Systematics of the Percomorph Fish Genus Polyprion Oken, 1817

Abstract

The genus Polyprion is revised on a world basis using comparative biology to interpret the evolutionary history of the populations, species and genus. A detailed approach, incorporating a wide range of morphological and biological information, proved successful in resolving problems concerning descriptive taxonomy, nomenclature, analytical taxonomy and classification. Analysis of data collected from fresh New Zealand specimens and preserved specimens sampled from most of the world's populations indicated that Polyprion Oken, 1817 is a valid genus comprising two widespread species; one with deep-bodied uniform coloured adults and mottled or spotted juveniles and named P. americanus (Bloch and Schneider, 1801), the other with shallow-bodied countershaded adults and banded juveniles and named P. oxygeneios (Schneider in Bloch and Schneider, 1801). One meristic character (count of tubed lateral line scales) and eleven morphometric characters showed divergence, but only mean width of body scales and body colour were diagnostic at all sizes. A key to the genus and each species is provided. Detailed synonymies of the two species are given and problems associated with the extensive nomenclature are discussed and resolved. Because of instability in the nomenclature and the absence of the original type specimens, a neotype was designated for each species. The type locality of the deep-bodied species, P. americanus, is in the Northwest Atlantic Ocean and the species is distributed in subtropical and temperate regions of the Mediterranean Sea, Atlantic Ocean, South Indian Ocean and Southwest Pacific Ocean. The type locality of the shallow-bodied species, P. oxygeneios, is in the area of Queen Charlotte Sound, Cook Strait, New Zealand, and the species is distributed in subtropical and temperate regions of the South Indian Ocean and South Pacific Ocean. Polyprion species are primary gonochorists and do not exhibit sexual dimorphism or sexual dichromatism. Sympatric populations of the two species were found to be reproductively isolated through differences in spawning time and habitat depth. Comparison of morphological, ecological and distributional characters with different species concepts and the predictions of various speciation models showed that the evolutionary species concept, not the biological species concept, best suits Polyprian, and that allopatric (vicariant) speciation by subdivision of a widely distributed ancestor is the most suitable speciation model. Ontogenetic character analysis indicated that the shallow-bodied species is more derived than the deep-bodied species. Phylogenetic relationships and classification were investigated cladistically and the following hypotheses were generated: Stereolepis Ayers is the sister genus of Polyprion Oken, Polyprion evolved from its common ancestor with Stereolepis in part by neoteny, Polyprion and Stereolepis alone comprise the monophyletic percomorph family Polyprionidae, and the series Percomorpha sensu stricto does not contain the Beryciformes and is a monophyletic taxon defined by the previously unrecognised synapomorphy of transforming cteni on the scales. Polyprion could not be aligned with the basal percomorph order Perciformes, suborder Percoidei and superfamily Percoidea because these three taxa could not be defined cladistically. However, it was confirmed that Polyprion is a highly plesiomorphic percomorph belonging to a plesiomorphic percomorph family. Assessment of basal percomorph character polarities using the "commonality principle" required a cautious choice of outgroup because several Polyprion characters were found to be uncommon in percomorphs, but nevertheless plesiomorphic when the outgroup was extended to include prepercomorphs. Formal characterisations of several percomorph suprageneric taxa, including Polyprionidae, are presented. The evolution of the Polyprionidae is discussed and some implications for the nature of basal percomorph evolution are considered. The wide morphological and ecological diversity of the most primitive basal percomorphs is interpreted to reflect an early radiation in the evolution of the lineage and it is suggested that neoteny may have played an important, but hitherto largely unrecognised role in percomorph evolution.