Is there a lesser of two evils? Seasonality and energetic impacts of castrating parasites on an estuarine mud crab along a salinity gradient

Estuarine organisms can evade biotic threats by utilizing low salinity extremes as refuge from natural enemies; however, they must also withstand osmotic stress. We investigated the spatiotemporal distribution of a mud crab host ( Rhithropanopeus harrisii ), and two castrating endoparasites (rhizocephalan Loxothylacus panopaei and entoniscid isopod Cryptocancrion brevibrachium ) in the Chesapeake Bay to elucidate if R. harrisii can utilize an environmental refuge against parasitic castration. Every two months over an 18-month period, we collected and dissected R. harrisii from nine Chesapeake Bay populations along three rivers with a salinity gradient of 1.38–20.14 ppt to capture host-parasite seasonality and quantify individual and reproductive stress via hepatosomatic and gonadosomatic (GSI) indices, respectively (N = 6530 R. harrisii total). Cryptocancrion brevibrachium occurred throughout R. harrisii ’s sampled salinity range (1.38–12.33 ppt), while L. panopaei occurred > 8.70 ppt. The prevalence of C. brevibrachium varied considerably across salinities (maximum 50%), while up to 82% of R. harrisii were infected with L. panopaei at higher salinities. Prevalence and probability of infection for both parasites was highest in spring and summer months, and R. harrisii between 7–8 mm carapace width were most likely to be infected. Based on GSI, L. panopaei castrated both female and male hosts, while C. brevibrachium only castrated female hosts. Thus, although R. harrisii can escape castration from L. panopaei by utilizing low salinity regions of the Chesapeake Bay, female crabs are still at risk from castration by C. brevibrachium , potentially limiting the crab’s ability to use low-salinity refuges to fully escape parasitic castrators.