Mitochondrial ribosomal gene sequences were used to investigate the status of several populations of hydromedusae belonging to the genus Turritopsis (family Oceaniidae). Several nominal species have been described for this genus, but most of them had been synonymized and attributed to one cosmopolitan species, Turritopsis nutricula. A recent revision based on morphological and reproductive characters, however, has shown that many different populations can be distinguished and that several of the nominal Turritopsis species are likely valid biological species. Our investigation using molecular sequence data of 16S mitochondrial gene confirms these results. The Mediterranean Turritopsis must be attributed to Turritopsis dohrnii and the Turritopsis of New Zealand must be referred to Turritopsis rubra. The situation of the Japanese Turritopsis is more complex, though all sampled populations are clearly distinct from T. nutricula, a species likely confined to the Western Atlantic. The Japanese Turritopsis fall into three widely separated lineages. One of them, corresponding likely to Turritopsis pacifica, is closely related to T. rubra. A second clade, which potentially represents an as yet undescribed species, produces smaller medusae than T. pacifica and is morphologically distinguishable from it. Finally, a third group was distinguished by a single haplotype sequence that is identical with a Mediterranean sample of T. dohrnii. It is postulated that the last group of Japanese Turritopsis is likely a recent introduction, most probably by human activity. A survey of all known and potentially valid Turritopsis species is given in table format to facilitate identifications and future revisory work.
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