Karplus, I., Aquaculture Research Unit, Institute of Animal Sciences, Volcani Center, Bet Dagan, Israel, Aquaculture Research Unit, Institute of Animal Sciences, Volcani Center, PO Box 6, Bet Dagan 50250, Israel

The wide range of sizes of sexually mature prawn populations and particularly of the males is very typical of Macrobrachium rosenbergii (De Man) and a major obstacle to increased profitability of prawn culture. Prawn size variation reflects a complex population structure composed of three major male morphotypes - the small males, the orange-claw males and the blue-claw males - which differ in their morphology, physiology and behaviour. Social interactions among juveniles and among sexually mature males affect their growth. Four different social mechanisms have been suggested to control growth in crustaceans: direct competition for food, appetite suppression, altered food-conversion efficiency and increased energy expenditure on motor activity. Since all growth-controlling mechanisms involve an aggressive behavioural component, our knowledge of prawn agonistic behaviour and prawn social organization has been reviewed. Present knowledge of the social mechanisms regulating growth among juveniles and among sexually mature male morphotypes as well as the sensory modalities involved was evaluated. The possible evolution of social growth-controlling mechanisms is discussed. Various ways for implementing our knowledge on social control of growth into prawn culture to increase profitability are suggested. © 2005 Blackwell Publishing Ltd.