Morphological and genetic analyses suggest that southern African crown crabs, Hymenosoma orbiculare, represent five distinct species

The crown crab, Hymenosoma orbiculare, occurs all along the coast of southern Africa and displays great morphological variation across this range. To determine whether the species comprises several distinct taxonomic units, H. orbiculare were collected from 18 estuaries and lagoons between Walvis Bay in Namibia and Kosi Bay in north-eastern South Africa. Open ocean individuals were also obtained from False Bay in south-western South Africa. Morphological and genetic (using mitochondrial DNA) comparisons were carried out between individuals from all locations. Five monophyletic clusters were identified on the basis of genetic data, each confined to specific portions of the distribution range. Morphological data supported the distinctness of each of these clusters. The typical H. orbiculare, characterized by large size (maximum carapace width 28 mm) and absence of the characteristic ornamentations of other morphotypes, occurred in estuaries and lagoons all along the west and south coasts. False Bay deep-water individuals were of two forms. The first small, granulose, and setose morph appears to represent the previously synonymized species, H. geometricum, and is distinguished by a long rostrum, a raised setaceous gastric region on the carapace, and post-branchial projections. The second False Bay form represents a new, undescribed species, identified primarily by genetic characters. Estuarine forms from the southeast and east coast formed two more clusters, distinguished by their small size (carapace width <10 mm), relatively large eyes, long walking legs, projections on the abdomen, and small anterior spikes on the coxae of the legs. Specimens from south-eastern sites differed from more northern samples by their longer second walking legs and darker colour. We propose that South African crown crabs in fact represent five distinct species: the true H. orbiculare, the form previously described as H. geometricum (to be re-established as a valid species), and three new species that remain to be formally described.