The effect of food conditioning on feeding and growth responses to variable rations in fast and slow growing spat of the Manila clam (Ruditapes philippinarum)

Two groups of clam spat (Ruditapes philippinarum) belonging to different growth categories of fast (F) and slow (S) growers were each conditioned to diets of low and high phytoplankton concentration (0.3 and 1.0 mg POM l− 1, respectively), and their components of energy balance measured across a range of food concentrations, in order to ascertain the combined contribution of endogenous and exogenous (food ambient) factors to growth performance. Analysis of the physiological traits indicates that both higher food acquisition and higher metabolic efficiencies were responsible for faster growth rates recorded with F spats in the tested range of food concentration (0.3 to 1.1 mg POM l− 1). In both types of clams acute response to increased food availability involved ingestion regulation by adjusting clearance rates was observed, thus helping to maintain nearly constant absorption efficiency values (around 70%) across food levels. Optimal ration (that providing maximum ingestion and scope for growth values) was found to rise from mid- (0.65 mg POM l− 1) to high food concentration (1.1 mg POM l− 1) following the change in conditioning from low to high food levels. Increased capacity to process food efficiently is accounted for by greater digestive investments in clams conditioned to high ration that results in increased metabolic costs of maintenance (recorded as rates of oxygen consumption after 4 days starvation), and associated to decline in growth performance during transitory exposure to poor food conditions.