Physiological processes modulate acute and chronic responses to dietary protein/energy ratio fluctuations in individuals and families of Manila clam (Ruditapes philippinarum) selected for variable growth rates

A range of phenotypes differing in growth rate were designed in the Manila clam by combining separate breeding families with size segregation within each family to constitute fast and slow growing groups. Physiological components of the energy budget and scope for growth (SFG) were then compared between these different phenotypes during the acute and chronic responses to two diets that were isocaloric but differed by 3-fold in their protein/energy (P/E) ratios. Both diets were based on the microalgae Rhodomonas lens obtained in either the exponential or the stationary phase of culture. The aims of the study were 1) to test the effects of these changes in food composition on growth rate, estimated as the balance of physiological processes of energy gain and loss integrated in the SFG; and 2) to assess the extent to which physiological adjustments to diet composition are modulated in order to fulfill the variable energy requirements posed by the occurrence of differential growth phenotypes. Growth performance improved with the high-protein (N+) diet for the different family * growth group combinations, with SFG values exceeding by 50% on average the values of the low-protein (N-) diet. Digestive constraints resulted in reduced absorption efficiency with the N-diet, which tended to cancel out the potential benefits of adjusting feeding rates in order to compensate for a low protein ration. Endogenous differences in growth rate associated with segregated phenotypes were mainly accounted for by differences in energy acquisition, with feeding rates differing by ~ 2-fold between fast and slow growers. Additionally, significant differences were recorded for the unitary metabolic costs (i.e., per unit of metabolizable energy), indicating that higher metabolic efficiency was also a component of faster growth.