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Tanaidacea (Crustacea: Peracardia) of the Gulf of Mexico. VI. On the Genus Mesotanais Dollfus, 1897 with Descriptions of Two New Species, M. longisetosus and M. vadicola

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Two new species of tanaidaceans belonging to the family Leptocheliidae were collected from the continental shelves and slopes of the eastem and Gulf coasts of the United States. Both of these species are assigned to the poorly known genus Mesotanais Dollfus, 1897. Mesotanais is unique within the Leptocheliidae because its members lack eyes. The second supposedly important generic character, uniramous uropods, reported in the original description is considered to be a misdiagnosis. The assumption of this error is supported by the rediscovery of the type-species, Mesotanais dubius Dollfus, 1897 (Sieg and Bird, in press). In the present study, the genus Mesotanais is rediagnosed and the two new species from the western Atlantic are described. Mesotanais longisetosus n. sp. can be distinguished from M. vadicola n. sp. by a variety of characters including the much longer terminal setae on antenna 1, the distinctly more elongate carpus of the cheliped, and the shape of the pars molaris of the mandible. Mesotanais longisetosus appears to be closely related to the type-species, M. dubius known from the eastern Atlantic. It is distinguished from M. dubius by having an elongate distal seta on the carpus and more styliform dactylus of the first peraeopod, and by the presence of two short, distal, tergal spines on the merus of peraeopods 4-6. Mesotanais vadicola occurred at depths of 22-865 m, while M. longisetosus was collected mainly from the upper continental slope at depths of 545-1386 m. Between 500 m and 900 m there is a broad area of overlap in the distribution of the two species, but with increasing depth M. vadicola is replaced by M. longisetosus.

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