Pea Crabs!
Pea crabs are parasitic and commensal crabs of the family, Pinnotheridae.
They live with a wide variety of hosts: in the mantle cavity of Bivalves, in the burrows of worms, in the cloaca of sea cucumbers, on the tests of echinoids, in the atria of Ascidians and with many other invertebrate hosts.
I have made hypotheses about the phylogeny of the Pinnotheridae, based on host affiliations. This project is on hold due to lack of specimens.