Interest in shellfish aquaculture, especially for culture of the native Eastern Oyster (Crassostrea virginica) has increased exponentially in the past several years. Part of this growth is due to the increased availability of triploid oyster seed for grow-out on shellfish leases.
Most organisms, including wild Eastern oysters, have two sets of chromosomes, one from each parent. These are known as Diploids (2N). Chromosomes are tiny, thread-like structures of DNA made up of many genes. The genes determine the specific traits of the oyster, such as shell height, or resistance to a certain disease. Although cross-breeding diploid oysters over many generations has been successful for selecting for advantages traits, the oyster is still fertile and as such, spends a lot of energy reproducing; creating a gonad that will be either eggs or sperm.
Enter the triploid oyster. Although triploids rarely occur naturally in the wild, the usual method is through a patented process that crosses a wild diploid female (2N) with a patented tetraploid male (4N). In this cross, nearly all offspring are triploid oysters (3N). The benefits of triploid oysters are that they are sterile and will typically grow faster than their diploid counterpart because they do not expend any energy in reproducing. The other benefit is that they are “fat” all year long as opposed to a diploid oyster that is “watery” during the summer as they expend energy spawning and reproducing. This makes triploid oysters available year-round in excellent quality for consumers.
Triploid oysters prompt a lot of questions to the division. Below are some common misconceptions:
- Triploids are GMO (Genetically Modified Organisms): FALSE. Triploids are genetic manipulations that result in a sterile Eastern oyster and can be found (though rarely) in nature. Other examples of manipulated triploid organisms are seedless watermelons and blueberries. GMO usually refers to a genetic modification that cannot take place in nature, even by mistake, such as splicing a gene from bacteria, Bacillus thuringiensis into seed corn to prevent corn borer insect damage. BT corn is now quite common and reduces the amount of pesticide needed in these crops.
- Triploid Eastern oysters are not native oysters: FALSE. Triploid oysters are the same species of our native oyster, just with an extra set of chromosomes.
- Triploid oysters outcompete native oysters: FALSE. Triploid oysters do not reproduce, but feed selectively the same as native oysters. Oysters eat microscopic plankton and algae and are very particular about the size food particles they eat. Diploid oyster larvae also eat phytoplankton and are not food for other oysters. Diploids will often settle and grow on cultured oysters.
- Chemically induced triploids cause illness: FALSE. Early methods to produce triploid oysters used a fungal toxin, Cytochalasin B, to induce triploidy in diploid oysters. This method did not affect the meat, however. In the United States., this method was replaced by the patented 4Cs method.
Find out more about North Carolina’s Shellfish Lease Programs in the N.C. Division of Marine Fisheries’ Habitat and Enhancement Section webpages.
Note: There is an increased health risk for consumption of raw or undercooked shellfish. If you have a compromised immune system or are on medication or treatments that lowers your immune system, you should always enjoy your oysters fully cooked.