Prevalence of a parasitic isopod, Probopyrus spp, on Palaemonid shrimp along a marsh estuarine gradient

Background/Question/Methods

The presence of parasites can negatively affect host fitness by causing reduced resource intake rates, cessation of reproduction, and even death. These effects have important consequences for host populations, species interactions, and food web dynamics. Branchial parasites of crustaceans, such as Probopyrus spp, have been shown to reduce host fitness by reducing oxygen intake and by castrating or feminizing males in the population.

In the Everglades, grass shrimp are an important resource for both fish and avian consumers. These decapod crustaceans are widely distributed throughout the freshwater, estuarine and marine regions of the ecosystem. Previous work has shown that five species of palaemonid shrimp segregate across freshwater marshes and estuarine mangrove regions: Palaemonetes paludosus, P. pugio, P. intermedius, Palaemon floridanus and Leander paulensis, and that their relative abundance along the transect varies seasonally, presumably in relation to freshwater inflow and salinity regimes.

In this study, we examined the  parasitic prevalence of Probopyrus spp on these five species of shrimp in the Shark Slough-Shark River, the main drainage of the southern Everglades. We collected shrimp from three marsh sites, five sites located on the marsh-mangrove ecotone, and three estuarine sites, using unbaited minnow traps deployed overnight. We repeated sampling in the wet, early and late dry seasons.

Results/Conclusions

Interestingly, of the five species collected, the riverine grass shrimp (P. paludosus) was the only one parasitized by Probopyrus spp. We also detected a marked pattern of incidence of the parasite in P. paludosus, where the majority of  shrimp collected at ecotonal sites were infected, but parasitism dropped to zero at the freshwater marsh sites. The mechanism for this pattern is unknown, possible related to the presence or abundance of intermediate hosts or to limiting abiotic conditions. This high parasitic prevalence should have a negative effect on populations of P. paludosus at the ecotone, and may act to limit its distribution. Fluctuation in shrimp populations due to parasitism may also have negative cascading effects on other trophic levels, and overall ecosystem structure and function; thus there is a need to better understand parasite-host dynamics, and how parasitism may be affected by hydrological disturbance.