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Rothschild (1941b,c) proposed increased host growth as the selective advantage of parasitic castration for trematodes castrating their first intermediate host, Peringia ulvae. She demonstrated a positive correlation between frequency of parasitism and host size, which led her to postulate that parasitized hosts grew faster and to a larger size than non-parasitized ones. This, she said, should result in increased fitness of the parasite, since the number of cercariae produced by a snail is proportionate to its size. However, an alternative hypothesis to that of increased host growth has been offered by Hartnoll (Hartnoll, 1967), to explain crustacean castration. He postulated that the typical sacculinization effects are produced by the parasite, which benefits from the resulting physiological and behavioral changes. These changes apparently result in all parasitized individuals, regardless of sex, behaving like ovigerous females; such individuals do not reproduce while they are parasitized, but they provide protection and ventilation to the parasites which occupy the external location usually 335

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