Biodiversity & Conservation

SS.SMx.SMxVS.AphPol

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Removal of the substratum would result in loss of its associated community and hence the biotope and an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Smothering
(View Benchmark)
The more mobile burrowing infauna, such as polychaetes, are likely to be able to relocate to their preferred depth following smothering with little or no loss of fitness, as long as the deposited sediment is similar to that already present.

Polydora ciliata is likely to tolerate smothering by 5 cm of sediment because the species inhabits a range of habitats including muddy sediment, larvae settle preferentially on substrata covered with mud (Lagadeuc, 1991) and worms can rebuild tubes close to the surface.

Emerson et al. (1990) examined smothering and burrowing of Mya arenaria after clam harvesting. Significant mortality (2 -60%) in small and large clams occurred only at burial depths of 50 cm or more in sandy substrata. However, they suggested that in mud, clams buried under 25 cm of sediment would almost certainly die. Dow & Wallace (1961) noted that large mortalities in clam beds resulted from smothering by blankets of algae (Ulva sp.) or mussels (Mytilus edulis). In addition, clam beds have been lost due to smothering by 6 cm of sawdust, thin layers of eroded clay material, and shifting sand (moved by water flow or storms) in the intertidal.

The siphons of epifaunal ascidians such as Ascidiella scabra would probably extend above 5cm of sediment, and together with Molgula manhattensis (see MarLIN reviews) could probably survive under sediment for a month (see benchmark). Therefore, most of the species within the biotope are likely to survive smothering by 5cm of sediment suggesting low intolerance. Nevertheless, deep burrowing bivalves such as Mya species may be adversely affected and experience a reduction in abundance, so that an overall intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

Increase in suspended sediment
(View Benchmark)
This biotope is probably exposed to the high levels of suspended sediment characteristic of estuarine conditions. Therefore, the resident species are probably adapted to high suspended sediment levels and an increase at the benchmark level are probably insignificant. An increase in suspended sediment may increase food availability to deposit feeders but increase the energy expenditure of suspension feeders (on clearance mechanism). Therefore, an intolerance of low has been recorded.
Decrease in suspended sediment
(View Benchmark)
This biotope is probably exposed to the high levels of suspended sediment characteristic of estuarine conditions and decrease at the benchmark level is probably insignificant. But food supply is probably important for rapid growing species such as ascidians and suspension feeding polychaetes (e.g. Polydora ciliata), and a reduction in food availability in the form of organic particulates may adversely affect the growth and reproduction of members of the biotope. Hence an intolerance of low has been recorded.
Desiccation
(View Benchmark)
This biotope occurs in the infralittoral so that only the upper extent of shallow examples of the biotope are likely to be emersed on extreme low tides. Most of the infaunal and burrowing species are likely to be protected from desiccation by their water logged habitat. However, epifauna such as Ascidiella scabra and Molgula manhattensis may be adversely affected. Other species such as Dendrodoa grossularia occurs in damp areas of the intertidal and can tolerant some desiccation. Similarly, the characteristic species Polydora ciliata and Mya arenaria occur in the mid to low intertidal, and would probably survive an increase in desiccation at the benchmark level. Therefore, an intolerance of low has been recorded.
Increase in emergence regime
(View Benchmark)
Shallow records of this biotope may be subject to emergence during extreme low water. An increase in emergence is likely to increase the risk of desiccation (see above) but also allow more intertidal species, e.g. barnacles and macroalgae (e.g. fucoids) to invade the biotope. The upper extent of the biotope may come to more closely resemble a typical intertidal mixed sediment community, and hence the upper extent of the biotope would be 'effectively' lost. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Decrease in emergence regime
(View Benchmark)
A decrease in emergence is likely to allow the biotope to extend its upper limit, where suitable substrata exist. Therefore, not sensitive* has been recorded.
Increase in water flow rate
(View Benchmark)
This biotope occurs in moderately strong tidal streams. The hydrographic regime is an important structuring factor in sedimentary habitats. Therefore, an increase in water flow from moderately strong to very strong (see benchmark) may have significant effects on the community due to changes in the sediment characteristics. An increase in water flow to very strong, is likely to mobilize the sediment, removing fine muds and muddy sands, increasing sediment scour and rolling the smaller pebbles and cobbles. Therefore, the biotope is likely to be replaced by a coarse sediment, gravel or cobble biotope, and an intolerance of high has been recorded.
Decrease in water flow rate
(View Benchmark)
The hydrographic regime is an important structuring factor in sedimentary habitats. This biotope has only been recorded from moderately strong tidal streams suggesting that water flow is an important habitat preference. In the highly turbid waters of estuaries, a reduction in water flow is likely to result in a significant increase in siltation, resulting in long-term smothering of epifauna and coarse substrata and leading to a change in the dominant substratum to muds or muddy gravel and increased dominance by infaunal polychaetes or bivalves (e.g. see £IMX.VsenMtru£ or £IMU.AphTub£). Therefore, the biotope as described would be lost and an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Increase in temperature
(View Benchmark)
Bamber & Spencer (1984) observed that Tubificoides and Caulleriella species, common species in the biotope, were dominant in the area affected by thermal discharge in the River Medway estuary. Murina (1997) categorized Polydora ciliata as a eurythermal species because of its ability to spawn in temperatures ranging from 10.6-19.9° C. Mya arenaria is reported from the White Sea, south to Portugal, while Mya truncata has a wider distribution. But the southern distribution of Mya arenaria may be restricted by a limit of 28 °C for both adults and larvae (Newell & Hidu, 1986; Strasser, 1999). Most organisms in the biotope are distributed to the north and south of Britain and Ireland and unlikely to be adversely affected by long term temperature change. In addition, subtidal and especially infaunal species are likely to be protected from acute temperature change. Therefore, not sensitive has been recorded at the benchmark level. Increased temperature may have indirect effects. For instance, higher temperatures have been implicated in the proliferation of trematode parasites which have caused mass mortalities in the snail Hydrobia ulvae (Jensen & Mouritsen, 1992). No other information has been found on tolerance of component species to increased temperature although it would be expected that the infauna in the biotope will be insulated from extreme changes of temperature. Nevertheless, an increase in temperature may indirectly affect some species as microbial activity within the sediments will be stimulated increasing oxygen consumption and promoting hypoxia (see 'Change in oxygenation' below). An intolerance of low is suggested but with a low confidence. Recoverability is likely to be rapid.
Decrease in temperature
(View Benchmark)
Polydora ciliata survived a drop in temperature from 11.5 to 7.5°C over the course of 15 hours (Gulliksen, 1977) and so it appears the species is tolerant of acute temperature decreases. During the extremely cold winter of 1962/63 when temperatures dropped below freezing point for several weeks, Polydora ciliata was apparently unaffected (Crisp, 1964). Over-wintering Mya arenaria survived temperatures as low as -2 °C in Alaska, persisted in the St. Lawrence estuary exposed to freezing winter air temperatures, and survived 60 days of ice in the severe 1995/1996 winter in the Wadden Sea (Strasser, 1999). However, severe winters have been known to cause mortality (Rasmussen, 1973; Strasser, 1999). This biotope is probably protected from freezing events and extremely low temperatures by its depth. Most of the characterizing species are distributed to the north and south of Britain and Ireland, and unlikely to affected by long-term temperature change. Therefore not sensitive has been recorded.
Increase in turbidity
(View Benchmark)
The absence of macroalgae in records of this biotope suggests that it exists in areas of high turbidity and hence very low light intensity. An increase in turbidity will probably reduce primary production in the water column and therefore reduce the availability of food to suspension feeders. In addition, primary production by the micro-phyto benthos on the sediment surface may be reduced, further decreasing food availability. However, phytoplankton will also immigrate from distant areas and so the effect may be decreased. As the turbidity increase only persists for a year (see benchmark), decreased food availability would probably only affect growth and fecundity and an intolerance of low is recorded.
Decrease in turbidity
(View Benchmark)
A decrease in turbidity is likely to allow subtidal algae such as Fucus spp. and Saccharina latissima, Chorda filum and red algae to colonize the hard substrata within shallower records of the biotope. The macroalgae would compete for space with epifauna, possibly reducing the abundance of ascidians. Few other species would be directly affected and the increased primary productivity and plant debris may benefit suspension feeders and detritivores. However, the biotope would probably come to resemble a macroalgal dominated mixed substrata biotope, e.g. £IMX.LsacX£ and the shallow extent of the described biotope would be lost. Therefore, an intolerance of intermediate has been recorded.
Increase in wave exposure
(View Benchmark)
This biotope was recorded from wave sheltered to very sheltered habitats. The hydrographic regimes is an important structuring factor in sedimentary habitats. An increase in wave action from wave sheltered to wave exposed (see benchmark) is likely to significantly alter the community by removing the finer sediments, increasing erosion of consolidated sediment and mobilizing the large coarser sediments increasing scour. Therefore, the biotope is likely to be lost, and replaced with a coarser sediment biotope. An intolerance of high has been recorded although recoverability is likely to be high (see additional information below).
Decrease in wave exposure
(View Benchmark)
This biotope was recorded from wave sheltered to very sheltered habitats. A decrease in wave exposure from very sheltered to extremely sheltered may increase the siltation rate although in the moderately strong tidal streams encountered in this habitat the effects are likely to be marginal. Therefore, not sensitive has been recorded.
Noise
(View Benchmark)
None of the species in the biotope are likely to be sensitive to noise or vibration at the benchmark level.
Visual Presence
(View Benchmark)
Most species will respond to the shading caused by the approach of a predator, however, their visual acuity is probably very low and none of the component species are likely to respond to visual presence at the benchmark level.
Abrasion & physical disturbance
(View Benchmark)
Both the epifaunal and the infaunal species in the biotope are likely to be sensitive to physical disturbance, such as a passing scallop dredge. Soft bodied epifauna, such as ascidians, are most vulnerable, and are likely to suffer high mortality. Erect epifaunal species are particularly vulnerable to physical disturbance. Veale et al.(2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Mobile gears also result in modification of the substratum, including removal of shell debris, cobbles and rocks, and the movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998). The removal of rocks or boulders to which species are attached results in substratum loss (see above). Despite their robust body form, bivalves are also vulnerable. For example, as a result of dredging activity, mortality and shell damage have been reported in Mya arenaria and Cerastoderma edule (Cotter et al., 1997). Overall, physical disturbance by passing scallop dredge, or mobile fishing gear is likely to result in loss of epifauna and hard substrata and a reduction in the abundance of infaunal species. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be rapid.
Displacement
(View Benchmark)
The infaunal species are active burrowers, unlikely to be adversely affect by displacement. Mya arenaria is a slow burrowing species: for example, Pfitzenmeyer & Droebeck (1967) reported that 62% of small clams (35-50mm), 39% of medium sized (51-65mm) and only 21% of large clams (66-75mm) had reburrowed within 48 hours, so that some individuals my be lost due to predation. But epifaunal ascidians and tubeworms are permanently attached to their substratum and cannot reattach and would probably be lost, unless transported with their substrata. While the characterizing species would probably survive displacement, overall the biotope would effectively be removed from its recorded location so an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).

Chemical Factors

Synthetic compound contamination
(View Benchmark)
The component species within the biotope vary in their tolerance to synthetic chemical contamination. For example:
  • Polydora ciliata was abundant at polluted sites close to acidified, halogenated effluent discharge from a bromide-extraction plant in Amlwch, Anglesey (Hoare & Hiscock, 1974).
  • Spionid polychaetes were found by McLusky (1982) to be relatively tolerant of distilling and petrochemical industrial waste in Scotland.
  • Scoloplos armiger exhibited 'moderate' intolerance to tributyl tin (TBT) anti-foulants (Bryan & Gibbs, 1991).
  • The polychaete Hediste diversicolor exhibited 100% mortality within 14 days when exposed to 8 mg/m² of the insecticide Ivermectin in a microcosm (Collier & Pinn, 1998). Ivermectin was found to produce a 10 day LC50 of 18µg ivermectin /kg of wet sediment in Arenicola marina and sub-lethal effects were apparent between 5 - 105 µg/kg (Cole et al., 1999). Cole et al. (1999) suggested that this indicated a high intolerance. Arenicola marina was also intolerant of ivermectin through the ingestion of contaminated sediment (Thain et al., 1998; cited in Collier & Pinn, 1998).
  • Beaumont et al. (1989) concluded that bivalves are particularly sensitive to tri-butyl tin (TBT), the toxic component of many antifouling paints. For example, when exposed to 1-3 µg TBT/l, Cerastoderma edule and Scrobicularia plana suffered 100% mortality after 2 weeks and 10 weeks respectively. Furthermore, there is evidence that TBT causes recruitment failure in bivalves, either due to reproductive failure or larval mortality (Bryan & Gibbs, 1991). Beaumont et al. (1989) also concluded that TBT had a detrimental effect on the larval and/or juvenile stages of infaunal polychaetes.
  • Waldock et al. (1999) reported that the species diversity of polychaete infauna, including Aphelochaeta marioni, Scoloplos armiger and Nephtys hombergii and the bivalve infauna (e.g. Mysella bidentata, Cerastoderma edule, and Abra alba) in the Crouch estuary increased in the three years after the use of TBT was banned within the estuary, suggesting that TBT had suppressed their abundance previously.
  • Rees et al. (2001) reported that the abundance of epifauna, including Ascidiella aspersa and Ciona intestinalis, had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. Rees et al. (2001) suggested that TBT inhibited settlement in ascidian larvae. This report suggested that epifaunal species (including, bryozoan, hydroids and ascidians) may be at least inhibited by the presence of TBT.
Overall, some polychaetes, ascidians and bivalves are probably intolerant of TBT contamination, while polychaetes and crustaceans are likely to be intolerant of ivermectin contamination. Therefore, synthetic contaminants are likely to at least reduce the abundance and probably recruitment in several members of the community and an intolerance of intermediate has been recorded. Recoverability is likely to be rapid.
Heavy metal contamination
(View Benchmark)
Evidence suggests that polychaetes are "fairy resistant" to the effects of heavy metals (Bryan, 1984). But Hall & Frid (1995) found that the four dominant taxa in their study ( including Tubificoides spp. and Capitella capitata) were reduced in abundance in copper-contaminated sediments and that recovery took up to one year after the source of contamination ceased. Aphelochaeta marioni is tolerant of heavy metal contamination occurring in the heavily polluted Restronguet Creek (Bryan & Gibbs, 1983) and it is also an accumulator of arsenic (Gibbs et al., 1983). Polydora ciliata occurs in an area of the southern North Sea polluted by heavy metals but was absent from sediments with very high heavy metal levels (Diaz-Castaneda et al., 1989). Hediste diversicolor has been found successfully living in estuarine sediments contaminated with copper ranging from 20 µm Cu/g in low copper areas to >4000 µm Cu/g where mining pollution is encountered e.g. Restronguet Creek in the Fal Estuary, Cornwall (Bryan & Hummerstone, 1971). Taking account of the variable salinity conditions that affect this biotope (in general, for estuarine animals, heavy metal toxicity increases as salinity decreases and temperature increases: McLusky et al., 1986), it seems possible that some polychaete species at least in the biotope might be adversely affected by high contamination by heavy metals.

Eisler (1977) exposed Mya arenaria to a mixture of heavy metals in solution at concentrations equivalent to the highest recorded concentrations in interstitial waters in the study area. At 0°C and 11°C (winter temperatures) 100% mortality occurred after 4-10 weeks. At 16-22°C (summer temperatures) 100% mortality occurred after 6-14 days, indicating greater intolerance at higher temperatures.

Overall, the dominant polychaetes within the biotope are probably tolerant of heavy metal contamination, while Mya arenaria (and by inference Mya truncata) is probably intolerant. Other component species probably vary in their heavy metal tolerance and species richness would probably decline. Therefore, an intolerance of high has been recorded to represent loss of an important characterizing species. Recoverability is probably high (see additional information below).
Hydrocarbon contamination
(View Benchmark)
The biotope is predominantly subtidal and component species are protected from the direct effects of oil spills by their depth but are likely to be exposed to the water soluble fraction of oils and hydrocarbons, or hydrocarbons adsorbed onto particulates. Suchanek (1993) reported that infaunal polychaetes were vulnerable to hydrocarbon contamination e.g. high mortality has been demonstrated in Arenicola marina (Levell, 1976). But some of the polychaetes in this biotope proliferate after oil spills: for instance Capitella capitata (Suchanek, 1993) and Aphelochaeta marioni (Dauvin, 1982, 2000). Cirratulids seem mostly immune probably because their feeding tentacles are protected by mucus (Suchanek, 1993).

In analysis of kelp holdfast fauna following the Sea Empress oil spill in Milford Haven the fauna present, including Polydora ciliata, showed a strong negative correlation between numbers of species and distance from the spill (SEEEC, 1998). After the extensive oil spill in West Falmouth, Massachusetts, Grassle & Grassle (1974) followed the settlement of polychaetes in this environmental disturbed area. Species with the most opportunistic life histories, including Polydora ligni, were able to settle in the area. This species has some brood protection which enables larvae to settle almost immediately in the nearby area (Reish, 1979).

Suchanek (1993) reported that sublethal concentrations may produce substantially reduced feeding rates and/or food detection ability in bivalves, probably due to ciliary inhibition. Respiration rates increased at low concentrations and decreased at high concentrations. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. Sublethal concentrations of hydrocarbons also reduce byssal thread production (thus weakening attachment) and infaunal burrowing rates. Mortality following oil spills has been recorded in Mya arenaria (Dow, 1978; Johnston, 1984), Ensis sp. (SEEEC, 1998) and Cerastoderma edule (SEEEC, 1998). However, the Abra alba population affected by the 1978 Amoco Cadiz benefited from the nutrient enrichment caused by the oil pollution (see nutrient enrichment, below).

Overall, hydrocarbon contamination is likely to adversely affect some members of the community, and low more tolerant or opportunistic species to increase in abundance, resulting in a reduction in species richness. Nevertheless, the biotope would probably survive and an intolerance of intermediate has been recorded to represent loss of some characterizing species.

Radionuclide contamination
(View Benchmark)
No information found.
Changes in nutrient levels
(View Benchmark)
Polydora ciliata is often found in environments subject to high levels of nutrients. For example, the species was abundant in areas of the Firth of Forth exposed to high levels of sewage pollution (Smyth, 1968) and in nutrient rich sediments in the Mondego estuary, Portugal (Pardal et al., 1993) and the coastal lagoon Lago Fusaro in Naples (Sordino et al., 1989). The abundance of the species was probably associated with their ability to use the increased availability of organic matter as a food source and silt for tube building. A 'Sewage Scheme' was introduced in the Firth of Forth (Read et al., 1983). Extensive growths of Polydora ciliata were recorded at West Ganton, in the Firth of Forth, but as water quality improved following introduction of the scheme these 'pollution tolerant' species disappeared providing space for colonization by other fauna (Read et al., 1983). However, Polydora ciliata can also occur in organically poor areas (Pearson & Rosenberg, 1978) and so is likely to have low intolerance to changes in nutrient concentrations.

Increased nutrients often derive from sewage inputs and presence of species such as Aphelochaeta marioni in such situations (for instance Broom et al., 1991) may reflect tolerance to high nutrients or to deoxygenated conditions or both. Similarly, an Abra alba population affected by the 1978 Amoco Cadiz oil spill benefited from the nutrient enrichment caused by the oil pollution.

Increased levels of nutrient may result in eutrophication, algal blooms and reductions in oxygen concentrations (see Rosenberg & Loo, 1988). Rosenberg & Loo (1988) reported mass mortalities of Mya arenaria and Cerastoderma edule following a eutrophication event in Sweden, although no direct causal link was established.

Overall, an increase in nutrient levels is likely to favour deposit feeding species, tolerant of increased hypoxia, and exclude suspension feeding invertebrates such as bivalves, resulting in a decline in species richness. The biotope is likely to come to resemble polychaete dominated biotopes (e.g. £IMU.AphTub£) and biotope as described will be lost. Therefore, an intolerance of high has been reported. Recoverability is likely to be high (see additional information below).

Increase in salinity
(View Benchmark)
This biotope occurs in variable salinity conditions but is unlikely to experience hypersaline conditions.
Decrease in salinity
(View Benchmark)
This biotope occurs in variable salinity. Polydora ciliata is a euryhaline species inhabiting fully marine and estuarine habitats. In an area of the western Baltic Sea, where bottom salinity was between 11.1 and 15.0psu Polydora ciliata was the second most abundant species with over 1000 individuals/m² (Gulliksen, 1977). Its intolerance to a decrease in salinity is therefore, expected to be low. Aphelochaeta marioni has been recorded from brackish inland waters in the Southern Netherlands with a salinity of 16 psu but not in areas permanently exposed to lower salinities (Wolff, 1973). However, it also penetrates into areas exposed to salinities as low as 4 psu for short periods at low tide when fresh water discharge from rivers is high (Farke, 1979). The distribution of Aphelochaeta marioni, therefore, suggests that it is very tolerant of low salinity conditions and would be tolerant of reduced salinity especially for short periods but a long term reduction from reduced to low salinity may affect some of the species in the biotope with possible losses and reduced viability. Mya arenaria is a euryhaline osmoconformer and has been reported from the west Atlantic coast in salinities of 4 psu (Strasser, 1999). However, Abra alba is typically found in full salinity conditions and is therefore likely to be intolerant of reductions in salinity in some way. The ascidian Ascidiella scabra occurs in reduced salinity conditions. Van Name (1945; quoted in Kott, 1985), noted that Molgula manhattensis occurred in salinities equivalent to 20 to 36 psu whilst Hartmeyer (1923; quoted in Tokioka & Kado, 1972) recorded Molgula manhattensis in brackish (16-30 psu) water of the Belt Sea. A fall in salinity from full to reduced would not therefore be expected to have an adverse effect on or Ascidiella scabra or Molgula manhattensis

Overall, the important characterizing species are likely to tolerate a short term change in salinity from e.g. variable to low salinity and a long term change from variable to reduced salinity. The species richness of the biotope may decline but the biotope will probably not be adversely affected. Therefore, an intolerance of low has been recorded.

Changes in oxygenation
(View Benchmark)
Sagasti et al. (2000) reported that epifaunal communities, especially tunicates, hydroids and anemones were equally abundant in the York River estuary exposed to brief hypoxic episodes and moderate hypoxia (0.5-2mg O2/l). The communities studied included the tunicate Molgula manhattensis and the polychaete Polydora cornuta. Their study suggests that estuarine epifaunal communities are relatively tolerant of hypoxia. In polluted waters in Los Angeles and Long Beach harbours Polydora ciliata was present in the oxygen range 0.0-3.9 mg/l and the species was abundant in hypoxic fjord habitats (Rosenberg, 1977). Other polychaetes may also tolerate hypoxia and many are facultative anaerobes (Diaz & Rosenberg, 1995). For example, Nephtys hombergi and Heteromastis filiformis) are noted by Diaz & Rosenberg (1995) as resistant to severe hypoxia, while Capitella capitata, Hediste diversicolor, Scoloplos armiger and Lagis koreni were considered to be resistant to moderate hypoxia. Tubificoides benedii has a high capacity to tolerate anoxic conditions (see Giere et al., 1999). Broom et al. (1991) found communities with polychaete species characteristic of this biotope in the Severn Estuary where the oxygenated layer was very thin probably as a result of sewage input and suggested that Aphelochaeta marioni was characteristic of faunal assemblages in the Severn Estuary with very poorly oxygenated mud.

Mya arenaria tolerates low oxygen concentration and the presence of hydrogen sulphide for several days or weeks. Fifty percent mortality was observed after 21 days at 10 °C exposed to 0.15 ml O2/l (0.21 mg/l) in the presence of H2S (Theede et al., 1969). At 0.5-1.0 ml O2/l(0.7-1.4mg/l), 8% survived in sediment for 32 days and 54% survived for 43 days (Rosenberg et al., 1991). Rosenberg & Loo (1988) reported mass mortalities of Mya arenaria and Cerastoderma edule in the 1980s in the Kattegat, which were associated with eutrophication and resultant low oxygen concentrations over several years (often <1 ml O2/l). However, Mya arenaria is probably tolerant of 2mg/l for a week (see benchmark).

Overall, most of the characterizing species are probably tolerant of hypoxia at the benchmark level and the biotope would probably not be adversely affected, although species richness may decline. Physiological tolerance and anaerobic metabolism incur extra energy demands, therefore, an intolerance of low has been recorded.

Biological Factors

Introduction of microbial pathogens/parasites
(View Benchmark)
Several parasites occur in Mya arenaria, e.g. cercaria of Himasthla leptosoma, the nemertean parasite Malacobdella sp. and the copepod Myicola metisciensis may be commensal (Clay, 1966). The protozoan, Perkinsus sp. has recently been isolated from Mya arenaria in Chesapeake Bay, USA (McLaughlin & Faisal, 2000). Mya arenaria is also known to suffer from cancers, disseminated neoplasia and gonadal tumours. Disseminated neoplasia, for example, has been reported to occur in 20% of the population in north eastern United States and Canada, and caused up to 78% mortalities in New England (Brousseau & Baglivo, 1991; Landsberg, 1996).
Little information was found regarding microbial infection of polychaetes, although Gibbs (1971) recorded that nearly all of the population of Aphelochaeta marioni in Stonehouse Pool, Plymouth Sound, was infected with a sporozoan parasite belonging to the acephaline gregarine genus Gonospora, which inhabits the coelom of the host. No evidence was found to suggest that gametogenesis was affected by Gonospora infection and there was no apparent reduction in fecundity.
The parasite loads of the bivalves discussed above have been proven to cause mortality and therefore a biotope intolerance of intermediate is recorded and there may be a minor decline in species richness in the biotope. Recoverability is recorded as high (see additional information below).
Introduction of non-native species
(View Benchmark)
The American hard-shelled clam, Mercenaria mercenaria, colonized the niche left by Mya arenaria killed after the cold winters of 1947 and 1962/63 in Southampton Water (Eno et al. 1997). The Mya arenaria populations had not recovered in this area by 1997 (Eno et al., 1997). Mya arenaria often occurs in the IMX.PolMtru biotope and therefore Mercenaria mercenaria may pose a threat of invasion.

Invasion by the slipper limpet Crepidula fornicata may switch the biotope to £IMU.CreAph£, as this IMX.PolMtru is often difficult to distinguish from reduced versions of £IMU.CreAph£ (Connor et al., 1997b), suggesting high intolerance as the original biotope would be lost. Species richness might decline as Crepidula may dominate the seabed. On the other hand, low densities of Crepidula might have no effect on species richness and add one species (Crepidula) to the community. Once established Crepidula fornicata is difficult to remove but should its numbers decrease then recoverability of IMX.PolMtru would probably be rapid.

Extraction
(View Benchmark)
It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope.

Additional information icon Additional information

Recoverability
The community is dominated by fast growing opportunistic polychaete and ascidian species and the community most likely reaches maturity within one year of space becoming available. In an experimental study investigating recovery of a range of species characteristically found in this biotope after copper contamination, Hall & Frid (1995) found that recovery took up to a year. However, Hall & Frid (1998) found that colonization by many of the polychaetes associated with this biotope did not vary significantly with season although recruitment of Tubificoides benedii and Ophyrotrocha hartmanni did vary significantly with season. Similarly Polydora ciliata is a short lived species that reaches maturity within a few months and has three or four spawnings during a breeding season of several months. For example, in colonization experiments in Helgoland (Harms & Anger, 1983) Polydora ciliata settled on panels within one month in the spring. The bivalve Abra alba demonstrates an 'r' type life-cycle strategy and is able to rapidly exploit any new or disturbed substratum available for colonization through larval recruitment, secondary settlement of post-metamorphosis juveniles or re-distribution of adults. For example, Abra alba recovered to former densities following loss of a population from Keil Bay owing to deoxygenation within 1.5 years, as did Lagis koreni, taking only one year (Arntz & Rumohr, 1986).

Mya arenaria has a high fecundity and reproductive potential but larval supply is sporadic and juvenile mortality is high so that, although large numbers of spat may settle annually, successful recruitment and hence recovery may take longer than a year. For example, Beukema (1995) reported that a population of Mya arenaria in the Wadden Sea, drastically reduced by lugworm dredging took about 5 years to recover.

Therefore, the polychaete infauna, ascidian and tube worm epifauna would probably colonize the habitat rapidly, producing a recognizable biotope within 1-2 year, while the abundance of some species, e.g. Mya sp. may take up to 5 years to recover.


This review can be cited as follows:

Tyler-Walters, H. 2002. Polydora ciliata, Mya truncata and solitary ascidians in variable salinity infralittoral mixed sediment.. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 02/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=114&code=2004>