Mitochondrial genome structure and evolution in the living fossil vampire squid, Vampyroteuthis infernalis, and extant cephalopods.

Complete nucleotide sequences of mitochondrial (mt) genomes of the "living fossil" cephalopod Vampyroteuthis infernalis (Vampyromorpha) and the cuttlefish Sepia esculenta (Sepiida) were determined. The V. infernalis mt genome structure is identical to the incirrate octopod Octopus vulgaris mt genome structure, and is therefore more similar to that of the polyplacophoran Katharina tunicata, than to that of the other "living fossil" cephalopod Nautilus macromphalus. The mt genome structure of S. esculenta is identical to that of Sepia officinalis. Molecular phylogenetic analyses based on the mt protein genes from the completely sequenced cephalopod mt genomes suggested the monophyletic relationship of two myopsid squids Loligo bleekeri and Sepiotheuthis lessoniana, and the monophyletic relationship of two oegopsid squids Watasenia scintillans, and Todarodes pacificus. Sepiida appeared as the sister group of Teuthida (Myopsida + Oegopsida). The phylogenetic position of Vampyromorpha appeared as the sister group of Octopoda, although the monophyly of Vampyromorpha and Decapodiformes cannot be rejected outright by our phylogenetic analyses. The hypothesis that Vampyromorpha is basal among the coleoid cephalopods can be rejected because of low statistical support. Therefore, it is reasonable to recognize three major groups in Coleoidea--Vampyromorpha, Octopoda, and Decapodiformes.