Triploid oysters have 3 sets of chromosomes as compared to ‘normal’ diploid oysters, which have 2 sets of chromosomes. Triploid oysters are sterile and grow faster than diploids. Although the commercial benefits of triploidy have been evaluated in the Pacific oyster, eastern oyster, Sydney rock oyster and European flat oyster, so far this technique has only been commercialised for Pacific oysters.
Commercial production of triploids on the West Coast of North America began in 1985. Since then production of triploids has greatly increased and the use of tetraploid (4 sets of chromosomes) males to fertilise eggs from diploids to produce batches of 100% triploids has been developed. In 1999/2000 triploid Pacific oysters made up 30% of all Pacific oysters farmed on the West Coast of North America. Most of these triploid were produced using sperm from tetraploids to fertilise eggs from diploids, instead of using older chemical methods of induction.
In Sydney rock oyster, farming of triploid oysters may help to overcome the seasonal drop in sales that normally occurs following spawning during summer/autumn. Triploid oysters are functionally sterile and don't spawn allowing them to keep their condition for longer than normal (diploid) oysters. They also grow faster than diploids because the energy that normal oysters put into reproduction can instead be used for growth in triploids.
Triploid Sydney rock oysters reach market size (40 – 60 g) 6 months earlier than the usual 3½ years for diploids, hold their meat condition longer in autumn and winter and death from winter mortality is reduced by more than half. The faster growth rate of triploids becomes most apparent over the second spring/summer-growing season. The increased weight gains achieved with triploidy are fully additive to those obtained with selective breeding, to that triploids produced from oysters selected for fast growth for 3 generations reach market size 9 months earlier than wild caught oysters. A slight brown discolouration of the gonad has been noted in some triploid Sydney rock oysters. Fortunately this is less noticeable during the cooler months of winter and spring when the superior condition of triploids over diploids make them most useful to oyster farmers.