Introduction
The Pacific oyster (Magallana gigas) , introduced to New Zealand in the 1950s, is central to a major aquaculture industry producing nearly 2,000 tonnes annually. After a decline due to the OsHV-1 µvar virus outbreak in 2010, the industry has been recovering, aided by selective breeding. Growth rate variability among individuals offers both challenges and opportunities for optimization, driven by physiological and genetic factors. Studies have linked growth differences to food acquisition, metabolic costs, and gill morphology—especially surface area and ciliary structure.
In mussels, fast growers have denser ciliary networks and larger gills, while slow growers exhibit elevated antioxidant activity, potentially signalling higher oxidative stress (Pérez-Cebrecos et al., 2022). Immune function is influenced by haemocytes, which are involved in immune defence and other functions like shell formation. Gene expression studies suggest that slow growers may have immune inefficiencies, affecting overall fitn ess (Prieto et al., 2019). Reproductive energy demands can also influence growth. Triploid oysters, which allocate more energy to somatic growth, provide a model for investigating these trade-offs. Previous field studies suggest that growth-rate phenotypes in triploids correlate with reduced reproductive investment, leading to more efficient somatic growth.
This study investigates the physiological, histological, and immunological aspects of growth variation in M. gigas, exploring how triploidy influences growth, immune function, and oxidative stress. By examining these factors, the study aims to inform selective breeding strategies and improve oyster aquaculture sustainability.
Materials and Methods
Triploid and diploid M. gigas oysters were produced from a biparental cross and reared under identical hatchery conditions . Growth phenotypes were classified into fast and slow growers within each ploidy group. Standard physiological measurements (clearance, ingestion, absorption, and respiration rates) were used to calculate scope for growth. Flow cytometry was applied to assess haemocyte activity, ROS production, and viability. Gill and digestive gland tissues were analysed for antioxidant enzyme activity (SOD, CAT), histopathology, and structural indices. Gametogenic stage was also evaluated histologically. All measurements were size-standardized and statistically analysed to test for effects of ploidy and growth phenotype.
Results
Fast-growing oysters were 51% heavier and 87% larger than slow growers in both diploid and triploid groups . Clearance rates (CR) were 23% higher in fast growers (3.60 ± 0.86 L·h⁻¹·g⁻¹) compared to slow growers (2.76 ± 0.48 L·h⁻¹·g⁻¹), while absorption efficiency (AE) was 11% higher in slow-growing oysters (0.74 ± 0.06) compared to fast -growing oysters (0.66 ± 0.15). Scope for growth (SFG) was 37% higher in triploid oysters (153.06 ± 47.33 J·h⁻¹) compared to diploids (96.05 ± 48.56 J·h⁻¹), with fast growers showing a 51% higher SFG (166.73 ± 73.58 J·h⁻¹) than slow growers (82.38 ± 22.31 J·h⁻¹). In immune responses, fast-growing diploid oysters exhibited 62% higher reactive oxygen species (ROS) production compared to triploid fast growers, with significant reductions in immune cell viability, especially in slow-growing triploids.
Catalase activity in the gills of triploid oysters (127.78 ± 9.88 µmol·min⁻¹·mg protein⁻¹) was 30% higher than in diploid oysters (89.80 ± 6.05 µmol·min⁻¹·mg protein⁻¹), while superoxide dismutase activity was 15% higher in fast-growing oysters (14.87 ± 1.75 U·min⁻¹·mg protein⁻¹) compared to slow-growing oysters (12.67 ± 2.03 U·min⁻¹·mg protein⁻¹).
Histopathological analysis revealed a higher prevalence of rickettsia-like organisms in triploid fast growers (0.88) compared to diploid fast growers (0.10). Slow growers exhibited a significantly lower gill structure index (0.66 ± 0.24) compared to fast growers (1.25 ± 0.23), and triploid oysters were more advanced in gamete development stages. Shell organic content was 27% higher in fast growers (15.31 ± 3.83) compared to slow growers (11.25 ± 3.05), while the shell-to-live weight ratio was 10% heavier in fast growers (0.48 ± 0.02) than slow growers (0.44 ± 0.05).
Discussion
Despite being from the same family and raised under identical conditions, fast-growing oysters were 52% heavier than slow growers, underscoring the importance of physiological traits in growth performance and aquaculture optimization.
Fast growers exhibited higher feeding rates without increased metabolic costs, suggesting enhanced metabolic efficiency, which supports their superior growth. This finding aligns with previous research showing that increased ingestion rates do not always correlate with higher metabolic costs (Babarro et al., 2000). Fast-growing oysters also had superior gill structure, a crucial factor in filtration efficiency. Their enhanced gill morphology likely supports their higher cleara nce rates and growth potential. In contrast, slow growers showed smaller, less viable haemocytes, and elevated ROS production, indicating compromised immune function and higher oxidative stress. This likely contributes to their lower SFG and reduced metabolic efficiency. The presence of basal cells in slow growers may reflect an increased immune turnover to compensate for the reduced viability of haemocytes.
Interestingly, triploid slow growers showed some compensatory advantages, with better gill structure and higher catalase activity than diploid slow growers . This suggests that triploids may invest more in antioxidant defences and somatic growth rather than reproduction. The higher organic shell content in slow-growing triploids, compared to diploid slow growers, further indicates a trade-off between growth and biomechanical protection. These oysters may allocate more energy to shell strengthening, enhancing resilience at the cost of slower growth.
Overall, the study demonstrates the complex trade-offs between growth, immune function, and oxidative stress. Fast-growing oysters exhibit superior growth, metabolic efficiency, and immune function, while slow growers are hampered by immune dysfunction and oxidative stress. Triploid oysters offer some advantages in immune defence. These findings emphasize the need to consider both physiological and immunological factors in aquaculture management and breeding programs aimed at optimizing oyster performance.
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