The parasite has somehow evolved to turn off the host's defenses, presumably by disarming the crab's immune response with some chemical trickery that fools the host into accepting the parasite as part of itself.... The adult parasite castrates the host, not by directly eating the gonadal tissue, but by some unknown mechanism probably involving penetration of the interna's roots around and into the crab's nervous system.

Triumph of the Root-Heads, p. 367 - Leonardo's Mountain of Clams and the Diet of Worms (1998)