Dominance and population structure of freshwater crabs (Potamonautes perlatus Milne Edwards)

Abstract

Although freshwater crabs, Potamonautes spp, are abundant (up to 23 crabs m^-2) in southern African freshwater systems and form a major part of the diet of many vertebrates, little is known about their biology. Understanding crab population dynamics and behaviour, for example, is important to understanding their role in the ecosystem more clearly. In this paper we report on dominance in Potamonautes perlatus and test the prediction that larger crabs are dominant to smaller ones, i.e. a linear hierarchy exists and it is size dependent. To understand how this would affect wild populations we also investigated the population structure (sex ratio, size distribution, density and population growth) of a wild population. Using Landau's index of linearity (h) we found three captive groups of P. perlatus to show moderate linearity, i.e. h = 0.9; 0.81 and 0.83. In all three groups the largest individual was the most dominant. There was a significant Spearman rank correlation between dominance rank and size of largest chelae in two of the groups, and a significant Spearman rank correlation between dominance rank and carapace width in only one of the three groups. Densities of crabs in the Eerste River, Western Cape Province, ranged from a mean of 2.89 ±2.11 to 15.57 ± 7.21 crabs m^-2. Unexpectedly the size class distribution of the crabs remained unchanged during the year. The lack of a significant increase in the mean size of the crabs can probably be ascribed to a year-round consistency in the availability of refugia for specific/different size classes. Intraspecific aggression, predation and refuge availability are probable strong selection pressures in determining population structures of wild populations of P. perlatus.

Keywords: Dominance. linear hierarchy, population structure, refuge availability, rivers, South Africa