In short, the root-head turns the crab into a Darwinian cipher, a feeding machine working entirely in the parasite's service. The castrated crab can make no contribution to its own evolutionary history; its "Darwinian fitness" has become flat zero. ...But ever so carefully, for the parasite must maintain the crab in constant and perfect servitude - not draining the host enough to kill this golden goose, but not letting the crab do anything for its own Darwinian benefit.
''leonardo's mountain of clams and the diet of worms'' (1998)