Hijacking the Host: Parasitic Manipulation of Estuarine Mud Crab Sexual Morphology

Parasites can alter the morphology of their hosts to enhance their own survival and reproductive success, manipulating host structures to better accommodate their life cycles. Parasitic barnacles (Rhizocephalans) can alter their crab hosts’ morphology through male feminization. We examined if male Chesapeake Bay mud crabs, Rhithropanopeus harrisii, increased their apron width when infected by the Rhizocephalan Loxothylacus panopaei. Crabs were dissected to confirm infection (N=495), and an image processing software measured the apron area and calculated the relative abdominal width (RAW) (e.g., ratio between the widths of apron segments 3 and 6). A low RAW score represents a broad apron, typically seen in females, while a high RAW score represents a narrow apron, typically seen in males. Infected males showed a significant increase in apron area (e.g., a decrease in RAW score) compared to uninfected males. Additionally, as the stage of infection progressed from virgin infection to sexually mature parasite, RAW scores in male crabs significantly decreased. In females, the RAW score and apron area did not change after infection. The feminization of the male apron may be correlated to the parasite’s need for protection of its egg case, which a male crab’s narrow apron does not provide.