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During their different life stages, parasites undergo remarkable morphological, physiological, and behavioral “metamorphoses” to meet the needs of their changing habitats. This is even true for ectoparasites, such as the monogeneans, which typically have a free-swimming larval stage (oncomiracidium) that seeks out and attaches to the external surfaces of fish where they mature. Before any obvious changes occur, there are ultrastructural differences in the oncomiracidium’s outer surface that prepare it for a parasitic existence. The present findings suggest a distinct variation in timing of the switch from oncomiracidia epidermis to the syncytial structure of the adult tegument and so, to date, there are three such categories within the Monogenea: (1) Nuclei of both ciliated cells and interciliary cytoplasm are shed from the surface layer and the epidermis becomes a syncytial layer during the later stages of embryogenesis; (2) nuclei of both ciliated cells and interciliary syncytium remain distinct and the switch occurs later after the oncomiracidia hatch (as in the present study); and (3) the nuclei remain distinct in the ciliated epidermis but those of the interciliary epidermis are lost during embryonic development. Here we describe how the epidermis of the oncomiracidium of  Discocotyle sagittata  is differentiated into two regions, a ciliated cell layer and an interciliary, syncytial cytoplasm, both of which are nucleated. The interciliary syncytium extends in-between and underneath the ciliated cells and sometimes covers part of their apical surfaces, possibly the start of their shedding process. The presence of membranous whorls and pyknotic nuclei over the surface are indicative of membrane turnover suggesting that the switch in epidermis morphology is already initiated at this stage. The body tegument and associated putative sensory receptors of subadult and adult  D. sagittata  are similar to those in other monogeneans.

Ultrastructural observations on the oncomiracidium epidermis and adult tegument of Discocotyle sagittata, a monogenean gill parasite of salmonids