The Pacific oyster, Japanese oyster, or Miyagi oyster (Magallana gigas ), is an oyster native to the Pacific coast of Asia. It has become an introduced species in North America, Australia, Europe, and New Zealand.
The genus Magallana is named for the Portuguese explorer Ferdinand Magellan and its specific epithet gígās is from the Greek for "giant". It was previously placed in the genus Crassostrea ; from the Latin crass meaning "thick", ostrea meaning "oyster", and Crassostrea gigas is considered by part of the scientific community to be the proper denomination as an accepted alternative in WoRMS,
Pacific oysters have a pelagic veliger larval stage which lasts from 14–18 days. In the hatcheries, they are kept at temperatures of 25–28 °C with an optimum salinity between 20 and 25%. Early-stage veligers (<120 nm shell length) are fed daily with flagellated algal species (Isochrysis galbana or Pavlova lutherii ) along with diatom species (either Chaetoceros calcitrans or Thalassiosira pseudonana ). The larvae are close to a settlement stage when dark eye spots and a foot develop. During this time, settlement materials (cultch), such as roughed PVC sheets, fluted PVC pipes, or shells, are placed into the tanks to encourage the larvae to attach and settle. However, particularly on the US West Coast, mature larvae are commonly packed and shipped to oyster farms, where the farmers set the oysters themselves.
The shell of M. gigas varies widely with the environment where it is attached. Its large, rounded, radial folds are often extremely rough and sharp. The two valves of the shell are slightly different in size and shape, the right valve being moderately concave. Shell colour is variable, usually pale white or off-white. Mature specimens can vary from 80 to 400 mm long.
M. gigas is an estuarine species, but can also be found in intertidal and subtidal zones. They prefer to attach to hard or rocky surfaces in shallow or sheltered waters up to 40 m deep, but have been known to attach to muddy or sandy areas when the preferred habitat is scarce. The Pacific oyster can also be found on the shells of other animals. Larvae often settle on the shell of adults, and great masses of oysters can grow together to form oyster reefs. The optimum salinity for Pacific oysters is between 20 and 35 parts per thousand (ppt), and they can tolerate salinities as high as 38 ppt; at this level, however, reproduction is unlikely to occur. The Pacific oyster is also a very temperature tolerant species, as it can withstand a range from −1.8 to 35 °C.
The larvae of the Pacific oyster are planktotrophic, and are about 70 µm at the prodissoconch 1 stage. The larvae move through the water column via the use of a larval foot to find suitable settlement locations. They can spend several weeks at this phase, which is dependent on water temperature, salinity, and food supply. Over these weeks, larvae can disperse great distances by water currents before they metamorphose and settle as small spat. Similar to other oyster species, once a Pacific oyster larva finds a suitable habitat, it attaches to it permanently using cement secreted from a gland in its foot. After settlement, the larva metamorphoses into a juvenile spat. The growth rate is very rapid in optimum environmental conditions, and market size can be achieved in 18 to 30 months. Unharvested Pacific oysters can live up to 30 years.
The Pacific oyster is one of the three main aquaculture species in New Zealand along with king salmon and the greenshell mussels. Pacific oyster aquaculture production has grown from an export value of $11 million in 1986 to $32 million in 2006. In 2006, the 23 Pacific oyster farms throughout New Zealand covered a total of 750 hectares of marine space and produced 2,800 tonnes of product per year. Annual production is now between about 3,300 and 4,000 tonnes. In 2005, the value of New Zealand's Pacific oyster production was $12 million domestically, and $16.9 million for export. New Zealand's main export markets are Japan, Korea, the US, the EU and Australia. However, research has demonstrated that changes in global ocean temperature and the advent of ocean acidification may alter the growth, reproduction, and development of this species with variable responses
