The partial sterility of triploid (3n) oysters enables year-round harvest, improved meat quality, and superior growth. Two main techniques are used to produce shellfish triploids: 1) ‘chemical induction’ method, blocking the release of polar body in embryos via chemical treatment and 2) ‘mated triploid’ method, widely used, where tetraploid (4n) males are crossed with diploid (2n) females to produce 3n offspring. There are growing concerns however that mated 3n may be more vulnerable to diseases or climate-driven stressors (e.g., temperature, salinity).
The aim of this research was to evaluate the long-term performance of 3n Pacific oysters (Crassostrea gigas) obtained using different induction methods to identify any potential trade-offs (e.g., impaired fitness, reproductive potential, ploidy status, or resilience to environmental stressors) from each method. Using stock of equivalent genetic background, one diploid control (2n) and three groups of triploids derived from chemical induction (3nC) or mating of 2n females with 4n males (3nT1 and 3nT2), were created in the hatchery.
Chemical induction (3nC) yielded 95% triploid larvae compared to 100% for mated triploid groups (3nT1 & 3nT2). Larval performances of mated 3n were comparable to those of 2n, whereas 3nC had lower spat yield than mated triploids.
Resistance to OsHV-1 was tested in the laboratory for each group, followed by a challenge to increasing temperatures to determine median lethal temperature (LT50). No difference in survival between groups were found, whereas thermotolerance of 2n and 3nC oysters was marginally lower than mated 3n. Remaining oysters were then deployed to multiple grow-out sites and their survival and growth assessed periodically. After 18 months, survival was high in all groups (≥ 84%) across all farm sites. We found however significant differences in live weights between groups, regardless of the farm environment (Fig. 1). Mated triploids (3nT1 and 3nT2) were on average 18% larger than 3nC, and 40% larger than their 2n counterparts.
Reproductive potential of each group was also examined; we found that mated 3n had a higher proportion of fecund animals than 3nC. This trend was exacerbated in oysters grown in warmer waters (e.g., Parengarenga). Collectively, these findings will inform stakeholders in assessing the feasibility of using tetraploidy for 3n oyster spat production moving forward.