The Coral Reef at Night

The dark of night provides cover for a host of reef creatures, allowing them to emerge from their daytime shelters, safe from the depredations of the animals that threaten them. In many instances, the tables are turned – the diurnal hunters of invertebrates are themselves at great risk from the deadliest reef predators, including morays and reef sharks. Although this is a dangerous period for many adult fishes, it is also the time when their larvae hatch and disperse into the plankton and, later, return to settle on the reef.
hour following dusk, giving them ten or eleven hours to ride the currents away from the reef and make themselves scarce before the huge populations of diurnal adult reef fish return. Hatching is controlled by light levels; if the eggs of sergeant majors are kept in artificial light, the embryos will delay their hatching. Mouthbrooders follow the same pattern, releasing their larval young from the safety of the buccal pouch soon after dusk.

What are the nocturnal fishes like?

Many daytime fish have their equivalent among the night-time assemblage, but these fish are very different in appearance. They have large eyes and their vision is boosted by a reflective layer within the eye that allows them to gather as much of the faint light as possible. Even so, most nocturnal fishes have no color vision and their sight is restricted to shades of gray. For this reason, nocturnal fish often lack the dazzling colors of their diurnal cousins, instead relying on communication by sound; the aptly named “drums,” such as the jackknife fish (Equetus lanceolatus) and highhat (Poreques acuminatus), are masters of this.

Squirrelfishes and cardinalfishes feed on the plankton, which is far more abundant at night. At this time, grunts migrate into the reef and the lagoons beyond to feed on the emerging invertebrates. Groupers and snappers patrol the reef in search of unwary fish prey, while morays seek out the shelters of the resting diurnal fish, mainly by smell. It is thought that the mucous sac produced by parrotfish, which holds in much of the fish's odor, is an adaptation to restrict the ability of moray eels to seek them out. Reef sharks use both their sense of smell and their exquisite electroreception abilities to detect the minute electrical impulses produced by their victims in the low light.

Why are so many different planktivores active at night?

Although there are many diurnal planktivorous fish, it is at night that invertebrate planktivores really come into their own. This is partly because many of their main predators are less active at night and partly because the plankton itself migrates over the course of the daily cycle, rising to the surface waters to feed as dusk gives way to night. Just before dawn, the plankton descend once more to the safety of deeper waters. The concentration of these tiny animals, including larval crustaceans and worms, is far higher on the reef at night and the size of each individual animal within the plankton tends to be higher during the night than during the day. This in turn makes the night-time plankton more nutritious. This upward migration of plankton from the deep is an extremely important part of the daily pattern of life on coral reefs and supports a huge range of different planktivorous reef organisms.

Individual coral polyps emerge from their limestone shelters to capture their food from this plankton bonanza. Featherstars and brittlestars scale the reef to reach good feeding spots, spreading their net-like arms to ensnare the tiny animals that drift in the water currents. Shrimps that spend the day scavenging among the reef join in the feast, resorting to the same sievelike appendages to harvest their food.

Why do so many demersal-spawning coral reef fishes hatch from their eggs at night?

The most likely reason is that this is when they stand the best chance of making it safely away from the reef to join the larvae of broadcast-spawners hunting in the swarms of plankton that will provide them all with both food and shelter during their early life. Damsels in both the Caribbean and in the Pacific tend to hatch in the first hour following dusk, giving them ten or eleven hours to ride the currents away from the reef and make themselves scarce before the huge populations of diurnal adult reef fish return. Hatching is controlled by light levels; if the eggs of sergeant majors are kept in artificial light, the embryos will delay their hatching. Mouthbrooders follow the same pattern, releasing their larval young from the safety of the buccal pouch soon after dusk.

When do larval fish settle onto the reef?

After their period of development drifting among the plankton, larval fish seek out a reef on which to settle and drift in from their offshore nursery on the ocean currents. This usually happens at night, again to avoid the battery of diurnal fish that would pick them off easily if they attempted to settle during the day. They arrive in numbers, like soldiers storming a beach. Having reached a suitable reef, the larvae undergo a rapid metamorphosis and settle as new juveniles, a process that once again tends to occur under cover of darkness.

What do adult diurnal fish do during the night?

For the most part, those fish that were active during the day seek shelter at night, in many cases using those places recently vacated by the emerging nocturnal fish. Nevertheless, during a full moon there are reports of some diurnal butterflyfish using the brighter conditions to continue foraging throughout the night.

However, the night brings danger in the shape of high predator activity. No longer equipped to detect danger by sight diurnal fish find themselves at a disadvantage, especially against predators that hunt by smell. Their best chance is to make themselves inconspicuous or to hide themselves well in the reef. Quite a few species achieve the former by changing color as night sets in. Moorish idols (Zondus comutus) and surgeonfish. for
darker colors to help them blend in. Hidden triggerfish lock their dorsal spines upwards, bracing themselves against the rock of their shelter. Once wedged in, it is almost impossible for a predator to extract them. Other fish, including some wrasses, hide beyond the reef, sheltering beneath the sand itself, only to emerge as dawn breaks.