Taxonomy, phylogenetic realationships and biogeography of the deep-sea hatchetfish genus Polyipnus (Stomiiformes: Sternoptychidae)

The tropical to subtropical mesopelagic fish genus Polyipnus (family Sternoptychidae) is the subject of a taxonomic, phylogenetic, and biogeographic study. Examination of specimens from most of the major collections leads to recognition of 29 species in the genus. A key is provided. New species are described from the South China Sea, off the northwestern coast of Australia, the Andaman Sea (eastern Indian Ocean), the western Indian Ocean off Kenya, and the tropical western North Atlantic. -- A study of phylogenetic relationships, based on mainly osteology and photophores, results in a well-resolved phylogeny. Instances of unresolved branching order are due to homoplasy and/or lack of osteological observations for rare species. Reductive homoplastic characters, such as small body size (within the omphus species group) and low photophore number (P. fraseri) are paedomorphic and likely derived. Monophyly of the genus and 4 species groups is indicated; the infrastructure of the genus is organized on the basis of these groups. Species that were previously members of the laternatus species complex do not constitute a monophyletic group and are placed in the asteroides and omphus species groups with their closest relatives. The remaining meteori and spinosus species groups contain species previously referred to the asteroides and spinosus complexes. -- Cladograms formed the framework for a vicariance biogeographic analysis. The history of the genus dates to pre-Miocene times (prior to 25 million years ago) when a widespread Tethyan ancestral biota existed. A chronological series of tectonic/climatic vicariant events is proposed which accounts for much of the observed endemism in the genus. The asteroides species group appears to be the only Polyipnus group that was present in the Atlantic as well as the Indo-Pacific in the early Miocene. Final closure of Tethys in the Mediterranean region, later in the Miocene, caused subdivision of a species ancestral to all Atlantic species and possibly a single species in the western Indian Ocean. Subdivision of an ancestral deep-sea fauna in the Indo-Pacific was effected by the early Miocene collision of the Australian and Asian Plates and subsequent tectonic activity which cut off deep water circulation between the eastern Indian Ocean and the western Pacific. Later in the Miocene, circulation between the South China Sea and the adjacent southern Sulu Basin was disrupted, possibly causing vicariance in three Polyipnus species groups