Sponge-dwelling snapping shrimps are often more heavily parasitized by bopyrid isopods than their free-living relatives, but little is known about how parasitism influences the behavior and morphology of these shrimps. In this study, we measured parasitism rates in the eusocial snapping shrimp Synalpheus elizabethae and tested whether parasitism by bopyrid isopods (Bopyrione sp.) affects shrimp activity levels and morphology. Using field surveys, we found that S. elizabethae suffer higher

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... tism rates when inhabiting the sponge Lissodendoryx colombiensis than do co-occurring pair-forming congeners (S. yano and S. dardeaui) in the same host. In laboratory behavioral assays, parasitized S. elizabethae showed 50% lower activity levels than unparasitized colony members, suggesting that infection by bopyrids decreases shrimp activity. Finally, parasitism influenced shrimp morphology; parasitized S. elizabethae individuals were larger, but had relatively smaller major chelae (25% reduction) than unparasitized individuals. Parasites comprised ~5% of the entire body weight of an individual shrimp. Together these data suggest that parasitism by bopyrid isopods may incur significant energetic costs for commensal sponge-dwelling shrimps such as Synalpheus, and may be especially severe for social species such as S. elizabethae.