Temperature and diet modified swimming behaviors of larval sand dollar

Swimming behaviors of marine invertebrate larvae play key roles in larval dispersal and survival and, hence, have important consequences for adult population dynamics. However, to date, insufficient quantitative information exists on larval swimming to understand and predict swimming movements in most marine invertebrate species. Previous work suggests that larvae swim more slowly at lower temperatures and, consequently, might have difficulty regulating depth when experiencing temperature changes. Improved diet quality in terms of essential fatty acid composition has been suggested to increase cold tolerance in many organisms. We used non-invasive videotracking techniques to quantify swimming in larvae of the sand dollar Dendraster excentricus, raised on 4 algal diets differing in their fatty acid profiles and then exposed to an ecologically relevant temperature decrease from 20 to 12°C. Differences in diet quality led to significant morphological differences by the 8-arm larval stage, and there were significant diet-temperature interaction effects on swimming patterns. While larval swimming speeds decreased as temperature decreased across all diet treatments, net vertical velocities of larvae did not decrease. Changes in helical geometries of larval swimming trajectories suggest that larvae compensate for reduced swimming speeds by reducing horizontal movement, thus preserving their ability to regulate depth. The observed compensatory mechanism effectively circumvents constraints on swimming due to lowered temperatures. More generally, video-tracking of free-swimming larvae can yield quantitative data to inform biophysically coupled models that better predict consequences of larval dispersal for adult population dynamics under current and future environmental conditions.