CHANGES IN BEHAVIOR AND OCELLAR STRUCTURE DURING THE LARVAL LIFE OF SOLITARY ASCIDIANS

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Abstract
Larvae of Ciona savignyi Herdman change swimming behavior during the course of development. Newly hatched larvae swim upward by negative geotaxis, accumulating beneath the water surface. Thirty minutes after hatching, the two-dimensional spread becomes more regionally restricted and the aggregated pattern looks like a swarm of mosquitoes. One and a half hours after hatching, larvae become photoresponsive, swimming upward for a short time immediately after shading. Meanwhile, the duration of swimming induced by shading becomes longer. Two and a half hours after hatching, larvae are weakly photonegative. Three and a half hours after hatching, swarming is abolished and photonegativity is much stronger than the previous stage. The larval ocellus differentiates after the tadpole hatches. In newly hatched larvae, the flat pigment cell contains sparsely scattered pigment granules. Several short tubular membranes derived from ciliary endings of photoreceptor cells are irregularly arranged. One hour after hatching, the pigment cell becomes roughly L-shaped. The originally tubular membranes become paddle-shaped and increase in number and size. The pigment cell then assumes a V- or J-shape and becomes loaded with densely packed pigment granules. Some of the paddle-shaped membranes are arranged into lamellae and increase greatly in number and length. Three and a half hours after hatching, the ocellus becomes fully differentiated. Using morphometrical parameters as regards the size of photoreceptor endings and the disposition of pigment granules, we show that changes in photic behavior coincides roughly with the course of differentiation of the ocellar elements.