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Pacific oyster

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Pacific oyster
Scientific classification
Kingdom:
Animalia
Phylum:
Mollusca
Class:
Bivalvia
Order:
Ostreoida
Family:
Ostreidae
Genus:
Crassostrea
Species:
C. gigas
Binomial name
Crassostrea gigas
Thunberg, 1793

The Pacific oyster or 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.

Etymology

The species name comes from the Greek crass meaning thick, [1] ostrea meaning oyster [2] and gígās meaning giant.[3]

Description

The shell of Crassostrea gigas varies widely with the environment where it is attached. There are large rounded radial folds that 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 and usually pale white or off-white. Mature specimens can be from 80 mm all the way up to 400 mm long.

Ecology

Habitat

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 40m 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 the Pacific oyster is between 20 and 25 ‰ (parts per thousand) and can tolerant salinities as high as 35‰ however at this level, reproduction is unlikely to occur. [4] The pacific oyster is also very a temperature tolerant species as it can withstand a range from -1.8 to 35°C. [4]

Biology

Sexuality

The Pacific oyster has separate sexes however hermaphrodites sometimes do exist. [5] The sex of the Pacific oyster can be determined by examining the gonads and can change from year to year normally during the winter months. [5] In certain environmental conditions one sex is favoured over the other. [5] Protandry is favoured in areas of high food abundance and protogyny occurs in areas of low food abundance. [5] In habitats with a high food supply, the sex ratio in the adult population tends to be favoured towards females and areas with low food abundances tend to have a larger proportion of male adults. [5]

Spawning

Spawning in the Pacific oyster occurs at 20°C. [5] 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. [5] Once released from the gonads, the eggs move through the suprabranchial chambers (gill), are then pushed through the gill ostia into the mantle chamber and finally they are released in the water forming a small cloud. [5] In males, the sperm is released at the opposite end of the oyster along with the normal exhalent stream of water. [5] 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 Cite error: A <ref> tag is missing the closing </ref> (see the help page).

Production Statistics

Global Pacific oyster production has increased from about 150 tonnes in 1950 to a peak of 750 tonnes in 1980.[6] By 2003, the global production of the Pacific oyster had increased to 4.38 million tonnes.[6] The majority of this production was in China, who produce 84% of the global production.[6] Japan, France and the Republic of Korea also are big Pacific oyster contributors, producing 261,000, 238,000 and 115,000 tonnes of produce respectively.[6] The other two major producers of the Pacific oyster are the United States of America (43 000 tonnes and Taiwan Province of China (23 000 tonnes).[6] In 2003, global pacific oyster production was worth $USD 3.69 billion, with Asia contributing over half of this amount.[6]

Example alt text
Global aquaculture production of Crassostrea gigas

Current issues

Virus management

Pacific oyster is a non-specific filter feeder which means they ingest any particulate matter in the water column. [7]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. [7] Globally, noroviruses are the most common cause of non-bacterial gastroenteritis and introduced into the water column by faecal matter either from sewage discharge or land runoff from nearby farmland. [7] There have been numerous gastroenteritis outbreaks in the world that have been directly caused by the consumption of shellfish from a polluted area. [8]

Heavy metal pollution

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. [5] These can accumulate in the tissues of the animal and leave it unharmed (bioaccumulation). [5] However, when the concentrations of the metals or biotoxins are high enough it can result in shellfish poisoning when consumed by humans. High copper concentration in Pacific oyster can be recognised by the meat turning a grass-green colour. [5] Most countries have strict water regulations and legislation to minimise such poisoning cases occurring. [9] [10] [11]

Diseases

Disease Agent Type Measures
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

[6]

Predators

There are numerous known predators of the Pacific oyster which can damage oyster stocks. [12]There are several crab species (Cancer magister, cancer productus, cancer gracilis), oyster drills and starfish species (Pisater ochraceus, Piaster brevispinus, Evasterias troschelii and Pycnopodia helianthoides) which can cause severe impacts to oyster culture. [12]

Productivity

Productivity of the Pacific oyster can be discussed as the amount of meat produced in relation to the amount of seed planted on cultch. [5] The productivity of a farm also depends on the interaction of biotic factors such as mortality, growth, oyster size as well as the quality of the seed and type of growing technique used (off bottom, bottom, suspended or floating culture). [5] Mortality of Pacific oysters can affect the overall productivity of the farm. There are four main causes of death in the Pacific oyster, these are;

  1. Natural mortality
    1. Age
    2. Predators
    3. Disease
    4. Environmental conditions (ice, freak winds)
  2. Competition for space: crowding of cultch
  3. Silting: sediment runoff from land
  4. Cluster separation: process of breaking up clusters of oysters to into as many individual oysters as possible.

Aquaculture in New Zealand

In New Zealand, the Pacific oyster was unintentionally introduced in 1950 most likely through ballast water and from the hulls of ships. [13] Aquaculture famers at the time noticed that the Pacific oyster out-competed the endemic species, Rock Oyster (Saccostrea commercialis) which naturally occurs in intertidal areas in the North Island. [14]The Rock oyster (Saccostrea commercialis) was experimented with in early cultivation procedures where they attached spat to cement covered sticks and laid them down in racks. [14] However the farmers noticed that the Pacific oyster (Crassostrea gigas) outgrew the endemic species in most areas and constantly was attaching to the Rock oyster collection sticks. A few years later, Pacific oyster was the dominant oyster 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.

Production Status

The pacific oyster is one of the three main aquaculture species in New Zealand along with King Salmon and the Greenshell mussel. [15] Pacific oyster aquaculture production has grown from an export value of $11 million in 1986 to $32 million in 2006. [15] In 2006, there were 23 pacific oyster farms throughout New Zealand which cover a total of 750 hectares of marine space and produce 2,800 tonnes of product per year. [13].The annual production of Pacific oyster has is now between about 3,300 and 4,000 tonnes .[14] In 2005, the domestic value of New Zealand's Pacific oysters was $12 million, the export value of New Zealand's Pacific oysters was $16.9 million, giving a total value of $28 million. [16] New Zealand’s main export markets are Japan, Korea, America, the European Union and Australia. [16]

References

  1. ^ Definition of crass at dictionary.com.
  2. ^ Definition of ostrea at dictionary.com.
  3. ^ Definition of giga at dictionary.com.
  4. ^ a b Pacific Oyster factsheet, Food and Agriculture Organization of the United Nations (FAO)
  5. ^ a b c d e f g h i j k l m n Quayle, D.B (1969). Pacific oyster culture in British Columba, p. 23. First Edition. Ottawa: The Queen’s Printer.
  6. ^ a b c d e f g The State of World Fisheries and Aquaculture (SOFIA)
  7. ^ a b c Greening, G.E., and McCoubrey D.J. 2010. Enteric Viruses and Management of Shellfish Production in New Zealand. Food Environ Virology, 2:167–175
  8. ^ [1] Arnold T. 2009.,Toxicity, Shellfish. Medical Director of Louisiana Poison Control Centre
  9. ^ [2] Scottish water quality regulations
  10. ^ [3] Irish water quality regulations
  11. ^ [4] American water quality regulations
  12. ^ a b [5] Non-indigenous aquatic species of concern for Alaska: Pacific oyster fact sheet
  13. ^ a b [6] Aquaculture.govt.nz
  14. ^ a b c [7] TeAra: The encyclopaedia of New Zealand
  15. ^ a b [8] Aquaculture.govt.nz: farmed species. Cite error: The named reference "NZgovt2" was defined multiple times with different content (see the help page).
  16. ^ a b [9] New Zealand Government, Blue Horizon document
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