Bat rays feed on clams, worms and crustaceans that live buried in mud and sand. To reach them the ray beats its pectoral fins against the bottom and jets water from its mouth, hollowing out a depression that can be deep enough to expose prey a hand's depth down. The pits remain visible on tidal flats after the ray has moved on.
The jaws are built for what comes next. The teeth are hexagonal plates locked edge to edge into a flat pavement in each jaw, forming a mill rather than a set of points. As the front row wears, it is shed and new rows move forward from behind, so the grinding surface is continually renewed.
The digging has a side effect worth noting: excavation exposes buried invertebrates that other animals then take, so foraging bat rays create feeding opportunities for fishes and birds around them.