ABSTRACT
Many anthomedusan jellyfish swim when ambient illumination is interrupted (Passano, Mackie & De Ceccatty, 1967; Ohtsu & Yoshida, 1973; Anderson & Mackie, 1977). This shadow response has been attributed to the ocelli, which are located at the margin of the bell. However, in some anthomedusae, it has been demonstrated that the motor neurones that innervate the swimming musculature are, themselves, directly photosensitive (Anderson & Mackie, 1977; Ohtsu, 1983). One species which possesses both classes of photoreceptor is Spirocodon (Ohtsu, 1983). In this animal, near ultraviolet light (u.v.) hyperpolarizes the swimming neurones and inhibits swimming. This effect is tonic, lasting long after the cessation of u.v. irradiation. However, this after-inhibition can be disinhibited by irradiation with visible light (u.v.-vis antagonism). To investigate the role of these two photoreceptor types in the behaviour of Spirocodon, the electroretinogram (ERG) of the ocelli was measured. The results described here are basically similar to those obtained from Sarsia ocelli (Weber, 1982), the only coelenterate photoreceptors investigated to date, but there are some differences.
