Diel vertical migration of the tunicate Salpa thompsoni in the Southern Ocean during summer

Abstract

Diel vertical migrations (DVM) of a pelagic tunicate, Salpa thompsoni, were investigated to help elucidate their life-history strategy in the Southern Ocean. S. thompsoni began migration to the subsurface at midday when the solar radiation is largest, stayed in the phytoplankton-rich and rather bright layer (30–120 m) for a relatively long time, and then moved up to the surface only during complete darkness. This DVM pattern would have an advantage in increasing the feeding opportunities for salps. Although the longer stay at a bright depth may also increase the risk of predation by potential visual predators, the overlap of the depths between salps and visual predators was considered to be low during the stay in the shallow layer. The unique DVM of S. thompsoni may be an adaptation to the oligotrophic environments of the oceanic Southern Ocean, and one of the characteristics that enable them to maintain large stocks in the ocean.