Grass shrimp parasites use complimentary life histories to avoid a conflict of interest

A conflict of interest occurs when parasites manipulate the behavior of their host in contradictory ways. In grass shrimp (Palaemonetes pugio), trematode parasites cause the shrimp to be more active than usual around predators, while bopyrid isopod parasites elicit the opposite response. Since these parasites are altering the host’s behavior in opposing directions, a conflict of interest should occur in doubly infected shrimp. Natural selection should favor attempts to resolve this conflict through avoidance, killing, or sabotage. In a field survey of shrimp populations in four tidal creeks in the Cape Fear River, we found a significant negative association between the two parasites. Parasite abundance was negatively correlated in differently sized hosts, suggesting avoidance as a mechanism. Subsequent mortality experiments showed no evidence of early death of doubly infected hosts. In behavior trials, doubly infected shrimp did not show significantly different behavior from other infection statuses, suggesting that neither parasite sabotages the manipulation of the other. Taken together, our results suggest that rather than sabotaging one another directly, bopyrid and trematode parasites reduce conflict by preferentially infecting differently sized hosts. Because grass shrimp exist at high biomass in salt marsh ecosystems and are infected at high prevalence, our findings have implications for ecosystem structure and function.