Ultrastructural study of Pisdnoodinium pillulare (Schäperclaus, 1954) Lom, 1981 with special emphasis on its attachment to the fish host

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Abstract
Trophonts of Pisdnoodinium pillulare (Schäperclaus, 1954), a common ectoparasite of freshwater aquarium fish, are attached to host cells by means of a specialized structure, the attachment disc. Unlike other dinoflagellate genera parasitic on fish and invertebrates, this disc features nail‐like organelles, the rhizocysts. Head‐parts of the rhizocysts are inverted in separate compartments, rhizothecas, in the sole of the disc while their long shafts are firmly embedded in the cytoplasm of cells of the host epidermis or gill epithelium. The attachment inflicts a serious injury on the host cells which may ultimately be destroyed. Rhizocysts originate in the subnuclear cytoplasm from where they migrate into the attachment disc. There are other specialized organelles and inclusions; fibrous vesicles, membraneous bodies, striated tubular bodies and paracrystalline bodies. Pisdnoodinium has well‐developed chloroplasts. While its cytological adaptations indicate a nutritional dependence on the host, there is no evidence of ingestion of host‐derived particulate material. Pisdnoodinium may derive an essential part of its nutrition from photosynthesis.