Common name:Japanese oyster
Scientific name:Crassostrea gigas
Alternative common names:
Pacific oyster, Miyagi oyster.
The Japanese oyster is native to the Pacific coast of Asia. It has been introduced to many parts of the world for aquacultural and ecological purposes. Introducing the species in vast quantities throughout the world has had a negative effect, and it is now considered an invasive species.
Where does this species come from?Japan and Southeast Asia.
What is its invasive status in South Africa?NEMBA Category 2.
Where in South Africa is it a problem?Western Cape, Eastern Cape and Northern Cape.
How does it spread?Its swimming stage, its capacity to survive in various environmental conditions and its ability to colonise new areas facilitate the dispersion of the species along coastal areas. The aggressive manner in which it spreads is detrimental to ecosystems.
Why is it a problem?These oysters are generally impossible to contain if environmental conditions are suitable. Their planktonic eggs and larvae facilitate natural dispersal, and this, in combination with high fecundity, allows for rapid population. In some areas it has become the dominant oyster species, displacing indigenous species.
What does it look like?Description: It is a cupped oyster species with a curved lower shell (which attaches to the substratum) and a flat upper shell. Its shell is elongated, with a maximum length of 400mm (average length 150-200mm), and it has at least one abductor scar, which is purple. No radial threads are present and valve margins generally show no colouration, with margins normally undulating. Habitat: The preferred habitat is a firm substrate, such as rocks, other shells or debris, although they can also be found on mud and sand. Breeding: They are protandrous hermaphrodites, meaning they are able to change gender (usually from male to female, although the reverse is also possible). When resources, such as food and space, are abundant, the sex ratio is skewed towards females, with males predominant when resources become scarce. The development of mature eggs and sperm takes place when the environmental conditions are appropriate for breeding, with a water temperature of approximately 16-34°C. An average female produces 50-200 million eggs in a single broadcast spawning event.