Action spectrum for the photophobic response of Ciona intestinalis (Ascidieacea, Urochordata) larvae implicates retinal protein

Ascidian tadpole larvae change swimming behavior during the course of devel opment. The photic behavior of the larvae of Ciona intestinalis was monitor ed by a computerized cell-tracking system with a time resolution of 0.1 s. Newly hatched larvae swim at an average speed of 1.4 mm/s but show no respo nse to light stimuli. The swimming speed of the larvae became slower (0.4 m m/s) 3 h after hatching and they were induced to swim more rapidly by a sud den decrease in light intensity 4 h after hatching. During the course of de velopment, the maximal speed of swimming behavior increased with time until 8 h after hatching and then plateaued. The action spectrum for the step-do wn photophobic response of the larvae was determined at around 8 h after ha tching and was fitted to Dartnall's nomogram with the absorbance maximum of the pigment located at 505 nm. These results suggest retinal proteins in t he ocellus of the larvae are! the photoreceptors for the photobehavior.