In situ swimming and feeding behavior of eight co-occurring hydromedusae

Abstract

The morphology, swimming kinematics and prey selection of hydromedusae have been shown to be closely related to hydromedusan foraging mode. In order to confirm this link, we examined the in situ foraging behavior of hydromedusae by video-recording and quantifying the in situ swimming activity and tentacle-extension behavior of 8 co-occurring hydromedusae from the waters surrounding Friday Harbor, Washington. The cruising-predatory medusa Aequorea victoria, Mitrocoma cellularia, Phialidium gregarium, and Eutonina indicans spent a greater proportion of their time swimming (on average from 74.1 to 92.1%) than the ambush-predatory medusae Aglantha digitale, Sarsia sp., Leuckartiara sp., and Stomotoca atra (on average from 19.5 to 47.0%). In addition, the cruising-predatory medusae were observed with their tentacles extended almost continuously, regardless of whether they were swimming or drifting. In contrast, the ambush-predatory medusae drifted with their tentacles extended but contracted their tentacles when swimming. These patterns indicate that cruising-predatory medusa can feed while either swimming or drifting, whereas the ambush-predatory medusae feed while drifting but not while swimming. Thus, swimming plays a distinctly different role within the behavioral repertoire of each of these medusa foraging modes. Furthermore, the swimming activity and tentacle-extension behavior we observed were consistent with strategies that optimized feeding by each of the predator types.