Vertical Migration of Pelagic Tunicates: Role of Feeding and Reproduction

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This investigation focuses on diel vertical migration by salps (pelagic tunicates) in the vicinity of the Bermuda Atlantic Time Series Station. Salps are efficient grazers on phytoplankton and other particulates; they are widely distributed and sometimes occur in dense populations. Several species migrate vertically, some as far as 800 m, while others are restricted to surface waters. Recent work in the Subarctic Pacific and near Bermuda has revealed intriguing new features to these migrations, which do not seem consistent with traditional theories for the advantages of migration. Salps in the Pacific make shallow migrations, but stop feeding while at the surface during the night. This behavior is believed to relate mainly to spawning and fertilization. Recently salps have been observed to consistently appeared at the surface at different times of day - morning, afternoon and night. This apparent migration is also inconsistent with the usual explanations. Dr. Purcell and collaborators will investigate the behavior of this extremely important group of pelagic herbivores. They will test two, non-exclusive hypotheses for the migratory patterns observed: 1) migration optimizes feeding at food-rich depths, or temporally partitions food among spatially co- occurring species; 2) migration concentrates salps at the surface and synchronizes spawning and fertilization. The investigations address basic questions of adaptation and ecology in migrating zooplankton, reproductive physiology of salps, and competition and niche separation. They are also critical for calculations of the role of grazing and migration by salps on vertical fluxes of particulate and dissolved nutrients.