Variations in life-history traits and statolith shape for Sepioteuthis spp. in the waters off southwestern Japan

Sepioteuthis lessoniana is a crucial squid species in fisheries and plays a critical role in marine ecosystems. At least two taxa of the squid species have been identified in the waters around Japan; however, the basic biological information of these taxa is limited. In this study, the taxa composition of Sepioteuthis was examined using molecular evidence, and the life-history traits and statolith shape of these taxa were analyzed to elucidate the potential population structure. A total of 142 squid samples were collected from six locations around Nagasaki (southwestern Japan) where high taxonomic diversity has been found previously. Three taxa (Sepioteuthis sp. 1, sp. 2, and sp. 2A) were identified based on the mitochondrial cytochrome oxidase c subunit I (COI) gene, among which Sepioteuthis sp. 2A was defined for the first time. Sepioteuthis sp. 2 was predominant in the region (average occurrence rate 84%), whereas Sepioteuthis sp. 1 was rare, with only two individuals found near Goto Islands. The mantle length composition, mantle length–body weight relationship, age structure, and average growth rates were similar between Sepioteuthis sp. 2 and sp. 2A. However, Sepioteuthis sp. 2 mainly hatched during summer (June–July), whereas Sepioteuthis sp. 2A hatched during winter (December–February). The statolith shapes of the squids were similar between the taxa, although slight differences were noted in the wing regions. The three taxa largely overlapped in a principal component analysis bi-plot based on variations extracted from five life-history traits. These results suggest that at least three taxa of Sepioteuthis inhabited the region, although few differences were found between them in life-history traits or statolith shapes that might be substantially influenced by regional oceanographic conditions. This study provides essential information on the population structure and connectivity of the squid, which is crucial in the context of conservation and management measures for fisheries.