THE ULTRASTRUCTURE OF PHOTORECEPTORS IN PSEUDODIPLORCHIS AMERICANUS AND NEODIPLORCHIS SCAPHIOPODIS (MONOGENEA: POLYSTOMATIDAE) zyxwvutsrqponmlkjihg J. CABLE* and R. C. TINSLEY School of Biological Sciences, Queen Mary and Westfield College, London University, Mile End Road, London El 4NS, U. K. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR (Received 4 July 1990; accepted 11 October 1990) .kbBtrPC~<ABLE J. and TINSLEY R. C. 1991. The ultrastructure of photoreceptors in Pseudodiplorchis americanus and Neodiplorchis scaphiopodis (Monogenea: Polystomatidae). International Journal for Parasitology 21: 81-90. The monogeneans Pseudodiplorchis americanus and Neodiplorchb scaphiopodis have two pairs of rhabdomeric eyespots, each composed of a cup-shaped supportive cell and a sensory component. The supportive cell is characterized by concentric rows of plates which reflect light onto the rhabdomere. Similar eyespots, with a reflecting supportive cup, have been described in four other, phylogenetically-diverse, invertebrate groups, but amongst platyhelminths they are recorded only in the Polystomatidae. The structure conforms to a multilayer quarter wavelength reflector which is adapted to low light intensities. By contrast with most other endoparasites, the eyes of P. americanus and N. scaphiopodis do not degenerate after host invasion but persist throughout life. The hosts (desert toads, Scaphiopus species) are strictly nocturnal during their brief activity season and spend the rest of the year in hibernation burrows. Whilst a very sensitive photoreceptor is likely to be important in night-time larval invasion, its role in the biology of the adult parasite is uncertain. INDEX KEY WORDS: Monogenea; Polystomatidae; Pseudodiplorchis americanus; Neodiplorchis scaphiopodis; ultrastructure; photoreceptors; rhabdomeric eyes. INTRODUCTION LIGHT is one of the most important factors employed by the free-living infective stages of parasites in the process of host location and many platyhelminth larvae are equipped with prominent photoreceptive organelles. The structure of these eyespots has been comprehensively reviewed by Foumier (198 1, 1984). In the simple rhabdomeric ocelli of monogeneans, there is usually a single cup-shaped cell, containing a dense screening pigment. This imparts an element of directional sensitivity to the receptor: only light entering the opening of the cup can be detected by the receptive region. In Polystoma integerrimum, and five other polystomatids previously studied, the whole of the supportive cup is adapted for light amplification (Fournier, Unpublished Ph.D. thesis, University of Perpignan, 1980; Zhang, 1987). Uniquely amongst platyhelminths, the cell enclosing the sensory cells is packed with rows of lamellae which reflect light onto the rhabdomere. The photoreceptors of P. inte- gerrimum are functional only in the infective stage and, as in the majority of helminth parasites, the receptors * Present address and address for correspondence: MRC Institute of Hearing Research, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. degenerate soon after invasion of the host (Foumier & Combes, 1978). The present study concerns the photoreceptors of Pseudodiplorchis americanus and Neodiplorchis scaphiopodis, and was stimulated by the unusual ecology of the host and parasite and the special adaptations for rapid and efficient transmission. These monogeneans infect the urinary bladder of desert toads, Scaphiopus species. The hosts are inactive underground for 10 months of the year, emerging at night after summer rainfall to spawn and replenish fat stores. Eggs produced by adult worms are retained and develop within a long uterus and by early summer there are usually about 100 infective larvae in utero. The eyespots first become apparent in the later stages of embryo development and can be seen through the uterus and tegumental wall of the adult as four bright silver spots. Parasite transmission is concentrated into a few nights at the start of the rainy season, an opportunity totalling less than 24 h per year. When the hosts enter temporary pools to spawn, encapsulated larvae are released and hatch almost immediately. Host invasion involves the migration of oncomiracidia over the toad’s skin to the nostrils and the subsequent development of the parasites in the respiratory tract before migration through the gut to the urinary bladder (Tinsley & Earle, 1983; Tinsley & Jackson, 81