Epizoic Ophiothela brittle stars have invaded the Atlantic It was thought that ophiuroids of the genus Ophiothela are confined to Pacific waters (Clark 1976). However, animals provisionally identified as Ophiothela mirabilis have founded emergent populations in the Atlantic. Further expansion of the range of Ophiothela could alter the appearance and the ecology of Atlantic coral reef habitats because ophiothelas, in multitudes, densely colonize gorgonians and sponges on Indo-West central Pacific and on tropical eastern Pacific reefs.

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Some species of sea urchins feature large variation in pigmentation. This variability may be the result of phenotypic plasticity or it may be associated with genetic divergence between morphs. Paracentrotus gaimardi exhibits five colour morphs (pink, brown, green, grey and black), which often occur side by side on the same rock. We studied genetic divergence between these morphs in three populations on the coast of Brazil. A fragment of the region encoding the mitochondrial ATPase 8 and 6 mitochondrial genes, a fragment of the intron of a nuclear histone and the entire nuclear gene coding for the sperm protein bindin were analysed. Mitochondrial DNA was differentiated between the pink and all other morphs, but the histone intron was similar in all colour morphs. In bindin, nine codons were found to be under positive selection and significant differences of allelic frequencies were observed in almost all pairwise comparisons between colour morphs. Although the molecular differentiation in bindin is not large enough to suggest reproductive isolation, some degree of assortative mating within morphs seems to be occurring in this species.