A direct and active influence of gravity on the behavior of a marine invertebrate larva

Larvae of the bryozoans Bugula neritina and Bugula stolonifera exhibit an apparent negative geotaxis under conditions of darkness and constant temperature. This behavior cannot be accounted for by buoyancy since the larvae are negatively bouyant, nor is it a consequence of gradients in the partial pressures of dissolved gases since the response occurs under conditions where the gradient is reversed or when experiments are conducted in chambers with interfaces of only glass and water. Pressure bomb experiments indicate that the behavior is not a barokinesis. Centrifuge experiments, however, showed that larvae of Bugula stolonifera orient directly and actively to gravity, while those of Bugula neritina have some other measure of geographic up. Since bryozoan larvae lack statocysts, the sensory apparatus mediating the gravity response in Bugula stolonifera is still unknown.