Ontogenetic changes in vertical distribution of an endemic amphipod, Jesogammarus annandalei, in Lake Biwa, Japan

We examined the seasonal variation in vertical distribution, gut fullness and diet of Jesogammarus annandalei, an endemic amphipod in Lake Biwa. The vertical distribution changed ontogenetically throughout the year and was restricted to depths with a water temperature < 12 °C. Newborn amphipods remained in the pelagic zone throughout the day while larger amphipods exhibited marked diurnal vertical migrations: individuals stayed on the lake bottom during the day and ascended to the pelagic zone at night. Gut fullness and contents of the migrating amphipods indicated that they fed upon plankton in the pelagic zone at night. Our results indicate that these migration patterns probably reduce the amphipod's predation risk from fish. As a result, J. annandalei may play an important role in the vertical translocation of material and energy in the Lake Biwa ecosystem.