Phylogenetic relationships within the primitive acanthomorph fish genus Polymixia, with changes to species composition and geographic distributions.

Abstract

The genus Polymixia is the only survivor of a Late Cretaceous marine fish radiation and is often said to be the most primitive living acanthomorph (i.e., Polymixia possesses the greatest number of primitive character states for Acanthomorpha). Recent studies, including this one, place Polymixia as the sister to all other Paracanthopterygii. Despite its importance, most species of Polymixia are extremely difficult to discriminate on the basis of morphology. As a result, the number of valid species is uncertain. Moreover, there has never been a phylogenetic analysis of the genus. Thus, a molecular phylogenetic study was needed to clarify species boundaries and to resolve relationships within the genus. Tissue or DNA samples backed by museum vouchers were obtained for most species, with additional samples from new geographic areas representing specimens with distinctively different meristics and uncertain identifications. Seven loci (five nuclear and two mitochondrial) were sequenced, from which Bayesian and maximum-likelihood trees were generated. Results reveal nine species-level clades, of which five represent previously known species (Polymixia berndti, P. japonica, P. longispina, P. lowei, and P. nobilis). Surprisingly, results also reveal four previously unknown species-level clades, one close to P. lowei, one close to P. nobilis, and two new species clades related to P. japonica. The species clades are distinguished by their phylogenetic histories, sequence differences, geographic distributions, and morphologies. The clade containing P. berndti is recovered as the sister to all other species of Polymixia. Its genetic variability suggests that it might contain two or more species and it is referred to here as a "species complex". Polymixia nobilis, the type species, was previously thought to be restricted to the Atlantic, but is now shown to be widespread in the Pacific and possibly in the Indian Ocean. Specimens from waters off Australia identified as P. busakhini actually belong to P. nobilis. In contrast, P. japonica is confirmed only in the area near Japan and the East China Sea; other more distant records are misidentifications. Wide (antipodal) geographic distributions are seen in several clades, including P. nobilis, the P. berndti species complex, and the P. japonica species group. The new phylogeny helps explain the evolution of some morphological characters previously used to distinguish groups of species, particularly dorsal-fin soft-ray count, shape of rows of scale ctenii, and number of pyloric caeca.