Diel patterns of migration, feeding, and spawning by salps in the subarctic Pacific

Abstract

The common pattern of feeding in marine zooplankton undergoing dlel vertical migrations is an alternation between not feed~ng at depth during the day and feeding near the surface at night. In contrast to this pattern, the pelagic tunicate Cyclosalpa bakeri fed actively during the day at depths of 30 to 60 m, but ceased to feed after nighttime migration into the surface layer (< 30 m). The cessation of feeding at night cannot be explained satisfactorily by current hypotheses that vertical migration serves to reduce visual predation, minimize photodamage, promote dispersion, reduce metabolic costs, or increase fecunlty. We offer evidence that the f e e l n g and migration pattern of C. baken is related to their reproductive behavior. Our data suggest that their nighttime ascent acts to concentrate the salps near the surface where spawning occurs, and filter-feeding ceases so that the swimming sperm are not trapped and ingested before they can enter the oviduct.