PERCEPTION OF LIGHT AT SETTLEMENT - A COMPARATIVE-STUDY OF 2 INVERTEBRATE LARVAE, A SCYPHOZOAN PLANULA AND A SIMPLE ASCIDIAN TADPOLE

The planula and the ascidian tadpole larva represent structurally and evolutionary opposites in a continum. In the planula, external stimuli are percieved at the cellular level and no sensory organs can be found while in the simple ascidian tadpole, external stimuli are perceived by well-defined sensory organs. A comparative experimental laboratory study was carried out to investigate effects of light intensity on phototactic and geotactic behaviour at settlement of the planulae of the scyphozoan Cyanea capillata L. and the tadpole of the solitary ascidian Ascidia mentula O.F. Muller. In the field, scyphistomae of Cyanea and juveniles of Ascidia are mostly found in shaded habitats and downward facing substrata most likely as a result of larval behaviour. Field measurements of light conditions were made in habitats where the two species commonly occur. The planula and the tadpole responded significantly to light as low as 2 x 10(17) Quanta . s(-1). m(-2) equalent to 10 Lux and 6 x 10(14) Quanta . s(-1). m(-2) equalent to 0.1 Lux, respectively. In both larval types, no effect of light intensity on negative geotaxis was evident. Ecological aspects of the larval behavioural patterns are discussed.