Molecular phylogeny of the prickly shark, Echinorhinus cookei, based on a nuclear (18S rRNA) and a mitochondrial (cytochrome b) gene.

The classification of the sharks is unclear. This is particularly true for the superorder Squalomorphii. The relationships between the squalomorphs and other superorders of sharks and the relationships between the different orders within the squalomorphs are a matter of debate. Here, we report a molecular phylogeny for a little known member of this superorder, the genus Echinorhinus. Echinorhinus is most commonly classified in either the family Echinorhinidae (Squaliformes) or the family Squalidae (Squaliformes). However, some authors have suggested a closer relationship to the order Hexanchiformes. In an attempt to shed light on this controversy, we have cloned, sequenced, and compared two genes widely used in molecular phylogeny studies, the cytochrome b and the 18S rRNA from the rare prickly shark, Echinorhinus cookei, and two potential relatives, the spiny dogfish Squalus acanthias (Squaliformes), and the sevengill shark, Notorynchus cepedianus (Hexanchiformes). The sequences of these genes for the prickly shark, the dogfish, and the sevengill shark were found to be equally divergent, suggesting that the prickly shark is no closer to the order Squaliformes than to the order Hexanchiformes.