Coral Reef at Night

Scuba divers know that the ocean after dark is a wondrous and entirely different place than under the warm rays of the sun. Many animals hide during daylight hours and come out only at night to feed and hunt.

Image Source G Howells KZN WIldlife
Image Source G Howells KZN WIldlife

The basic structure of many reefs is the build-up of calcium carbonate secreted by corals. Looking at hard corals during the day, you might be misled into thinking they were not alive. But if you visit at night, you will see the soft tentacles of the coral polyps emerge into the ocean current, picking up zooplankton. If you were to compare the appearance of a brain coral in daylight to what you see at night, you wouldn’t believe it was the same creature.

Like all ecosystems on planet Ocean, coral reefs depend on energy from the sun to fuel photosynthesis, which then trickles up the food chain to higher trophic levels. When the sun goes down, photosynthesis abruptly ends. Dramatic changes, physical, chemical, and biological, take place at night on a coral reef. Zooplankton rise from the nooks and crannies of the reef and drift past an ocean of mouths that includes reef corals, sea anemones, brittlestars, and basketstars.

These Demersal zooplankton arise from the reef substrate as the sun goes down and include a host of different invertebrates, with copepods and polychaetes often numerically dominant. Capturing particles from the water is called suspension feeding and it is one of the most ancient forms of feeding by animals. Both active (pumping) and passive (using ocean currents) suspension feeders occur in high abundance near the coral habitat. Corals are passive suspension feeders. The tentacles of the expanded polyps catch particulate prey.

Image Source G Howells KZN Wildlife
Image Source G Howells KZN Wildlife

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.

Depending on when they are most active, nocturnal fish have developed certain physical and behavioural characteristics. In general, they are not designed to swim as fast as diurnal fish. They tend to have larger eyes. They tend to be more solitary, rather than living in large schools. They are often more shy and during the day are found hiding in caves or under overhangs. They are often red or yellowish brown in coloration since these colours are absorbed by the water and are the first colours of the spectrum to become indistinguishable as light levels drop. They tend to be carnivores versus diurnal fish that are often herbivores or omnivores. They may have a well developed lateral line, like the Dwarf Scorpionfish, which allows them to sense water movement as an aid in finding prey in the dark or low light.

Sea turtles lay their eggs at night and the hatchlings emerge at night. Turtle hatchlings are led by the moonlight reflecting on the water. They crawl from their nests to the shallow water at the shoreline. As a result of increased human presence on coastlines, artificial light can disorientate the hatchlings and cause them to move toward buildings and streetlights, away from the ocean. Turtle hatchlings are very clumsy on land, but as soon as they reach the water, their natural swimming abilities take over.

Leatherback Turtle
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Lobsters hide in nooks and crannies in the reef during the day and come out at night to hunt for food. They eat a variety of other sea creatures, including clams, urchins, crabs, and fish. When threatened, lobsters move very quickly by contracting their abdomens, causing the tail to move up and down, but they move backward, tail first.

Another fascinating creature more likely seen at night is the octopus. Octopuses are shy creatures whose first defence is to hide. In the darkness, they emerge to hunt. In contrast to squid and cuttlefish, octopuses have no internal skeleton and they can modify their shapes in amazing ways. Their other defences are quickly swimming away, ejecting a cloud of ink to confuse predators and changing colour to match their environment. If you follow along behind a swimming octopus at night, you will see it change colour each time it moves over a new patch of rock or sand.

Moray eels are really fish. They have one long fin on top of the body that extends from behind the head all the way to the tail. Morays have poor eyesight, but an excellent sense of smell. The characteristic that most people notice first is a lot of sharp teeth. Eels tend to hide away during the day in cracks and crevices and do their hunting at night.

 Article courtesy of Glynne Howels, KZN Wildlife

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