Abstract
Estuaries exhibit pronounced fluctuations in the abiotic environment, and salinity strongly structures species distribution, physiology, and biotic interactions, including hosts and their parasites. The white-fingered mud crab Rhithropanopeus harrisii is host to two macroparasites: the invasive rhizocephalan barnacle Loxothylacus panopaei and a recently described entoniscid isopod Cryptocancrion brevibrachium . To investigate how estuarine conditions shape this host-parasite system, we quantified the distribution, prevalence, and seasonality of both parasites over 18 months between 2021 and 2022 across nine sites spanning salinities ~ 1–20 in three tributaries of the Chesapeake Bay, USA. We further assessed parasitic impacts on host energetic and reproductive investment using hepatosomatic (HSI) and gonadosomatic (GSI) indices. Out of the 6,530 R. harrisii examined, the crabs only were found at sites with average salinities of ≤ 12.3. Cryptocancrion brevibrachium occurred at all sites with R. harrisii , where prevalence ranged from 0.4% to 39%, and was not influenced by salinity. In contrast, L. panopaei occurred at sites with salinities ≥ 8.7, and prevalence increased with salinity, reaching 80% at higher-salinity sites. Coinfections occurred only where the parasites’ salinity ranges overlapped (≥ 8.7). Prevalence and probability of infection for both parasites were highest between August and October, and R. harrisii between 7 and 8 mm carapace width were most likely to be infected. Both parasites significantly reduced host condition via lowered HSI and GSI, indicating substantial energetic and reproductive suppression. Our results demonstrate that salinity is a key driver of parasite distributions in R. harrisii , with distinctive patterns and impacts of each parasite on their shared host along a salinity gradient.