Sensitization and habituation of the swimming behavior in ascidian larvae to light

Ascidian larvae of Ciona intestinalis change their photic behavior during the course of development. Newly hatched larvae show no response to a light stimulus at any intensity. At 4 hr after hatching, larvae were induced to start to swimming upon the cessation of illumination, and to stop swimming upon the onset of illumination. At a weaker light intensity (5.0x10(-3) J/m(2).S), the larvae showed similar responses to either a single stimulus or repeated stimuli of onset and cessation of light until 10 hr after hatching. At a stronger light intensity (3.2x10(-1) J/m(2).s), when the stimulus was repeated, they showed sensitization and habituation of the swimming response.
At 3 hr after hatching the larvae failed to show any response to an initial stimulus at any intensity of light, but after several repeated stimuli (sensitization) they showed a swimming response at light intensities above 4.0x10(-2) J/m(2).s. At 5 hr and with intensity above 1.0x10(-2) J/m(2).s, the larvae showed photoresponses to the first stimulus, but after several repetitions the larvae failed to stop swimming upon the onset of light (habituation). A repeated series of stimuli at stronger intensities of light caused greater habituation; this habituation was retained for about 1 min. Since the larval central nervous system in Ciona is comprised of only about 100 neurons, learning behavior in ascidian larvae should provide insights for a minimal mechanism of memory in vertebrates.