Influence of social factors on the nocturnal activity pattern of the noble crayfish, Astacus astacus (Crustacea, Decapoda) in recirculating aquaculture systems

The social environment plays a major role in the control of growth in many crustaceans, but little is known on the mechanisms involved. Two- and three-year-old noble crayfish, Astacus astacus are strictly nocturnally active. In the present article, we studied social effects on the pattern of nocturnal activity, represented by the time which the animals spent outside shelters. For this purpose, the activity of groups of uniformly large (dominant') and uniformly small (subordinate') individuals were recorded at different densities both separately and in mixed set-ups. In crowded laboratory cultures, the presence of larger (dominant) conspecifics was found to reduce the time which smaller (subordinate) individuals spent outside shelters in search of food and feeding by about one-third. It is suggested that this asymmetric inhibitory effect is an element of the functional chain which ultimately results in an extremely wide variation in individual growth rates of communally reared A.astacus. The relevance of the findings to crayfish aquaculture is discussed.