Are there coral reefs in the dark and cold, and what do they eat?
Reefs of living coral grow in the dark and cold of the deep sea, where fishermen found them 250 years ago, but scientists only began to understand them once mini-submarines could reach them.
▶ Start the storyYes: there are coral reefs in the dark and cold, and they live by catching food that drifts past. Deep-water corals, also called cold-water corals, live in deeper, darker parts of the ocean than tropical corals, from near the surface down beyond 2,000 metres where the water may be as cold as 4 degrees Celsius. They are still animals of the same group as tropical corals, but they do not need . They feed on and rely on ocean currents to bring them food.
Fishermen found them about 250 years ago, which got scientists interested. Early scientists were unsure how such reefs could live in seemingly barren, dark northern water. It was not until crewed mini-submarines reached enough depth that scientists began to understand them. The first live video of a large one came in July 1982, when Statoil surveyed a reef 15 metres tall and 50 metres wide, 280 metres down off northern Norway.
Without algae to feed them, deep corals grow slowly. , a common species, extends its polyps about 10 millimetres a year, while branching shallow corals like Acropora may exceed 10 to 20 centimetres. Some living deep-water corals are estimated to be at least 10,000 years old.
Tropical corals
- Shallow, sunlit water
- Algae partners () feed them
- Branching types may grow 10-20 cm a year
Deep-water corals
- Dark water, down past 2,000 m, as cold as 4 °C
- No , they catch
- grows about 10 mm a year
They can make homes for a lot of life: a reef can host up to 1,300 species of fish and invertebrates. The largest known Lophelia complex, the Røst Reef off Norway, lies 300 to 400 metres down. They are under threat: scientists estimate that has damaged or destroyed 30 to 50 percent of the Norwegian shelf coral area.
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Recap
Deep reefs live on drifting food, grow far more slowly than sunlit branching corals, and are easily broken by trawls.
💡 A trick to remember it · No sunshine, no algae: a deep reef is a slow buffet that waits for the current to bring dinner.
Surprising fact · Some living deep-water corals are estimated to be at least 10,000 years old.
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No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.