A comparative study of epibionts and age structure of two east coast populations of American horseshoe crabs

Epibionts were identified from adult horseshoe crabs from Delaware Bay, New Jersey and Jamaica Bay, New York during the peak of the spawning season from 1997–2003 and 2012–2021. Acorn barnacles, bryozoans, slipper shells (Crepidula fornicata), polychaetes (Sabellaria vulgaris), and mussels (Mytilus edulis) were the most frequently occurring epibionts; barnacles, Crepidula, and Mytilus were more common in the Jamaica Bay population, whereas bryozoans and Sabellaria were more common in Delaware Bay. However, epibiont species composition was insufficiently distinctive between the two populations to be useful as a biological tag. Four previously unreported epibionts were found: the urochordate Molgula manhattensis, sponge Clathria (Microciona) prolifera, alga Gracilaria sp., and an unidentified chiton. Males were more heavily encrusted than females, which may be related to their mating behavior, in which the males cover a portion of the female's opisthosoma while they are attached to her. Jamaica Bay crabs had significantly more of their carapace encrusted than crabs from Delaware Bay, and the carapace condition index, an indicator of post-molt age, was significantly worse for Jamaica Bay animals. The preponderance of older aged adults in Jamaica Bay may be indicative of poor recruitment into this population and/or emigration of younger adults from Jamaica Bay.