Phylogenetic characterisation of the genus idiosepius (Cephalopoda; Idiosepiidae)

Abstract

The family Idiosepiidae is an atypical cephalopod group; the member species are the smallest cephalopods in body size, and their phylogenetic position with regard to the other cephalopods as well as the relationships within the genus Idiosepius remain controversial. Currently, 8 recognized species belong to Idiosepius, although the taxonomic position of I. macrocheir and I. thailandicus is uncertain: their diagnostic characters closely overlap with those of I. biserialis. To provide further information on the phylogenetic relationships of Idiosepius, 4 mitochondrial loci (12S rRNA, 16S rRNA and cytochrome c oxidase subunits I and III) were analysed for all Idiosepius species and several populations. I. macrocheir and I. thailandicus nested within the African and the Indo-Pacific group of I. biserialis, respectively. This indicates that both species were incorrectly assigned to a single species, and that they rather represent junior synonyms of I. biserialis. Furthermore, the species I. biserialis itself exhibits considerable genetic variability: an African and Indo-Pacific region population was evident. Our results revealed 2 new aspects of Idiosepius: a population of I. biserialis from Japan appeared to be closely related to I. paradoxus, a species with 4 rows of suckers on the tentacular club. In contrast, I. paradoxus from Okinawa Island, also considered a 4-rowed species, showed strong congruence on the phylogenetic and taxonomic level with the 2-rowed species I. biserialis. This congruence leads to the hypothesis that climatic conditions, rather than habitat preference or geographical barriers, support the structuring of Idiosepius populations.