Test morphologies were examined for both living and denuded specimens of all established species of Diadema Gray, 1825 and Echinothrix Peters, 1853, including two colour morphs of Echinothrix calamaris (Pallas, 1774). Twenty-eight morphological characters were measured and analysed using ordination by multi-dimensional scaling (MDS) and cluster analysis. Specific test features were found which differentiated the species. Of these the apical systems, markings on the genital plates, features of the median areas of the interambulacra and distribution of iridophores were particularly useful forms of taxa distinction. Well defined associations in test morphologies were found between Diadema antillarum Philippi, 1845, D. ascensionis Mortensen, 1909, D. mexicanum A. Agassiz, 1863, D. savignyi Michelin, 1845 and D. paucispinum A. Agassiz, 1863. Diadema setosum (Leske, 1778) and particularly D. palmeri Baker, 1967 appeared to be outlying species within the genus; D. palmeri being the only species within the genus Diadema to have a monocyclic apical system. Test morphologies of Echinothrix diadema (Linnaeus, 1758) and E. calamaris were distinctly different, with marked differences in test size and shape, plate arrangements, features of the apical system and structures of the median areas of the interambulacra. The different colour morphs of E. calamaris demonstrated similar test features; however, differences were found which suggest that the brown colour morph may be a separate species or subspecies.
Echinodermata, sea urchin, morphology, Diadema, Echinothrix.