Reproductive strategies of the parasitic flatworm Thaparocleidus vistulensis (Siwak, 1932) (platyhelminthes, monogenea) infecting the European catfish Silurus glanis Linnaeus, 1758

The life cycle of Thaparocleidus vistulensis (Siwak, 1932), a host-specific monogenean parasite of European catfish (Silurus glanis Linnaeus, 1758), was investigated by detailed observation of infection dynamics, egg development, hatching rate and in vitro survival rates of the parasite at different life stages at 23 °C. A total of 30 naive fingerlings were infected in three exposure trials by co-habitation with donor fish carrying adult parasites. Two fish were dissected every two days during the 10-day experimental period to explore the development of larvae and juvenile parasites on the host gills. Freshly laid eggs by adult monogeneans were collected and observed daily under a light microscope until hatching. A total of 445 eggs were collected and distributed into wells of 96-well microtiter plates containing filtered fish tank water to determine their hatching rates. A similar method was used to investigate the survival rates of isolated parasites at different developmental stages (larvae, juveniles, and adults). T. vistulensis populations on the European catfish in fish tanks increased markedly within ten days, dependent on the severity of the initial infection levels of the donor fish. The first eggs hatched three to four days after oviposition, and the hatching rate peaked on the fifth day (89.7%). The survival rate for freely swimming oncomiracidia without host was 7.4% after five days, whereas isolated juvenile and adult parasites showed a higher dependence of host contact (survival rates three days post-isolation of 0.9% and 1.6%, respectively). The data allows prediction of parasite-host dynamics and may improve control of gill-disease in cultured European catfish stocks in fish farms.