Photo: Alice Barbé
Oysters are bivalve molluscs. They are hermaphrodites, i.e., they have both male and female reproductive organs. When a flat oyster functions as a male it sheds it milt into the surrounding water by means of the exhalant current. When functioning as a female the eggs are shed into the mesh work of the gill (sieve-like organ). If the inhalant current of water is carrying sperm from a nearby male, it fertilises the eggs. The resultant embryos remain for up to two weeks in the female's cavity before they are expelled to the water outside. When the embryos, which are now called larvae, are shed to the outside by the ‘mother oyster’ they already have a delicate bivalve shell and a large ciliated structure, called a velum, which is a swimming and feeding organ. The larvae swim upwards in the water, then sink slowly and swim upward again. By the end of the ‘swimming period’, the larvae have increased their weight up to fivefold and their bodily structure has become more complex. Their behaviour also changes; they now try to attach to a solid surface. Once they have settled onto the surface, they loose their larval features and within three to four days they take on the typical adult form. They are now known as spat. Within three to five years, the spat will grow to adulthood. However, the larger majority of the spat dies before reaching adulthood.
Fluctuations
During the past two decades, the Dutch flat oyster population has been subject to strong fluctuations. In the Eastern Scheldt, the flat oyster is nearly extinct. The population of Pacific oysters in this area has increased strongly during the past decade. These fluctuations are partly caused by climatological conditions and partly by diseases and pests. These pests and diseases pose no threat to human consumption.