Ultrastructure of pigmented and unpigmented photoreceptors in cercariae of Trichobilharzia ocellata (Plathelminthes, Trematoda, Schistosomatidae): evidence for the evolution of parasitism in Neodermata.

The lens-covered pigment cup ocelli in free-swimming cercariae of Trichobilharzia ocellata were re-examined and the differentiation of the lenticular elements was studied in cercariae still enclosed in sporocysts. Each eye consists of a single rhabdomeric sensory cell, a single cup cell harboring pigment granules and the lens. This lens is developed step by step by the fusion of numerous dark platelets not enclosed by a bordering membrane. The material is presumably proteinaceous. There is no evidence that the lenses are of mitochondrial origin. Besides the pigment cup ocelli, a special type of unpigmented rhabdomeric photoreceptor was discovered. Three unicellular photoreceptors arranged in a three-dimensional configuration exist. These light-sensing organs show a principle of construction similar to that of phaosomous photoreceptors. We hypothesize that both types of photoreceptors are relevant for distinct reactions in the host-finding behaviour of the cercariae. The special type of lensing in the pigmented eyes seems to be restricted to Trichobilharzia species, whereas the phaosomous-like receptors may be more widespread in members of the Schistosomatidae. The obvious absence of any mitochondrial lensing in T. ocellata, and probably all other species of the Digenea, may be correlated with the evolution of endoparasitism in the Trematoda.