Abstract
Dramatic, rapid, population declines of the native North American burrowing shrimp Upogebia pugettensis (Dana, 1852) are associated with intense infestations by the introduced Asian bopyrid isopod parasite, Orthione griffenis Markham, 2004. However, expected host weight losses with increasing parasite weights do not occur, even among apparently castrated females. The prevailing assumption that energetic losses cause host castration have thus remained open to question, and the mechanism(s) resulting in castration and consequent population declines of U. pugettensis have remained unclear. Proposed alternative explanations for these declines, which have been based on a dramatically greater prevalence of O. griffenis among U. pugettensis females, include parasite induced sex change, increased male mortality, and differential tidal exposure of sexes to settling O. griffenis larvae. We examined 508 O. griffenis infestations from 2,014 shrimp collected from 26 stations in 5 Oregon estuaries to test these alternative hypotheses. We expected greater infestation frequencies among females than among males and a close association of O. griffenis infestations with intersex shrimp in the overall population if feminization occurs. We also expected covariation in sex ratio with tide exposure if O. griffenis settlement is sex linked. Instead, we found an overall 1:1.07 sex ratio, a lack of association of intersex U. pugettensis with O. griffenis infestations, and an unchanging sex ratio with tidal exposure, precluding parasite induced sex change, male mortality, or tidal immersion effects on infestations. The most likely mechanism driving U. pugettensis declines thus remains castration due to host energetic losses. This energetic interaction is likely to be resolved quantitatively through controlled experiments and increasingly detailed field surveys over time.
Highlights
The native burrowing mud shrimp Upogebia pugettensis (Dana, 1852) once ranged from Morro Bay, California, to Prince William Sound, Alaska
We found an overall 1:1.07 sex ratio, a lack of association of intersex U. pugettensis with O. griffenis infestations, and an unchanging sex ratio with tidal exposure, precluding parasite induced sex change, male mortality, or tidal immersion effects on infestations
Twenty-six percent of the 2,015 U. pugettensis collected were infested by O. griffenis
Summary
The native burrowing mud shrimp Upogebia pugettensis (Dana, 1852) once ranged from Morro Bay, California, to Prince William Sound, Alaska. The striking variation in U. pugettensis declines between Washington, Oregon, and California after the arrival of O. griffenis (Chapman et al 2012) reflect persistent differences among northeast Pacific estuary ecosystems, that are altering the local hostparasite population dynamics and responses of U. pugettensis populations to O. griffenis invasions. Resolving how those regional differences occur is critical for conservation and requires deep knowledge of how the host-parasite population dynamics occur. The interactions of this non-coevolved marine bopyrid and its new hosts are of particular interest for invasion ecology because native bopyrid isopod/host interactions presumed in other systems may not apply to O. griffenis in North America
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