FAMILY : SACCULINIDAE?
Sacculina sp?

A parasitic species which chemically castrates it's crab host. It changes the
host's body and it's habits to the point where the crab will be infertile and
care for the parasite's eggs as if they were it's own. Contaminated male crabs
can even be controlled to act as females and also care for the eggs.

Below is the externa, or egg-sac of the parasite, situated in the area where
the crab would normally carry it's own eggs.