been found in areas with sedimentary bed forms that suggest current velocities in the range of 0.5 m/s to 1.0 m/s (Mistakidis, 1956; Jones et al., 2000; Davies et al., 2009).  In the southern North Sea close to the coast of England, Sabellaria spinulosa reefs have been recorded in areas exposed to peak spring tidal flows of 1.0 m/s (Pearce et al., 2014). Davies et al. (2009) also found through laboratory experiments with Sabellaria spinulosa in tanks that increasing the water flow to an average of 0.03 m/s is adequate to begin distribution of the sediment rain from the airlift throughout the tank and that doubling the water flow to almost 0.07 m/s further improved particle distribution throughout the tank.  It is therefore likely that Sabellaria spinulosa will exist in habitats with a water flow anywhere above 0.07 m/s so that particles are suspended and distributed for the use of tube building and feeding. 

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of the congener 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 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 not directly applicable to Sabellaria spinulosa this data suggests that tube-building Sabellariids are able to occur within a broad range of current speeds.

In cases of reduced water flow, Sabellaria spinulosa is likely to suffer a reduction in the supply of suspended food and particles that are integral for growth and repair.  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 impact.

Sensitivity assessment. The range of flow tolerances recorded (0.5 m/s to 1 m/s cited by Jones et al., 2000; Braithwaite et al., 2006; Davies et al., 2009) suggest that the worms have a broad tolerance of different flow levels.  Tillin (2010) modelled optimal flow speeds of 0.5-1.22 m/s for the congener Sabellaria alveolata.  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 range.  Similarly, a reduction in flow may reduce the supply of tube-building materials and food but again, given the range of reported tolerances a change at the pressure benchmark 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 subtidal habitats.

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

No empirical evidence was found to assess this pressure.  Intertidal Sabellaria spinulosa is directly exposed to waves but wave exposure can also potentially affect subtidal Sabellaria spinulosa reefs.  At depth, the motion from surface waves becomes oscillatory and produces back-and-forth water movement at the seabed (Dubois et al., 2006).  In sublittoral habitats water movements are likely to provide sand and food particles that are necessary for Sabellaria spinulosa to build tubes, feed and subsequently grow and develop.  

Sensitivity assessment. As Sabellaria spinulosa reefs are robust, stable structures that are present subtidally in naturally disturbed environments and areas with high water flow, changes (decrease or increase) in wave height at the pressure benchmark are not considered to affect reefs.  All biotopes within this group are therefore considered to have ‘High’ resistance to this pressure, resilience is assessed as ‘High’ (no impact to recover from) and all subtidal reef biotopes are considered to be ‘Not Sensitive’.  Intertidal populations of Sabellaria spinulosa would be more exposed to the impacts of wave exposure but the corresponding biotopes for these habitats are not included in this assessment.  

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

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

Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. No information was found regarding Sabellaria spinulosa tolerance to changes in oxygenation and this pressure is not assessed due to lack of evidence.

Nutrient enrichment

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

Evidence

No direct evidence was found to assess this pressure. As the reefs are circalittoral increased nutrient enrichment (eutrophication) would not stimulate the overgrowth of macroa algae on reefs as light penetration is too limited (especially in turbid areas) to allow growth. Enhanced phytoplanton production may increase food suply and increased siltation and deoxygenation from algal blooms is likely to be mitigated by water movements in the areas most suitable for Sabellaria spinulosa reefs. 

Sensitivity assessment. At the pressure benchmark, which refers to the maintenance of 'good' status according to the Water Framework Directive, Sabellaria spinulosa reefs are considered 'Not sensitive'. 

Organic enrichment

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

Evidence

Limited direct evidence was found to assess the effects of this pressure.  Sabellaria spinulosa was reported to show enhanced growth adjacent to a sludge dumping area in Dublin Bay (Walker & Rees 1980).  Hence, Sabellaria spinulosa reef biotopes are probably resistant to a high level of organic enrichment.  Information on the levels of organic matter in Dublin Bay were not provided and so it is unclear how the levels experienced relate to the pressure benchmark. 

Sabellaria spinulosa reefs are found in areas of high water movement of up to 1 m/s (Pearce et al., 2014, see change in water flow for further details) that would naturally disperse some organic matter preventing accumulation and siltation.  In larger, dense colonies of Sabellaria spinulosa, sand, detritus, and finer faecal materials collect in between worm tubes.  These detritus layers do not interrupt the normal growth of the individuals or of the colony as a whole (Schafer, 1972).  Taking into consideration these points it seems likely that Sabellaria spinulosa are resistant to the deposition of a fine layer of organic materials.

Indirect effects arising from inputs of organic matter are possible where habitat quality and species interactions are altered.  In the Wadden Sea large subtidal areas of Sabellaria spinulosa reefs have been completely lost since the 1920s.  This decline has been partly attributed to an increase in coastal eutrophication that has favoured blue mussel beds (Dörjes, 1992; Hayward & Ryland, 1998; Benson et al., 2013).  However, a direct causal link has not been established and it is possible that the decline of Sabellaria spinulosa reefs was due to physical damage from fishing activities rather than competitive interactions (Jones et al., 2000).

Sensitivity assessment. Little 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 spinulosa and the associated species assemblage (which typically includes attached filter feeders from a number of phyla) is likely to be able to consume extra organic matter.  This conclusion is supported by the enhanced growth rates 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).  All Sabellaria spinulosa reef biotopes within this group are considered to be ‘Not Sensitive’ at the pressure benchmark. 

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.

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 artificial structures and have been found on a gas pipeline off the coast of Aberdeen (Mistakidis, 1956).  An increase in the availability of artificial substratum may support reef development in areas where sedimentary habitats were previously unsuitable for colonisation.  

Where rock substrata are overlain by sediments the impact on habitat suitability for Sabellaria spinulosa will depend on the sediment changes that result.  Sabellaria spinulosa reefs can develop on stable or coarse mixed substrata but mud and sandy muds are considered unsuitable, based on the lack of records of reefs occurring on these sediment types which 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.  It should be noted that the basis of the sensitivity assessment for this pressure is the sensitivity of the biotope to changes in substratum type, rather than the sensitivity of Sabellaria spinulosa to the mechanism by which this occurs (e.g. complete smothering of the biotope, then any recovery, if possible). A permanent change in substratum type to coarse mixed sediments would alter the character of the biotope and lead to re-classification to a Sabellaria spinulosa reef on mixed sediment biotope (SS.SBR.PoR.SspiMx).  An increase in fine sediments to the degree that sediments are re-classified as mud or sandy mud would severely reduce habitat suitability and is considered to prevent subsequent recovery of the reef.  The resistance of the biotope to this pressure 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.

Physical change (to another sediment type)

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

Evidence

Not relevant to biotopes occurring on bedrock.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Sabellaria spinulosa reefs on rock would be sensitive to the removal of surface layers of the reef and the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Sabellaria spinulosa reef biotopes are directly exposed to physical damage that affects the surface.  The effects of abrasion coupled with penetration of sub-surface layers are described in a separate section below, this section describes abrasion of the surface only.

Gibb et al. (2014) found no direct evidence for impacts of the surface only for Sabellaria spinulosa.  Studies of intertidal reefs of the congener Sabellaria alveolata (Cunningham et al.,1984) have found that the reef recovered within 23 days from the effects of trampling, (i.e. treading, walking or stamping on the reef structures) by repairing minor damage to the worm tube porches.  Severe damage caused by kicking and jumping on the reef structure, resulted in large cracks between the tubes, and removal of sections (ca 15 x15 x10 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. 

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 and, although the trawl had left impressions, 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. 

Sabellaria spinulosa reefs are suggested to be more fragile than Sabellaria alveolata (B. Pearce, pers comm, cited from Gibb et al., 2014) and therefore surface abrasion may lead to greater damage and lower recovery rates than observed for Sabellaria alveolata.  Sabellaria spinulosa reefs are often only approximately 10cm thick, surface abrasion can, therefore, severely damage and/or remove a reef (see also evidence from penetration and disturbance of the sunbstratum, below).No direct observations of reef recovery, through repair, from abrasion were found for Sabellaria spinulosa.

Sensitivity assessment. Based on the evidence discussed above, abrasion at the surface of Sabellaria spinulosa reefs is considered likely to damage the tubes and result in sub-lethal and lethal damage to the worms.  Resistance is therefore assessed as ‘Low’ (loss of 25-75% of tubes and worms within the impact footprint).  Resilience is therefore assessed as ‘Medium’ (within 2 years) and sensitivity is therefore assessed as ‘Medium’.  This assessment is relatively precautionary and it should be noted the degree of resilience will be mediated by the character of the impact.  The recovery of small areas of surficial damage in thick reefs is likely to occur through tube repair and may be relatively rapid. 

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

Sabellaria spinulosa reef biotopes are directly exposed to physical damage that affects the surface layers (abrasion) and penetrates deeper beneath the surface of the reef.   No quantitative studies were found and although Vorberg (2000) is widely cited (see above in the abrasion section) the study used a light shrimp trawl on Sabellaria alveolata reefs and the relevance to Sabellaria spinulosa and the use of heavy fishing trawls is questionable.

Sabellaria spinulosa reefs in the Wadden Sea suffered great losses in the 1950s which are thought to be due to heavy anchor chains being trawled over grounds in association with shrimp fishing (Reise &Schubert, 1987; JNCC, 2013).  It is believed that local fishermen targeted areas of Sabellaria spinulosa reef due to their association with the brown shrimp Crangon crangon, and that deliberate attempts to remove the reefs were made so that fishing gear was not snagged and damaged  (Defra, 2004; JNCC, 2013).  Similar activity has been reported by fishermen at Ramsgate on Sabellaria spinulosa reefs in the Thames sea area but no direct evidence has been identified (Fariñas-Franco, 2014). 

Other studies have found significant evidence of trawl scars from unspecified fisheries through Sabellaria spinulosa reefs (Collins, 2003; Pearce et al., 2007) indicating that damage from fishing gear is a real possibility (Hendrick et al., 2011).  Obvious evidence of the destruction of Sabellaria spinulosa reef clumps by a beam trawler has been reported off the coast of Swanage, Dorset (Collins, 2003; cited from Benson et al., 2013).  The loss of reefs within a monitoring zone may have been due to bottom trawling based on the presence of trawl scars within the survey area, although the loss cannot be directly attributed to this activity based on the lack of direct observation (Pearce et al., 2011a).

Sabellaria spinulosa reefs remain extensive despite clear damage from bottom trawling at Hastings Shingle Bank (Cooper et al., 2007; Pearce et al., 2007) and at the Thanet offshore wind farm site (Pearce et al., in press).  However, in other areas such as the Wadden Sea (Riesen & Reise, 1982) and Morecambe Bay (see references in Holt et al., 1998), reefs which have been thought to have been trawled have disappeared and have not recovered.  It is acknowledged that the limited evidence available does not allow these losses to be directly attributed to fishing.

At the Hastings site, a newly developed reef (six months old) demonstrated the same multivariate community structure of fauna inhabiting a nearby reef that had been observed over the past 5 years (Pearce et al., 2007).  This suggests that the epifauna community associated with Sabellaria spinulosa reefs could also recover from fishing activity quickly, but it should be noted that the older reef had experienced on-going fishing activity and so the associated assemblage may be at a relatively early successional stage  (Pearce et al., 2007). The quick recovery of the reef and associated biota was not seen in the Wadden Sea after shrimping activity in the 1950’s.  Instead, together with a loss of mussel beds and seagrass, community composition in the subtidal zone changed and a significant decline in sessile species was observed (Reise et al., 1989; Buhs & Reise, 1997; Reise & Buhs, 1999; Reise, 2005).

Sensitivity assessment. Structural damage to the seabed sub-surface is likely to damage and break-up tube aggregations leading to the loss of reef within the footprint of direct impact.  Sabellaria spinulosa is assessed as having a resistance of 'None' to this pressure (removal of >75% of the reef in the pressure footprint).  Based on evidence (Pearce et al., 2007; Pearce et al., 2011a) resilience was assessed as ‘Medium’.  Therefore, the sensitivity of Sabellaria spinulosa biotopes within this group is considered to be ‘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 spinulosa do not rely on light penetration for photosynthesis, it is also believed that visual perception is limited and that this species does not rely on sight to locate food or other resources.  In a recent review of the sensitivity of Sabellaira spinulosa reefs to anthropogenic disturbance, Fariñas-Franco et al. (2014) concluded that impacts on Sabellaria spinulosa due to a decrease in water clarity resulting from an increase in suspended solids (inorganic or organic) are unlikely although no thresholds regarding tolerance or intolerance were found.  Decreases in suspended particles that reduce the supply of food or tube-building materials may, however, negatively impact this species (Davies et al., 2009; Last et al., 2011).

Sabellaria spinulosa relies on a supply of suspended solids and organic matter in order to filter feed and build protective tubes and so they are often found in areas with high levels of turbidity.  Davies, et al. (2009) and Last, et al. (2011) developed Vortex Resuspension Tanks (VoRT) which are able to test the effects of a change in the composition of suspended sediment on benthic species.  This laboratory experiment manipulated turbidity and current flow and demonstrated the susceptibility of Sabellaria spinulosa to a decrease in suspended particulate matter (SPM).  A clear erosion of tubes was observed in the absence of SPM and subsequent starvation of tube building materials.  At high and intermediate sediment regimes (high SPM ~71 mg / l) conditions were comparable to what might be expected within only a few hundred meters distance of a primary aggregate extraction site and Sabellaria spinulosa maintained a cumulative growth rate at these rates of SPM.  This supports the view that availability of suspended particles is necessary for Sabellaria spinulosa development and that tolerance of elevated levels is likely (Davies et al., 2009).

Indirect evidence for the tolerance of Sabellaria spinulosa for changes in turbidity is provided by the persistence of reefs on the outskirts of aggregate dredging areas (Pearce et al., 2007, 2011a) which appear unaffected by extraction which is likely to have led to sediment plumes.  Such plumes, however, are short-lived (Tillin et al., 2011) and therefore the long-term effect depends on the duration of dredging activities.

Tillin (2010) used logistic regression to develop statistical models that indicate how the probability of occurrence of the congener 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.  Although not directly applicable to S. spinulosa this data suggests that tube-building sabellariids are tolerant to very high levels of suspended sediment.  Fine sediments such as mud may clog the gills and feeding tentacles of some polychaetes and therefore the potential impact will be mediated by the character of the pressure.

Sensitivity assessment. The benchmark for this pressure refers to a change in turbidity of one rank.  Sabellaria spinulosa do not photosynthesise and do not rely on sight to locate resources and therefore no effects are predicted for reef biotopes from an increase or decrease in clarity resulting from a change in one rank on the water framework directive scale.  Experiments (Davies et al., 2009) and predictive modelling (Tillin, 2010) indicate that tube building sabellariids can tolerate a broad range of suspended solids. Resistance to this pressure is therefore assessed as ‘High’ and resilience as ‘High’ (no impact to recover from). 

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 spinulosa are often found in areas of high water movement with some degree of sediment transport essential for tube-building and feeding.  Sabellaria spinulosa reefs adjacent to aggregate dredging areas appear unimpacted by dredging operations (Pearce et al., 2007; Pearce et al., 2011a).  Evidence suggests that given the dynamic sedimentary environments in which sabellariids live, their populations can certainly persevere in turbid conditions in spite of ‘typical’ natural levels of burial (Last et al., 2011) and that recovery from burial events is high.  

Direct evidence for the effects of siltation on Sabellaria spinulosa is limited to the experiments undertaken by Last et al. (2011).  Last et al. (2011) buried Sabellaria spinulosa worms (isolated into artificial tubes), under three different depths of sediment – shallow (2 cm), medium (5 cm) and deep (7 cm).  The results indicate that Sabellaria spinulosa can survive short-term (32 days), periodic sand burial of up to 7 cm.  Last et al. (2011) suggested that the formation of ‘emergence tubes’ (newly created tubes extending to the surface) under sediment burial allowed Sabellaria spinulosa to tolerate gradual burial and that perhaps this mechanism allows for continued adult dispersal.  This mechanism occurred most rapidly throughout the 8-day burial at ~1mm per day (Last et al., 2011) but even though tube-growth still seems possible under burial, it is likely that a dumping of fine and coarse material will block feeding apparatus and therefore worm development will be curtailed.

The evidence above suggests that Ross worm reefs are sensitive to damage from siltation events (Hendrick et al., 2011).  However, at the pressure benchmark, the depth of burial is likely to be similar or less than that experienced during natural storm events that move sediments and, in areas of high water movement, deposits of fine sediments are likely to be remobilised and moved.

Sensitivity assessment. In areas of high water flow dispersion of fine sediments may be rapid and this will mitigate the magnitude of this pressure by reducing the time exposed.  Based on the experiments by Last et al. (2011) which are considered relevant to the pressure benchmark, resistance and resilience are assessed as ‘High’ and 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 spinulosa are often found in areas of high water movement with some degree of sediment transport essential for tube-building and feeding (Jackson & Hiscock, 2008).  Given their preference for turbid waters their tolerance to the suspension and/or settlement of fine material during adjacent dredging activity may be high (Tyler-Walters, 2007; Jackson & Hiscock, 2008; Tyler-Walters, 2008).  Sabellaria spinulosa reefs adjacent to aggregate dredging areas appear unimpacted by dredging operations (Pearce et al., 2007; Pearce et al., 2011a).  Evidence suggests that given the dynamic sedimentary environments in which sabellariids live, their populations can certainly persevere in turbid conditions in spite of ‘typical’ natural levels of burial (Last et al., 2011) and that recovery from burial events is high.  The congener S. alveolata was reported to survive short-term burial for days and even weeks in the south west as a result of storms that altered sand levels up to two meters, they were, however, killed by longer-term burial (Earll & Erwin 1983).

Direct evidence for the effects of siltation on Sabellaria spinulosa is limited to the experiments undertaken by Last et al. (2011).  The experimental conditions do not, however, relate to the pressure benchmark (30 cm of siltation in a single event).  Last et al. (2011) buried Sabellaria spinulosa worms (isolated into artificial tubes), under three different depths of sediment – shallow (2 cm), medium (5 cm) and deep (7 cm).  The results indicate that Sabellaria spinulosa can survive short-term (32 days), periodic sand burial of up to 7 cm.  Last et al. (2011) suggested that the formation of ‘emergence tubes’ (newly created tubes extending to the surface) under sediment burial allowed Sabellaria spinulosa to tolerate gradual burial and that perhaps this mechanism allows for continued adult dispersal.  This mechanism occurred most rapidly throughout the 8-day burial at ~1mm per day (Last et al., 2011) but even though tube-growth still seems possible under burial, it is likely that a dumping of fine and coarse material will block feeding apparatus and therefore worm development will be curtailed.

A Sabellaria spinulosa reef off the coast of Dorset has shown periodic burial from large sand waves (Collins, 2003).  The displacement of some colonies that had established themselves on a gas pipeline 1km off the coast of Aberdeen was also associated with burial (Mistakidis, 1956; cited by Holt et al., 1998).  Furthermore the loss of a 2 km² area of Ross worm reef in Jade Bay, North Sea was attributed to burial as a consequence of mud deposition, although fishing activity may have contributed to the decline (Dörjes, 1992, cited from Hendrick et al., 2011).

The evidence above suggests that Sabellaria spinulosa reefs are sensitive to damage from siltation events (Hendrick et al., 2011).  However, recovery is likely to be rapid given that larval dispersal is not interrupted and new reefs are likely to be able to establish themselves over old buried ones as postulated by (Fariñas-Franco et al., 2014).

Sensitivity assessment. No direct evidence was found for the length of time that Sabellaria spinulosa can survive beneath 30 cm of sediment.  In areas of high water flow dispersion of fine sediments may be rapid and this will mitigate the magnitude of this pressure by reducing the time exposed.  However, this mitigating effect was not taken into account as it depends on site-specific conditions.  Resistance was assessed as ‘None’ due to the depth of overburden. Resilience was assessed as ‘Medium’ (2-10 years) and sensitivity was therefore 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

Not relevant.

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

No direct evidence was found to assess this pressure. As the larvae of Sabellaria spinulosa are planktonic and are transported by water movements, barriers that reduce the degree of tidal excursion may alter the supply of Sabellaria spinulosa to suitable habitats from source populations. However, the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  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’. It should be noted that offshore circalittoral habitats are unlikely to be exposed to this pressure.

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 spinulosa and the associated species found within the habitat complex are not farmed or translocated and 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

No direct evidence relating to the impacts of the introduction of non-indigenous species on Sabellaria spinulosa reefs was found by Gibb et al. (2014). There are no records to suggest that their distribution overlaps with Sabellaria spinulosa reefs for many of the non-indigenous species that are found in UK seabed habitats,

The oyster drill, Urosalpinx cinerea, is not known to predate on polychaetes (Brown & Richardson, 1988), therefore, their introduction is not considered a threat to Sabellaria spinulosa. There is, however, some overlap between the environmental niche of Sabellaria spinulosa and the oysters that Urosalpinx cinerara selectively feed on (Brown & Richardson, 1988). Japanese wireweed Sargassum muticum and green sea fingers Codium fragile have the potential to compete for space where Sabellaria spinulosa reefs occur intertidally, however, intertidal biotopes are not included in this assessment and these species are unlikely to impact deeper subtidal reefs. No records of the reef-building serpulid Ficopomatus enigmaticus, the colonial sea squirt Perophora japonica, or Japanese kelp Undaria pinnatifida, suggest these species occur on or near Sabellaria spinulosa reefs. However, the further spread may impact subtidal Sabellaria spinulosa reefs through smothering or competition, although this is entirely speculative. Sabellaria spinulosa reefs support a variety of attached epifauna including species of bryozoans, hydroids and sponges. As Sabellaria spinulosa reefs are known to support encrusting organisms without apparent adverse effects, the stalked sea squirt Styela clava, as a solitary sea squirt is considered unlikely to have greater negative impacts than native species.

Two species that potentially pose a threat to Sabellaria spinulosa reefs are the Pacific oyster Magallana gigas and the slipper limpet Crepidula fornicata. Reefs of Sabellaria alveolata in the bay of Mont Saint Michel, France are becoming increasingly colonized by the Pacific oyster Magallana gigas (Dubois et al., 2006). Given the high filtration rates of Magallana gigas, it is believed that they can out-compete Sabellaria alveolata for feeding resources (Dubois et al., 2006). In the Wadden Sea, Magallana gigas has replaced blue mussels (Foster-Smith, 2000) suggesting that Magallana gigas may impact filter-feeding, reef-forming organisms in general. The reasons underlying the species shift from Mytilus edulis to Magallana gigas have not been elucidated, however, and may be due to recent changes in climatic conditions (Thieltges, 2005) rather than competitive interactions. It should be noted that even though Magallana gigas is distributed throughout UK waters following an initial introduction in 1926 (Linke, 1951) there is currently no evidence, in the absence of any targeted studies, that this species is impacting native Sabellaria spinulosa or Sabellaria alveolata reefs (Crisp, 1964; Hendrick, et al., 2011).

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

When the slipper limpet Crepidula fornicata settles in an area it can increase the amount of pseudofaeces and subsequently, the substratum may be altered from hard substratum to soft sediment which will reduce the substratum available for settlement by other species. This was observed when a 28-30% mortality of Mytilus edulis occurred after the introduction of Crepidula fornicata to mussel beds (Thieltges, 2005). Crepidula fornicata has been recorded in association with Sabellaria spinulosa reefs at Hastings Shingle Bank (up to 66 individuals per grab, Pearce, 2007) and in lower numbers in the East Coast REC area (maximum 4 per grab (Pearce et al., 2011a)). The relationship between Crepidula fornicata and Sabellaria spinulosa has not been investigated. However, potential impacts on Sabellaria spinulosa reefs could occur through changes to substratum suitability or other interactions. 

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).

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).  

Sensitivity assessment. No evidence was found that non-indigenous species are currently significantly impacting Sabellaria spinulosa reef biotopes. However, it should be noted that Crepidula fornicata and Crassostrea gigas may pose a potential threat in terms of competition for food and space. The circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata due to the wave exposed to moderately wave exposed conditions, in which wave action and storms may mitigate or prevent the colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). However, the presence of mixed substrata, including pebble and cobble (JNCC, 2015), in some examples of the biotope, could provide suitable hard substrata for the colonization by Crepidula, but there is no evidence to suggest Crepidula would establish. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required. 

Didemnum vexillum has not been recorded colonizing Sabellaria reefs or associating with Sabellaria spinulosa. No evidence was found on the potential effects of Didemnum sp. on Sabellaria. Nevertheless, Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This Sabellaria biotope occurs on rock, boulders, cobbles and pebbles, that could provide suitable hard substratum for colonization by Didemnum sp. 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 in tide-swept conditions. Didemnum vexillum regresses as temperatures decline in winter so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold' it might overgrow, smother or cause mortality 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 could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

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 was found for adverse impacts of microbial pathogens on Sabellaria spinulosa.

Removal of target species

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

Evidence

Sabellaria spinulosa may be directly 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. The sensitivity assessment for this pressure considers any biological effects resulting from the removal of target species on Sabellaria spinulosa biotopes.

Sabellaria spinulosa has no economic value and is not commercially harvested. Gibb et al. (2014), evaluated the evidence regarding potential biological effects of the removal of other target species on the Sabellaria spinulosa reef biotopes. Experimental laboratory work reported that scallop shells, especially Pecten maximus, induced Sabellaria spinulosa larvae to settle (Earll & Erwin 1983). However; the settlement-inducing property of Pecten maximus shells related mostly to the upper valve which was covered in sand grains (an existing requirement of larvae settlement) and given the diverse range of substrata that Sabellaria spinulosa have been reported in (see physical change pressure below) it is unlikely that the removal of scallops will have a significant negative impact on larval recruitment.

Gibb et al. (2014) suggest that the removal of target species that prey on Sabellaria spinulosa could potentially be beneficial to this species and Sabellaria spinulosa reefs appear to be important nursery areas for commercially targeted flat fish including Dover sole (Bryony Pearce, pers comm). Assessment of this indirect effect is limited by the lack of empirical evidence for predator-prey relationships. Stomach analysis of fish by Pearce (2001) found that juvenile flatfish captured in reef areas including Dover sole, dab and plaice fed preferentially on Sabellaria spinulosa. Where these species removed as target species (or as by-catch) then predation rates on Sabellaria spinulosa could be reduced. However, as the rate of predation on Sabellaria spinulosa and impacts on reefs through population effects and the rate of removal of the predator species are not known, the impact of this potentially beneficial effect could not be assessed.

Sensitivity assessment. Sabellaria spinulosa has no economic value and is not commercially harvested. The Sabellaria spinulosa biotopes are not, therefore directly impacted by this pressure and all biotopes within this group are considered, by default, to be ‘Not Sensitive’. The removal of target species that predate on Sabellaria spinulosa may have a potentially beneficial effect and the biotope is not considered to be sensitive to ecological effects resulting from their removal. 

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 spinulosa 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. Evidence for ecological interactions between Sabellaria spinulosa and other species is limited. The removal of Sabellaria spinulosa predators as bycatch may be beneficial. Pearce et al. (2011b) found that as well as the commercially targeted species described for the removal of target species pressure, butterfish Pholis gunnelis and dragonet Callionymus lyra predated on Sabellaria spinulosa. Previous studies have also shown that Carcinus maenas feeds on Sabellaria spinulosa (Taylor, 1962; Bamber & Irving, 1997). Other invertebrates such as Pandalus montagui and Asterias rubens found in association with S. spinulosa reefs may also be feeding on the worms or on species associated with the reefs rather than Sabellaria spinulosa. Due to the limited information available on predator-prey relationships, the impact of predator removal on Sabellaria spinulosa reef biotopes cannot be assessed.

Dense aggregations of the brittle star, Ophiothrix fragilis, have been suggested to compete with Sabellaria spinulosa for space and food and potentially to consume the gametes inhibiting recruitment (George & Warwick 1985). Removal of this species as by-catch could potentially be beneficial to the reef biotopes.

A further potential interaction has been identified between Sabellaria spinulosa and the sand mason Lanice conchilega. It has been observed that sand stabilised by the sand mason Lanice conchilega is stable enough for colonization by Sabellaria alveolata (Larsonneur et al., 1994). It is believed that the same may also be possible for Sabellaria spinulosa, as Lanice conchilega and Sabellaria spinulosa are sometimes found together (Holt et al., 1997). However, a decline in Sabellaria spinulosa numbers was coincident with an increase in Lanice conchilega numbers (see Limpenny et al., 2010 and references therein). It is not clear from the available evidence therefore whether removal of Lanice conchilega would have any impact on Sabellaria spinulosa reefs (Foster-Smith 2001a).

Sensitivity assessment.The biogenic structure created by the Sabellaria spinulosa worms is the key characterizing feature of this biotope group. Removal of the worms and tubes as by-catch would remove the biotope and hence this group is considered to have a resistance of 'None' to this pressure and to have ‘Medium’ resilience. Sensitivity is, therefore ‘Medium’.