Parasitic castrators take the bulk of their energy from their host's reproductive tissues, often reducing the host's fitness to zero. While the host is alive, and appears well, it is dead from an evolutionary perspective. This strategy is not commonly recognized in the veterinary and medical fields, because large vertebrates generally lack parasitic castrators. In many other systems, however, parasitic castrators may be common: cestodes can castrate their fish and invertebrate hosts, larval trematodes usually castrate their molluskan first intermediate hosts, and parasitic barnacles castrate their crustacean hosts. Parasitic castrators are large with respect to their hosts, but ironically, they are generally best modeled as microparasites because their effects on the host are intensity independent.
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