A barnacle that lives inside a crab and shows only as a soft sac under the folded abdomen.
A smooth, soft, yellow-brown or cream sac bulging from under the abdominal flap on the underside of a crab, up to about three centimetres across, with no plates, no limbs and no opening that is obvious from outside. Nothing about it resembles the plated cone of an acorn barnacle, and a crab carrying one is usually walking and feeding normally. The sac is only the reproductive part of the animal. The rest is a system of fine roots spread through the host’s body, and it has no gut of its own.
The group is placed among the barnacles on the evidence of the larvae, which are the nauplii and cyprids the rest of the class also produces. A female cyprid settles on a crab, injects a mass of cells through the cuticle, and the parasite grows internally before pushing the sac out under the abdomen; a male cyprid then settles on the sac. The effects on the host are consistent and visible. An infected crab stops moulting and stops growing, it does not reproduce, and an infected male develops the broad abdominal flap of a female and tends the parasite’s sac with the same behaviour it would give an egg mass of its own.
On muck ground it turns up on swimming crabs, on hairy rubble crabs, on porcelain crabs and on hermit crabs, and it is found by looking at the underside of a crab rather than at the top. Identification stops well above species level, since the externa carries few characters and the host is often the only clue. Recorded across the depth range the hosts use.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.