The kinematics of phototaxis in larvae of the ascidian Aplidium constellatum

Although phototaxis has an important influence on the vertical distribution and settlement of marine invertebrate larvae, few studies have explored the mechanisms of taxis in larvae at the organismal level. We examined how phototaxis changes over ontogeny in larvae of the ascidian Aplidium constellatum and experimentally tested hypotheses about the kinematics of oriented swimming. By video recording their swimming movements at regular intervals over their ontogeny, we found that larvae switched from positive to negative phototaxis. We tested hypotheses about the kinematics of phototaxis by recording the three-dimensional movement of larvae in response to a change in the direction of illumination and by tracking the tail motion of tethered larvae in response to sinusoidal changes in light intensity. Larvae swimming with negative phototaxis changed their rate of rotation about their antero-posterior and dorso-ventral axes in response to a change in the direction of illumination. These changes in the rates of rotation caused the axis of the helical trajectory to orient away from the light source. Tethered larvae oscillated their tails at a constant tail beat frequency and with a slow periodicity that was correlated with the stimulus frequency. These findings suggest that ascidian larvae orient by changing tail motion in proportion to perceived changes in light intensity. This method of orientation predicts that larvae achieve the switch from positive to negative phototaxis by changing the delay of their kinematic response to changes in perceived light intensity. [PUBLICATION ABSTRACT]