STROTHER, J. A.; MCHENRY, M. J.; Univ. of California, Berkeley; Harvard University: How do marine invertebrate larvae orient to light? A numerical simulation of phototaxis in ascidian larvae.

A diversity of marine invertebrate larvae orient their swimming to light. By swimming towards or away from the light, a larva can move up or down in the water column, thereby affecting its dispersal by ambient currents and its settlement. Despite an abundance of research that has quantified ontogenetic changes in phototaxis, the behavioral mechanisms of phototaxis in marine invertebrate larvae are poorly understood. To study a mechanism of orientation relative to a directional light source, we used the tadpole larvae of ascidians, which swim in a helix that exposes a single off-center ocellus to a time-varying light intensity. In order to address the complexity of three-dimensional swimming, we constructed a mathematical model that considers the hydrodynamic forces and mass distribution of an ascidian larva (Botrylloides sp.) and allows the model larva to change its position and orientation by beating its tail. This approach allowed us to test behavioral algorithms of phototaxis by controlling the motion of the tail in response to changes in light intensity on the ocellus. The resulting three-dimensional trajectories were compared with measured trajectories in order to verify our model. Our results support the hypothesis that ascidian larvae orient by helical klinotaxis and that the direction they swim with respect to light depends on the phase lag between the oscillating light intensity on the ocellus and the undulatory beating of the tail.