Northward expansion of the rhizocephalan Loxothylacus panopaei (Gissler, 1884) in the northwest Atlantic

This report describes the first occurrence of the rhizocephalan Loxothylacus panopaei (Gissler, 1884) in North America, north of Virginia and the Chesapeake Bay. Of the panopeid mud crabs suitable for infection by L. panopaei (Panopeus herbstii, Rhithropanopeus harrisii, etc.), the parasite only infected Eurypanopeus depressus. In particular, L. panopaei disproportionately infected small E. depressus (20.5% of crabs with carapace width (CW) of 5-10 mm and 11.8% of crabs with 10-15 mm CW) but did not infect any crabs over 15 mm CW. Analysis of genetic data (COI sequences) suggests this Long Island Sound population could have derived from a range expansion from the mid-Atlantic, where the parasite is invasive. Given the parasite's disjunct distribution, human-mediated dispersal is the most likely vector for this northward range expansion.