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The Pacific oyster, Japanese oyster or Miyagi oyster (Crassostrea 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 shell of Crassostrea 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 mm to 400 mm long.
Crassostrea 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 25 parts per thousand (ppt), and they can tolerant salinities as high as 35 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 Pacific oyster has separate sexes, but hermaphrodites sometimes do exist. Their sex can be determined by examining the gonads, and it can change from year to year, normally during the winter months. In certain environmental conditions, one sex is favoured over the other. Protandry is favoured in areas of high food abundance and protogyny occurs in areas of low food abundance. In habitats with a high food supply, the sex ratio in the adult population tends to favour females, and areas with low food abundances tend to have a larger proportion of male adults.
Spawning in the Pacific oyster occurs at 20°C. This species is very fecund, with females releasing about 50-200 million eggs in regular intervals (with a rate at 5-10 times a minute) in a single spawning. Once released from the gonads, the eggs move through the suprabranchial chambers (gills), are then pushed through the gill ostia into the mantle chamber, and finally are released in the water, forming a small cloud. In males, the sperm is released at the opposite end of the oyster, along with the normal exhalent stream of water. A rise in water temperature is thought to be the main cue in the initiation of spawning, as the onset of higher water temperatures in the summer results in earlier spawning in the Pacific oyster.
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.
Crassostrea gigas was named by a Swedish naturalist, Carl Peter Thunberg in 1795. It originated from Japan, where it has been cultured for hundreds of years. It is now the most widely farmed and commercially important oyster in the world, as it is very easy to grow, environmentally tolerant and is easily spread from one area to another. The most significant introductions were to the Pacific Coast of the United States in the 1920s and to France in 1966. In most places, the Pacific oyster was introduced to replace the native oyster stocks which were seriously dwindling due to overfishing or disease. In addition, this species was introduced to create an industry that was previously not available at all in that area. As well as intentional introductions, the Pacific oyster has spread through accidental introductions either through larvae in ballast water or on the hulls of ships. In some places in the world, though, it is considered by some to be an invasive species, where it is outcompeting native species, such as the Olympia oyster in Puget Sound, Washington, the rock oyster, Saccostrea commercialis in the North Island of New Zealand and the blue mussel, Mytilus edulis, in the Wadden Sea.
Numerous methods are used in the production of Pacific oysters. These techniques depend on factors such as the seed supply resources, the environmental conditions in the region and the market product, i.e., whether the oysters are sold in a half shell, or shelled for meat extraction. Production can either be entirely sea-based or rely on hatcheries for seed supply.
Most of the global Pacific oyster spat supply comes from the wild, but some is now produced by hatchery methods. The seed from the wild can either be collected by the removal of seaweed from beaches or by hanging shell (cultch in suspension from long lines in the open water. The movement towards hatchery-reared spat is important, as wild seed is susceptible to changeable environmental conditions, such as toxic algal blooms, which can halt the supply of seed from that region. In addition, several pests have been noted as considerable dangers to oyster seed. The Japanese oyster drill (Ocenebra japonica), flatworm (Pseudostylochus osterophagus), and parasitic copepod (Mytlilcola orientalis) have been introduced accidentally to aquaculture areas, and have had serious impacts on oyster production, particularly in British Columbia and Europe.
Pacific oyster broodstock in hatcheries are kept in optimum conditions so the production of large amounts of high quality eggs and sperm can be achieved. Pacific oyster females are very fecund, and individuals of 70-100g live weight can produce 50-80 million eggs in a single spawn. Broodstock adults are held in tanks at 20-22°C, supplied with cultured algae and with salinities of 25-32 ppt. These individuals can be induced to spawn by thermal shock treatment. Yet, it is more common for the eggs from a small sample of females (about six) to be stripped from the gonads using Pasteur pipettes and fertilized by sperm from a similar number of males.
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 flagellate algae 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. It is common, however, particularly on the US West Coast, for the mature larvae to be packed and shipped to oyster farms, where the farmers set the oysters themselves.
Pacific oyster spat can be grown in nurseries by sea-based or land-based upwelling systems. Nursery culture reduces mortality in small spat, thus increasing the farm’s efficiency. Sea-based nursery systems are often located in estuarine areas where the spat are mounted on barges or rafts. Land-based nursery systems have spat mounted on barges in large saltwater tanks, which either have a natural algae supply or are enriched with nutrients from fertilizers.
This stage of oyster culture is almost completely sea-based. A range of bottom, off-bottom, suspended and floating cultures are used. The technique used depends on site-specific conditions, such as tidal range, shelter, water depth, current flow and nature of substratum. Pacific oysters take 18– 30 months to develop to the market size of 70-100 g live weight (shell on). Growth from spat to adults in this species is very rapid at temperatures of 15-25°C and at salinities of 25 to 32 ppt.
In 2000, the Pacific oyster accounted for 98% of the world’s cultured oyster production, and is produced in countries all over the world.
Global production has increased from about 150 tonnes in 1950 to 750 tonnes in 1980. By 2003, global production had increased to 4.38 million tonnes. The majority was in China, which produced 84% of the global production. Japan, France and the Republic of Korea also contributed, producing 261 000, 238 000 and 115 000 tonnes produce, respectively. The other two major producers are the United States (43 000 tonnes) and Taiwan (23 000 tonnes). In 2003, global Pacific oyster production was worth $ 3.69 billion, with Asia contributing over half of this amount.
Pacific oysters are nonspecific filter feeders, which means they ingest any particulate matter in the water column. This presents major issues for virus management of open water shellfish farms, as shellfish like the Pacific oyster have been found to contain norovirus strains which can be harmful to humans. Globally, noroviruses are the most common cause of nonbacterial gastroenteritis, and are introduced into the water column by faecal matter, either from sewage discharge or land runoff from nearby farmland. Numerous gastroenteritis outbreaks in the world have been directly caused by the consumption of shellfish from polluted areas.
Pacific oysters, like other shellfish, are able to remove heavy metals, such as zinc and copper, as well as biotoxins (microscopic toxic phytoplankton), from the surrounding water. These can accumulate in the tissues of the animal and leave it unharmed (bioaccumulation). However, when the concentrations of the metals or biotoxins are high enough, shellfish poisoning can result when they are consumed by humans. High copper concentration in Pacific oyster can be recognised by the meat turning a grass-green colour. Most countries have strict water regulations and legislation to minimise the occurrence of such poisoning cases.
|Denman Island disease||Mikrocytos mackini||Protozoan parasite||Restricted modified culture practices|
|Nocardiosis||Norcardia crassoteae||Bacterium||Modified culture practices|
|Oyster velar virus disease (OVVD)||Unknown||Virus||None known|
|Herpes-type virus disease of C.gigas larvae||Unknown||Virus||None|
Numerous predators are known to damage Pacific oyster stocks. Several crab species (Metacarcinus magister, Cancer productus, Metacarcinus gracilis), oyster drills and starfish species (Pisater ochraceus, P. brevispinus, Evasterias troschelii and Pycnopodia helianthoides) can cause severe impacts to oyster culture.
Productivity of the Pacific oyster can be discussed as the amount of meat produced in relation to the amount of seed planted on cultch. The productivity of a farm also depends on the interaction of biotic factors, such as mortality, growth, and oyster size, as well as the quality of the seed and the growing technique used (off bottom, bottom, suspended or floating culture). The main causes of mortality in the Pacific oystere are: natural mortality (age), predators, disease, environmental conditions (ice, freak winds), competition for space (crowding of cultch), silting (sediment runoff from land) and cluster separation (process of breaking up clusters of oysters to into as many individual oysters as possible).
In New Zealand, the Pacific oyster was unintentionally introduced in 1950s, most likely through ballast water and from the hulls of ships. Aquaculture farmers at the time noticed the Pacific oyster outcompeted the endemic species, the Sydney rock oyster (Saccostrea glomerata), which naturally occurs in intertidal areas in the North Island. Early experiments in rock oyster cultivation procedures attached spat to cement-covered sticks and laid them down in racks. The farmers noticed, however, the Pacific oyster outgrew the endemic species in most areas, and constantly was attaching to the rock oyster collection sticks. A few years later, Pacific oysters were the dominant species in the farms, as it grew three times faster than the rock oyster, produced a reliable and constant supply of spat, and had an already established market overseas. In 1977, the Pacific oyster was accidentally introduced to the Marlborough Sounds, and farming began there in the 1990s. Marlborough farmers developed a different method of cultivation in comparison to the North Island method of racks; they instead suspended their oysters on longlines.
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.