The phylogeny and evolutionary history of the South American electric fishes (order Gymnotiformes)

Abstract

Although considerable physiological, anatomical and pharmacological information on various aspects of the electronic and electrosensory systems (EES) in gymnotiform electric fishes has been generated during the last few decades, little is known about the evolutionary history of these fishes. In this dissertation, I used mitochondrial DNA sequences from the 12S and 16S ribosomal RNA subunits to address evolutionary aspects of the gymnotiform natural history. Approximately a total of 48 genera of the series Otophysis, including 19 Gymnotiformes, 16 Siluriformes, 10 Characiformes, and 3 Cypriniformes. Estimations evolutionary relationships were obtained by three methods: maximum parsimony, distance and maximum likelihood. Chapter two deals with the evolution of the gymnotiforms as a group. Molecular data in conjunction with morphological evidence provide a congruent phylogenetic hypothesis for the Otophysi. Cypriniformes is the sister group of Characiphysi (Characiformes + Siluriformes + Gymnotiformes), and Characiformes is the sister group of Siluriformes + Gymnotiformes. Adopting this hypothesis as the correct alternative, and taking into account fossil evidence and the timing of Pangea fragmentation, the following scenario is proposed: Cypriniformes evolved in Asia during the early Cretaceous. The separation between Africa and South represented a vicariant event for characiforms, and the differentiation of gymnotiforms and siluriforms occurred in the isolated South America. Siluriformes were apparently able to disperse through salt water, attaining a cosmopolitan distribution by the early-middle Tertiary. Gymnotiformes did not leave South America, which may be related to the functional constraints placed upon the efficiency of the EES by water of low resistivity. Chapter three addresses the phylogeny within the gymnotiforms. By combining molecular phylogeny, morphological studies and physiological information about the gymnotiform electrogenic system, three proposals are made: the family Sternopygidae is an unnatural group, and Sternopygus represents an unique lineage within the order; the family Hypopomidae is not monophyletic; and the order Gymnotiformes is composed by at least six natural clades: Sternopygidae, in a new sense, containing only the genus Sternopygus, a new clade (family Eigenmanniidae) consisting of the remaining sternopygids, family Apteronotidae, families Hypopomidae + Rhamphichthyidae, family Electrophoridae, and family Gymnotidae.