Despite their appeal to both researchers and the general public, the biology of giant deep-sea squids remains enigmatic. The robust clubhook squid Onykia robusta (Verrill, 1876) is one of the largest living deep-sea squids. It is distributed across the northern Pacific Ocean between Japan and California, including the Bering Sea. Although this species has been frequently caught by fishermen, its biology remains largely unknown. In this paper, I report biological data from one specimen, which was collected from the Sanriku Coast of northeastern Japan. The individual was a sexually mature female (total length: 246 cm, mantle length: 125 cm, wet body weight: 26 kg) and with oocytes that may have been large enough to begin yolk accumulation. Low genetic diversity within this species was implied by comparative analysis of the mitochondrial DNA (mtDNA) cytochrome oxidase I (COI) gene (n = 5; additional four sequences sourced from GenBank). The low nucleotide diversity (π = 0.0044) was similar to that of the deep-sea giant squid Architeuthis dux, which is known from mtDNA analysis to lack discernible population structure, showing no genetic differentiation among samples from different regions of the Pacific. Architeuthis dux has been hypothesized to be highly migratory, probably through a pelagic paralarval stage. My data suggest the presence of a common dispersal strategy among the deep-sea giant squids.
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