Electric organs in skates: Variation and phylogenetic significance (Chondrichthyes: Rajoidei).

Abstract

A total of 63 species of skates (Chondrichthyes: Rajoidei) were surveyed, along with three species of the outgroup (Chondrichtyes: Rhinobatoidei) for electric organs along the sides of the tail. All skate specimens examined possessed what appeared to be functional electric organs, and the three species of the outgroup lacked evidence of electric organs. The electric organs were tail-positive and arranged into horizontal columns divided by transverse septa. The electrocytes varied considerably within and among supraspecific taxa (subgenera and genera), but they could be broadly classified into cup-shape, modified cup-shape, intermediate-shape, and disc-shape cells, provided that the distinction was partially based on position of the electrocytes within their connective tissue chambers. The survey, in part, corroborates a phylogenetic hypothesis of skates and in some respects further resolves the hypothesis. The supraspecific taxa Atlantoraja and Rioraja have similar derived-type electrocytes, as do the five supraspecific taxa of Rajini, and Cruriraja and Anacanthobatis, and to a lesser extent the supraspecific taxa Arhynchobatis, Psammobatis, and Sympterygia, and the supraspecific taxa Notoraja, Pavoraja, and Pseudoraja, corroborating the hypothesis. The supraspecific taxa Amblyraja, Rajella, Leucoraja, Breviraja, and Dactylobatus were unresolved in the phylogenetic hypothesis, but the electrocyte survey suggested that Leucoraja, Breviraja, and Dactylobatus were derived with respect to Amblyraja and Rajella. © 1994 Wiley-Liss, Inc.