The pseudotympanum in the Gymnotiformes (Teleostei, Ostariophysi, Otophysi): homology and evolution of a previously unexplored system in Neotropical electric fishes

Abstract

The pseudotympanum, a reduction of the hypaxialis muscle in the body wall lateral to the anterior portion of the swim bladder, is unique to some members of the Otophysi, the largest clade of primarily freshwater fishes. Prior studies documented the presence of the pseudotympanum in a number of groups within the Characiformes, to a lesser extent in subunits of the Siluriformes, and in a few taxa of the Cypriniformes, but with only one record of the aperture in the Gymnotiformes - the Neotropical electric fishes. Surveys across the five families and c. two-thirds of the genera in the Gymnotiformes revealed the occurrence of a pseudotympanum in all examined taxa. Pronounced variation in the expanse and details of the morphology of the opening and its relationship to the anterior elements of the axial skeleton was documented amongst Neotropical electric fish. Details of pseudotympanal morphology in the different families of the Gymnotiformes are described and the variation evaluated within the context of the two alternative hypotheses of higher-level phylogenetic relationships within the order. © 2015 The Linnean Society of London