The Coral Reef During the Day

The coral reef during the day is a dazzling collage of colors and activity. Huge numbers of fish and invertebrates bustle around the reef in a way reminiscent of a human city. In the tropics, day and night are of similar length, so before retiring to the relative safety of their sleeping areas, reef inhabitants have a great deal to pack into 12 hours: searching for food, fighting, dodging predators, and mating.

The different types of feeders gather into specific areas; grazers and invertebrate hunters hug the substrate, while plankton-feeders head into the current to reap the drifting prey particles. But not all reef fish are active during the day; nocturnal species await the coming dusk and the start of their “day.”

Why are most reef fish active during the day?

During daylight hours, the reef is flooded with bright sunlight. Fish with good eyesight can exploit these conditions to seek out invertebrate food or harvest algae when it is at its most nutritious. They are also able to spot approaching danger sooner; it can be hard for predators of reef fishes to sneak up on their quarry in these conditions. The importance of vision to diurnal fish is reflected by the bright, so-called “poster” colors of many reef fishes, which promote species recognition and carry information variously for territorial rivals, schoolmates and breeding partners. Diurnal coral reef fishes have excellent color vision and their eyes are protected from excessive brightness by a layer of dark pigment called melanin.

When are reef invertebrates most active?

Many reef invertebrates, including corals, worms, and plankton, are preyed upon by fish. For this reason, they tend to lie low throughout the day, waiting for night to fall before exposing vulnerable body parts in order to feed. Even sea urchins, with their forbidding spines, tend to seek shelter during the day. By retreating into tight holes and crevices on the reef, they protect their undersides, while presenting a bristling array of spines to any curious fish. Invertebrates that want to be active by day must be well protected from hungry fishes; the highly poisonous sea slugs are a good example.

At what time of day do reef fishes spawn?

There are two daily peaks of activity for spawning in coral reef fishes. Those that broadcast drifting, pelagic eggs, such as angelfish and surgeonfish, spawn at dusk. For fish such as the Japanese angelfish (Centropyge interruptus), the window of opportunity is extremely narrow; these fish carry out the majority of their breeding in a 15-minute slot straddling sunset. Spawning at this time gives the eggs the best possible chance of avoiding predators, as most of the diurnal, planktonic-feeding animals have left the water column to seek shelter by this time, and the nocturnal feeders have yet to emerge.

In contrast demersal-nesting fishes (those that lay eggs on the bottom), such as the sergeant major (Abudefduf saxatilis), usually spawn early in the morning. Again, there are generally fewer egg-predators around at dawn than later in the morning, and the eggs tend to hatch at dusk a few days later, allowing the newly emerged embryos their best chance of escaping from the battalions of predators on the reef
during the change-over period that happens every night at twilight.

When is the peak feeding time for reef fish?

Different feeding guilds of fish reach their peak activity levels at various times of the day. Out-and-out fish hunters can find it difficult to feed during the main part of the day when their prey is highly active and alert. Their response is to concentrate their activity to the twilight periods. Fish that forage on invertebrates, including goatfish and boxfish that forage in the sandy beds of the lagoon and among the coral heads, feed more or less continuously throughout the day, starting shortly after dawn and continuing until late afternoon.

Similarly, seahorses keep up a fairly steady level of feeding activity, holding on to seagrasses with their prehensile tails and snapping at tiny invertebrates as they drift past. But the availability of these prey — and of the plankton upon which many other reef fishes, including some damsels, wrasses, and basslets, feed — depends to an extent on oceanic currents and the time of year. If food is abundant, the fish may be able to satiate themselves rapidly, then move out of the main feeding grounds in the water column and back to the safety of the reef. There, they are less at risk from the surprise attacks of fast pursuit predators, such as jacks.

Herbivores, too, may feed throughout the day. However, the brightest sunlight produces the richest algae, so the best time to feed is during late morning and early afternoon and it makes sense for the grazers to pack in as much feeding as possible at this time.

What do nocturnal fishes do during the day?

While the brightly colored diurnal fishes bustle around the habitat, their nocturnal counterparts spend the day concealed. Fish such as cardinalfish, squirrelfish, and soldierfish that feed on plankton at night, seek shelter in huge schools, sometimes numbering tens of thousands of individuals.

These tightly packed gatherings can be seen underneath coral outcrops and in underwater caves. The fish on the periphery of the school usually face outwards in order to spot danger early. Although these fish are resting, they do not actually sleep as we understand it. Instead, they simply conserve energy by seeking areas where there is little current to fight against and where they can benefit from the safety of the group. Similarly, moray eels spend the day hidden from view. Although they are known to emerge to forage occasionally during the day, most of their hunting is carried out at night.