UC San Diego

Many benthic marine invertebrates have a planktonic larval stage in their life cycle. Larval growth is partly determined by environmental factors that ultimately affect reproductive success of adults. The goal of this dissertation was to understand the effects of small-scale biological and physical environmental factors using both laminar and turbulent flow experimental approaches on grazing, growth, and swimming in larvae of the white urchin, Lytechinus pictus, and purple urchin, Strongylocentrotus purpuratus. Predator-prey encounter models predict that ingestion increases for fluid shear levels greater than G_cr, the critical shear level above which encounters due to shear are greater than those resulting from behavior alone. Ingestion rates in larvae exposed to laminar shear representing dissipation rates, [Epsilon], of 0 - 1 cm² s⁻³, using simple Couette flow matched model predictions incorporating feeding current rather than swimming speed. Shear had no effect on ingestion rate for shear levels < G_cr. Model predictions were also tested using turbulent rather than laminar flow, in which flow is intermittent and vorticity occurs. There was no difference in ingestion rates for dissipation rates lower than the critical level, as predicted. Turbulent flow results were similar to those using a laminar flow approach, supporting the prediction that suspension feeders such as urchin larvae do not benefit from low to moderate levels of turbulence present in the ocean. The swimming ability of larvae was investigated using laminar flow to test the hypothesis that urchin larvae do not behave as passive particles in small-scale turbulence. L. pictus larvae, which have longer arms than S. purpuratus, exhibited swimming stability at low shear levels while all larvae behaved as passive particles at higher shear levels. As swimming speed is correlated with arm length, these results suggest that swimming stability scales with swimming speed. Overall, these results support the new paradigm that larval behavior plays an important role in small-scale biological-physical interactions involved in grazing and growth. These processes are important in feeding success, duration of the larval period, dispersal potential, and recruitment success