A parasitic isopod inside a shrimp's gill chamber, seen from outside as a bulge in the carapace.
What is seen is not the isopod but the shape of it. One side of a shrimp’s carapace is pushed out into a smooth rounded swelling, often pale against the animal’s own colour, and in a clear-bodied host the parasite shows through as a flattened outline with a dark margin. The animal inside is a female isopod, five to twenty millimetres long, whitish or yellow, asymmetrical and twisted to fit the gill chamber, with a dwarf male a fraction of her size attached to her underside. Neither leaves the host once settled.
The infestation is common and is easiest to notice in the transparent and pale shrimps that a muck site is full of: cleaner shrimps, anemone shrimps, palaemonids on hosts of their own, and hingebeak shrimps under ledges. Hermit crabs and small crabs carry related parasites in the same position or under the abdomen. The isopod feeds on the host’s blood. Infested animals usually survive and continue to feed, but commonly do not reproduce, and they do not shed the parasite at a moult because the chamber is recolonised as the new shell hardens.
The life cycle runs through a copepod as an intermediate host before the larva reaches a decapod, which is why infestation rates vary a great deal between sites and seasons. Identification stops at the family in almost every case, since the characters are on an animal enclosed inside another one, and the record from a dive is properly of an infested host rather than of a named parasite. It is worth logging: a photograph taken for the shrimp often turns out to contain this as well.
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.