Biodiversity & Conservation

SS.IMX.Oy.Ost

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Ostrea edulis cements its lower valve to the substratum permanently. Loss of the substratum would result in loss of the oyster bed and its associated community and hence the biotope. Therefore an intolerance of high has been recorded.
Loss of the substratum would also result in loss of the epifauna and infauna and, hence a major decline in species richness.
Recovery is dependant on larval recruitment since adult Ostrea edulis are permanently attached and incapable of migration. Recruitment of Ostrea edulis is sporadic and dependant on the local environmental conditions, hydrographic regime and the presence of suitable substratum, especially adult shells or shell debris, and has probably been inhibited by the presence of competition from non native species (see additional information below). Since the biotope is dependant on the presence of Ostrea edulis a recoverability of very low has been suggested.
Smothering
(View Benchmark)
Smothering by 5 cm of sediment would prevent the flow of water through the oyster that permits respiration, feeding and removal of waste. Ostrea edulis is permanently fixed to the substratum and would not be able to burrow up through the deposited material. Ostrea edulis can respire anaerobically, and is known to be able to survive for many weeks (Yonge, 1960) or 24 days (Korringa, 1952) out of water at low temperatures used for storage after collection. However, it is likely that at normal environmental temperatures, the population would be killed by smothering. Yonge (1960) reported death of populations of Ostrea edulis due to smothering of oyster beds by sediment and debris from the land as a result of flooding. Therefore, an intolerance of high has been recorded.

Smothering will probably also kill the sessile, fixed members of the epifauna, unless large enough to protrude above the deposited layer, e.g. Ascidiella sp. However, burrowing infauna will probably burrow to the surface. Death of the oyster bed will exacerbate changes in the sediment surface and nutrient levels in the long term, so that the characterizing species may be replaced by others. Therefore, species richness is likely to decline markedly. Recruitment in Ostrea edulis is potentially good due to its high fecundity and high dispersal potential, however, dependency of the hydrographic regime, and environmental conditions of (e.g. temperature, food availability), high larval and juvenile mortality, competition for settlement space with native species results in sporadic recruitment, which together with competition for suitable substratum with non native species such as Crepidula fornicata results in a potentially long recovery time (see additional information below). In addition, a layer of settled material of 1-2 mm in depth was reported to prevent satisfactory oyster sets, i.e. settlement, reducing effective recruitment (Galtsoff, 1964, cited in Wilbur, 1971). Therefore, a recoverability of very low has been recorded.

Increase in suspended sediment
(View Benchmark)
Oysters respond to an increase in suspended sediment by increasing pseudofaeces production with occasional rapid closure of their valves to expel accumulated silt (Yonge, 1960) both of which exert an energetic cost. Korringa (1952) reported that an increase in suspended sediment decreased the filtration rate in oysters. Suspended sediment was also shown to reduce the growth rate of adult Ostrea edulis and to result in shell thickening (Moore, 1977). Reduced growth probably results from increased shell deposition and an inability to feed efficiently. Hutchinson & Hawkins (1992) reported that filtration was completely inhibited by 10mg/l of particulate organic matter and significantly reduced by 5mg/l.
Ostrea edulis larvae survived 7 days exposure to up to 4 g/l silt with little mortality. However, their growth was impaired at 0.75 g/l or above (Moore, 1977).
Yonge (1960) and Korringa (1952) considered Ostrea edulis to be intolerant of turbid (silt laden) environments. However, oyster beds are found in the relatively turbid estuarine environments and the values of suspended sediment quoted above are high in comparison to the benchmark value. Therefore, a change in suspended sediment at the benchmark level may only result in sub-lethal effects. However, Moore (1977) reported that variation in suspended sediment and silted substratum and resultant scour was an important factor restricting oyster spat fall, i.e. recruitment. Therefore, an increase in suspended sediment may have longer term effects of the population by inhibiting recruitment, especially if the increase coincided with the peak settlement period in summer.

The other suspension feeders characteristic of this biotope are probably tolerant of a degree of suspended sediment but an increase, especially of fine silt, would probably interfere with feeding mechanisms, resulting in reduced feeding and a loss of energy through mechanisms to shed or remove silt.

Overall, an increase in suspended sediment at the level of the benchmark for a period of a month, may not adversely affect the biotope. Therefore, an intolerance of low has been recorded. However, high levels of suspended sediment or a protracted increase may be detrimental. Recovery will depend on clearance of filtration apparatus and return to condition, which will probably be relatively rapid.
Decrease in suspended sediment
(View Benchmark)
In areas of high suspended sediment, a decrease may result in improved condition and recruitment due to a reduction in the clogging of filtration apparatus of suspension feeders and an increase in the relative proportion of organic particulates. However, a decrease in suspended sediments in some areas may reduce food availability resulting in lower growth or reduced energy for reproduction. Therefore, an intolerance of low has been recorded at the level of the benchmark.
Desiccation
(View Benchmark)
Beds of the native oyster Ostrea edulis may occur low on the shore and are exposed for a proportion of the tidal cycle. Ostrea edulis is known to be able to survive aerial exposure at low temperatures during storage and are known to be capable of anaerobic respiration (Korringa, 1952; Yonge, 1960), which suggests that they can tolerate aerial exposure. In addition, in the mariculture of oysters (native and introduced species) oyster trays are position in the low intertidal, and regularly exposed to the air. Therefore, an increase in desiccation in this biotope, is unlikely to result in death of the oysters themselves at the level of the benchmark. However, exposure to the air prevents feeding, and anaerobic respiration usually results in an oxygen debt, an energetic cost that the organism must make up on return to aerated water, resulting in reduced growth and reproductive capacity.

The associated epifauna may be more intolerant, such as ascidians (e.g. Ascidiella spp.) and Asterias rubens. Burrowing infauna are likely to be protected from desiccation by their infaunal habit and species such as Lanice conchilega, Myxicola infundibulum and Chaetopterus variopedatus may be found at low water. Mobile epifaunal species would probably move to deeper water while delicate hydroids and bryozoans may be damaged or killed by desiccation.

Therefore, oyster beds may tolerate an increase in desiccation at the benchmark level and an intolerance of low has been recorded, while the epifauna and the species richness reduced. Recovery will depend on return to original condition by the oysters and recolonization by epifauna, both of which are likely to be rapid.
Increase in emergence regime
(View Benchmark)
The adult oyster can close the valves of its shell tightly when exposed. Some populations are found in the lower intertidal. A change of one hour in emergence would mean that the valves are kept shut for a greater time, resulting in less time available for feeding, and hence reduced growth and reproductive capacity, and an increased risk of desiccation. However, the epifauna are likely to be more intolerant of increases in emergence, resulting in loss of some species and a reduction in species richness. The infauna species are likely to be protected by their burrowing habit. Overall, therefore, the biotope may suffer a decrease in the diversity of epifauna but the oyster bed would not be markedly affected at the level of the benchmark. Therefore an intolerance of low has been recorded. The oysters would probably recover condition rapidly, and the epifauna will probably also recolonize available habitat quickly.
Decrease in emergence regime
(View Benchmark)
This biotope is subtidal so that an increase in emergence is unlikely to have an adverse effect on the community. However, increased emergence may allow the oyster bed to spread further up the shore, although at a slow rate. Therefore, the biotope may benefit from the factor.
Increase in water flow rate
(View Benchmark)
This biotope occurs in weak to very weak tidal streams. An increase in water flow from, for example weak to strong is likely to remove (erode) fine particulates, leaving coarser substrata and making more hard substratum available for settlement by oysters and other members of the community, e.g. Ascidiella spp. and epifauna.

The effects of increased water flow are most likely to be in reducing the time oysters are able to feed. Oysters may be swept away by strong tidal flow if the substratum to which they are attached is removed. Therefore, a proportion of the oyster bed may be lost, depending on the nature of the substratum, and an intolerance of intermediate has been recorded. Overall, the nature of the biotope is likely to change significantly.

Recruitment in Ostrea edulis is sporadic and dependant of the hydrographic regime and local environmental conditions but will be enhanced by the presence of adults and shell material. Therefore, a recoverability of low has been recorded (see additional information below).
Decrease in water flow rate
(View Benchmark)
The biotope is found in weak to very weak tidal streams, so that any further decrease is unlikely.
Increase in temperature
(View Benchmark)
Filtration rate, metabolic rate, assimilation efficiency and growth rates of adult Ostrea edulis increase with temperature. Growth was predicted to be optimal at 17°C or, for short periods, at 25°C (Korringa, 1952; Yonge, 1960; Buxton et al., 1981; Hutchinson & Hawkins, 1992). Huchinson & Hawkins (1992) noted that temperature and salinity were co-dependant, so that high temperatures and low salinity resulted in marked mortality, no individuals surviving more than 7 days at 16psu and 25°C, although these conditions rarely occurred in nature. No upper lethal temperature was found, although Kinne (1970) reported that gill tissue activity fell to zero between 40-42°C, although values derived from single tissue studies should be viewed with caution. Buxton et al. 1981 reported that specimens survived short term exposure to 30°C. Ostrea edulis and many of the other species in the biotope occur from the Mediterranean to the Norwegian coast and are unlikely to be adversely affected by long term changes in temperatures in Britain and Ireland.

Spärck's data (1951) suggest that temperature is an important factor in recruitment of Ostrea edulis, especially at the northern extremes of its range and Korringa (1952) reported that warm summers resulted in good recruitment. Spawning is initiated once the temperature has risen to 15-16°C, although local adaptation is likely (Korringa, 1952; Yonge, 1960). Davis & Calabrese (1969) reported that larvae grew faster with increasing temperature and that survival was optimal between from 12.5 - 27.5°C but that survival was poor at 30°C. Therefore, recruitment and the long term survival of an oyster bed is probably affected by temperature and may benefit from both short and long term increases.

Most of the other characterizing species within the biotope have a wide distribution in Europe suggesting that they are able tolerate a wider range of temperatures than found in British waters. Delicate species may not be so tolerant and mobile species may leave the biotope temporarily resulting in a decline in species richness. However, an overall biotope intolerance of low has been recorded to represent the effects of temperature on feeding and growth.
Once the temperature returns to normal limits the characterizing species will probably regain their condition rapidly.
Decrease in temperature
(View Benchmark)
Hutchinson & Hawkins (1992) suggested that Ostrea edulis, the dominant species in this biotope, switched to a reduced, winter metabolic state below 10 °C that enabled it to survive low temperatures and low salinities encountered in shallow coastal waters around Britain. Davis & Calabrese (1969) also noted that larval survival was poor at 10 °C. Korringa (1952) reported that British, Dutch and Danish oysters can withstand 1.5°C for several weeks. However, heavy mortalities of native oyster were reported after the severe winters of 1939/40 (Orton, 1940) and 1962/63 (Waugh, 1964). Mortality was attributed to relaxation of the adductor muscle so that the shell gaped, resulting in increased susceptibility to low salinities or to clogging with silt.

Low temperatures and cold summers are correlated with poor recruitment in Ostrea edulis, presumably due to reduced food availability and longer larval developmental time, especially at the northern limits of its range. Therefore, a reduction in temperature may result in reduced recruitment and a greater variation in the populations of Ostrea edulis.

The severe winters of 1939/40 and 1962/63 (Orton, 1940; Waugh, 1964) also resulted in the death of associated fauna, e.g. Sabella pavonina and other polychaetes died in great numbers, Crepidula fornicata incurred about 25% mortality and Ocenebra erinacea died in large numbers, while only small Carcinus maenas remained on the beds (Orton, 1940; Waugh, 1964). However, starfish, crabs such as Hyas araneus and Urosalpinx cinerea and Ascidiella aspersa were little affected (Orton, 1940; Waugh, 1964).

Decreases in temperature experienced in a severe winter are more extreme than our benchmark. However, long term decreases in temperature could potentially effects overall recruitment and other members of the community are intolerant of short term acute decreases in temperature. Therefore, an overall biotope intolerance of intermediate intolerance has been suggested at the benchmark level.
Recruitment in Ostrea edulis is sporadic and dependant of the hydrographic regime and local environmental conditions but will be enhanced by the presence of adults and shell material. Therefore a recoverability of low has been recorded (see additional information below).

Increase in turbidity
(View Benchmark)
The native oyster has no dependence on light availability so changes in turbidity would have no effect. However, increased turbidity may decrease primary production by phytoplankton and hence food availability. The characteristic red algae found in this biotope will suffer reduced primary production and growth but are probably shade tolerant but may be lost from deeper examples of this biotope. Therefore, an intolerance of low has been recorded. Once conditions returned to prior levels condition would probably be recovered rapidly.
Decrease in turbidity
(View Benchmark)
A decrease in turbidity and hence increased light penetration may result in increased phytoplankton production and hence increased food availability for suspension feeders, including Ostrea edulis. Therefore, reduced turbidity may be beneficial. However, increased fouling by red algae may result and compete with juveniles and settling spat for space.
Increase in wave exposure
(View Benchmark)
This biotope is found in sheltered to extremely sheltered conditions. Although subtidal, wave action in shallow water results in oscillatory water flow, the magnitude of which is greatest in shallow water and attenuated with depth. While the oysters' attachment is permanent, increased wave action may result in erosion of its substratum and the oysters with it. Areas where sufficient shell debris has accumulated may be less vulnerable to this disturbance. However, a proportion of the bed is likely to be displaced by an increase in wave action. Similarly, infaunal species, burrowing polychaetes and epifauna are characteristic of wave sheltered conditions and may be lost, e.g. Ascidiella sp. The biotope may be replaced by communities characteristic of stronger wave action and coarser sediments. Therefore, an intolerance of high has been recorded. Recruitment in Ostrea edulis is sporadic and dependant of the hydrographic regime and local environmental conditions but will be enhanced by the presence of adults and shell material. Therefore a recoverability of very low has been recorded (see additional information below).
Decrease in wave exposure
(View Benchmark)
This biotope is found in sheltered to extremely sheltered conditions. Therefore, a further reduction in wave exposure is unlikely to have any adverse effects.
Noise
(View Benchmark)
The majority of invertebrates within this biotope are probably unable to distinguish noise at the level of the benchmark.
Visual Presence
(View Benchmark)
While most invertebrates can detect light and react to shading as indicative of an approaching predator, they have very limited visual acuity or range and are unlikely to be adversely affected by visual presence.
Abrasion & physical disturbance
(View Benchmark)
Abrasion may cause damage to the shell of Ostrea edulis, particularly to the growing edge. Regeneration and repair abilities of the oyster are quite good. Power washing of cultivated oysters routinely causes chips to the edge of the shell increasing the risk of desiccation. This damage is soon repaired by the mantle. Oysters were often harvested by dredging in the past, which their shells survived relatively intact. However, a passing scallop dredge is likely to remove a proportion of the population. On mixed sediments, the dredge may remove the underlying sediment, and cobbles and shell material with effects similar to substratum loss above.

Polychaetes and other segmented worms were reported to be badly affected by oyster dredging while any bivalves were displaced (Gubbay & Knapman, 1999). In addition, the epifauna associated with horse mussel beds (Modiolus modiolus) was found to be particularly sensitive to abrasion due to scallop dredging (see £MCR.ModT£; Service & Magorrian, 1997). Therefore Ostrea edulis and the other characterizing species are probably sensitive to physical disturbance at the benchmark level and a biotope intolerance of intermediate has been recorded. See 'extraction' below for the effects of fishing on native oyster populations.

Recovery will depend on recolonization by the epifaunal and infaunal species, most of which are widespread with dispersive pelagic larvae. However, recruitment in Ostrea edulis is sporadic and dependant of the hydrographic regime and local environmental conditions but will be enhanced by the presence of adults and shell material. Therefore a recoverability of moderate has been recorded (see additional information below).
Displacement
(View Benchmark)
Although individuals are cemented to the substratum removal from the substratum (provided it does not damage the shell) will have little effect. Often oysters attach to non-fixed objects like old shells. Oyster beds used to be maintained by raking and harrowing. More modern methods involve keeping oysters in trays or bags, placing them in suitable conditions, sorting them and generally moving them around. Such disturbance may restrict feeding and affect the timing of spawning but suggests a low intolerance to displacement.

Displacement of Ostrea edulis will affect the associated epifauna, and probably result in loss of several of the more delicate species with a resultant loss of species richness. Burrowing species will probably be able to reburrow relatively quickly. Epifauna will probably recolonize the shells of Ostrea edulis rapidly. Therefore, an overall intolerance of low has been recorded.

Chemical Factors

Synthetic compound contamination
(View Benchmark)
Suspension feeding organisms process large volumes of sea water and remove organic and inorganic particulates from the water column. Therefore, they are vulnerable to both water soluble contaminants and contaminants adsorbed onto particulates. The effect of pollutants on oysters has been extensively studied. Particular examples follow:
  • Crassostrea virginica was found to be intolerant of halogenated by-products of chlorinated power station cooling waters, larval growth was adversely affected, up to 20% larval mortality occurred at 0.05 mg/l (LC50 48 hrs of 1 mg/l. (Cole et al., 1999).
  • Bromoform reduced feeding and gametogenesis at 25 µg/l in Crassostrea virginica (Cole et al., 1999).
  • Various detergents, previously used to treat oil spills, were shown to halve the normal development rate of Ostrea edulis larvae over the range 2.5 -7.5 ppm, depending on the type of detergent (Smith, 1968). An increase in development time is likely to increase larval mortality prior to settlement (see Mytilus edulis review).
  • Rees et al. (2001) suggested that TBT contamination may have locally reduced population sizes of Ostrea edulis.
  • In Ostrea edulis, TBT has been reported to cause:
    • reduced growth of new spat at 20 ng/l, a 50% reduction in growth at 60 ng/l;, although older spat grew normally at 240 ng/l for 7 days, and
    • the prevention of larval production in adults exposed to 240 and 2620 ng/l for 74 days (Thain & Waldock, 1986; Bryan & Gibbs, 1991).
  • Adults bioaccumulate TBT. Thain & Waldock (1986) and Thain et al. (1986) noted that TBT retarded normal sex change (male to female) in Ostrea edulis.
TBT also has marked effects on other marine organisms. For example TBT causes imposex in prosobranch gastropods, especially the neogastropods such as Nucella lapillus, Ocenebra erinacea and Urosalpinx cinerea resulting in markedly reduced reproductive capacity and population decline. Ascidian larval stages were reported to be intolerant of TBT (Mansueto et al., 1993 cited in Rees et al., 2001).

Beaumont et al. (1989) investigated the effects of tri-butyl tin (TBT) on benthic organisms. At concentrations of 1-3 µg/l there was no significant effect on the abundance of Hediste diversicolor or Cirratulus cirratus (family Cirratulidae) after 9 weeks in a microcosm. However, no juvenile polychaetes were retrieved from the substratum and hence there is some evidence that TBT had an effect on the larval and/or juvenile stages of these polychaetes. However, no information concerning the polychaetes characteristic of this biotope were found. However, recent surveys of the Crouch estuary suggested that benthic epifauna were recovering since a reduction in TBT contamination suggesting that populations of several epifaunal species, including Ascidiella sp., had previously been reduced (Rees et al., 1999; 2001).

While loss of predatory neogastropods (which are particularly intolerant of TBT) may be of benefit to Ostrea edulis populations, TBT has been show to reduce reproduction and the growth of spat. Spärck, (1951) demonstrated marked fluctuations and decreases in oyster population in the Limfjord due to variation in recruitment success, therefore any factor that significantly interferes with reproduction and recruitment in Ostrea edulis is likely to result in a marked effect on the population. Therefore, an overall intolerance of high has been recorded.
Rees et al. (1999; 2001) reported that the epifauna of the inner Crouch estuary had largely recovered within 5 years (1987-1992) after the ban on the use of TBT on small boats in 1987. Increases in the abundance of Ascidiella sp. and Ciona intestinalis were especially noted. Ostrea edulis numbers increased between 1987 -1992 with a further increase by 1997. However, they noted that the continued increase in Ostrea edulis numbers and the continued absence of neogastropods suggested that recovery was still incomplete at the population level. Clark (1997) suggested that temperate water benthos would probably take 6-10 years to recover after pollution had stopped. Overall, therefore, given the sporadic nature of recruitment in Ostrea edulis and its exclusion from otherwise suitable sediment by competitors (e.g. Crepidula fornicata) (see additional information below) a recoverability of very low has been recorded.
Heavy metal contamination
(View Benchmark)
In heavily polluted estuaries, e.g. Restronguet Creek in the Fal estuary, oyster flesh is known to turn green due to the accumulation of copper Cu. (Yonge, 1960; Bryan et al. 1987). Bryan et al. (1987) noted that in the Cu and Zn were accumulated in the tissues of Ostrea edulis, estimates ranging form ca 1000 to ca 16,500 µg/g dry weight, which would probably toxic for human consumption. Ostrea edulis is therefore tolerant of high levels of Cu and Zn and is able to survive in the lower reaches of Restronguet Creek, where other species are excluded by the heavy metal pollution. However, larval stages may be more intolerant, especially to Hg, Cu, Cd and Zn. Bryan (1984) reported at 48 hr LC50 for Hg of 1-3.3 ppb in Ostrea edulis larvae compared with a 48 hr LC50 for Hg of 4200 ppb in adults.
Little information on the tolerance of ascidians or sponges was found. However, polychaetes are though to be relatively tolerant of heavy metal pollution, even though some heavy metals may suppress reproduction (Bryan, 1984). Similarly, Bryan (1984) suggested that adult gastropod molluscs were also relatively tolerant of heavy metal pollution. Therefore, most other characteristic species in this biotope may be relatively tolerant of heavy metal pollution.

Although the adult Ostrea edulis may be tolerant of heavy metal pollution the larval effects suggest that recruitment may be impaired resulting in a reduction in the population over time, and hence a reduction in the associated fauna. Therefore, an overall intolerance of intermediate has been recorded. Recruitment in Ostrea edulis is sporadic and dependant of the hydrographic regime and local environmental conditions but will be enhanced by the presence of adults and shell material. Therefore a recoverability of low has been recorded (see additional information below).

Hydrocarbon contamination
(View Benchmark)
This biotope will probably be partly protected from the direct effects of an oil spill by its subtidal position. However, in sheltered areas oil is likely to persist, and reach the shallow sea bed adsorbed to particulates or in solution. Oil and its fractions has been shown to result in reduced feeding rates in bivalves (e.g. Crassostrea sp.) (Bayne et al., 1992; Suchanek, 1993). Oils and their fractions have also been shown to cause genetic abnormalities in Crassostrea virginica. Oysters and other bivalves are known to accumulate hydrocarbons in their tissues (Clark, 1997). Polyaromatic hydrocarbons were show to reduce the scope for growth in Mytilus edulis and may have a similar effect in other bivalves.
Polychaetes, bivalves and amphipods are generally particularly affected by oil spills in infaunal habits, and echinoderms are also particularly intolerant of oil contamination (Suchanek, 1993). Hydrocarbons in the environment probably also affect growth but no information concerning their effects on reproduction were found.
Overall, hydrocarbon contamination would probably affect growth rates of juveniles and adult Ostrea edulis, while an oil spill is likely to kill a proportion of the associated community. Therefore, an intolerance of intermediate has been recorded in the absence of further evidence, to represent the effects on the infauna and epifauna rather than the oysters themselves.
Recovery will depend on recolonization of the sediments by infauna and epifauna once the hydrocarbons levels have returned to normal levels, and is likely to rapid, although oil will persist in sheltered sediments for some time. Therefore, an overall recoverability of moderate has been recorded.
Radionuclide contamination
(View Benchmark)
Insufficient information
Changes in nutrient levels
(View Benchmark)
Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for all the suspension feeders within the biotope. Therefore, an intolerance of 'not sensitive*' has been recorded. However, long term or high levels of organic enrichment may result in eutrophication and have indirect adverse effects, such as increased turbidity, increased suspended sediment (see above), increased risk of deoxygenation (see below) and the risk of algal blooms. Ostrea edulis has been reported to suffer mortality due to toxic algal blooms, e.g. blooms of Gonyaulax sp. and Gymnodinium sp. (Shumway, 1990). The subsequent death of toxic and non-toxic algal blooms may result in large numbers of dead algal cells collecting on the sea bottom, resulting in local de-oxygenation as the algal decompose, especially in sheltered areas with little water movement where this biotope is found. Ostrea edulis may be relatively tolerant of low oxygen concentrations other species within the community may be more intolerant (see below).
Increase in salinity
(View Benchmark)
This biotope is found subtidally in full to variable salinity waters and is unlikely to experience increased salinity waters. Hyper-saline effluent may be damaging but no information concerning the effects of increased salinity on oyster beds was found.
Decrease in salinity
(View Benchmark)
Ostrea edulis is euryhaline and colonizes estuaries and coastal waters exposed to freshwater influence (Yonge, 1960). Yonge (1960) reported that the flat oyster could not withstand salinities below 23 psu. However, Hutchinson & Hawkins (1992) noted that scope for growth was severely affected below 22psu, probably because the oyster's valves were closed, but that 19 -16 psu could be tolerated if the temperature did not exceed 20°C. At 25°C animals did not survive more than 7 days at 16psu. Hutchinson & Hawkins (1992) noted that at low temperatures (10°C or less) the metabolic rate was minimal, which would help Ostrea edulis survive in low salinities associated with storm runoff in the winter months.
Several of the characterizing species in this biotope are commonly found in estuarine and full salinity waters and are probably tolerant of reduced salinity, e.g. Lanice conchilega and Ascidiella aspersa will tolerate salinities as low as 18psu (Fish & Fish, 1996). However, this biotope has only been recorded from full salinity habitats, therefore, a proportion of the epifauna and infauna may not tolerate a reduction in salinity and may be lost. Predatory starfish and other echinoderms are generally not able to tolerate low salinity are may be excluded.

Overall, therefore, the oyster bed itself may not be adversely damaged by a decrease in salinity comparable to the benchmark, and can probably tolerate short term acute reductions in salinity due to runoff, and an intolerance of low has been recorded. However, the diversity of the oyster bed will be reduced. Recovery will depend on recolonization by the associated fauna and flora and will probably be rapid (see additional information below).

Changes in oxygenation
(View Benchmark)
Oysters were considered to be tolerant of long periods of anaerobiosis due to their ability to survive out of water during transportation for long periods of time, and many weeks at low temperatures (Korringa, 1952; Yonge, 1960). For example, Lenihan (1999) reported that Crassostrea virginica could withstand hypoxic conditions (< 2mg O2 /l ) for 7-10 days at 18 °C but last for several weeks at <5 °C. However, Lenihan (1999) also suggested that many days (26) of hypoxia, contributed to the high rate of mortality observed at the base reefs at 6m depth together with poor condition, parasitism and reduced food availability. In addition, a prolonged period of hypoxia in the River Neuse (North Carolina) resulted in mass mortality of oysters (Lenihan, 1999).

Members of the characterizing species that occur in estuaries e.g. Ascidiella aspersa are probably tolerant of a degree of hypoxia and occasional anoxia. Similarly, most polychaetes are capable of a degree of anaerobic respiration (Diaz & Rosenberg, 1995). However, periods of hypoxia and anoxia are likely to result in loss of some members of the infauna and epifauna within this biotope. Overall, oysters are probably tolerant of hypoxia at the level of the benchmark and an intolerance of low has been recorded, although the biotope is likely to experience a decrease in species richness. Recovery will depend on recolonization by the associated fauna and flora and is likely to be rapid.

Biological Factors

Introduction of microbial pathogens/parasites
(View Benchmark)
Numerous diseases and parasites have been identified in oysters, partly due to their commercial importance and partly because of incidences of disease related mass mortalities in oyster beds. Diseases in oysters and other commercial bivalve species may be caused by bacteria (especially in larvae), protists, fungi, coccidians, gregarines, trematodes, while annelids and copepods may be parasite. The reader should refer to reviews by Lauckner (1983) and Bower & McGladdery (1996) for further detail. For example The following species have caused mortalities in Ostrea edulis populations in the UK:
  • Polydora ciliata burrows into the shell, weakening the shell and increasing the oysters vulnerability to predation and physical damage, whereas Polydora hoplura causes shell blisters;
  • boring sponges of the genus Cliona may bore the shell of oysters caused shell weakening, especially in older specimens;
  • the flagellate protozoan Heximata sp. resulted in mass mortalities on natural and cultivated beds of oysters in Europe in the 1920-21, from which many population did not recover (Yonge, 1960);
  • The parasitic protozoan Bonamia ostreae caused mass mortalities in France, the Netherlands, Spain, Iceland and England after its accidental introduction in 1980's resulting a further reduction in oyster production (Edwards, 1997);
  • another protozoan parasite Marteilia refingens, present in France has not yet affected stocks in the British Isles, and
  • the copepod parasite, Mytilicola intestinalis, of mussels, has also been found to infect Ostrea edulis potentially causing considerable loss of condition, although in most infections there is no evidence of pathology.
No information on the effects of diseases and parasites on the associated species was found. However, various diseases are associated with mass mortality in oyster beds and an overall intolerance of high has been recorded.

Recovery is dependant on larval recruitment since the adults are permanently attached and incapable of migration. Recruitment is sporadic and dependant on the local environmental conditions, hydrographic regime and the presence of suitable substratum, especially adult shells or shell debris, and has probably been inhibited by the presence of competition from non native species (see additional information below). Therefore, a recoverability of very low has been suggested.

Introduction of non-native species
(View Benchmark)
The slipper limpet Crepidula fornicata was introduced with American oyster between 1887-1890 and has became a serious pest on oyster beds. Crepidula fornicata competes for space with oyster, and the build up of its faeces and pseudofaeces smothers oysters and renders the substratum unsuitable for settlement (Blanchard, 1997; Eno et al., 1997, 2000). Where abundant, Crepidula fornicata may prevent recolonization by Ostrea edulis.

The American oyster drill Urosalpinx cinerea was first recorded in 1927 and occurs in south east and south west of the UK. Urosalpinx cinerea is a major predator of oyster spat and was considered to be a major pest on native and cultured oyster beds (Korringa, 1952; Yonge, 1960) and contributed to the decline in oyster populations in the first half of the 20th century.

The above species may cause marked effects on UK oyster beds, especially Crepidula fornicata that may change the entire biotope, to produce a Crepidula fornicata dominated biotope (see £IMX.CreAph£). Therefore, an intolerance of high has been recorded. The loss of the oyster population will result in loss of the biotope and many of its associated species.

Recovery is dependant on larval recruitment since the adults are permanently attached and incapable of migration. Recruitment is sporadic and dependant on the local environmental conditions, hydrographic regime and the presence of suitable substratum, especially adult shells or shell debris, and has probably been inhibited by the presence of competition from non native species (see additional information below). Therefore, a recoverability of very low has been suggested.

Extraction
(View Benchmark)
The introduction of oyster dredging in the mid 19th century developed the oyster beds into a major fishery. However, by the late 19th century stocks were beginning to be depleted so that by the 1950s the native oyster beds were regarded as scarce (Korringa, 1952; Yonge, 1960; Edwards, 1997). This biotope is still regarded as scarce today. Over-fishing, combined with reductions in water quality, cold winters (hence poor spat fall), flooding, the introduction of non-native competitors and pests (see above), outbreaks of disease and severe winters were blamed for the decline (Korringa, 1952; Yonge, 1960; Edwards, 1997). As a result, although 700 million oysters were consumed in London alone in 1864, the catch fell from 40 million in 1920 to 3 million in the 1960s, from which the catch has not recovered (Edwards, 1997).

Loss of the Ostrea edulis population would result in loss of the associated biotope. Therefore, while over-fishing was not the sole cause of the overall decline of UK Ostrea edulis population it was nevertheless a major contributing factor. Hence, while the benchmark would otherwise result in an intolerance of intermediate, due to the demonstrable potential effects of fishing on this biotope, an intolerance of high has been recorded.

Recovery is dependant on larval recruitment since the adults are permanently attached and incapable of migration. Recruitment is sporadic and dependant on the local environmental conditions, hydrographic regime and the presence of suitable substratum, especially adult shells or shell debris, and has probably been inhibited by the presence of competition from non native species (see additional information below). Therefore, a recoverability of very low has been suggested.

Additional information icon Additional information

Recoverability
Recovery of Ostrea edulis populations is dependant on larval recruitment, since newly settled juveniles and adults cement themselves to the substratum and are subsequently incapable of migration.
Recruitment in Ostrea edulis is sporadic and dependant on local environmental conditions, including the average summer sea water temperature, predation intensity and the hydrographic regime. Spärck, (1951) reported marked changes in population size due to recruitment failure. In unfavourable year stocks declined naturally (in the absence of fishing pressure) and the population in the Limfjord became restricted to the most favourable sites. In favourable years the stock increased and the population slowly spread from the most favoured locations. He concluded that a long series of favourable years was required for recovery, for example after closure of the oyster fishery in 1925, stocks did not recovery their fishery potential until 1947/48, ca 20 years. However, the Limfjord population of Ostrea edulis is at the northern most extent of its range where recruitment may be more dependant on summer temperatures than more southerly temperate populations. Rees et al. (2001) reported that the population of native oysters in the Crouch estuary was increasing (between 1992 -1997) since the reduction in TBT concentration in the water column. Nevertheless, Spärck's data (1951) suggest that several years of favourable recruitment would be required for a Ostrea edulis population to recover. Native oyster beds were considered scarce in Europe as early as the 1950s (Korringa, 1952, Yonge, 1960) and are still regarded as scarce today (Connor et al., 1999a). Dominance of other species such as Crepidula fornicata following loss of the oyster population can prevent re-establishment, through changes to the environment and competition, and together with introduced and native predators has probably inhibited recovery of natural populations. Recovery is likely to be slow even within or from established populations (see Spärck's, 1951). However, since larvae require hard substratum for settlement with a significant preference for the shells of adults, where the adult population has been removed, especially where shell debris has also been removed, recovery is likely to be very slow, i.e. 10 -25 years or more.

Other species
The other characterizing species are widespread species, with pelagic larvae, potentially capable of wide dispersal and are therefore, likely to be able to recolonize available substratum rapidly. Although the ascidian tadpole larva is short lived and has a low dispersal capability, fertilization is external in the most conspicuous ascidians in the biotope, Ascidiella sp., which are widespread in distribution and probably capable of rapid recolonization from adjacent or nearby populations.

This review can be cited as follows:

Tyler-Walters, H. 2001. Ostrea edulis beds on shallow sublittoral muddy sediment. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 24/04/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=69&code=1997>