Parasitic castrator

Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part. For example, Hemioniscus balani, a parasitic castrator of hermaphroditic barnacles, feeds on ovarian fluid, so that its host loses female reproductive ability but still can function as a male. This would be a case of direct parasitic castration (feeding on host gonads). Indirect strategies are also seen such as diverting host energy from gonad development or secreting castrating hormones.

The parasitic castration strategy is used by some larval trematode parasites of snails and some isopod and barnacle parasites of crustaceans. For example, 18 species of trematodes are known to parasitically castrate the California horn snail, Cerithidea californica.

A parasite that ends the reproductive life of its host theoretically liberates a significant fraction of the host's resources, which can now be used to benefit the parasite. Lafferty points out that the fraction of intact host energy spent on reproduction includes not just gonads and gametes but also secondary sexual characteristics, mate-seeking behavior, competition, and care for offspring. Poulin suggests that prolonged host life may also result from parasitic castration, benefiting the parasite.

The evolutionary pressure of parasitic castrators on a potential host population is in the direction of resistance to being infected rather than on post-infection recovery. Once fertility has been lost or reduced, the host gains much less evolutionary advantage by surviving the parasite than it would have by avoiding it.

Once the host's reproductive ability is lost, any future changes in host behavior cannot directly benefit the host's reproductive fitness but may benefit the parasite's. Several cases have been described where infection with a parasitic castrator causes the host to change its behavior in ways that benefit the parasite.

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