Species identification and genetic relationships in the squid family Gonatidae (Teuthida, Cephalopoda) based on partial sequencing of mitochondrial and nuclear genes

Squids of the family Gonatidae are key components in oceanic communities. However, issues related to correct species identification, number of species, and their genetic relatedness remain. To address these issues, sequences from three mitochondrial (cytochrome c oxidase subunit I [CO1], 16S rRNA, and 12S rRNA) and two nuclear (18S and 28S) genes were analysed in the Gonatidae. Four of the five sequences (12S rRNA, 16S rRNA, 28S, and CO1) yielded rather similar patterns of genetic relationships among the species. Molecular evidence suggested intra-familial subdivision into two major groups of species having either five or seven longitudinal rows of teeth in the radula. The former group included all species of the genus Gonatus and two sister-species of Gonatopsis s. str. suggesting that all gonatid species with five rows of radular teeth represent a single taxonomic unit of a genus or subfamily level. Species with seven rows of radular teeth formed several ‘species’ clusters. Sequence analysis also addressed species identification issues in the Gonatidae. Two genetically divergent groups were found among squid which conformed to the description of Gonatus berryi. Molecular evidence suggested sister-species relationships between ‘large’ and ‘small’ forms of Boreoteuthis borealis with size-at-maturity as the only reported difference between these two cohorts. Sequence variation was observed within Gonatus pyros. Inclusion of gonatid sequences from the GenBank into the analysis suggested probable species misidentification in several cases. Combined use of several mitochondrial and nuclear sequences served as a valuable tool for species identification and provided a solid background for unravelling molecular genetic and taxonomic relationships in the Gonatidae.