Gravitactic Swimming of the Planula Larva of the Coral Acropora: Characterization of Straightforward Vertical Swimming.

Vertical migration as well as horizontal dispersion is important in the ecological strategy of planktonic larvae of sedentary corals. We report in this paper unique vertical swimming behavior of planulae of the reef-building coral Acropora tenuis. Several days after fertilization, most of the planulae stayed exclusively at either the top or the bottom of the rearing tank. A good proportion of the planulae migrated almost vertically between top and bottom with fairly straight trajectories. Planulae sometimes switched their swimming direction via a sharp turn between the opposite directions. Quantitative analyses demonstrated that planulae kept constant speed while swimming either upward or downward, in contrast to frequent changes of direction and speed in horizontal swimming. Statistical comparison of propulsive speeds, estimated from swimming speeds and passive sedimentation, revealed gravikinesis of planulae, where the propulsive speed was significantly greater in downward swimming than upward swimming. The larval density hydrodynamically estimated was 0.25% lower than sea water density, which might be explained by the large quantity of lipids in planulae. Also, the deciliated larvae tended to orient oral end-up during floatation, presumably due to asymmetrical distribution of the endogenous light lipids. Plasticity of the larval tissue geometry could easily cause relocation of the center of forces which work together to generate gravitactic-orientation torque and, therefore, abrupt changing of the gravitactic swimming direction. The bimodal gravitactic behavior may give a new insight into dispersal and recruitment of coral larvae.