Triploids (3n) are organisms with three sets of chromosomes, while the normal diploid (2n) organisms have only two sets. Over the past two decades, triploids have been studied in more than 20 species of molluscs. The main interests of those studies have primarily focused on their sterility and improved growth rates. These are mainly due to that in the normal diploid commercial stocks the animals expend considerable energy on gametogenesis and become watery in the summer months making them less desirable in the market. In Pacific oysters, for example, they release approximately 50% of their body mass during spawning, affecting their meat quality and acceptance by consumers for extended period of time. In a sterile stock, on the other hand, they could partially redirect this energy to growth and maintain their meat quality during these periods, allowing them to be marketed year round.  Triploids, in most species studied so far, experience significantly higher growth rates than their diploid siblings. In bivalve species triploids have showed 10 to 80% faster growth than their diploid siblings (Guo, 1999). However, the expression of the triploid advantage in growth can be influenced by genetic and environmental factors. In addition, production of triploids from normal diploid stock directly is technically difficult in most bivalve species; often resulting in inconsistent percentage of triploid and high larval mortality in the first few days. The commercial use of triploids may, therefore, ultimately depend on the development of tetraploids, which can produce 100 % pure triploids simply by mating with normal diploids.

The establishment of tetraploid breeding stock however, is still a major challenge in most molluscan species. Prior to 1997, tetraploid bivalve spats have only been produced in three species: mussels (17.2% in one month old, Scarpa et al. 1993); Manila clam (3 spat detected, Allen et al. 1994); and Pacific oysters (67% in juveniles, Guo and Allen 1994).  The tetraploid mussels and clams were induced using gametes from diploid males and females and the tetraploid Pacific oysters were produced using eggs from triploid females and sperm from diploid males.  

The main objectives of this project were to evaluate and develop techniques for the production of tetraploid broodstock in Pacific oysters and to investigate the potential to produce triploids by crossing tetraploids with diploids. Throughout this project (March 1995 to February 1998), most techniques developed to induce tetraploids from diploid stock in fish and shellfish, have been attempted; new techniques were investigated; and tetraploid spats were produced. The techniques published by Guo and Allen (1994) was not tried because no mature triploid broodstock were available in South Australia.

Keywords: Pacific oyster, Crassostrea gigas, ploidy manipulation, tetraploid.