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.
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 is 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.
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.
|Parasite group||Parasite species||Host group||Host species||Remarks|
|Protista Sporozoa||Mackinnonia tubificis||Annelida Oligochaete||Tubifex tubifex||Destroys gonad|
|Protista Haplosporidia||Urosporidium charletti||Cestoda||Catenotaenia dendritica||"Hypercastrator" (a hyperparasite that castrates the parasite it parasitizes)|
|Platyhelminthes Trematoda||Bucephalus mytili||Mollusca Bivalvia||various species||Destroys gonad, host grows larger|
|Platyhelminthes Cestoda||various species||Pisces Cyprinidae||various species||Destroys gonad, behavioral changes|
|Arthropoda Isopoda||Hemioniscus balani||Arthropoda Cirripedia||various barnacles||Drains ovarian fluid of hermaphrodite, but spares male function|
|Arthropoda Cirripedia||Sacculina||Arthropoda Decapoda||various crabs||Destroys gonad, behavioral changes|
|Arthropoda||Strepsiptera twisted-wing flies||Arthropoda Hymenoptera or Hemiptera||various species||Males feminized, females produce no eggs but instead disperse eggs of parasite|
|Platyhelminthes Cestoda||Flamingolepis liguloides||Arthropoda||Artemia spp.||Destroys gonad, behavioral changes|
|Arthropoda||Crematogaster sjostedti||Plant||Acacia drepanolobium||Ant removes axillary meristems, sterilizing trees.|