Coral Reefs at Dawn and Dusk

Dawn and dusk are incredibly active times on coral reefs. Together, these twilight, or crepuscular, periods take up only about an hour of each day, which amounts to only about 4% of the 24-hour cycle, but they are extremely important to all reef life. This is a time of transition between diurnal and nocturnal reef species and can be a highly dangerous period; a predator's eyes are well suited to the twilight. Many reef fishes spawn at dusk, thereby exposing themselves to great risk, but at the same time giving their eggs the best chance of reaching safety.

What changes happen during dawn and dusk?

The daily twilight periods are when fish and invertebrates emerge or retreat to their refuges. Diurnal fish stir from their overnight resting places and swarm out onto the reef at dawn, returning to these shelters at dusk, while nocturnal fish follow the opposite pattern. At dusk, the smaller species of fish migrate down to the reef first. Some species migrate from their daytime feeding grounds to their night shelters along predictable migration routes, which can cover surprisingly long distances. In the Red Sea, brown surgeonfish (Aconthurus nigrofuscus) can travel over a half mile between these sites, using learned landmarks along the way to navigate. Often, fish return to exactly the same place to rest night after night.

How do animals know when to emerge or hide?

At dusk, light levels decrease from around 100 lux just before sunset to 0.01 lux only 30 minutes later Although the fish use these light cues to prepare to switch their activity patterns, they are also guided by their internal clocks and start to make the transition well ahead of actual dawn and dusk. If fish in an aquarium are switched from a consistent night and day regime to one where the light is held constant over time, they will continue to switch from night to day behavior and vice versa at the correct time for quite a few days. After this, the absence of anything to reset their internal clocks will cause the timings to go awry.

What it meant by the "quiet time"?

Between one set of fishes going into shelter and their counterparts emerging to feed, there is a period of about 20 minutes when the reef seems absolutely devoid of all life. This slightly eerie period is sometimes referred to as the "quiet time." One of the reasons for this dead zone in the reef's daily transitional period is the abundance of predators at this time. Dawn and dusk are when hunters are at their most effective, because their eyes are attuned to the half-light and this gives them the edge. Predators capture a substantial proportion of their daily food during the twilight periods. Many predators, including groupers and snappers, keep close to the reef, maintaining a watch on the waters above. Even at twilight their prey are silhouetted against the surface, while the prey themselves find it difficuft to see the danger lurking against the growing gloom of the reef below. If a fish does swim above it a predator can launch a surprise attack from below and capture its quarry before it can react.

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