Twilight Transition

About an hour before sunset, the plankton feeders up in the water column signal the approach of evening. With the coming of twilight, they appear uneasy. Still feeding, cardinalfish, manta rays, and sweepers gradually sink lower and lower toward the reef and its waiting shelter, crowding closer to one another as they descend. Although the sun may still seem bright at the beginning of the descent, the angles of the rays as they strike the water create a sharp silhouette. It is a critical period for survival on the reef, which explains why observers witness a heightened sense of nervous activity at this time, not related to feeding.

Schooling migrators-—parrotfish, goatfish, and wrasse—-are by now back in the vicinity of their individual homes. The smallest species take cover first. Larger fish, such as surgeons and parrots, often mill about in groups before taking cover. Fish seem to be defending their sleeping sites. Many fish live out their lives in one defended space, and so, of course sleep there. Damselfish find bedtime a simple routine, and many territorial damsels are among the last to retire. Small blennies, gobies, and hawkfishes quietly slip into their holes. Others, such as butterflyfish and wrasses chase away intruders from their favorite nooks, which usually are just large enough to fit into.

Some fish protect themselves in various ways while sleeping. Triggerfish squeeze their compressed bodies into rocky crevices and lock themselves in for the night by erecting their triggers—strong dorsal spines—so that they cannot be extracted. Some parrotfish excrete transparent mucus envelopes about their bodies at night, thought to block olfactory cues from predators. Parrotfish seem to be among the soundest sleepers on the reef and exactly how parrots arrive at the decision to sleep in a mucus envelope is not fully understood. Other species bury themselves in sand or gravel, having snouts adapted to rapid burrowing.

Just as dawn has a brief period when nearly all the reef inhabitants seem absent, so does twilight. Within 10 minutes after sunset, most daylight fishes are hidden from view. Few fish stir and those that do seem to do so quietly. During this time surgeonfish, butterflyfish, and moorish idols, already dimming their colors, and a few damselfishes are still about, but close to their shelters. As they disappear, small cardinalfish begin to emerge from their caves. Next to appear are squirrelfish and the grunts whose colors change to faded stripes and blotchy spots that disguise them for their nocturnal activities.

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