Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part, to its own benefit. This is one of six major strategies within parasitism.
Evolutionary strategy
The parasitic castration strategy, which results in the reproductive death of the host, can be compared with the parasitoid strategy, which results in the host's death. Both parasitoids and parasitic castrators tend to be similar to their host in size, whereas most non-castrating parasites are orders of magnitude smaller than the host. In both strategies, an infected host is much less hospitable to new parasites than an uninfected one.
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. 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. Infected hosts may have a different appearance, lacking said sex characteristics and sometimes even devoting more energy to growth, resulting in gigantism. The evolutionary parasitologist Robert Poulin suggests that parasitic castration may result in prolonged host life, benefiting the parasite.
Parasitic castration may be direct, as in Hemioniscus balani, a parasite of hermaphroditic barnacles which feeds on ovarian fluid, so that its host loses female reproductive ability but still can function as a male. Parasitic castration may equally be indirect, as when a parasite diverts host energy from developing gonads or secretes 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 parasitically castrate the California horn snail, Cerithidea californica.
Certain other effects of a parasite on its host may appear similar to parasitic castration, such as the host's immune system diverting energy from reproduction in response to numerous parasites that singly would have no impact on fecundity or fertility, or parasitoids that may consume reproductive organs first.
Mechanisms
Parasitic castration occurs via physiological and/or developmental mechanisms that destroy or impair the reproduction capabilities of the host organisms. Many parasites, such as Cercaria batillariae, completely destroy the gonads of their host. Other parasites may interfere with the normal development of gonads/reproductive organs.
There are also indirect mechanisms, which divert the energy usually dedicated to reproduction, to the parasite's growth and reproduction. The redirection of nutrients will hinder the hosts reproduction, even if their gonads are still intact. Parasites may also alter hormones, in turn affecting reproductive health/function. There may also be a combination of both gonad replacement and redirection.
Taxonomic range
Parasite group Parasite species Host group Host species RemarksProtista Sporozoa Mackinnonia tubificisAnnelida Oligochaete Tubifex tubifexDestroys gonadProtista Haplosporidia Urosporidium charlettiCestoda Catenotaenia dendritica"Hypercastrator" (a hyperparasite that castrates the parasite it parasitizes)Platyhelminthes Trematoda Bucephalus mytiliMollusca Bivalvia or Gastropoda various speciesDestroys gonad, host grows largerPlatyhelminthes Cestoda various speciesPisces Cyprinidae various speciesDestroys gonad, behavioral changesArthropoda Isopoda Hemioniscus balaniArthropoda Cirripedia various barnaclesDrains ovarian fluid of hermaphrodite, but spares male functionArthropoda Cirripedia SacculinaArthropoda Decapoda various crabsAtrophies gonads, behavioral changes, partially feminizes males and stops regeneration of crab legsArthropoda Strepsiptera twisted-wing fliesArthropoda Hymenoptera or Hemiptera various speciesMales feminized, females produce no eggs but instead disperse eggs of parasite Platyhelminthes Cestoda Flamingolepis liguloidesArthropoda Artemia spp.Destroys gonads, behavioral changes Arthropoda HymenopteraCrematogaster sjostedtiPlantAcacia drepanolobiumAnt removes axillary meristems, sterilizing trees.
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