Most of what we know about coral reefs comes from the top 30 metres — the depth range comfortably reached by recreational scuba divers. Below that, the reef keeps going. On steep slopes and walls, living coral communities extend down into a blue twilight that only a few specialised divers and robotic vehicles have explored. These deep reefs have turned out to be extensive, diverse and full of surprises. This guide explains what the deep reef is, how corals survive in such low light, what lives there, why scientists are so interested in its potential as a refuge, and what threatens it.

The mesophotic zone explained

The deep reef occupies the middle ground between the sunlit shallows and the lightless deep sea.

Below the bright, busy fore reef, sunlight fades and shifts toward blue as water absorbs the red and yellow parts of the spectrum. At around 30 metres, most of the light that powers shallow reefs is gone, yet enough remains for specially adapted corals. This is the start of the mesophotic zone. Scientists often divide it into an upper mesophotic zone, roughly 30 to 60 metres, where many shallow-reef species still occur, and a lower mesophotic zone, roughly 60 to 150 metres, where communities become dominated by deep specialists. The exact depths vary with water clarity: in murky coastal water, mesophotic conditions begin much shallower.

Below the lower mesophotic zone, light becomes too dim for corals with symbiotic algae. Researchers have described a further “rariphotic” zone beyond about 130–150 metres, where reef fish communities change again before giving way to the deep sea. Mesophotic reefs have been found throughout the tropics, from the Caribbean and Gulf of Mexico to Hawaii, the Red Sea and the Great Barrier Reef. Notable examples include Pulley Ridge off south-west Florida, and the extensive deep coral beds of Hawaii’s ‘Au‘au Channel.

How corals survive in the twilight

Deep-reef corals are masters of light harvesting, reshaping their colonies and their biology to capture every photon.

Reef-building corals depend on the zooxanthellae living in their tissue, which photosynthesise and share food with the coral. In dim light, the challenge is to capture as much light as possible. Many mesophotic corals solve this by growing as thin, flat, horizontal plates or overlapping tiers, exposing maximum surface area to the faint light from above — a growth form that would be smashed by waves in the shallows. Corals in the genus Leptoseris and related agariciids are characteristic of the deepest coral communities, and some can grow at remarkable depths.

Deep corals also adjust their physiology. Some contain pigments that help channel blue light to their algae, and many supplement photosynthesis by capturing plankton, relying more on feeding than shallow corals do. Their skeletons grow more slowly, and colonies may be very old. Because the water is calm, these fragile plates can grow wide and thin, forming extensive coral “shingles” that carpet steep slopes.

Life on the deep reef

Despite the dimness, mesophotic reefs can be surprisingly rich, with communities found nowhere else on the reef.

On some deep slopes, coral cover rivals that of healthy shallow reefs. Alongside the corals, sponges become increasingly dominant with depth, from large barrel sponges to colourful encrusting forms, and they play a major role in filtering water and cycling nutrients. Gorgonian sea fans, sea whips and black corals grow on walls and ledges. Calcified green algae such as Halimeda can form dense beds, and coralline algae continue to build reef structure.

Fish communities change with depth too. Many shallow-reef species extend into the upper mesophotic zone, but deeper down, colourful specialists appear, including certain basslets, anthias, butterflyfish and wrasses that are rarely or never seen by recreational divers. Exploration of mesophotic reefs has led to the discovery of many species new to science. Commercially important fish such as snappers and groupers also use deep reefs, and some spawn there. For more on how animals divide up the reef, see coral reef zones and animals and sponges.

What shapes conditions on the deep reef

Light, temperature, water movement and sediment all change with depth, and together they decide what can live where.

Light is the master factor. Every few metres of depth removes more of the energy available to coral’s symbiotic algae, so the deepest coral communities occur only where water is exceptionally clear, such as on oceanic islands and remote atolls. Near river mouths or turbid coasts, the same light levels may be reached at a fraction of the depth, and mesophotic-type communities can begin in water only 15 or 20 metres deep.

Temperature is generally cooler and steadier at depth, because mesophotic reefs often lie below the warm surface mixed layer. On some slopes, internal waves — large underwater waves travelling along density boundaries in the ocean — periodically push cooler, nutrient-rich water up the reef, which may help shield deep corals from surface heat and supply extra food. Water movement is gentle, allowing fragile growth forms to persist, but sediment sliding down steep slopes can bury corals. The shape of the slope matters too: gentle terraces and ledges accumulate sediment and may support algae and sponges, while steep walls shed it and favour plating corals and sea fans. These factors combine differently on every reef, which is why mesophotic communities vary so much from place to place.

The deep reef as a refuge

Could deep reefs reseed damaged shallow reefs? The answer appears to be “sometimes, partly.”

The deep reef refugia hypothesis proposes that because mesophotic reefs are cooler and shielded from waves, they may escape some of the disturbances that devastate shallow reefs — marine heatwaves, storms and many local human impacts. If deep populations of corals and fish survive, they could supply larvae that drift upward and help shallow reefs recover. For species found at both depths, there is evidence that deep populations can be connected to shallow ones.

But research over the past two decades has added important caveats. Many mesophotic species, especially in the lower zone, are deep specialists that do not occur in the shallows, so they cannot reseed shallow reefs. Coral populations at different depths can be genetically distinct, limiting exchange. And the deep reef is not immune to heat: during severe marine heatwaves, bleaching has been recorded at mesophotic depths in some regions. The refuge effect is therefore real but limited, strongest for some species in the upper mesophotic zone. Protecting deep reefs is valuable in its own right, but it is not a substitute for protecting shallow reefs. See coral reef bleaching and can reefs recover from bleaching.

Exploring and protecting the deep reef

Deep reefs remain among the least explored ecosystems on Earth — and they face real threats.

Reaching mesophotic depths requires technical diving with mixed gases and long decompression, closed-circuit rebreathers, or remotely operated vehicles and submersibles. Because of these challenges, mesophotic reefs were long overlooked, and many remain unmapped. Organisations such as NOAA’s Ocean Exploration programme and research teams worldwide have made deep reefs a priority in recent decades, revealing communities far more extensive than once thought.

Deep reefs face several threats. Sediment from land and dredging can settle on flat, plating corals that cannot easily shed it. Fishing, including bottom fishing and traps, can damage slow-growing corals and sponges and remove large fish. Anchors and cables can scar deep slopes. Invasive lionfish in the western Atlantic hunt down to mesophotic depths. Storms can send rubble cascading down slopes, and marine heatwaves can reach deeper than once believed. Because deep reefs grow slowly, damage may take a very long time to repair. Including mesophotic reefs in marine protected areas and reducing sediment and fishing pressure are the main protective measures. See local threats to coral reefs.

See also: The Coral Reef Ecosystem.

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