The Ecological Distribution of Grapsid and Ocypodid Shore Crabs (Crustacea: Brachyura) in Tasmania

Brachyuran crabs of the families Grapsidae and Ocypodidae usually have a rectangular, more or less straight-sided carapace with a wide anterior edge and prominent eyestalks reaching to the lateral angles. They are typically intertidal animals and in Tasmania are the most abundant and diverse crabs of this zone. Nine species of grapsid crabs and two ocypodids occur in Tasmania. At many localities several of the eleven species can be found on the one area of beach flat under the same boulder or under adjacent stones or in adjacent burrows. Ovigerous (egg-bearing) females of most species can be found together in summer. Some species seem to be equally abundant on a number of different types of shore whereas others are found on one or, at most, two types of shore. Some of the now familiar questions arising from such observations are the following. What factors are determining the distributions of the species? Are there any differences between the species' potential distributions and their observed distributions, i.e., are their distributions importantly limited by interspecific competition? If not, how is competition avoided in those situations where several species occur together. It is with answers to these kinds of questions that this study is concerned. The term 'competition' is used here in the sense preferred by Birch (1957) to describe the situation where a number of animals (of the same or different species) utilize common resources the supply of which is short, or if the resources are not in short supply, when the animals seeking those resources nevertheless harm one another in the process-see also Andrewartha & Browning (1961), Browning (1962) and Park(1962). The term 'niche' is used here as defined by Hutchinson (1958) to mean the multidimensional hypervolume delimited by all the environmental states which allow that species to exist indefinitely. In this paper the ecological distributions and general behaviour of the species are described from field observations, and their feeding habits and important morphological adaptations are briefly considered. The temporal fluctuations in intensities of breeding are briefly described, the number and relative volume of the gills, the structure of the mouthparts and arrangement of hairs on them are considered in relation to each species' behaviour and the habitat in which it occurs. Finally, the correlations between the distribution in nature and each species' behaviour towards certain habitats under laboratory conditions are described in an effort to determine whether there are differences which might be due to competition for space or food. The nine grapsids occurring in Tasmania are listed below by their scientific name and their common name (where one is available).