How disc-shaped stingrays create sediment flow for effective burying

Disc-shaped stingrays are dorsoventrally flattened fishes with enlarged and flexible pectoral fins that are used to power locomotor behaviours along the floor of marine and freshwater ecosystems, making them an intriguing model system for advancing our understanding of aquatic movement along the substrate. These fishes commonly bury into the substrate, possibly to hide from predators and to station hold in high current flow. To bury, the animal displaces itself downwards and covers itself with sediment, yet we do not fully understand how the flows that move sediment are generated and propagated to affect burying. Burial events of the motoro stingray were analyzed via video. Rather than digging, stingrays functioned like a piston pump with flapping fins, whereby the head repeatedly pumped up and down to fluidize the sediment, and the fins folded up and over to direct a vortex of sediment onto the dorsal side of the fish. An increase in head pump and finbeat displacement and speed induced greater sediment coverage of the dorsal surface. Sediment coverage ranged from 60-97%, and the eyes and the barbed stinger at the tip of the tail always remained exposed. Accordingly, we argue that a disc shape can function to promote effective control of sediment displacement during burying.