Navigational behaviour of the sea slug Hermissenda crassicornis in natural wave-affected flow conditions

Navigation behaviour of aquatic animals has often been studied in unidirectional flow. Yet many aquatic habitats have highly variable flow directions, particularly those affected by waves. We used field measurements of natural flows and underwater video to survey how the nudibranch Hermissenda crassicornis navigates in wave-affected flow conditions. We compared navigation tracks relative to feeding events in low and high prey densities and low and high flow direction variability. Regardless of flow direction variability or prey density, we found evidence of chemical and flow cue-based navigation towards prey. Evidence of flow-based navigation was only significant when flow direction was less variable and calculated over time scales of wave-affected flow rather than tidal flow. Slugs oriented parallel to flow (either upstream or downstream) before feeding, possibly orienting to flow oscillations from wave action when prey is temporarily upstream (and thus using a form of chemical-gated rheotaxis). In higher flow direction variability, slug track metrics still suggested a navigational strategy was used to find prey, but the apparent absence of any orientation to flow suggested the slugs use a different strategy (possibly chemotaxis) in conditions that could produce highly mixed chemical cues. Finally, when slugs were distant from feeding, they moved cross-stream, which may increase the chances of intercepting prey chemical cues. Collectively, these findings possibly provide further evidence of context-dependent switching of gastropod navigational strategies. In addition, they also suggest that chemical-gated rheotaxis is a viable navigation strategy in habitats with either unidirectional or wave-affected flow.