Published online by Cambridge University Press: 06 April 2009
Apophallus brevis metacercariae localize in parasite-produced cysts mainly in the myotomes of Perca flavescens. The host response induced by this parasite is very intense and leads to the formation of an elaborate host-formed capsule of material which is very similar if not identical to host bone. This capsule is pierced by two canals situated at opposite poles in the cyst and enables the parasite to escape from this host capsule on being ingested by a definitive host. Successive layers of melanocytes and connective tissue, containing lipid droplets, are also present around the bony capsule. Histochemically this bony capsule and the fish bone are virtually identical but there are slight organizational differences in the capsule tissues compared to the fish bone. It is postulated that the formation and arrangement of the capsule may be influenced by the parasite and may not therefore be a defensive development by the host.