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Species

European flat oyster

European flat oyster ostrea edulis. Picture: Renate Olie

Species description

The European flat oyster (Ostrea edulis) is a bivalve mollusc found mostly with exterior colours such as pale green, yellow, off-white and several tints of brown.1-4 The inside however has been known to have a pearlescent colour such as white or bluish-grey.1 The oyster itself consists of two differently shaped halves, one that is shaped concave and is used to mount itself to the substrate while the other is completely flat and fits inside the convex half.1 This flat side is also the feature from which the flat oyster derives its name.2 Flat oyster can reach ages of up to 20 to 30 years, becoming sexually mature at an age of 3-4 years old.1-2 The first year and a half of a Flat oyster are also a time of rapid growth, which then drops to a constant 20g per year before slowing down when nearing the age of five.1 They can grow up to sizes of 15-20 cm, but most oysters become mature when they are 5 cm and reach 11 cm at the end of their lifespan.1-3

Habitat and distribution

The European flat oyster is mostly know to settle on hard and sheltered but not too muddy substrates such as gravel, rocks, silt and other oyster shells.1 4-6 Flat oysters need a certain amount of hard substrate to settle but existing reefs have been known to expand on to soft substrate as long as the seabed is stable.7 Most flat oyster can be found no deeper than 10 meters, but there have been sightings and records of flat oysters appearing at depths of 30-50 meters and sometimes even 80 meters.1 4-6 In comparison to its Japanese competitor (Crassostrea gigas) the flat oyster is better at settling at these depths causing it to be found more in deeper waters.8 It should however be noted that there currently is a lack of knowledge on the ecological impact that deep-sea oyster beds provide.9 The flat oyster prefers locations with a water velocity between 0,25 and 0,8 meters per second and a temperature between 7 and 25 degrees Celsius.7

The flat oyster is distributed along the coast of western Europe, the Mediterranean, the Black sea, the Norwegian sea and several regions around the British Isles such the Solent, Thames estuary and River Fal.1-6 There are currently only a few of wild flat oysters currently present in the Dutch part of the North sea and very few places with suitable hard substrate. Combined with the relatively short mobility phase after spawning with only a maximum reproduction range of 10km with favourable conditions makes the reintroduction of the flat oyster a challenge. 10 11

Food and predators

Flat oysters feed themselves by pumping large quantities of water through a gill chamber which filters out any suspended organic particles. A single adult oyster can filter between 140 and 240 litres of sea water.1-4 This filtration method also gives the flat oyster the important role of filtering and storing other harmful substances such as nitrogen from the water column which prevents the growth of toxic algae blooms.2-4 Flat oysters are preyed upon by numerous species such as starfish, crabs and a variety of snails such as a invasive species know as the American oyster drill who was accidentally brought over with the introduction of the American oyster.1 3 7

Reproduction

Flat oysters are so called protandrous alternating hermaphrodites. This means they start out as males but later on turn in to females and then back in to male and so forth.1 3-5 During March or April the maturation of gamete cells begins, this moment is partially decided by the water temperature at the time.1 11 In the months June to August the gamete release takes place in which the sperm is released into the water and drawn in to the female mantle cavity.5 In here the fertilization takes place and the larvae are brooded for 6-10 days before being released in to the water column to mature for another 6-10 days.1 5 After their time as plankton the larvae oysters settle on hard substrate, these larvae are known as ‘competent’ larvae.7 A singular oyster can spawn up to a million oyster larvae, but more mature oyster can spawn up to 2 million by changing genders mid-spawning season.1 4 10 11

Status and protection

Until the mid 19th century the flat oyster was a commonly found and largely spread species across multiple regions in the North sea and beyond.2 5 8-10 But by the end of the 19th century overexploitation occurred and the hard substrate habitats of the flat oyster were destroyed beyond recovery.5 8-10 This is due to the targeted fishing of larger oyster which as previously mentioned produce the largest amount of offspring.2

It is currently estimated that 85% of the original flat oyster population worldwide have been lost due to overexploitation and pollution.2 5 As such the flat oyster has been classified as ‘threatened’ by the OSPAR convention.3 6 However, existing oyster beds create a positive feedback so by providing enough locations with hard substrate, suitable conditions and a healthy starting colony the flat oyster population will increase on its own.5 Supporting the increase of flat oyster populations is not only benefitting for just the oyster industry but can also increase the biodiversity of other marine wildlife such as crabs, lobsters and herring.11

Literature

1. Perry, F., Jackson, A., & Garrad, S. L. (2017). Ostrea edulis Native oyster. Marine Life Information Network: Biology and Sensitivity Key Information Reviews. https://www.marlin.ac.uk/species/detail/1146

2. Federal Agency for Nature Conservation. (2017). Restoration of the European flat oyster. Hope for the European Flat Oyster. https://www.bfn.de/en/activities/marine-nature-conservation/artenschutzprojekte/restoration-of-the-european-flat-oyster.html

3. Native Oyster Network. (n.d.). EUROPEAN NATIVE OYSTER. Retrieved September 20, 2021, from https://nativeoysternetwork.org/

4. Gamble, C., & Zoology Society of London. (2020). European native oyster guide: where it’s found, and why it’s an ecosystem engineer. Discover Wildlife. https://www.discoverwildlife.com/animal-facts/insects-invertebrates/facts-about-european-native-oysters/

5 Maathuis, M. A. M., Coolen, J. W. P., van der Have, T., & Kamermans, P. (2020). Factors determining the timing of swarming of European flat oyster (Ostrea edulis L.) larvae in the Dutch Delta area: Implications for flat oyster restoration. Journal of Sea Research, 156. https://doi.org/10.1016/j.seares.2019.101828

6. The OSPAR commision. (2009). Background document for Ostrea edulis and Ostrea edulis beds. https://www.ospar.org/work-areas/bdc/species-habitats/list-of-threatened-declining-species-habitats/invertebrates/flat-oyster

7. Smaal, A., Kamermans, P., Kleissen, F., van Duren, L., & van der Have, T. (2017). European flat oysters on offshore wind farms: additional locations : opportunities for the development of European flat oyster (Ostrea edulis) populations on planned wind farms and additional locations in the Dutch section of the North Sea. https://doi.org/10.18174/418092

8. van Duren, L. A., Gittenberger, A., Smaal, A. C., van Koningsveld, M., Osinga, R., Cado van der Lelij, C. A., & de Vries, M. B. (2016). Rijke riffen in de Noordzee.

9. Kamermans, P., Walles, B., Kraan, M., van Duren, L., Kleissen, F., van der Have, T., Smaal, A., & Poelman, M. (2018). Offshore Wind Farms as Potential Locations for Flat Oyster (Ostrea edulis) Restoration in the Dutch North Sea. Sustainability, 10(11). https://doi.org/10.3390/su10113942

10. Smaal, A. C., Kamermans, P., van der Have, T. M., Engelsma, M., & Sas, H. J. W. (2015). Feasibility of Flat Oyster (Ostrea edulis L.) restoration in the Dutch part of the North Sea.

11. van Onselen, E., Kamermans, P., & Sas, H. (2020). Flat Oyster Reproduction – A memo.

Author: The Rich North Sea

Year: 2021