Comparative feeding and physiological energetics of diploid and triploid Sydney rock oysters, Saccostrea commercialis. II. Influences of food concentration and tissue energy distribution

This study determined the potential roles of food concentration and distribution of energy between body components in the greater growth rates of triploid Sydney rock oysters (Saccostrea commercialis) compared to diploids. Laboratory studies were conducted using rehydrated microalgae, Spongiococcum excentricum, up to 20 mg l⁻¹, to assess food concentration effects on the feeding and physiological energetics of juvenile and adult diploid and triploid S. commercialis. 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). The cost of respiration and excretion was significantly higher for adult diploid oysters across a range of food concentrations (3 to 20 mg l⁻¹). Juvenile diploids also used more energy in excretion across this range of food concentration. These differences in energy expenditures contribute to marginal differences in scope for growth (SFG). Triploid S. commercialis have higher SFG than their diploid siblings at low food; however, they have less ability to handle higher food concentrations than diploid oysters. Because of this inability of triploids to outperform their diploids at high food concentrations, feeding and physiological energetics probably play only minor roles in the observed differences in growth of diploid and triploid S. commercialis. Analyses of the energy content of soft and shell tissues in adult oysters revealed major differences between the ploidy conditions. Triploid oysters have total energy content that is much higher than diploids of the same shell length, and their soft tissue content is relatively higher. In adult triploids, either or both of two things are occurring to enhance their growth advantages over diploids: (1) the proportion of energy devoted to soft tissue production (flesh and reproductive tissue) is greater or (2) the stunted reproductive cycle of triploids helps them maintain energy levels stored in the soft tissue, whereas diploids expend and must replenish these levels following a reproductive episode