A CLOSER LOOK AT THE RELATIONSHIP BETWEEN A NEW PINNOTHEROID PEA CRAB FROM THE MIOCENE ST. MARYS FORMATION, AND ITS PREFERRED HOST 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 has been previously described to exhibit preferential symbiosis with the surf clam Mactrodesma subponderosa. The unique paleoecological relationship between the crab and its host clam acts as a taphonomic filter, resulting in exceptional preservation of these rare pea crabs. However, the nature of the symbiotic relationship between the crab and clam, whether it be mutualism, commensalism, or parasitism is unclear. It is apparent that the crab receives the benefit of protection and a food source from the clam, similar to modern pinnotheroids; however, it is not apparent if the clam receives any benefit or detriment from the crab. To determine if the M. subponderosa host clams were negatively impacted by the presence of the pea crabs, several measurements, including shell height the maximum distance from the umbo to the ventral side and shell thickness, were taken from clams with and without pea crabs present. The results of these measurements showed that M. subponderosa clams found in association with the pea crabs were smaller, on average approximately 56mm in height, compared to 71mm for their contemporaries that did not contain pea crabs. In addition, the shells of M. subponderosa clams with pea crabs were also approximately 1mm thinner than their contemporaries that lacked pea crabs. The smaller and thinner shells of the M. subponderosa clams found in association with pea crabs compared to their contemporaries that lacked pea crabs is possible evidence that the clams were negatively impacted by the presence of the crab. This indicates that the relationship between the pea crabs and M. subponderosa clams was parasitism, as the crabs received benefit from its relationship to the clam, while the clam was negatively impacted by the presence of the crab. Future research will explore other metrics to document this relationship and attempt to account for other factors such as ontogeny of the clam.