Diurnal migrations of the echinoid Diadema setosum (Leske)

Abstract

Millot (1954) has performed a series of experiments on the responses of Diadema antillarum Philippi to directional light and to changes in light intensity, concerning movements both of the whole animal and of isolated organs. In a study of the effect of change in right intensity on the movement of the whole animal, he has demonstrated that the response depends on the light intensity to which the urchin has been subjected for the period immediately preceding the change. Thus a dark-adapted animal avoids light of increased intensity, and a light-adapted one moves away from shade when the intensity of light is reduced. The response is most marked in dark-adapted urchins. Millot suggests that the nervous system, which he has shown to be directly excited by light, is shielded during the day by pigment in superficial melanophores and relatively exposed at night, the animal then responding to very small changes in light intensity Observations and experiments of the related The island of Suakin stands in a lagoon connected to the open sea by a narrow channel atmost a mile long. The natural harbour thus formed is no longer used except by a few small native craft during the pilgrim season. The low-lying island is bounded by a cliff which drops half a metre to the water surface and a further one to three metres to the bottom. A search of the sea bed near the island during the day reveals only very occasional specimens, but the tips of the spines of many individuals can be seen in the crevices of the cliff face. At night, the urchins emerge on to the adjacent sea bed, and their dark shapes can be spotrighted with a powerful lamp, when they move out of the beam immediately.