Structure, Development, and Behaviour of New Strigeatoid Metacercariae from Subtropical Fishes of South America

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Abstract
Eighteen species of metacercariae of the order Strigeatoidea (Trematoda, Digenea) are described from tropical and subtropical freshwater fishes of Argentina. Sixteen of them belong to the superfamily Strigeoidea and two to Clinostomatoidea. Ten are new species, five belong to previously known trematode species, and three are not specifically named.On the basis of the morphology of these metacercariae, probable life cycles and host–parasite relations are discussed. A general conclusion is drawn that the South American trematodes are more primitive than their counterparts from other continents.Different groups of fish hosts are recognized, each with a different history of association with their present environment. Originally marine fishes adapted to life in fresh water with the aid of increased endocrine activity, primarily thyroid activity. The high levels of endocrine secretions affected the parasites of these fishes, causing metacercariae to develop neotenically. The hosts which were originally freshwater appear to exert less influence on their parasites. Their metacercariae have a more or less normal course of development.Attention is drawn to the evolution in behaviour of strigeoid metacercariae. Primitive species infect less protected sites in the host. With progressive evolution, increasingly more protected sites are selected by the parasite. The evolution culminates in infection of the lens of the eye, which offers the best protection to the parasite against destructive influences of the environment.It is suggested that the metacercariae are guided to their selected infection site by an "engram," or a specific memory, developed and fixed in the course of many generations. New research into parasite behaviour is urged.

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