Fish Diseases and Management

4.1.2.2 Digenea

The fish parasitic digeneans are oviparous and the eggs laid by adults hatch to release a small ciliated free-swimming miracidium larva. This are alive for a few hours, bythen if it could locate the first intermediate host of a gastropod or bivalve mollusc, it develops into free swimming cercariae after an asexual reproduction phase. These larvae survive for up to about 24 hours within which it finds a suitable second host. In some cases cercariae penetrate into a fish and mature directly to adult stage. Sometimes fish may act as an intermediate host and the cercariaencysts within the fish to form the metacercarial stage. Metacercariae maysurvive for several years in fish and when another fish or any final host eats this it develops into adult.