Native oysters are described as protandrous alternating hermaphrodites. This means that when they reach maturity, they function first as a male and then they go into alternate cycles of female and male stages for the rest of their life. The number of cycles each year depends on the length of the breeding season, so in the UK native oysters usually spawn twice during the summer, once as a male and once as a female. A minimum water temperature of 16oC is required before they spawn. The males release sperm into the surrounding sea water which is taken in through the inhalent siphon of the female. The eggs produced by the female are fertilised inside the parent’s shell and the larvae are brooded within the mantle cavity until they have a fully-formed shell (at around 0.170 mm). This usually takes about 10 days. The parent oyster releases the larvae into the sea water where they drift in the plankton and feed on natural phytoplankton. The number of larvae released is related to the size of the parent oyster.
After 2 to 3 weeks, depending on local environmental conditions, the larvae are mature and they develop a foot. At this stage, they are called pediveligers. They sink to the seabed and explore the sediment surface with their foot until they find a suitable surface on which to settle permanently. The pediveligers cement firmly to a hard surface such as shell or stone. Next, they go through a series of morphological and physiological changes, a process known as metamorphosis(which takes 3 to 4 days), to become ‘immature’ adults called spat (or seed).
LIFE CYCLE OF PACIFIC OYSTERS
As a species not native to the UK, the Pacific oyster seldom reproduces naturally in the wild. Sea water temperatures around the UK are generally too low to support maturation or successful larval development and settlement. All juvenile oysters used for culture are produced by specialist hatchery facilities.
Pacific oysters are alternating hermaphrodites. On maturity, they can function as either a male or female. The following season, they change sex and then have alternate cycles of female and male stages for the rest of their life. However, a few remain as true hermaphrodites, acting as both male and female at the same time.
In their native environments, Pacific oysters release eggs and sperm into the sea water where fertilisation occurs. The larvae are plank tonic, drifting in the surface layers for approximately 2 to 3 weeks before they sink to the bottom to take up a sessile lifestyle attached to shell, stones or rocks.