Behavior plasticity in free swimming coral larvae via long term tracking microscopy

The larval stage of coral is the only motile stage in its developmental cycle. As a result, the ability of coral larvae to navigate towards reefs and find a suitable settlement location is crucial to their survival. While the effect of external cues such as chemicals, light or sound on larval swimming has been studied, and order of magnitude estimates of swimming speeds have also been measured, observing the free-swimming behavior of larvae at ecologically relevant scales has remained a technological challenge. Here, we use a novel scale-free vertical-tracking microscope (Krishnamurthy et al. 2020) to resolve the swimming behavior of the coral species Acropora millepora at the meter scale. Our experimental results quantify the behavioral modes of larvae both as a function of their diverse shapes and as a function of their developmental cycle. We observe dramatic, short time-scale shape change during swimming that is coupled to a change in swimming velocity. Finally, we also observe that the introduction of symbiotic algae significantly affects the gravitaxis behavior of coral larvae.