Occurrence and life cycles of Dermocystidium species (Mesomycetozoa) in the perch (Perca fluviatilis) and ruff (Gymnocephalus cernuus) (Pisces: Perciformes) in Finland and Estonia

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In Finland Dermocystidium percae Reichenbach-Klinke was first recorded by Pekkarinen in the fins of a perch in 1988. Because it is a poorly known parasite, its occurrence and life stages are studied here. In occasional sampling during 1995-1998 and 2001, and more systematic sampling during 1999 and 2000 in two different lakes (one oligotrophic, the other slightly eutrophic), it was found to occur almost continuously, although sometimes very sparsely, in both lakes. The life cycle is maintained from year to year by overwintering cysts. Young perch acquire first infections in their first summer of life, and fish over 3 years possibly develop some immunity. In Estonia, in the Kasari River and the Matsalu Bay, the parasite seems to be very rare. Cysts of D. percae, capable of sporogenesis, can occasionally occur in the ruff, too. In perch in Finland, cysts were found differing from the longish and thin-walled cysts typical of D. percae. These roundish to dumbbell-shaped cysts with thicker walls are here suggested to belong to a different Dermocystidium species, called D. sp. Cysts of D. percae occurred in the skin of all fins, but most often in abdominal fins and rarely elsewhere. In ruff, cysts of D. percae were also found in the gills. Of the fins D. sp. favoured the first dorsal fin, but also occurred elsewhere, e.g. in the head region of perch. Cysts of D. percae develop from thin-walled, round plasmodium mother cells, about 10 µm in diameter, by elongating and increasing in size. The nucleus of the plasmodium mother cell degenerates, and a reticular chromatin-containing structure with dense centra spreads out between conspicuous lipid droplets in the plasmodium. Nuclei reappear during fragmentation of the plasmodium. The sporonts thus formed divide to form sporoblasts, which in addition to small lipid droplets acquire a non-lipid 'central' inclusion. The inclusion grows in size to the typical inclusion of the ultimate Dermocystidium spore. Sporogenesis both in D. percae and D. sp. takes place mostly in summer, and at least in D. percae can continue until autumn. In addition, both species can produce numerous zoospores from their spores within 2 days in water at 25°C and at slower rates at lower temperatures. The body of the zoospore is about 1.2-2.2 µm in length and the flagellum is about six times the body length. The zoospores may then slightly grow in size and transform into amoebae. Small cysts, which possibly originated from an experimental infection by zoospores of D. sp., developed in 0-group perch kept in an aquarium at 17°C. The two Dermocystidium species here discussed can be grouped together with some other species, in which nuclei reappear and the plasmodium divides late in development and in which sporogenesis, unlike that in D. vejdovskyi Jírovec and D. cyprini Červinka and Lom, does not take place in compartments.