Tiny snapping shrimp amidst the oysters make a punctuated vibration that sounds to human ears like a snapped finger or a dropped pebble. Collectively they make it possible to hear the reef’s structure as a vertically layered crackling. And, we learned later upon analysis of field recordings, we were also hearing the oysters themselves. As each individual organism filters food from the water passing through its membranes, it expels excess material by rapidly closing its valves, emitting a low-pitched metabolic rumble.