Native oyster (Ostrea edulis)

NBN Interactive07-07-2007

Map accurate at time of writing. Visit NBN or OBIS to view current distribution

Researched byAngus Jackson Refereed byThis information is not refereed.
AuthorityLinnaeus, 1758
Other common names- Synonyms-

Summary

Description

Ostrea edulis is a bivalve mollusc that has an oval or pear-shaped shell with a rough, scaly surface. The two halves (valves) of the shell are different shapes. The left valve is concave and fixed to the substratum, the right being flat and sitting inside the left. The shell is off-white, yellowish or cream in colour with light brown or bluish concentric bands on the right valve. Ostrea edulis grows up to 110 mm long, rarely larger. The inner surfaces are pearly, white or bluish-grey, often with darker blue areas.

Recorded distribution in Britain and Ireland

Widely distributed around the British Isles but less so on the east and north-east coasts of Britain and Ireland. The main stocks are now in the west coast of Scotland, the south-east and Thames estuary, the Solent, the River Fal, and Lough Foyle.

Global distribution

Found naturally from the Norwegian Sea south through the North Sea down to the Iberian Peninsula and the Atlantic coast of Morocco. Found in the Mediterranean Sea and extends into the Black Sea.

Habitat

Ostrea edulis is associated with highly productive estuarine and shallow coastal water habitats on firm bottoms of mud, rocks, muddy sand, muddy gravel with shells and hard silt. In exploited areas, suitable habitat is/has been created in the form of 'cultch' - broken shells and other hard substrata.

Depth range

0-80

Identifying features

  • Shell inequivalve, lower (left) valve convex and upper valve flat sitting within the lower.
  • Periostracum thin, dark brown.
  • Outer surface rough and scaly with concentric sculpture and fine radiating ridges.
  • Yellowish or cream in colour with light brown or bluish concentric bands on the right valve.
  • Hinge line without teeth in the adult.
  • Adductor muscle scar white, or slightly discoloured.

Additional information

Also commonly known as the flat oyster and European oyster.

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Biology review

Taxonomy

PhylumMolluscaSnails, slugs, mussels, cockles, clams & squid
ClassBivalviaClams, cockles, mussels, oysters, and scallops
OrderOstreoidaOysters, scallops & saddle oysters
FamilyOstreidae
GenusOstrea
AuthorityLinnaeus, 1758
Recent Synonyms

Biology

Typical abundanceHigh density
Male size range2-110mmMale size at maturity50mm
Female size range50mmFemale size at maturity
Growth formBivalvedGrowth rate20 g/year
Body flexibilityNone (less than 10 degrees)Mobility
Characteristic feeding methodActive suspension feeder, Active suspension feeder
Diet/food source
Typically feeds onSuspended organic particles
Sociability Environmental positionEpifaunal
DependencyHost for.
SupportsIndependent

the protozoan Bonamia ostreae, and the copepodMytilicola intestinalis.

Is the species harmful?No

Biology information

  • There is some evidence that reduced growth, weight and poor conditions are a consequence of high population densities (300 per square yard). Size and shape can be extremely variable. Because the oyster cements itself to the substratum, growth of neighbouring individuals may result in competition or space and distort the usual shell shape.
  • Feeding is carried out by pumping water through a filter in the gill chamber removing suspended organic particles. Particulate matter which is resuspended from the bottom material by tidal currents and storms is likely to be an important food source (Grant et al., 1990). Growth rates of Ostrea edulis are faster in sheltered sites than exposed locations, however this is thought to be attributed to the seston volume rather than flow speed or food availability (Valero, 2006).
  • Growth is quite rapid for the first year and a half. It then remains constant at around 20 grams per year before slowing down after five years. In the British Isles, the main growing season is from April to October. The oyster faces serious competition from the introduced species Crepidula fornicata, the slipper limpet. Brought over from the United States this species can occur in very high densities competing for space and food. The slipper limpet deposits pseudo faeces which forms 'mussel mud' changing the substratum and hindering settlement. Native oysters are preyed on by a variety of species including starfish and Ocenebra erinacea, the sting winkle or rough tingle. Buccinum undatum, the common whelk also feeds on oysters but not as exclusively as the sting winkle. Urosalpinx cinerea, the American oyster drill, was accidentally introduced to the British Isles with American oysters and lives on oyster beds feeding almost entirely on oyster spat.

Habitat preferences

Physiographic preferencesOpen coast, Sea loch / Sea lough, Ria / Voe, Estuary
Biological zone preferencesLower circalittoral, Lower eulittoral, Lower infralittoral, Sublittoral fringe, Upper circalittoral, Upper infralittoral
Substratum / habitat preferencesBedrock, Cobbles, Gravel / shingle, Large to very large boulders, Mud, Muddy gravel, Muddy sand, Pebbles, Small boulders
Tidal strength preferences
Wave exposure preferencesExposed, Extremely sheltered, Moderately exposed, Sheltered, Very sheltered
Salinity preferencesFull (30-40 psu), Variable (18-40 psu)
Depth range0-80
Other preferencesNo text entered
Migration PatternNon-migratory / resident

Habital Information

The native oyster has also been introduced and is cultivated in North America, Australia and Japan. Cultivated populations may be encouraged through the use of an artificial substratum (limed tiles) used preferentially for larval settlement. Oysters are found on a wide variety of substrata but typically where the seabed is hard. Can form into dense 'beds' of oyster shells.

Life history

Adult characteristics

Reproductive type Protandrous hermaphrodite Reproductive frequency Annual protracted
Fecundity (number of eggs) >1,000,000 Generation time Insufficient information
Age at maturity 3 years Season June - September
Life span 5-10 years

Larval characteristics

Larval/propagule type - Larval/juvenile development Planktotrophic
Duration of larval stage 11-30 days Larval dispersal potential Greater than 10 km
Larval settlement period Insufficient information

Life history information

A life span of 5-10 years is probably typical as the majority of individuals in populations are 2-6 years old. However, they may reach in excess of 15 years old. Oysters are protandrous alternating hermaphrodites. This means that they start off as males producing sperm then switch to egg producing females, back to males and so on. The native oyster starts life as male, becoming mature at around 3 years of age. After spawning the oyster becomes a functional female. Larvae are seldom produced by oysters under 50 mm. Gamete maturation begins in March or April and is in part temperature dependent. Gametogenesis may be continuous in warmer conditions e.g. California. On the west coast of Ireland there is at least one spawning in each sexual phase during the summer. There may be some periodicity in spawning with peaks during full moon periods. Fecundity may be as high as 2,000,000 in large individuals. The eggs are around 150 microns in diameter. Eggs produced during the female stage are held in the gills and mantle cavity. The eggs are fertilized by sperm drawn in by the inhalant water flow used for feeding and respiration. The fertilized eggs are retained for 7-10 days until the veliger stage is reached, at which point they are released. This is called larviparous or incubatory development.

Sensitivity reviewHow is sensitivity assessed?

Physical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High Very low / none Very High Low
This species typically cements itself to the substratum on metamorphosis so loss of the substratum would cause death of the population. The native oyster does have a pelagic larval phase which can disperse over large distances to re-establish populations. It is also highly fecund and spawns regularly. However, 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. Because the adults are cemented to the substratum, adult immigration is not possible. Native and introduced predators can also restrict re-establishment. Habitat management may be required in order to allow oysters to re-colonize an area.
High Very low / none Very High Low
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 culture. 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 after flooding due to exceptionally high tides in 1953. Even small increases in sediment deposition have been found to reduce growth rates in Ostrea edulis (Grant et al., 1990). Therefore, an intolerance of high has been recorded.
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 1-2mm 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.
Low Very high Very Low Low
Oysters can reject unwanted particles (Yonge, 1926) and 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. This study is supported by Grant et al. (1990) who found declining clearance rates in Ostrea edulis in responds to an increase in suspended particulate matter. Suspended sediment was also shown to reduce the growth rate of adult Ostrea edulis and results 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 10 mg/l of particulate organic matter and significantly reduced by 5 mg/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).

Ostrea edulis is less well adapted to silted conditions than other species, e.g. Crassostrea virginica (Yonge, 1960). Yonge (1960) and Korringa (1952) considered Ostrea edulis to be intolerant of turbid environments. For example, Yonge (1960) reported smothering of oyster beds after flooding (see above). 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 and an intolerance of low has been recorded. Moore (1977) reported that variation in suspended sediment and silted substratum and resultant scour was an important factor restricting oyster spatfall, 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.
Once 'normal' conditions are restored then normal feeding will resume.
No information
Intermediate Low High Low
The adult oyster can close the valves of its shell tightly so minimising desiccation. Some populations are found in the lower intertidal. Subtidal populations exposed for one hour would simply close up for this time and intertidal populations would close up for longer. This would limit the amount of time available for feeding. Individuals at the limit of their desiccation exposure tolerance would die under further increases in desiccation. The native oyster does have a pelagic larval phase which can disperse over large distances to augment populations. It is also highly fecund and spawns regularly. However, dominance of other species such as Crepidula fornicata following reduction in oyster populations can restrict re-establishment of former levels, through changes to the environment and competition. Native and introduced predators can also restrict re-establishment. If populations have been reduced considerably then the standing stock may be insufficient to ensure synchronous and successful spawning. Because the adults are cemented to the substratum, adult immigration is not possible.
Intermediate Low High Low
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 exposure would mean that the valves are kept shut for a greater or lesser time. Increases in emergence would result in less time available for feeding. Individuals already at the limit of their emergence tolerance would die under further increases in emergence. The native oyster does have a pelagic larval phase which can disperse over large distances to augment populations. It is also highly fecund and spawns regularly. However, dominance of other species such as Crepidula fornicata following reduction in oyster populations can restrict re-establishment of former levels, through changes to the environment and competition. Native and introduced predators can also restrict re-establishment. If populations have been reduced considerably then the standing stock may be insufficient to ensure synchronous and successful spawning. Because the adults are cemented to the substratum, adult immigration is not possible.
Tolerant* Not relevant Not sensitive*
A decrease in emergence regime may allow the oyster beds to extend their range up the shore in suitable conditions. Therefore, tolerant is recorded.
Low Very high Very Low Low
Hydrodynamic currents supply food and oxygen to Ostrea edulis. Increases in water flow may improve the availability of suspended particles on which the oyster feeds. With increased water flow rate the oysters filtration rate increases, up to a point where the oysters are unable to remove more particles from the passing water (Walne, 1979). However increases in water flow rate may interfere with settlement of spat. Growth rates of Ostrea edulis are faster in sheltered sites than exposed locations, however this is thought to be attributed to the seston volume rather than flow speed or food availability (Valero, 2006). Decreased water flow may result in increased siltation and consequential changes in substratum type. This may result in reduced weight, condition and fecundity. Therefore intolerance is assessed as low. Once 'normal' conditions are restored then normal feeding will allow condition to be restored, hence recovery is very high, yielding a very low sensitivity value.
No information
Low Very high Very Low Moderate
Temperature and salinity are the most significant abiotic factors affecting Ostrea edulis (Valero, 2006).
Filtration rate, metabolic rate, assimilation efficiency and growth rates of adult Ostrea edulis increase with temperature and 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; Grant et al., 1990). 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. However, Ostrea edulis occurs from the Mediterranean to the Norwegian coast and is unlikely to be adversely affected by long term changes in temperatures in the UK.
Spärck's data (1951) suggest that temperature is an important factor in recruitment, 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), and minimum temperatures required for spawning in France are 14-16°C, with gametogenesis occurring at 10°C (FAO, accessed 2009). 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 long term increases.
Once the temperature returns to normal limits the characterizing species will probably regain their condition rapidly.
Intermediate Low High
Growth rates are usually slower, mortality increased and spawning less frequent and reliable with low temperatures (Valero, 2006).
Hutchinson & Hawkins (1992) suggested that Ostrea edulis 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. Korringa (1952) also reported Ostrea edulis form waters of -1°C. 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 as the ice melted or to clogging with silt.
Low temperatures and cold summers are also correlated with poor recruitment, 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. Hence an 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).
Low Very high Very Low Low
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. Therefore, an intolerance of low has been recorded. Once conditions returned to prior levels condition would probably be recovered rapidly.
No information
Intermediate Low High Moderate
The native oyster occurs in areas with wave exposure ranging from exposed to extremely sheltered. Increases in wave exposure to levels greater than this are likely to cause death. Settlement of spat may be hindered, young oysters may be damaged or displaced by the wave action. The native oyster does have a pelagic larval phase which can disperse over large distances to augment populations. It is also highly fecund and spawns regularly. However, dominance of other species such as Crepidula fornicata following reduction in oyster populations can restrict re-establishment of former levels, through changes to the environment and competition. Native and introduced predators can also restrict re-establishment. If populations have been reduced considerably then the standing stock may be insufficient to ensure synchronous and successful spawning. Because the adults are cemented to the substratum, adult immigration is not possible.
Tolerant Not relevant Not sensitive Low
Decreases in wave exposure are unlikely to have any effect on the population.
Tolerant Not relevant Not sensitive Low
This species probably has very limited ability for noise detection.
Tolerant Not relevant Not sensitive Low
This species probably has very limited ability for visual perception.
Intermediate Low High Moderate
The native oyster has a calcareous shell that can get very thin in older individuals. The shell may be brittle. Abrasion may cause damage to the shell, 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. 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. Therefore, an intolerance of intermediate has been recorded. See 'extraction' below for the effects of fishing on native oyster populations.
The native oyster does have a pelagic larval phase which can disperse over large distances to augment populations. It is also highly fecund and spawns regularly. However, dominance of other species such as Crepidula fornicata following reduction in oyster populations can restrict re-establishment of former levels, through changes to the environment and competition. Native and introduced predators can also restrict re-establishment. If populations have been reduced considerably then the standing stock may be insufficient to ensure synchronous and successful spawning. Because the adults are cemented to the substratum, adult immigration is not possible.
Low Very high Very Low Moderate
Although individuals are cemented to the substratum removal from the substratum (provided it does not damage the shell) will have no effect. Often the 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 timing of spawning. Once 'normal' conditions are restored then normal behaviour will resume.

Chemical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High Very High High
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 0.05mg/l (LC50 48hrs of 1mg/l. (Cole et al., 1999).
  • Bromoform reduced feeding and gametogenesis at 25 µg/l (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
  • . TBT has been reported to cause:
    • reduced growth of new spat at 20ng/l, a 50% reduction in growth at 60ng/l;, although older spat grew normally at 240ng/l for 7 days;
    • prevented the production of larvae in adults exposed to 240 and 2620ng/l for 74 days
    Adults bioaccumulate TBT. Thain & Waldock (1986) and Thain et al. (1986) noted that TBT retarded normal sex change (male to female) in Ostrea edulis.
Therefore, given the evidence of mortality, the effects on reproduction and suppression of population sizes above an intolerance of high has been recorded.
Rees et al. (2001) reported that Ostrea edulis numbers increased between 1987 -1992 after the ban on the use of TBT on small boats in 1987 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 (Waldock et al, 1999; Rees et al, 2001). 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
Intermediate Low High Moderate
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 Cu and Zn were accumulated in the tissues of Ostrea edulis, estimates ranging from ca 1000 to ca 16,500 µg/g dry weight, which would probably be toxic for human consumption. However, 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 48hr LC50 for Hg of 1-3.3 ppb in Ostrea edulis larvae compared with a 48hr LC50 for Hg of 4200ppb in adults. Therefore, although the adults 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. Therefore an intolerance of intermediate has been recorded.
Recovery will depend on recruitment, which is sporadic (see additional information below) and a recoverability of low has been recorded.
Hydrocarbon contamination
Low Very high Very Low Low
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 shown to reduce the scope for growth in Mytilus edulis and may have a similar effect in other bivalves. Therefore, an intolerance of low has been recorded.
Radionuclide contamination
No information No information No information Not relevant
Oysters, as active suspension feeders are known to be highly efficient bioaccumulators. Radionuclides may be accumulated in the oyster tissue. The actual effects of this bio-concentration are not known.
Changes in nutrient levels
Tolerant* Not relevant Not sensitive* Not relevant
The species can do well in estuarine environments which frequently have higher levels of nutrients than the open coast. Nutrient concentration may have no effect on the oysters themselves. However, the oysters may benefit indirectly through the enhanced growth of microalgae (on which they feed) with increased levels of nutrients.
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
Low Very high Very Low Low
Temperature and salinity are the most significant abiotic factors affecting Ostrea edulis (Valero, 2006).
Ostrea edulis is found subtidally in full to variable salinity waters and is unlikely to experience increased salinity waters. Therefore intolerance is assessed as low. Recovery would be very high, yielding a very low sensitivity value. Hyper-saline effluent may be damaging but no information concerning the effects of increased salinity on oyster beds was found.
Low Very high Very Low Moderate
Ostrea edulis is euryhaline and colonizes estuaries and coastal waters exposed to freshwater influence (Yonge, 1960), although the species has a preference for more fully saline conditions (Laing et al. 2005), and low salinity results in a cessation of feeding (Korringa, 1952). Yonge (1960) reported that the flat oyster could not withstand salinities below 23 psu. 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. Further, in low salinity conditions, the mortality rate of spat is lower at 5°C than at 10°C. (RÖdstrÖm and Jonsson, 2000). Ostrea edulis larva may grow at salinities of 20 psu, but can survive salinities as low as 15 psu (FAO, accessed 2009). However, larvae do not survive at very low salinity although they will settle in low salinity waters, otherwise they could not colonize estuarine waters (Yonge, 1960). Therefore, an intolerance of low has been recorded.
Intermediate Low High Low
Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. No information is available regarding the tolerance of oysters to changes in oxygen concentration. Reduced oxygen levels may be problematic if combined with reduced salinity where the valves are kept closed as much as possible limiting respiration. The native oyster does have a pelagic larval phase which can disperse over large distances to augment populations. It is also highly fecund and spawns regularly. However, dominance of other species such as Crepidula fornicata following reduction in oyster populations can restrict re-establishment of former levels, through changes to the environment and competition. Native and introduced predators can also restrict re-establishment. If populations have been reduced considerably then the standing stock may be insufficient to ensure synchronous and successful spawning. Because the adults are cemented to the substratum, adult immigration is not possible.

Biological pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High Very High Moderate
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. However, the following examples has 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 causing 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 populations did not recover (Yonge, 1960);
  • The parasitic protozoan Bonamia ostreae caused mass mortalities in 2-3 year old oysters in France, the Netherlands, Spain, Iceland and England after its accidental introduction in 1980's resulting in a further reduction in oyster production (Edwards, 1997);
  • another protozoan parasite Marteilia refingens, present in France has not affected stocks in the British Isles yet (U.K. BAP, 1999), but may cause reoccurring mortalities. ;
  • 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;
  • Alsio the fungus Ostracoblabe implexa causes shell malformations which may eventually lead to mortality (FAO, 2009).
Therefore, 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.
High Very High Moderate
The slipper limpet Crepidula fornicatawas introduced with an American oyster between 1887-1890 and has became a serious pest on oyster beds. Crepidula fornicate competes for space and food 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 fornicate may prevent recolonization by Ostrea edulis.
The American oyster drill Urosalpinx cinerea was first recorded in 1927 after accidental introduction with American oysters, and occurs in south east and south west of the UK. Urosalpinx cinerea is a major predator of oyster spat (feeding almost exclusively on them) 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.
Given the marked effects on UK oyster beds attributed to the above species, an intolerance of high has been recorded. The loss of the oyster population will result in loss of the biotope and 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.
High Very High Moderate
British native oyster beds (characteristic of this biotope) were exploited in Roman times. However, the introduction of oyster dredging in the mid 19th century, and the accompanying improvement in rail transport developed the oyster beds into a major fishery. 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. Overfishing, combined with reductions in water quality, cold winters (hence poor spatfall), flooding, the introduction of non-native competitors and pests (see above), outbreaks of disease and severe winters was 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). Therefore, while overfishing was not the sole cause of the overall decline of UK Ostrea edulis population it was nevertheless a major contributing factor. Therefore, although the benchmark would otherwise result in an intolerance of intermediate, due to the demonstrable potential effects of fishing on this species, 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.
Not relevant Not relevant Not relevant Not relevant
No species associated with oyster beds are known to be subject to extraction.

Additional information

Recovery
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 years, 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. However, 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 recover 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., 1997a). 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.

Importance review

Policy/legislation

UK Biodiversity Action Plan Priority
Species of principal importance (England)
Species of principal importance (Wales)
Scottish Biodiversity List
OSPAR Annex V
Features of Conservation Importance (England & Wales)

Status

National (GB) importanceNot rare/scarceGlobal red list (IUCN) category-

Non-native

Native-
Origin- Date Arrived-

Importance information

Native oyster fisheries are subject primarily to UK shellfisheries conservation legislation; the species is not named in any national or international nature conservation legislation or conventions. However, Ostrea edulis is included in a Species Action Plan under the UK Biodiversity Action Plan (Anon, 1999b) and naturally occurring native oyster beds are a nationally scarce habitat (see IMX.Ost).

Commercial native oyster transplantation has been recorded as a dispersal mechanism for non-native species. Oysters exported for market to the Netherlands from Britain may have attached to them plants of wireweed Sargassum muticum. The copepod mussel parasite Mytilicola intestinalis has also been recorded in native oysters on the S.W. and E coasts of Britain. Infection of oysters does not readily occur in the presence of mussels. Infection levels of up to 9.5 percent and 4 parasites per oyster have been recorded. Translocations of oysters could serve as a dispersal mechanism for this parasite into areas where it currently does not occur. The copepod can breed in female stage oysters larger than 50 mm. Infestation is unlikely to occur in oysters less than 15 mm. Consequently the maximum size limit for transported oyster seed has been set at 12 mm. A parasitic protozoan, Bonamia ostreae, causing the disease Bonamiasis has caused massive mortalities in France and has been introduced to some English populations. Care is required to prevent the infection of British Isles populations with the parasitic protozoan Marteilia refringens which is present in other European countries. There is a closed season from 14 May to 4 August during the main spawning season. Efforts are being made to reintroduce oysters to old, now derelict grounds. Settlement areas have been degraded by species such as Crepidula fornicata.

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Citation

This review can be cited as:

Jackson, A. 2007. Ostrea edulis Native oyster. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1146

Last Updated: 07/07/2007