A NEW PINNOTHEROID PEA CRAB (DECAPODA, BRACHYURA, PINNOTHEROIDEA) FROM THE MIOCENE ST. MARYS FORMATION EXHIBITING PREFERENTIAL SYMBIOSIS WITH THE SURF CLAM MACTRODESMA SUBPONDEROSA

A new fossil pinnotheroid crab from the late Miocene (Tortonian) Little Cove Point Member of the St. Marys Formation of Maryland, marking the first occurrence of a pinnotheroid from the Chesapeake Group, is herein described on the basis of several well preserved specimens. They are only known to occur in a highly-localized, finely-bedded lens, which contains other fragile, well-preserved fossils, including the spider crab Libinia amplissimus, an undescribed species of hermit crab, and the brittle star Ophiura marylandica. The pinnotheroids exhibit remarkable preservation, many of which are fully articulated, and at least one specimen retains preserved pleopods and gills. These crabs also appear to exhibit preferential association with the surf clam Mactrodesma subponderosa, with all but one being found in association with its host clam, and none being found in association with any other species. The exceptional preservation of these pea crabs can be attributed in part to this symbiotic relationship, as the host clam serves to protect the crabs from being destroyed by taphonomic processes. Modern pea crabs, Zaops ostreus, found in the Chesapeake Bay, preferentially inhabit oyster shells, Crassostrea virginica. Although modern pea crabs are often regarded as parasites, since they consume the food obtained by their host oyster, the oysters are seemingly unaffected as the food is adequately abundant to feed both the host and the symbiont. Thus, the symbiotic relationship between the two may represent commensalism.