Genetic structure among populations of Potamonautes perlatus (Decapoda : Potamonautidae) from the Olifants River system in the Western Cape, South Africa

Populations of Potamonautes perlatus were sampled at various points along t he Olifants Rivers (220 km), in the Western Cape to examine the degree of g enetic variation. Allozyme electrophoresis revealed a near fixed allelic di fference at one locus and extensive differentiation at another locus betwee n populations from the upper and lower reaches of the Olifants River system . Limited hybridization in the region of population sympatry was observed. The narrow hybrid zone and the low occurrence of hybrids suggest selection against hybrids. These results provide evidence for two genetically distinc t freshwater crab populations. We concluded that the genetic divergence bet ween the two population groups represents a species boundary between the tw o populations.