Biodiversity & Conservation

Ostrea edulis beds on shallow sublittoral muddy sediment

SS.SMx.IMx.Ost


IMX.Ost

Image Bernard Picton - Ostrea edulis beds on shallow sublittoral muddy sediment. Image width ca 25 cm.
Image copyright information

Distribution map

SS.SMx.IMx.Ost recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)


  • EC_Habitats
  • OSPAR

Ecological and functional relationships

Oyster beds are dominated by suspension feeding invertebrates. Ostrea edulis is an active suspension feeder on phytoplankton, bacteria, particulate detritus and dissolved organic matter (DOM) (Korringa, 1952; Yonge, 1960). The production of faeces and pseudofaeces enriches the underlying sediment, providing a rich food source for infauna detritivores, deposit feeders, meiofauna and bacteria.

Dense beds of suspension feeding bivalves are important in nutrient cycling in estuarine and coastal ecosystems, transferring phytoplankton primary production and nutrients to benthic secondary production (pelagic-benthic coupling) (Dame, 1996). A model food web for an oyster reef (based on intertidal Crassostrea sp. beds) was presented by Dame (1996).

Other suspension feeding epifauna include the ascidians (e.g. Ascidiella aspersa, Ascidiella scabra and Dendrodoa grossularia) and sponges (e.g. Halichondria bowerbanki), hydroids, barnacles (e.g. Balanus balanus), and tube worms such as Pomatoceros triqueter and Polydora ciliata.

Infaunal suspension feeders include the tube worms Chaetopterus variopedatus, Sabella pavonina, Myxicola infundibulum, and Lanice conchilega and where present Abra sp. and the tellinids Angulus tenuis and Fabulina fabula.

Lanice conchilega, Fabulina fabula and Polydora ciliata are also surface deposit feeders on organic particulates and detritus.

The enriched sediment probably supports a diverse meiofauna, including nematodes and polychaetes (e.g. Scoloplos armiger and terebellids).

The sediment may also support amphipods such as Bathyporeia guilliamsoniana and Ampelisca brevicornis, which have been recorded in native oyster beds (Millar, 1961).

Hermit crabs such as Pagurus bernhardus and the common whelk Buccinum undatum may be scavengers on the bed.

A variety of predators feed in oyster beds. Asterias rubens is a general predator occasionally taking oyster spat and oysters but with a preference for mussels and, in their absence, Crepidula fornicata and the American oyster drill Urosalpinx cinerea. Young Asterias rubens feeds on barnacles in preference to oyster spat (Hancock, 1955). Hancock (1955) suggested that Asterias rubens fed significantly more on predators and competitors of the native oyster than on the oysters themselves. However, he also noted that the starfish was still likely to cause severe damage on highly cultivated areas with a high abundance of oysters and their spat. Asterias rubens is itself preyed on by the sun star Crossaster papposus (Hancock, 1958).

Predatory gastropods such as the native Sting winkle Ocenebra erinacea and the introduced American oyster drill Urosalpinx cinerea prey on small oysters and oyster spat. For example, 55 -58% of the oyster spat settling in 1953 in Essex oyster beds were destroyed by Urosalpinx cinerea. The dog whelk Nucella lapillus may occasionally take oyster spat (Korringa, 1952; Hancock, 1954; Yonge, 1960). However, only 10% of adults of 3 years of age were taken by Urosalpinx cinerea (Hancock, 1954), suggesting that the risk of predation decreases with increasing oyster size. A similar size refuge from predation is seen in other bivalve beds e.g. Mytilus edulis and the horse mussel Modiolus modiolus.

Crabs, such as Carcinus maenas and Hyas araneus are mobile omnivores that prey on oysters and their spat and also on the other fauna associated with oyster beds, including the drills, whelks and starfish (Yonge, 1960).

Predatory fish may also enter the bed to feed on the associated species, although Yonge (1960) suggested that fish were not a significant predator of the oysters themselves.

Several species compete with the oyster spat for settlement space on the shells of adult oysters, especially those species that breed at the same time of the year. Ascidiella sp. are know to settle at the same time as oyster spat, competing for the available hard substratum such as oyster shells (living or dead), and subsequently overgrowing spat that are able to settle. However, this may only seriously affect the oyster recruitment where the ascidians occur in any abundance.

Barnacles (e.g. Balanus balanus and Eliminius modestus), the tube worm Pomatoceros triqueter and the ascidian Dendrodoa grossularia were also reported to compete for settlement space, especially the barnacles (Korringa, 1952; Yonge, 1960; Millar, 1961).

The introduced slipper limpet Crepidula fornicata competes with the oyster for space and food, and its pseudofaeces may smother the oyster. Where Crepidula fornicata has become abundant the oyster beds have been lost (see sensitivity to introduced species) (Blanchard, 1997).

Seasonal and longer term change

Fish and crabs predators probably migrate further offshore in winter months, reducing predation pressure. Changes in the average summer temperature may have significant effects on recruitment (see recruitment processes below). In addition, Spärck (1951) noted marked changes in the populations of Ostrea edulis in the Limfjord, Denmark between 1852 and 1949. In periods of poor recruitment and the absence of fishing pressure, populations gradually declined, becoming restricted to the most favourable areas of the Limfjord. In some areas there was a 90% decrease in stock. Temperature was probably the most important controlling factor in recruitment in the Limfjord population (see recruitment) (Spärck, 1951). Korringa (1952) noted that while temperature was probably the most important factor in populations at their northern most range of the species, other factors were important in more temperate waters. However, Spärck (1951) demonstrated the importance of recruitment in natural populations of the native oyster and the potential for large fluctuations in population size over time.

Habitat structure and complexity

Oyster beds provide hard substratum for settlement in an otherwise sedimentary habitat and therefore support a diverse range of invertebrates. The oyster bed also modifies the sediment, increasing the amount of shell debris and organically enriching the sediment with faeces and pseudofaeces. Ostrea edulis preferentially settles on adult of the same species (i.e. it is gregarious) resulting in layer upon layer of oysters in the absence of fishing pressure. The layer of living and dead oyster shell probably alters the water flowing over the sediment surface and protects it from erosion. The layers of shell debris and living oyster provide interstices for other organisms. For example, the American oyster Crassostrea virginica can form extensive reefs several metres in height that have been shown to affect the local hydrodynamics and hence larval dispersal and settlement and hence community composition (Lenihan, 1999; Peterson et al., 2000). While, no information concerning the scale of native oyster reefs was found, it is likely that they also affect the local hydrographic regime to some extent.
  • Oyster beds support a diverse epifauna consisting of protozoa, sponges, hydroids, the benthic stages of Aurelia sp., flatworms, ribbon worms, nematodes polychaetes, amphipods and ostracod crustaceans, crabs, sea spiders, gastropod molluscs, ascidians, bryozoans, starfish and sea urchins (Korringa, 1951; Yonge, 1960). Korringa (1951) also noted that the flanges or flaps of the oyster shell provided refuges for some species. Although the exact fauna found will depend on locality, a detailed account of the epifauna of oyster beds in the Oosterschelde is given by Korringa (1951).
  • The sediment surface may be punctuated by burrowing tube worms such as Chaetopterus variopedatus, Sabella pavonina, Myxicola infundibulum, and Lanice conchilega.
  • Burrowing amphipods may occupy the top few cm of the sediment e.g. Bathyporeia guilliamsoniana and Ampelisca brevicornis.
  • The sediment below the oyster bed is enriched by faeces and pseudofaeces and usually contains shell debris accumulated from dead oysters. The infauna will vary with nature of the underlying sediment and the relative proportions of shell debris and faecal deposits. However, macroinfauna probably includes burrowing polychaetes, nematodes, and bivalves (see ecological relationships above)

Productivity

Dame (1996) suggested that dense beds of bivalve suspension feeders increase turnover of nutrients and organic carbon in estuarine (and presumably coastal) environments by effectively transferring pelagic phytoplanktonic primary production to secondary production in the sediments (pelagic-benthic coupling). Increased microbial activity within the enriched sediments underlying the beds, increases the rate of nutrient turnover and hence the productivity of the ecosystem as a whole (Dame, 1996).
Epifloral macroalgae provide some primary productivity to the ecosystem, however, the majority of production with the biotope is secondary production, with organic carbon derived from phytoplankton and organic particulates consumed by suspension feeders, especially the oysters. No estimate of the overall productivity of native oyster beds was found. However, before over-fishing and disease (see importance) oysters beds supported fisheries, suggesting that there are potentially highly productive.

Recruitment processes

The flat oyster
In Ostrea edulis, spawning occurs in the summer months (June to September) and is coincident with spring tides (and the new or full moon) (Korringa, 1952; Yonge, 1960). Spawning is thought to require a minimum temperature (which also probably controls gametogenesis) of 15-16°C (Yonge, 1960) although the exact temperature probably varies with area and local adaptation (Korringa, 1952). Eggs are fertilized internally and incubated to the veliger stage (7-10 days) at which point they are released into the plankton.

Ostrea edulis is highly fecund producing an average of between 91,000 to up to 2 million eggs with increasing age and size. However, good fertilization efficiency requires a minimum population size, so that in small populations not all the eggs may be fertilized (Spärck, 1951). The larvae are pelagic for 11-30 days, providing potentially high levels of dispersal, depending on the local hydrographic regime. Subsequent recruitment however, is dependant on larval growth and mortality due to predation in the plankton, the availability of settlement sites and post-settlement and juvenile mortality.

Good recruitment (settlement) is associated with warm summers whereas poor recruitment occurred in cold summers in the Oosterschelde and Limfjord (Spärck, 1951; Korringa, 1952), and is probably related to larval food availability and developmental time. Widdows (1991) states that any environmental or genetic factor that reduces the rate of growth or development of Mytilus edulis larvae will increase the time spent in the plankton and hence significantly decrease larval survival, which may also be true of most pelagic bivalve larvae.

In areas of strong currents larvae may be swept away form the adult populations to other oyster beds or to areas of unsuitable substratum and lost. Oyster beds on open coasts may be dependent on recruitment from other areas. Oyster beds in enclosed embayments may be self recruiting. Due to the high numbers of larvae produced, a single good recruitment event could potentially significantly increase the population. Oyster larvae will settle on available hard substrata but are gregarious and prefer to settle on adult shells, especially the new growth. However, competition for space (substratum for settlement) from other species that settle at the same time of year e.g. barnacles and ascidians (see ecological relationships), results in high levels of larval and juvenile mortality. Newly settled Ascidiella sp., are known to overgrow and hence kill oyster larvae. In addition, newly settled spat and juveniles are subject to intense mortality due to predation, especially by the oyster drills (Urosalpinx cinerea and Ocenebra erinacea) and starfish. For example, in the Oosterschelde, Korringa (1952) reported 90% mortality in oyster spat by their first winter, with up to 75% being taken by Urosalpinx cinerea, while Hancock (1955) noted that 73% of spat settling in summer 1953 died by December, 55 -58% being taken by Urosalpinx cinerea.

Overall, recruitment in Ostrea edulis is sporadic and dependant of 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. However, he concluded that a long series of favourable years was required for recovery of stocks after depletion. 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. Nevertheless, Spärck's data (1951) suggest that several years of favourable recruitment would be required for an Ostrea edulis population to recover.

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.

Time for community to reach maturity

Korringa (1951) noted that many of the Ostrea edulis epifauna were dependant on the oyster for substratum. It is also likely that some burrowing species are dependant of the conditions provided by the bed of Ostrea edulis. Therefore, the time taken for the community to reach maturity will depend primarily on the time taken for the oyster bed to develop (see recruitment processes above), after which recolonization will probably be rapid, and in the order of 1-2 years.

Additional information

No text entered

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 24/11/2014]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=69&code=2004>