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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Owenia fusiformis and Amphiura filiformis in offshore circalittoral sand or muddy sand

17-09-2018

Summary

UK and Ireland classification

UK and Ireland classification

Description

Areas of slightly muddy sand (generally <20% mud) in offshore waters may be characterized by high numbers of the tube building polychaete Owenia fusiformis often with the brittlestar Amphiura filiformis. Whilst Owenia fusiformis is also found in other circalittoral or offshore biotopes it usually occurs in lower abundances than in SSA.OfusAfil. Other species found in this community are the polychaetes Goniada maculata, Pholoe inornata, Diplocirrus glaucus, Chaetozone setosa and Spiophanes kroyeri with occasional bivalves such as Timoclea ovata and Thyasira equalis. The sea cucumber Labidoplax buskii (syn. Labidoplax buski) and the cumacean Eudorella truncatula are also commonly often found in this biotope.(Information taken from the revised Marine Habitat Classification, Version 04.05: Connor et al., 2004.)

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Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

SS.SSa.CMuSa.AbraAirr occurs in shallow, circalittoral, non-cohesive muddy sand (typically less than 20% silt/clay). The biotope occurs in a range of hydrographic regimes, including wave exposures ranging from very sheltered to sheltered, moderately exposed and exposed sites, with weak of very weak tidal streams (Connor et al. 2004). Abundant populations of the brittlestar Acrocnida brachiata (syn. Amphiura brachiata) may occur, with other echinoderms such as Astropecten irregularis, Asterias rubens, Ophiura ophiura and Echinocardium cordatum. Other infaunal species typically include Kurtiella bidentata, Lanice conchilega and Magelona filiformis. This biotope is likely to form part of the non-cohesive/cohesive muddy sand communities, which make up the 'offshore muddy sand association' described by other workers (Jones, 1951; Mackie, 1990). Records of the community, such as those provided by Jones (1951) suggested that although there seemed to occur a gradual transition in the community from shallow outwards towards deeper depth, a sufficient number of species was common to the whole to characterize the community, suggesting that the denominated species in this biotope, Acrocnida brachiata, Astropecten irregularis and other echinoderms such as Asterias rubens, Ophiura ophiura and Echinocardium cordatum consistently occurred throughout. For this reason, these species are considered the key characterizing species in this biotope and are the focus of this sensitivity assessment. In addition, Acrocnida brachiata has been recorded in all examples of the biotope, often in abundance (up to >780 individuals/m2 (Keegan & Könnecker, 1980)), with Astropecten irregularis reported as a potential predator (Fish & Fish, 1996). Astropecten generally tends to be found partially or completely buried in the sediment, but when foraging, they roam the sediment and are known to be voracious predators, behaviour which can have a profound influence in the structure of benthic communities (Freeman et al., 2001). As a result, Astropecten is considered as key functional species of SS.SSa.CMuSa.AbraAirr.

SS.SSa.OSa.OfusAfil here is considered to represent an offshore example of SS.SSa.CMuSa.AbraAirr. However, no records have been found and this sensitivity assessment is based upon the description of the biotope given by Connor et al. (2004). SS.SSa.OSa.OfusAfil occurs in areas of slightly muddy sand (generally <20% mud) in offshore waters and may be characterized by high numbers of the tube building polychaete Owenia fusiformis often with the brittlestar Amphiura filiformis, both of which are associated with and live buried in muddy sands. Whilst Owenia fusiformis is also found in other circalittoral or offshore biotopes, such as SS.SSa.OSa.MalEdef, it usually occurs in lower abundances than in SS.SSa.OSa.OfusAfil. This suggests that the occurrence of Owenia fusiformis and Amphiura filiformis in offshore muddy substrata define this biotope. For this reason, these are considered the characterizing species of this biotope and will be the focus of this sensitivity assessment. Furthermore, as an infaunal tube building polychaete, Owenia fusiformis are known to be good ecosystem engineers as a result of building their tubes for protection from hydrodynamics and predators, which in turn provide stability to the benthic soft sediment, and influence the structure of the benthic community with regard to diversity, abundances and spatial distribution (Dauer et al., 1982; Zuhlke, 2001, cited in Noffke et al., 2009). Other species found in this community are the polychaetes Goniada maculata, Pholoe inornata, Diplocirrus glaucus, Chaetozone setosa and Spiophanes kroyeri with occasional bivalves such as Timoclea ovata and Thyasira equalis. The sea cucumber Labidoplax buskii (syn. Labidoplax buski) and the cumacean Eudorella truncatula are also commonly often found in this biotope.

Resilience and recovery rates of habitat

The fauna characterizing these biotopes occur buried in muddy sands. Brittlestar Acrocnida brachiata displays the characteristic brittlestar body plan with a flat central disc (up to 12 mm diameter) and five very long, slender arms, up to 15 times the diameter of the disc (Fish & Fish, 1996). Acrocnida brachiata is known to spawn during summer and it has been suggested that it has a brief pelagic phase (Fish & Fish, 1996). Zakadjian (1990) studied the reproductive strategy of Acrocnida brachiata from the Bay of Seine and suggested a well-defined annual reproductive cycle, with gonad development beginning in late summer to autumn and spawning occurring in May and June. The authors also suggested that individuals did not reach sexual maturity until the second or third year of life with most individuals spawning at least two or three times in their lifetimes of up to 4-5 years (Zakadjian, 1990). Reproduction did not seem to coincide with annual temperature peak. Acrocnida brachiata, like other brittlestars, has been reported to be able to tolerate a significant level of sub-lethal predation, with large portions of populations having been observed to be regenerating arms (Bourgoin & Guillou, 1994). Acrocnida brachiata may benefit from the buried position it occupies in the sediment to strategically rotate arms between suspension feeding and burial in the sediment to allow arm regeneration (Makra & Keegan, 1999).

Astropecten irregularis has a stiff flattened body and can grow up to 20 cm in diameter. The sexes are separate and breeding apparently takes place during the summer months. It has a bipinnaria larva but no brachiolaria in the life-cycle (Fish & Fish, 1996). Freeman et al. (2001) studied the seasonal trends in abundance, spatial distribution, spawning and growth of a population of Astropecten irregularis of the coast of North Wales. The authors observed that Astropecten irregularis population varied seasonally, with maximum and minimum abundances in summer and winter respectively, suggesting that the starfish might migrate offshore to deeper, more stable waters during winter. The higher densities in the summer may coincide with spawning aggregations, which on the study site occur during late spring, early summer. In north-eastern Europe, Astropecten irregularis displays a marked annual reproductive cycle, with frequent spawning episodes throughout the summer months. Like most starfish, fertilization takes places externally, which is likely to benefit from population aggregations and synchronized spawning (Freeman et al., 2001). The authors suggested a lifespan of approx. 3.5 years, although under laboratory conditions Astropecten irregularis have been reported to live for up to about 10 years (Christensen, 1970, cited in Freeman et al., 2001).

Owenia fusiformis lives in a tough, flexible tube, which it builds by selectively collecting grain particles from its environment (Noffke et al., 2009). The sexes are separate and the larvae have a planktonic life of about four weeks. On the south coast of England, breeding occurs during June and July. Length of life is four years, with breeding occurring every year (Fish & Fish, 1996; Rouse & Pleijel, 2001). Owenia fusiformis has a polymodal population structure of three to five year classes (Menard et al., 1990). The mortality rate increases gradually with age but suddenly increases in the fourth year of life (Menard et al., 1990). Growth is rapid in summer, slows in the autumn and is negligible in winter, resuming in April each year. The maximum recorded density was 4660 individuals/m² but this fluctuated over each year with mortality and massive larval settlement (Menard et al., 1990). Maturity is size-dependent and all worms 6 cm long or more are mature but some individuals reach maturity at 2.4 cm. Some individuals may breed in their first year if they can grow fast enough (Gentil et al., 1990). In the southern North Sea, spatfall occurs from spring to early summer (Hartmann-Schöder, 1996, cited in Noffke et al., 2009) and in the English Channel the maximum of spat is in mid-May (Thiebaut et al., 1992). Larval settlement depends on the portion of fine sediment (mud) in the sediment (Wilson, 1932), with juvenile settlement strongly decreasing where mud portion was <5%. Furthermore, if the fine fraction is missing in the sediment, initial tube building is strongly restricted and the survival of the juveniles is thus negatively influenced (Pinedo et al., 2000; Noffke et al., 2009). These post-settlement processes are thought to have more influence on the macrobenthic community than processes in the pelagic phase.

Amphiura filiformis is a small brittlestar, disc up to 10 mm in diameter, with very long arms (10x disc diameter) which lives buried in muddy sand. Muus (1981) showed the mortality of new settling Amphiura filiformis to be extremely high with less than 5% contributing to the adult population in any given year. Sköld et al. (1994) also commented on the high mortality and low rates of recruitment in this species. In Galway Bay populations (O'Connor et al., 1983), small individuals make up ca. 5% of the population in any given month, which also suggests the actual level of input into the adult population is extremely low. Muus (1981) estimated the lifespan of Amphiura filiformis to be 25 years based on oral width (which does not change with gonadal growth) with recruitment taking place at the 0.3 mm disc size. In very long-term studies of Amphiura filiformis populations in Galway Bay, a lifespan of some 20 years is possible (O'Connor et al., 1983). Amphiura filiformis reaches sexual maturity after 2 years, breeds annually and, in the UK, one period of recruitment occurs in the autumn (Pedrotti, 1993). The species is thought to have a long pelagic life. Sköld et al. (1994) estimated the time lag between full gonads and settlement to be 88 days. This duration is comparable to the time period when pelagic larvae have been recorded in the plankton from July to November in one prior study and August to December in another prior study (Fosshagen, 1965; Thorson, 1946, respectively, cited in Sköld et al., 1994). A long planktonic life stage means this species is predicted to disperse over considerable distances.

Resilience assessment: Minor damage to individual brittlestars, such as Acrocnida brachiata and Amphiura filiformis, and starfish Astropecten irregularis is likely to be repaired, and recovery from impacts with a small spatial footprint may occur through migration of adults. Where the majority of the population remain (resistance is High or Medium), and/or recruitment by adult mobility is possible recovery (resilience) is likely to be High. Where populations are removed or significantly reduced over large areas then recovery will be through recruitment of juveniles and will depend on the supply of new larvae. The characterizing species in these biotopes reproduce annually, so recovery through juvenile recruitment may occur within two years. However, recruitment rates may be low in places and are dependent on favourable hydrodynamic conditions that allow settlement of new recruits. So where impacts remove a significant proportion of the population (resistance is Low or None), recovery is likely to be Medium (2-10 years). Within this time period it is likely that most species could have re-established biomass and age structured populations. 

NB: The resilience and the ability to recover from human induced pressures is a combination of the environmental conditions of the site, the frequency (repeated disturbances versus a one-off event) and the intensity of the disturbance. Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales including, but not limited to, local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations. Full recovery is defined as the return to the state of the habitat that existed prior to impact.  This does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognizable as the initial habitat of interest. It should be noted that the recovery rates are only indicative of the recovery potential.

Hydrological Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
High High Not sensitive
Q: Medium
A: Medium
C: High
Q: High
A: High
C: High
Q: Medium
A: Medium
C: High

The characterizing species in these biotopes are widely distributed, from Norway to Morocco and the Mediterranean (Fish & Fish, 1996; Sabatini, 2008; Neil & Avant, 2008; Hill & Wilson, 2008). Kröncke et al. (2011) reported the increase in abundance and regional changes in the distribution of various species with a southern distribution in the North Sea in 2000, suggesting the changes were largely associated with an increase in sea surface temperature, primary production and, thus, food supply. The authors suggested that the increase of annual average was of about 1.1°C. Brittlestar Acrocnida brachiata was reported to be amongst these species, suggesting the brittlestar may benefit from warmer sea temperatures. On the other hand, Amphiura filiformis was among the species observed to have decreased. Zakadjian (1990) studied the reproductive strategy of Acrocnida brachiata from the Bay of Seine, where the annual temperature variations in the study site were from 6 to 22°C, with no link suggested between reproduction and temperature.

Freeman et al. (2001) observed that spawning in Astropecten irregularis in theirs studies coincided with an increase in seawater temperature from approx. 8°C to 15°C. Furthermore, the authors noted that, in the laboratory, the shallow burrowing species adjusted the depth at which they burrowed into the sediment to seawater temperature, burrowing deeper at lower temperatures.

In Galway Bay, long-term recordings of water temperature at a site of high density aggregations of Amphiura filiformis showed the species is subject to annual variations in temperature of about 10°C (O'Connor et al., 1983). Increases in temperature may affect growth and fecundity. Muus (1981) showed that juvenile Amphiura filiformis are capable of much higher growth rates in experiments with temperatures between 12 and 17°C.

Owenia fusiformis is found in waters from -1 to 30°C (Dauvin & Thiebaut, 1994) globally. In the Bay of Seine, where there is a large population of Owenia fusiformis, the temperature varies between 5 and 20°C (Gentil et al., 1990).

Sensitivity assessment: The characterizing species of these biotopes are widely distributed and likely to occur both north and south of the British Isles. Furthermore, the evidence presented suggests that these species are likely to potentially benefit from an increase in temperature at the pressure benchmark level, with increased distribution range, growth and fecundity. Resistance and resilience are therefore assessed as High and the biotopes considered Not Sensitive to an increase in temperature at the benchmark level.

Low Medium Medium
Q: Medium
A: Medium
C: High
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

The characterizing species in these biotopes are widely distributed, from Norway to Morocco and the Mediterranean (Fish & Fish, 1996; Sabatini, 2008; Neil & Avant, 2008; Hill & Wilson, 2008). Zakadjian (1990) studied the reproductive strategy of Acrocnida brachiata from the Bay of Seine, where the annual temperature variations in the study site were from 6 to 22°C, with no link suggested between reproduction and temperature. Holme (1967) reported the absence of Acrocnida brachiata from samples taken from Weymouth Bay and Poole Bay, England, after severe winter temperatures (4 and 5°C, respectively, below the mean for about a month). The abundance of Amphiura filiformis was also reported to have decreased.

Freeman et al. (2001) observed that, in the laboratory, Astropecten irregularis adjusted the depth at which it burrowed into the sediment to seawater temperature, burrowing deeper at lower temperatures. Furthermore, the authors noted that locomotory activity in Astropecten irregularis was inhibited at low seawater temperatures (<6°C), and that temperature is likely to be an important factor influencing the abundance and distribution of the species in coastal waters, as it has been suggested that individuals migrate offshore during the winter (Freeman et al., 2001)

In Galway Bay,  long-term recordings of water temperature at a site of high density aggregations of Amphiura filiformis showed the species is subject to annual variations in temperature of about 10°C (O'Connor et al., 1983). Increases in temperature may affect growth and fecundity. Muus (1981) showed that juvenile Amphiura filiformis are capable of much higher growth rates in experiments with temperatures between 12 and 17°C. However, echinoderms, including Amphiura filiformis, of the North Sea seem periodically affected by winter cold. A population at 27 m depth off the Danish coast was killed by the winter of 1962-63 (Muus, 1981) and a population at 35-50 m depth in the inner German Bight was killed in the winter of 1969-1970 and a new population not re-established until 1974 (Gerdes, 1977). Ursin (1960, cited in Gerdes, 1977) suggested that Amphiura filiformis does not occur in areas with winter temperatures below 4°C although in Helgoland waters it could tolerate temperatures as low as 3.5°C.

Owenia fusiformis is found in waters from -1 to 30°C (Dauvin & Thiebaut, 1994) globally. In the Bay of Seine, where there is a large population of Owenia fusiformis, the temperature varies between 5 and 20°C (Gentil et al., 1990).

Sensitivity assessment: The characterizing species of these biotopes are widely distributed and likely to occur both north and south of the British Isles. However, the evidence presented suggests that low temperatures are likely to be a limiting factor for activity and breeding of the characterizing species. Furthermore, the species seem to be affected by extreme low temperatures and some mortality in shallower examples of the biotopes may occur as a result of a temperature change in winter at the benchmark level. Resistance is therefore assessed a Low and resilience as Medium and the biotopes considered to have a Medium sensitivity to a decrease in temperature at the benchmark level.

Low Medium Medium
Q: Low
A: Low
C: Low
Q: High
A: Medium
C: Medium
Q: Low
A: Low
C: Low

Echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Stickle & Diehl, 1987; Russell, 2013). A review by Russell (2013) confirmed that none of the echinoderm species relevant in this assessment occur in hypersaline conditions. Pagett (1981) suggested that localised physiological adaption to reduced or variable salinities may occur in nearshore areas subject to freshwater runoffs. Records indicate that SS.SSa.CMuSa.AbraAirr mainly occurs in full (30-35 ppt) salinity, but that it may also be found in variable (18-35 ppt) salinity (Connor et al., 2004). This suggests that the species in this biotope may experience variable salinities, and resident species perhaps may be adapted to variation in salinity, as suggested by the results given by Pagett (1981). On the other hand, records indicate that SS.SSa.OSa.OfusAfil only occurs in full salinity and is a circalittoral habitat, so is less likely to experience variable salinities, and resident species, therefore, less likely to be adapted to variation in salinity, as suggested by the results given by Pagett (1981).

Sensitivity assessment: There is little direct evidence of the effects of hypersaline conditions on the characterizing species of these biotopes. However, echinoderms are generally considered to be stenohaline (Stickle & Diehl, 1987; Russell, 2013). The biotopes mainly experience full salinity conditions (Connor et al., 2004) and the species found are unlike to be adapted to increases in salinity. Therefore, an increase in salinity to >40 psu is likely to result in mortality of the characterizing species. Resistance is assessed as Low but with low confidence. Resilience is probably Medium so that sensitivity is therefore assessed as Medium.

Medium High Low
Q: Medium
A: Medium
C: High
Q: High
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

Echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Stickle & Diehl, 1987; Russell, 2013). However, there are examples where characterizing species in these biotopes have been recorded in hyposaline conditions. For example, Amphiura filiformis was recorded in the Sado estuary in Portugal (Monteiro-Marques, 1982 cited in Russell, 2013) where the salinity is 25.5‰, and in the Black Sea where it tolerated 8.9‰ (Russell, 2013). Furthermore, adult and juvenile Astropecten irregularis were exposed to varying salinities of 16-32‰ and mortality was observed to have occurred at 26‰ (Russell, 2013). Pagett (1981) suggested that localised physiological adaption to reduced or variable salinities may occur in nearshore areas subject to freshwater runoffs. Records indicate that SS.SSa.CMuSa.AbraAirr mainly occurs in full (30-35 ppt) salinity, but that it may also be found in variable (18-35 ppt) salinity (Connor et al., 2004). This suggests that the species in this biotope may experience variable salinities, and resident species perhaps may be adapted to variation in salinity, as suggested by the results given by Pagett (1981). On the other hand, records indicate that SS.SSa.OSa.OfusAfil only occurs in full salinity and is a circalittoral habitat, so is less likely to experience variable salinities, and resident species, therefore, less likely to be adapted to variation in salinity, as suggested by the results given by Pagett (1981).

Owenia fusiformis is found in front of river outlets in the Mediterranean (Somaschini, 1993) and English Channel (Gentil et al., 1990) so is, therefore, likely to experience variable salinities.

Sensitivity assessment: Echinoderms are generally considered to be stenohaline (Stickle & Diehl, 1987; Russell, 2013). However, the evidence suggests that the characterizing species in these biotopes are likely to resist a decrease in salinity to 18-30 psu. Astropecten irregularis is the only characterizing species that may suffer some mortality so resistance is therefore assessed as Medium and resilience as High. Sensitivity is therefore assessed as Low.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Tyler & Banner (1977) studied coastal hydrodynamics and echinoderm distributions in Oxwich Bay in the Bristol Channel. The authors suggested a positive correlation between the distribution of Acrocnida brachiata and the percentage of fine sediments, and that wave and tidal-current energy played a major role in determining the distribution of echinoderms on the seabed by influencing the composition of the sediment. Acrocnida brachiata occurred mainly in the muddier areas of the bay, where maximum bottom spring-tidal flood currents of 0.41 m/s were recorded. Owenia fusiformis was also common in the study site.

Amphiura filiformis respond rapidly to currents by extending their arms into the water column to feed. Under laboratory conditions, they were shown to maintain this vertical position at currents of 0.3 m/s (Buchanan, 1964). Amphiura filiformis feed on suspended material in flowing water but will change to deposit feeding in stagnant water or areas of very low water flow (Ockelmann & Muus, 1978). Acrocnida brachiata and Amphiura filiformis have been recorded in biotopes experiencing moderately strong (<0.5 -1.5 m/s), and very weak to moderately strong (negligible - 1.5m/s) water flow strengths, respectively (Connor et al., 1997a&b, 2004). Food requirements probably set a lower limit on the current regime of areas able to support brittlestars.

Astropecten irregularis has been reported to migrate offshore during winter, probably to avoid being displaced during storm surges, which has been observed, suggesting that Astropecten irregularis is probably likely to be displaced by increased tidal streams (Freeman et al., 2001).

Noffke et al. (2009) reported that Owenia fusiformis occurred in abundance in a study site characterized by highly variable flow velocity and direction due to tides. It was reported to have adapted feeding strategies depending on the flow conditions (Dales, 1957, cited in Noffke et al., 2009) and sediment dynamics. Owenia fusiformis is found in front of river outlets in the Mediterranean and can be subject to a wide range of water velocities. Increase in water flow rate will most likely cause winnowing of the sediment, but the tubes of Owenia fusiformis can stabilize the sediment and reduce water movement related stresses on the benthos (Somaschini, 1993).

Both Amphiura filiformis and Owenia fusiformis have been reported in the Northumberland coast, the UK, where tidal currents ranged between surface speeds of 0.65 m/s at springs to 0.4 m/s at neaps, on a flood tide. Bottom residual currents were much weaker than near-surface, reaching a maximum of 0.7 m/s (Jones, 1979, cited in Birchenough & Frid, 2009)

Sensitivity assessment: The biotopes are found in weak to very weak tidal streams (Connor et al., 2004). The evidence presented suggests the characterizing species of these biotopes appear to have behavioural adaptations to changes in water flow. An increase in water flow rate may inhibit suspension feeding in the biotopes and alter the character of the soft-sediment but species may be able to switch to deposit feeding. A decrease in water flow may result in increased siltation, which could be associated with deposition of organic particles. The characterizing species are likely to be able to utilize the additional deposits and burrow up through the deposited sediment. A change in water flow rate at the pressure benchmark level is considered to fall within the range of flow speeds experienced by mid-range populations. Resistance and resilience are therefore assessed as High and the biotope considered Not Sensitive to a change in water flow at the pressure benchmark level.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes in emergence are Not Relevant to the biotopes, which are restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

The brittlestar species that characterize these biotopes, Acrocnida brachiata and Amphiura filiformis, occur in a range of wave exposures from very sheltered to extremely exposed, and extremely sheltered to moderately exposed, respectively (Tillin & Tyler-Walters, 2014). Tyler & Banner (1977) studied coastal hydrodynamics and echinoderm distributions in Oxwich Bay in the Bristol Channel. The authors suggested a positive correlation between the distribution of Acrocnida brachiata and the percentage of fine sediments, and that wave and tidal-current energy played a major role in determining the distribution of echinoderms on the seabed by influencing the composition of the sediment. Acrocnida brachiata occurred mainly in the muddier areas of the bay. Owenia fusiformis was also common in the study site.

Astropecten irregularis have been suggested to migrate offshore in the winter to avoid storms, and have been reported being washed ashore during strong wave surges (Rees et al., 1977, cited in Freeman et al., 2001). The communities described by Jones (1951) which are thought to represent SS.SSa.CMuSa.AbraAirr (Connor et al., 2004), occurred in fairly exposed conditions, with the author suggesting the wave action was felt at the depth of 45 m (25 fm), depth at which the community was reported to occur.

Wells et al. (1981) reported that Owenia fusiformis in the intertidal and shallow subtidal are likely to be buried as a result of wave action but can survive this by working its way up through the sediment in its flexible tube. However, the effect of being washed out of the sediment by wave action was not commented on. In this situation, Owenia fusiformis would probably have to rebury in the sediment and construct a new tube. However, Owenia fusiformis only builds one tube in its lifespan (Noffke et al., 2009). Although tube building by polychaetes is normally a relatively fast process of a few hours (Ziegelmeier, 1952; Hempel, 1957; Myers, 1972), this is unlikely to occur quickly enough to avoid predation by flatfish and opportunistic predators. A decrease in wave exposure is likely to cause increased siltation which adult Owenia fusiformis can probably survive (Dauvin & Gillet, 1991; Wells et al., 1981). However, juveniles cannot construct tubes in sediments with a grain size <63 µm (mud). Therefore if there is a lot of clay and silt deposited ar