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Douglass F. Hoese
Xenisthmids are restricted to the tropical regions of the Indian Ocean and western and central Pacific oceans. Five genera are currently recognised — Allomicrodesmus, Paraxenisthmus, Rotuma, Tyson and Xenisthmus — and 10 described species. All except Paraxenisthmus and Xenisthmus are regarded as monotypic. In Australia, three genera and seven species have been recorded. The family is currently under study by A.C. Gill and it is likely that the number of species will exceed 30.
Members of the Xenisthmidae are characterised by having a projecting lower jaw and, apparently, the habit of diving head first into sand. They are all known from coral reefs from the intertidal to depths of over 30 m and are small sized, rarely exceeding 3.5 cm in length.
Relationships of the Xenisthmidae are uncertain, but they have been studied by several workers including Springer (1983, 1988), Hoese (1984) and Gill & Hoese (1993); based on these studies xenisthmids are believed to be closest to eleotrids. Thacker & Hardman (2005), however, provided evidence, based on molecular work, that the group is closest to Gobiidae (as Gobiinae).
Gill, A.C. & Hoese, D.F. 1993. Paraxenisthmus springeri, new genus and species of gobioid fish from the west Pacific, and its phylogenetic position within the Xenisthmidae. Copeia 1993: 1049-1057
Hoese, D.F. 1984. Gobioidei: Relationships. 588-591 in Moser, H.G. et al. (eds). Ontogeny and Systematics of Fishes. American Society of Ichthyologists and Herpetologists. Special Publication 1: 1-760
Springer, V.G. 1983. Tyson belos, new genus and species of western Pacific fish (Gobiidae, Xenisthminae), with discussions of gobioid osteology and classification. Smithsonian Contributions to Zoology 390: 1-40
Springer, V.G. 1988. Rotuma lewisi, new genus and species of fish from the southwest Pacific (Gobioidei, Xenisthmidae). Proceedings of the Biological Society of Washington 101(3): 530-539 figs 1-8
Thacker, C.E. & Hardman, M.A. 2005. Molecular phylogeny of basal gobioid fishes: Rhyacichthyidae, Odontobutidae, Xenisthmidae, Eleotridae (Teleostei: Perciformes: Gobioidei). Molecular Phylogenetics and Evolution 37: 851-871
Common Name References
Myers, R.F. 1999. Micronesian Reef Fishes. A comprehensive guide to the coral reef fishes of Micronesia. Guam : Coral Graphics vi 330 pp. 192 pls.  (Wrigglers)