Native Oyster

The European flat oyster / Native Oyster Ostrea edulis is a habitat-building bivalve mollusc. Young oysters have a planktonic phase, after which they preferentially settle out on adult oysters (Figure 1). Once settled, oysters fuse their shells to the underlying substrate and can therefore form dense aggregations, termed an oyster reef. Oyster reefs provide food and habitat for numerous species and may serve as nursery grounds for some fish species (Figure 2). 

Figure 1: Life cycle of Ostrea edulis. Image courtesy of Fitzsimmons et al. 2019, see Restoration Guidelines for Shellfish Reefs
Figure 2: Functions provided by intact oyster reef habitats. Image courtesy of Fitzsimmons et al. 2019, see Restoration Guidelines for Shellfish Reefs

Oysters are filter feeders, and a single oyster can filter up to 240 litres of seawater per day. Their filtering activity can improve water quality on local scales (Figure 2). This is not only because oysters remove particles from the water, but also because they then deposit them on the sediment, where conditions for bacteria that break down pollutants such as nitrates are better. This results in enhanced rates of denitrification, a process why which nitrites and nitrates are transformed into inert di-nitrogen gas. By removing particles from the water column the oysters can also increase light penetration to the sediment, and promote the recovery of seagrasses, another threatened and valuable coastal habitat. The draw down of sediments together with the stabilizing effect of the reef, can also result in reefs acting as carbon sinks, although this ecosystem benefit is complicated (read more here…).

European Native oyster reproduction
By Eline van Onselen

European flat oysters are so called “protandrous hermaphrodites”, meaning an oyster starts life as male and may after several years change to female. Within one season the sex change may occur back-and-forth. A male oyster releases sperm into the water column, fertilising up to 1 million eggs in the pallial cavity of the female. Older oysters can spawn twice during one spawning season: once as a male and once as a female. Sperm cells are filtered out of the water phase by the females, and combined with egg cells in their shell cavity. The larvae are released from the female into the water after 8 to 10 days (depending on temperature), spending another 8 to 10 days in the water column before settling on a suitable surface. The metamorphosis from mobile larvae to sessile spat probably mainly depends on food availability and can take up to two weeks. Because of the relatively brief mobile phase, dispersal of the European native oyster occurs over relatively small distances, generally up to 1 km but sometimes ranging to 10 km. The lifecycle of the native oyster is summed up in Figure 1. 

The timing of spawning varies between years and locations with water temperature generally accepted as the most important factor for inducing reproductive activity. Using a technique call the “temperature sum” the larval peak can be predicted for a specific location. The temperature sum, also known as growing degree-days, heat units or thermal time, is the sum of the (average) water temperature per day, if the temperature is higher than a threshold temperature. A study in Lake Grevelingen and the Oosterschelde in the Dutch Delta showed that spawning is not only dependent on temperature sum but also on lunar cycle, chlorophyll a concentration (food abundance), day-in-year and mean temperature. This can make prediction of spawning somewhat complicated. Hence, a prediction model based on local temperature sum is currently the most practical method of predicting the larval peak.

Profile of the European flat oyster

Name:European flat oyster / European native oyster, Huître plâte, Europäische Auster
Scientific name:Ostrea edulis
Size:up to approx.15 cm
Age:up to 30 years; sexually mature at 3-4 years
Appearance:Shape roundish to oval. Left / lower valve convex, right / upper valve almost flat and fitting inside the left valve to close it. The genus Ostrea ´flat oysters` takes its name from its shape.
Habitat:Estuaries and sea lochs as well as open coastal seas to ~50m depth. Primarily subtidal, colonizing mixed hard substrates, in particular shell material.
Range: The native range is pan-European, including the northeast Atlantic from the south of Norway through to the Mediterranean Sea, as far as the Black Sea (See native range map above, data from Ocean Biogeographic Information System (OBIS)).
Status:Now rare in Europe due to overfishing, impacts of bottom towed gears, and pollution.
Legal status across Europe: At an international scale, the European flat oyster is included in the OSPAR List of Threatened and/or Declining Species and Habitats for the North-East Atlantic (Region II – Greater North Sea and Region III – Celtic Sea). It is also included in Ramsar as “shellfish reefs”, and by some member states as “Reef” in the Habitats Directive.