Swimming and Vulnerability in Pelagic Development

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Early swimming in planktonic development provides a system for testing hypotheses on advantages of the water column rather than the benthos as a habitat for animals early in their life histories. Diverse marine animals release eggs into the water to develop as solitary planktonic embryos that swim early, often while still in embryonic stages. The widespread existence of early swimming stages suggests advantages of the water column as a habitat at early and vulnerable life history stages. Solitary embryos deposited on the bottom may be at greater risk. Benthic embryos receive greater parental protection, further indicating that the benthos is a risky place for solitary embryos. This project will answer three questions. Does early swimming remove early embryonic stages from benthic predators, despite turbulent mixing? Does early swimming reduce capture by common pelagic predators? Are animals at early stages less vulnerable to pelagic than to benthic predators?These questions will be addressed through a combination of lab and field studies that assay sinking and swimming velocities under turbulent and non-turbulent conditions. Because turbulence turns swimmers away from vertical orientation, experiments will also compare vertical bias in turbulence with vertical bias in still water to estimate effectiveness of reorientation to the vertical in turbulence. Temperature commonly varies with depth, so this project will also measure the effect of temperature on rates of sinking and swimming. Swimming faster than sinking could increase encounters with ambush predators, so velocities of sinking and swimming will be compared with those of predators relative to water, and to predicted turbulence generated shear within predators' perception volumes. The hypothesis that swimmers displace water at smaller distances, and thus produce a smaller signal to predators than do passive sinkers, will also be tested. Quantifying capture and ingestion of embryos before and after they begin to swim will test the hypothesis that swimming decreases successful captures per encounter. Predators will include fish larvae, copepods, medusae, and ctenophores. Results on vulnerability of preswimming and swimming stages will be combined with published data on abundance and clearance rates of pelagic and benthic predators to test the hypothesis that animals at early stages of development are safer from predation in the water column than near the bottom. Two graduate students will participate in the proposed research in their dissertation research, and several undergraduates will participate in the research.