An allozyme study of the blue swimmer crab, Portunus pelagicus (Crustacea : Portunidae), in Australia: stock delineation in southern Australia and evidence for a cryptic species in northern waters

Allozyme analysis was used to examine the species-level systematics and stock structure of the Australian blue swimmer crab Portunus pelagicus. Fifty-seven crabs from eight sites were screened in an overview study for allozyme variation at 35 loci. This overview study revealed the presence of two species, differing at a Nei D of 0.14 (2% fixed differences), in the Darwin region of northern Australia. One of these species corresponds to the common P. pelagicus found throughout Australia, whereas the other is most likely either an undescribed ‘cryptic’ species, or the east-Asian species P. trituberculatus. In total, 609 P. pelagicus from 11 sites covering three regions in South Australia and two regions in the Northern Territory were then genotyped at seven polymorphic loci and these data assessed, using goodness- of-fit and F-statistics, for the existence of subpopulations. Four discrete subpopulations could be discerned, namely West Coast, Spencer Gulf, and Gulf St Vincent in South Australia, and Darwin–Gove in the Northern Territory. No evidence of population substructuring among sites within each subpopulation was evident from the allozyme data. The results support the current recognition of the three South Australian regions as separate stocks, and suggest that a taxonomic revision of Indo-Pacific Portunus is warranted.