A phylogenetic analysis of the American knifefishes (Teleostei: Gymnotoidei).

Abstract

The evolutionary relationships of the American knifefishes (Teleostei: Gymnotoidei) are documented from examination of 507 museum lots containing 2454 knifefish specimens, representing 80 of the 92 recognized species and 13 additional undescribed species. Observations of neural tissues were made for 31 knifefish species, including representatives of 20 recognized genera. The data recovered from these surveys were compiled into a matrix of 219 characters, including 39 characters of the central nervous system, for 46 operational taxonomic units. This review of the Gymnotoidei includes a brief history of gymnotoid systematics and a discussion on the implications of the phylogenetic hypothesis proposed on the evolution of gymnotoid morphology, ecology, and biogeography. Diagnoses for 87 gymnotoid clades are provided, including 27 monophyletic genera and and five monophyletic families. Two new apteronotid genera, Parapteronotus and Compsaraia, are diagnosed. Two new species are described; a species of Gymnotus from the Pacific drainages of Middle America, and a species of Sternopygus from the Rio Xingu of Brazil. Morphological, meristic, and pigmentation features are discussed, and keys to the species of both genera are provided. A data matrix of 52 characters for the four recognized species of Sternopygus and representative outgroup taxa is presented, and characters thought to be plesiomorphic for Sternopygus are discussed. Whole brains of 35 gymnotoid species were examined, and 25 species were prepared for histological examination. Drawings of nuclei and tracts were compiled into an atlas of 12 transverse sections for species representing each major gymnotoid lineage. Homologous features between the electrosensory systems of siluroid and gymnotoid fishes are summarized. The use of neurohistological data from museum specimens is discussed along with a review of the nomenclature of gymnotoid neural structures. The phylogenetic and spatial distribution of 33 neural characters is analyzed to test several models of the evolution of nervous systems. Diversification in the gross morphology of gymnotoid brains does not conform to the hypothesis that the sensory and motor peripheries exhibit greater variability than do characters of the central nervous system. The most variable regions are association nuclei of the dorsal telencephalon, primary sensory and motor nuclei of the rhombencephalon, and structures developing from the dorsolateral series of ectodermal placodes.