The morphology and occurrence ofGyrodactylus mediusKathariner, 1894 (Monogenoidea) fromOnos mustelus(L.)

Summary The morphology and occurrence of Gyrodactylus medius Kathariner, 1894 from the gills and skin of the five-bearded rockling, Onos mustelus (L.), from the inter-tidal zone at Mumbles Head, Swansea, is described. The percentage incidence and the mean number of parasite per infested fish increases with the size or age of the host fish and also undergo seasonal fluctuations with biannual peaks in infestation in May and September–October. These variations may be partly influenced by temperature, tidal movements and the habits and breeding cycle of the host fish.