Except for the opening and closing of their valves, the sessile adult oyster shows no externally observable behavior. Valves open for respiration, feeding and spawning. Valves close when oysters are exposed to air at low tide which helps prevent their desiccation. Vibrations and shadows produced by potential predators induce rapid shell closure. The more active larvae display a greater repertoire of behavior. Larvae are ciliated and capable of swimming. Larval behavior changes with age. As larvae age, they swim less and congregate near the bottom.