Epidermal Uptake of Nutrients in an Unusual Turbellarian Parasitic in the Starfish Coscinasterias calamaria in Tasmanian Waters

¹ Department of Pure and Applied Biology, Baines Wing, University of Leeds, Leeds LS2 9JT, England

The parasitic turbellarian Acholades asteris (Neorhabdocoela: Acholadidae) lives encysted on the ambulacral tube-feet of the starfish Coscinasterias calamaria in Tasmanian waters. Cysts, each containing one worm, generally occur singly as spindle-shaped protrusions from near the bases of the tube-feet; the cyst wall consists of tube-foot epidermis, with nervous tissue, and part of the outer layer of the tube-foot's two-layered connective tissue sheath. The inner layer, musculature, and coelomic epithelium are not involved, and the tube-foot remains functional. The turbellarian lies within the split outer layer with its anterior towards the base of the tube-foot. It lacks mouth, pharynx, and intestine and feeds via its ciliated epidermis on the surrounding collagenous and cellular materials. These are partially digested by enzymes from epidermal and subepidermal glands, products are pinocytosed, and digestion is completed in epidermal phagosomes formed by fusion of pinocytotic vesicles. Food uptake occurs over the entire body surface but especially anteriorly where the epidermis is thicker, deeply invaginated, and underlain by concentrations of gland cells. In approximately 25% of flatworms examined, the epidermis and adjacent parenchyma contained trophozoites and other stages of Monocystella sp., an aseptate gregarine whose other, known, species occur in turbellarian alimentary systems.