Seasonal occurrence and sexual maturation of Japanese pygmy squid (Idiosepius paradoxus) at the northern limits of their distribution

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Abstract

The distribution of aquatic animals is limited by water temperature. However, little is known about migration patterns, overwintering, and reproduction at the extremities of their distribution. To investigate the sexual maturation of Japanese pygmy squid (Idiosepius paradoxus) and their ability to survive during winter at the northern limits of their distribution, we collected samples monthly at Usujiri in northern Japan and carried out experiments on low-temperature tolerance. Squid were collected during 4 months in autumn and early winter. In autumn, all squid were large and sexually mature, and many egg masses were found on the surface of seagrass, whereas in early winter, only small immature squid were collected. A warm north-flowing current reached the Usujiri area during autumn, so the squid were likely transported by the current and some of the small squid were recruits that hatched there. Field data and the results of experiments on tolerance of the species to low temperatures showed that it cannot survive at the low temperatures around Usujiri in winter. The migration pattern of I. paradoxus may differ from the traditional pattern for marine resources, where young fish are transported passively and do not reproduce at the extremities of their distribution.

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