These are the eyes of the twilight hunters. During the day these fish shun the light of the bright sun, hiding in the dark holes, and crevices of the reef. But as dusk approaches, they begin to emerge, and they are ready to eat. To hunt successfully, these fish have little use for color vision. And for this their eyes are perfectly adapted.
Predation is only one of the reefs behavioral changes that occur at dusk.
With fading light, and the exit of voracious daytime feeders, a host of invertebrates begin to emerge from the secluded safety of the reef. Sea urchins, crinoids, and other echinoderms, rise from the protective nooks and crannies of the coral and begin their nocturnal habits. Crabs, shrimp, lobster, and other crustaceans, cloistered in coral crevices by day, also come into the open, where they are easily visible to twilight snorkelers. And without the beaks of marauding parrotfish, the coral animals, retracted by day, extend their tentacles to capture small planktonic prey. The predominant factor, obviously, is that snorkelers enter the water late in the day with ample light to see, but exit the water in what can be nearly complete darkness.