Molluscan Aquaculture and the Triploid Oyster

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She wheeled her wheelbarrow
Through streets broad and narrow,
Crying cockles, and mussels,
Alive, alive--o.
--From an Irish Folksong

Many people in the Pacific Northwest enjoy a nice bowl of steamed Penn Cove mussels from time to time. The small, indigo-colored shellfish grown in Puget Sound have become a mainstay on the menus of many top quality restaurants. Thanks to assistance from UW fisheries professor Kenneth Chew and his students, a whole new industry has grown up in Washington State around the culture of mussels.footnote 1

Chew also has developed methods to enhance Manila clam culture, one of the most important commercial steamer clams on the West Coast of the U.S. His 1992 manual on the subject has been used extensively by commercial shellfish growers and private beach owners in the region.footnote 2

As a consequence of research carried out by Chew and colleagues, Washington State has become one of the nation's top three producers of oysters. Washington State oysters are sold throughout the country, even in areas like the Chesapeake Bay and Gulf states, where the local oyster industry has been reduced by overfishing, disease, or pollution.

Chew is perhaps best known for developing the triploid Pacific oyster, which doesn't become soft and "mushy" during the summer spawning season. Production of the triploid oyster in Washington State has grown tremendously since it was developed in the early 1980s. During 1994, for instance, Washington's largest oyster grower, Coast Seafoods, produced over 22 billion oyster larvae, of which 50% were triploids. The technology has been adopted by aquaculturists in many other regions and other countries.

Triploid oysters have been genetically altered so that they are reproductively inactive—virtually sterile. As a result, they attain larger size than normal, or diploid, strains. And because triploids don't undergo the transformations associated with reproduction that make diploid oysters unpalatable during the summer months, they can be marketed all year round. The old adage about not eating oysters in months without an "r" in their name--which probably arose from the fact that diploid oysters become "mushy" during the reproductive phase--doesn't apply to triploids.

Triploids are genetically altered to have three sets of chromosomes in each cell instead of the normal two. The technique involves interrupting the process of maturation of the oyster egg cell using controlled applications of heat, pressure, or a chemical, so that the egg retains two sets of chromosomes, to which the sperm contributes another for a total of three. Ordinarily, in the process called meiosis, the egg cell ultimately reduces its set of two chromosomes to one before uniting with the sperm chromosome, producing the normal complement of two chromosomes that are found in diploid organisms.

Triploid oysters grow normally overall but have poorly developed gonads, and therefore do not divert much of their energy into reproduction. The result is a fast-growing, firmer, more palatable product.

Research on triploid oysters was originally carried out at the University of Maine on a different type of oyster, the American oyster, but it failed to catch on in the Northeast where hatcheries were small, and the characteristics of the American oyster made the effects of triploidy less dramatic. But in the Pacific Northwest, hatcheries were well-established and poised to adopt the new technology. Hatchery methods had long been used in the Northwest because its coastal waters are generally too cold for the Pacific oyster--imported originally from Japan in the 1920s--to spawn naturally. Furthermore, because the Pacific oyster is more fecund than the American variety, it is a less marketable product when sexually mature, creating a stronger driving force in the Northwest than on the east coast for the adoption of the triploid oyster.

In 1983, Chew and researchers Standish Allen and Sandra Downing initiated studies on the triploid oyster at the UW. Soon the researchers were applying their research to large-scale production with support from the Washington Sea Grant Program and the Pacific Coast Oyster Growers Association.

Besides being a boon to the molluscan aquaculture industry in Washington State, the triploid oyster is famous as the subject of a landmark court case for what would have been the first patent on a genetically manipulated animal. The case did not win, but it did lead the U.S. Patent Office and the U. S. Court of Appeals to declare that genetically altered higher animals can indeed be patented, a ruling of far-reaching implications.footnote 3comma footnote 6

  1. Mussel Aquaculture in Puget Sound, D. Skidmore, K. K. Chew, Washington Sea Grant Program, University of Washington, Seattle, 1985.
  2. Guide to Manila Clam Culture in Washington, D. R. Toba, D. S. Thompson, K. K. Chew, G. J. Anderson, and M. B. Miller, Washington Sea Grant Program, University of Washington, Seattle, 1992.
  3. Sexless Oysters and Self-Tipping Hats, Adam Woog, Sasquatch Books, Seattle, 1991.
  4. Remote Setting and Nursery Culture for Shellfish Growers: Workshop Record, February 19, 1991, Washington Sea Grant Program, University of Washington, Seattle.
  5. Hatchery Manual for Producing Triploid Oysters, S. K. Allen, S. L. Downing, K. K. Chew, Washington Sea Grant Program, University of Washington, Seattle, 1989.
  6. Ex parte Allen, 2 U.S.P.Q. 2d, p. 1425, Board of Patent Appeals and Interferences, 1987.

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