Comparative feeding and physiological energetics of diploid and triploid Sydney rock oysters, Saccostrea commercialis

This study assessed the possible physiological bases for the greater growth rates in triploid Saccostrea commercialis over diploid cohorts. Laboratory studies were conducted using rehydrated microalgae, Spongiococcum excentricum, at 2 mg l⁻¹ to assess the effects of body size (dry soft tissue weight) on the feeding and energetics of juvenile and adult S. commercialis of 2n and 3n ploidy. The parameters clearance rate (CR), absorption efficiency (abs. eff.), pseudofaeces production (PF), respiration (R) and ammonia excretion (E) were measured to determine the energy available for growth and reproduction, defined as scope for growth (SFG). Both diploids and triploids of juvenile and adult S. commercialis were found to have similar SFG, being 4.56 J h⁻¹ for diploid and 4.81 J h⁻¹ for triploid 0.01-g oysters, and 21.3 J h⁻¹ for diploid and 20.8 J h⁻¹ for triploid 1-g oysters. The only significant differences between the ploidy conditions were in PF and E in adult oysters. However, since PF and E were only minor components of the energy budget, they did not result in any significant difference to SFG. Abs. eff. was inversely related to size in triploid oysters, but not related to size in diploids. CR, PF, absorbed energy (AE), R and E were significantly related to size in exponential relationships. For CR, AE and R, the exponent was low, about 0.3–0.4, indicating marked declines in size-specific rates with increasing oyster size, e.g. AE declined from 605 J g−1 h⁻¹ for 0.01-g to 31.6 J g−1 h⁻¹ for 1-g diploid oysters. The exponents were negative for PF, indicating a marked decline in relative production of pseudofaeces with size, e.g. PF declined from 23% of filtered particles by 0.01-g oysters to 6% by 1-g diploid oysters. The inorganic content of the pseudofaeces (25–33%) was significantly higher than in the food (5.6%) indicating selectivity during the feeding and ingestion processes. While this study found no differences in the influence of size on SFG in diploid and triploid S. commercialis at a standard level of food, it is possible that the feeding and energetics of diploid and triploid oysters vary according to food regime. There is also the possibility that diploids and triploids partition their SFG energy in different proportions between the compartments of the body. These two possible differences between the ploidy conditions as bases for different growth rates are considered in a companion study