Early dispersal of ayu during marine stages as inferred from geographic variation in the number of vertebrae

ABSTRACT:   Marine environments often allow interbreeding of individuals over the species range, and analyses using neutral molecular markers may lose extant genetic boundaries laid between geographic majorities. Ayu Plecoglossus altivelis has a typical amphidromous life history, migrating between rivers and the sea. In order to clarify reproductive elements of the species, migrants from 64 rivers and streams sampled over a wide latitudinal range were examined for their vertebral number (VN) and dorsal pterygiophore number (DPN) as morphological markers to estimate the temperature history. The irregular variability in VN without a geographic cline suggests that the ascending schools of fish are composed of conspecifics sharing incubation temperature or hatching site. The intersample difference in DPN indicates that larvae and juveniles of ayu spend their marine life inside a water body with a distinct temperature. Site tenacity during the marine stage may be helpful to enhance the opportunity to return to the river where they hatched. Each assemblage of fish ascending to rivers and streams is deemed to mostly represent a reproductive element, and therefore, each acts as an evolutionarily significant unit within a metapopulational structure.