Résumé
A central question in marine ecology today is to understand the spatial scales over which populations are connected by larval dispersal. Although coral reef fish larvae develop strong behavioral capabilities during the processes of dispersal (e.g., vertical migration, swimming), the influence of these capabilities on survival depends on the ability of larvae to orient in the open ocean. Yet, behavioral mechanisms by which reef fish larvae achieve successful recruitment from blue waters to coral reefs are still unknown. We describe a novel system designed to detect and quantify the orientation of larval coral reef fish in the pelagic environment, where they have no apparent frame of reference to navigate. The Orientation With No Frame Of Reference (OWNFOR) system is deployed at sea and drifts while videotaping the movement of a larva placed within a clear, circular arena. The data are then treated with a combination of open-source programs that track particles and analyze orientation using circular statistics. We demonstrate that (1) this system successfully detects orientation behavior and (2) the shape of the behavioral arena and appropriate statistical treatments minimize the impact of the semi-enclosure on quantifying larval bearing. This observational approach provides a means of assessing the abilities of larvae to orientate during ontogeny and of testing cues, representing a breakthrough in the field of larval ecology. Such behavioral data will provide critical inputs to a new generation of biophysical larval dispersal models that are vital to achieve a better understanding of larval connectivity in marine systems.