Aggressiveness and life underground: the case of burrowing crayfish

Abstract

This study compared aggression in two morphologically similar neotropical burrowing crayfishes, Parastacus pilimanus, a primary burrower, and Parastacus brasiliensis, a secondary burrower. Intraspecific pairs were formed, with a maximum 15% difference in carapace and chelae length within each pair. Pairs were allowed to interact for 20 min, during which they were recorded, and the agonistic behaviour was then analyzed throughout these recordings. The species were compared with respect to mean bout duration, first bout duration, number of bouts, latency period, frequency of highly aggressive behaviours, frequency of low aggressive behaviours, as well as the number of approaches, antennal whips and chelae punches. The proportion of interactions that resulted in formation of a clear hierarchy was also compared. Parastacus brasiliensis was the more aggressive species, showing statistically higher values for all parameters except latency, as well as number of bouts and antennal whips (among winners); while P. pilimanus performed more chelae punches. In general terms, both species showed low aggression (due to the absence of clearly escalated fights and other behaviours), which differs from the pattern expected for crayfish. The formation of dominance relationships was more frequent in P. pilimanus than in P. brasiliensis. Due to possible pressure for co-existence and reduced competition for resources, the burrowing habit appears to influence aggression, with the more fossorial species being less aggressive. It is assumed that these differences are related to: (i) phylogenetic distance from the open water species; (ii) the burrowing habitat and the related morphological adaptations and (iii) a reduced need to acquire and defend resources other than their burrows.