Analysis of marine hybridization between two species of the genus cyclograpsus (Crustacea: Brachyura) in South-Eastern Australia

The structure, extent and maintenance of two zones of overlap, with hybridization, between the grapsid crab species Cyclograpsus audouini and C. granulosus were determined. A modified hybrid index technique was used for analysis. The hybrids were found to be the most abundant group in the central regions of sympatry. In these areas hybrids were found in the central midlittoral zone with C. audouini above them in the supralittoral fringe and C. granulosus in the central and lower midlittoral. The number of hybrids present on shores north and south of the centre of sympatry decreased markedly. The observations led to the conclusions that the interaction was ecologically controlled and that the two interacting taxa were separate species. The geographic distribution of C. audouini and the hybrids was limited by the effect of seawater temperatures on the survival of their planktonic larval stage (a pre-settling factor). Conversely the geographic distribution of C. granulosus was limited by the action of air temperature on the adult crabs (a post-settling factor).