Surface ultrastructural characteristics of Dictyocotyle coeliaca Nybelin, 1941 (Monopisthocotylea: Monocotylidae), an endoparasitic monogenean of rays.

Scanning and transmission electron microscopical observations were made of the surface topography of Dictyocotyle coeliaca (Nybelin, 1941) (Monopisthocotylea: Monocotylidae), a unique endoparasitic monogenean from the body cavity of the ray Amblyraja radiata (Elasmobranchii: Rajidae). Scanning investigation show the presence on the ventral side of the anterior body of smooth areas with pit-like depressions and shallow ridges, whereas the tegument of the middle and posterior regions of the body is extensively folded, and on the dorsal surface the tegument is smooth but interrupted by deep depressions. Transmission observation revealed the presence of invaginations of varying irregularity and depth, which form the various depressions of the tegumental surface. Non-ciliated, dome-shaped papillae occur singly or in groups and are common around the mouth and both the genital and vaginal pores, but exhibit no particular orientation. The haptor is much reduced, compared with that of ectoparasitic monocotylids, and is covered ventrally with an irregular array of 40-70 shallow loculi. Internal differences occur in the thickness of the syncytial tegumentary layer of the haptoral loculi and septa and also in the number of cytoplasmic inclusions. The locular surface has distinct ultrastructural characteristics, the most obvious of which are a honeycomb arrangement of small pockets measuring 0.6 × 0.75 to 1.0 × 1.7 μm in diameter, a terminal web beneath the surface plasma membrane and large outgrowths on the surface of the loculi filled with lysosome-like bodies. The surface specializations of the body and haptoral tegument of D. coeliaca are discussed in relation to their being adaptations to an endoparasitic environment.