Dine-in or take-away? Scavenging strategies in predaceous diving beetles at different water depths

Predaceous diving beetle adults scavenge for carrion in freshwater habitats. We identified two scavenging strategies used by adult Dytiscus sinensis Feng (Coleoptera: Dytiscidae) to locate and consume food at the bottom of water when needing to surface for air store renewal: dine-in then relocate the food (dine-in strategy) and carry the food to the surface (take-away strategy). In 5-cm-deep water, a higher proportion of individuals successfully located food within an 8-min test window than in 25-cm-deep water. Their latencies to locating food did not differ among depths. Dytiscus sinensis adults preferred the take-away strategy in 5-cm-deep water and a higher proportion of them adopted the dine-in strategy in 25-cm-deep water. Further, a 10-cm-deep test showed that the success rate or efficiency of relocating food did not increase with the experience of locating it the first time. This might explain why D. sinensis preferred the take-away strategy in shallower water, in which it had lower risks of losing the food. The present study revealed a direct bottom-up benefit for dytiscids foraging in shallower water where finding and securing food were easier. Our findings demonstrate that the surfacing requirement of dytiscid adults was reflected in foraging strategies with depth-dependent preferences.