Long-term field survey reveals salinity as key determinant of estuarine host infection by a non-native castrating parasite

Long-term ecological studies are essential for understanding community structure and change. In estuaries, dynamic environmental gradients drive spatiotemporal shifts in populations through complex abiotic and biotic interactions. Biological invasions add novel relationships, including those between hosts and parasites. Estuarine environments often provide low-salinity refugia that reduce parasite pressure, slowing host resistance evolution by maintaining a reservoir of susceptible individuals. When naive hosts emerge from these refugia, they may support host recovery and supply new targets for parasites, diluting resistance in the population. Using a 12-year field survey across a salinity gradient in Chesapeake Bay, MD, USA, we examined how environmental conditions (salinity, temperature) and host demographics (% gravid females, total abundance and size) correlate with infection prevalence of the introduced castrating parasite Loxothylacus panopaei in native, white-fingered mud crabs (Rhithropanopeus harrisii). Infection prevalence varied from 0 to 75.9% (n = 102 632) and peaked with elevated salinity, temperature and host reproduction. Larger crabs (>9 mm carapace width) showed higher infection rates, especially under high salinity and temperature. Salinity was the most consistent predictor of infection, with low salinity (<10 ppt) providing refuge. These findings show how fluctuating environmental conditions structure parasite prevalence across scales, with implications for host population dynamics under climate change. This article is part of the theme issue 'Managing infectious marine diseases in wild populations'.