Population Structure of Todarodes pacificus (Cephalopoda: Ommastrephidae) in the Northwest Pacific: Insights from Integrated Genetic and Statolith Morphology Analyses

Accurate identification of population structure is crucial for the sustainable management of cephalopod fisheries. This study integrated genetic and morphological approaches to investigate the population structure of Todarodes pacificus in the northwest Pacific. Specimens were collected from the northern Yellow Sea, northern East China Sea, and southern East China Sea between 2019 and 2024. Population genetic analyses—based on two mitochondrial genes (COI and 16S rRNA) and one nuclear gene (ODH)—revealed no significant genetic differentiation among geographical regions or between summer- and autumn-spawning cohorts, suggesting high gene flow and possible panmixia. In contrast, geometric morphometric analysis of statoliths revealed significant differentiation between spawning cohorts, particularly in the posterior indentation region. Random Forest classification achieved high discriminatory accuracy (85.0–94.8%), supporting the robustness of statolith shape as a phenotypic marker. Interannual variation in statolith morphology was also detected, potentially linked to climatic events. Although no winter-spawning cohort was identified in the commercial samples examined, the summer-spawning cohort appears to constitute a major component of the Western Pacific stock. These findings demonstrate that statolith morphology can serve as a reliable tool for cohort discrimination even in the absence of genetic differentiation, offering critical insights for the cohort-specific management and conservation of T. pacificus resources.