Notes on some factors affecting aggressive behaviour in Carcinus mediterraneus

The relationships between aggressive behaviour and animal density and between aggressive behaviour and metabolic excretion have been investigated in Carcinus mediterraneus Czerniavsky. Increasing the crab density by increasing the number of crabs or reducing the available area revealed that the aggressive interactions perindividual did not increase. The frequency of interactions reached a maximum in several minutes following the introduction of crabs. Individuals isolated between 6 and 24 h were more aggressive than non-isolated ones. The water in which a group of crabs had been held had an inhibitory effect (not abolished by commercial aquarium filters) on aggressiveness of pairs of crabs grater than plain water or water in which single crabs had been kept. No home-group effect was observed. The effect of group water was enhanced by increasing the concentration of crabs in the duration of confinement, and it is probably due to the accumulation of catabolites in the medium. Crabs kept in groups had a greater excretion than isolated crabs and the nitrogen oupput decreased with an increase in the number of crabs or whith a decrease in the size of the tank (with a constant volume of water and number of crabs).