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

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 s olar radiation is largest, stayed in the phytoplankton-rich and rather brig ht layer (30-120 m) for a relatively long time, and then moved up to the su rface only during complete darkness. This DVM pattern would have an advanta ge in increasing the feeding opportunities for salps. Although the longer s tay at a bright depth may also increase the risk of predation by potential visual predators, the overlap of the depths between salps and visual predat ors was considered to be low during the stay in the shallow layer. The uniq ue DVM of S. thompsoni may be an adaptation to the oligotrophic environment s of the oceanic Southern Ocean, and one of the characteristics that enable them to maintain large stocks in the ocean.