Effects of a newly invasive parasite on the burrowing mud shrimp, a widespread ecosystem engineer

Abstract

The burrowing mud shrimp Upogebia pugettensis is an important ecosystem engineer in Pacific bays and estuaries from Alaska to Baja California. Recently, mud shrimp populations throughout this range have declined precipitously. This decline has followed invasion by a bopyrid isopod, Orthione griffenis, which parasitizes and removes hemolymph from the shrimp's gill cham- bers. However, a causal link between this invasive parasite and the decline of its host has yet to be demonstrated. I used observed allometric relationships between the parasite and its host, spatial pat- terns of parasite prevalence across host demographics, and simulation modeling to explore potential mechanisms leading to this causal link. Results here, considered with other work, suggest that this parasite may hamper host reproduction by increasing metabolic demands. First, shrimp are more likely to become infected with increasing exposure to parasite-laden water, which varies both with the proportion of time that tides cover individual shrimp burrows and with each shrimp's size-specific pumping rate of water through its burrow. Second, parasites impose a metabolic burden on their host that is proportional to parasite size, which itself increases with time since infection. Third, increasing metabolic costs in turn reduce reproductive capacity, potentially through feminization of male hosts. Fourth, increased metabolic costs of infection are most detrimental when feeding time is limited, which occurs in higher intertidal areas. These conclusions should be taken as testable hypotheses that, if examined, will elucidate this host-parasite interaction and clarify if this new invasive parasite is capable of reducing and eliminating this important ecosystem engineer.