Phylogeny of cephalopods inferred from mitochondrial DNA sequences

Sequences of partial mitochondrial cytochrome oxidase III gene (533 bp) were obtained for 17 species of cephalopods, 14 decapods, 2 octopods, and 1 vampyromorph. This study aimed to: (1) compare partial COII and COIII amino acid sequences of three species of cephalopods with other invertebrates in terms of base composition and phylogenetic relationships. Cephalopod sequences are closer to Katharina tunicata sequences than to Mytilus edulis (marine bivalve) and Albinaria turrita (terrestrial gastropod); (2) investigate the phylogenetic relationships among cephalopods. Nucleotides as well as amino acid sequence variations of a COIII fragment were examined with parsimony and distance methods. Our data confirm the monophyly of the Decapoda and of each family. A high hierarchical rank for sepiolids, already suggested by other authors, appears justified. The order Sepioidea comprising five families should be abandoned, with Spirula being excluded. The surprising position of Idiosepius as the sister group of some oegopsids and not of sepioids or sepiolids is confirmed. Vampyroteuthis is the sister group of octopods. However, the data do not allow resolution of the phylogenetic relationships between the orders of Decapoda; consequently we suspect a rapid radiation.