Parasitoidal Crustaceans
The subphylum Crustacea includes a surprising range of parasitoidal species and strategies. As with many other parasitoids, the killing of the host often is incidental. For example, in the family Ergasilidae, the "gill lice", most adult females live as parasites in the gills of fish. The harm they do the host is incidental to the parasitism, but it often is fatal or at least debilitates the fish so badly as to prevent breeding.
A particularly startling genus of Cirripedia, or barnacles is Sacculina. It literally injects itself into a crab of a suitable species and by complex processes converts itself into an egg-laying bag. In the process it disrupts the reproductive system of the host, an act of parasitic castration that qualifies it for classification as a parasitoid rather than just a parasite.
Those examples are just a few of many among the Crustacea.
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