On this rainy day, I decided to fly down to the Florida Keys and go snorkeling for pinnotherid crabs in the nearshore Halemida beds (i.e., sort through a box of old slides).
Pinnotherid crabs (pea crabs) are ectosymbionts of a variety of marine invertebrates. The genera Dissodactylus and Clypeasterophilus live on echinoderms of the Orders Clypeasteroida and Spatangoida. If you look closely on this sea biscuit, Clypeaster rosaceus, you may see one or two small crabs, Clypeasterophilus rugatus (formerly Dissodactylus rugatus). Their size and colouration mimic the bits of Halemida (a calcareous alga) that stick to the urchin.
Here they are flushed out in the open. The brown-banded legs help break up the "crab" outline, which works well on an urchin, less well in a petri dish.
Here is a related species, Dissodactylus latus, which spends much of its time hunkered down in the food grooves on the undersides of sand dollars of the family Mellitidae. There are about nine species of Dissodactylus and four species of Clypeasterophilus. Both crabs and hosts are distributed throughout subtropical Atlantic and Pacific American waters.
The degrees of commensalism/parasitism between crabs and echinoderm hosts are not well understood, and seem to differ among crab species, but at least some crabs are known to consume echinoderm spines, as seen in the digestive tract of the crab in the radiograph above (Clypeasterophilus juvenilis, an Atlantic species, host Clypeaster sp.).
Obviously these little crustaceans require more research. Time to book a flight. Ahh, Florida.