Population distribution and group structure of the invasive shrimp Palaemon macrodactylus (Caridea: Palaemonidae) living in a fouling community

Abstract

Behaviour traits like sociality have been identified as factors promoting invasiveness. Studying the manner in which animals are distributed in their natural environment relative to conspecifics and other species provides basic information about behavioural aspects, including social tendencies, aggressiveness, nesting and mating system, among others. The invasive shrimp Palaemon macrodactylus has shown aggressive behaviour in laboratory conditions, whereas in natural conditions it seems to live aggregated, sharing patchy habitats with other species such as the small goby fish Gobiosoma parri. In this study we investigated the abundance, density, group structure (i.e. size, sex, reproductive condition), and the possible coexistence of P. macrodactylus with G. parri in the fouling community of Mar del Plata harbour (Argentina), where this shrimp has invaded and established a stable population since 2000. Samples taken included all individuals present in each fouling patch. The results show that P. macrodactylus, inhabiting in the fouling community, are in groups of up to 20 individuals, without sex or size structure and that a positive correlation exists with the goby fish. This suggests a gregarious behaviour, and tolerance to other species using the same area, at least in an environment with high availability of refuges, traits that may be crucial for a successful invader. The persistence of such behaviour in different contexts and the effect of refuge should be investigated in order to fully understand and make predictions of new invasions.