Abstract
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.
Highlights
Most monogeneans are ectoparasitic while a few genera are endoparasites of fish or other vertebrates
The tegument of adult Discocotyle sagittata is composed of a surface syncytial cytoplasmic layer separated from underlying tegumental cell bodies by a basal lamina complex and muscle blocks
Previous studies have shown that the two-layered tegumental structure of monogeneans develops from a primitive epithelium in which a nucleated apical layer becomes connected via cytoplasmic processes to underlying subtegumental cells (Lyons 1973; Smyth and Halton 1983; Bereiter-Hahn et al 1984; Cable and Tinsley 1992)
Summary
Most monogeneans are ectoparasitic while a few genera are endoparasites of fish or other vertebrates. In addition to nuclei and multilayered whorls, the interciliary layer contains Golgi bodies (Fig. 3b, g), numerous mitochondria, glycogen granules, granular endoplasmic reticulum, ribosomes, and oval-shaped and circular, membrane-bound vesicles which are either translucent or moderately electron dense
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