Marine Biology & Ocean Science

A Coral Reef Presumed Dead for 60 Years Is Alive Off Benin

Surveyors spotted hints of a deep-water coral reef off the coast of Benin in the 1960s, assumed it had died, and never looked again. Six decades later, underwater drones found it thriving — a "mesophotic" reef that lives in a depth band where corals were not supposed to survive in the tropics.

• The reef lies at roughly 52 to 56 metres below the surface off the Benin-Togo continental shelf.
• It was first flagged during 1963-1964 fishing-area surveys by ORSTOM, the French overseas research agency.
• The ecosystem type is "mesophotic" — tropical coral life at mid-depths where light is too weak for most shallow-water corals.

Most people picture coral reefs as sunlit shallow gardens, metres under the waves. That is only half the story. Mesophotic coral reefs live in a twilight band, typically somewhere between 30 and 150 metres deep, where photosynthesis still happens but barely. The algae that feed many corals have adapted pigments tuned to the faint green and blue light that reaches those depths. Finding a healthy mesophotic reef means the corals there are running on a different energy budget than their shallow cousins, and the population dynamics are largely unknown.

The Benin reef's rediscovery was accidental. Modern surveyors were using sonar and drone camera systems to map fish diversity and identify trawlable areas, not hunting for corals. The deep-sea garden showed up as a signal that did not fit the expected seafloor, and when the drone cameras were pointed down, a living reef was there. The original 1960s survey had recorded exactly the same signature and simply concluded the corals must be long dead — an assumption that lasted six decades until someone looked again with better tools.

That matters for climate reasons. Shallow tropical reefs have been bleaching and dying across the globe as ocean temperatures rise. Mesophotic reefs, bathed in cooler deeper water, have been suspected for years of acting as a refuge population — a nursery that can reseed the shallower reefs above if they ever recover. Confirming that such reefs exist, and are healthy, strengthens the case that the full vertical range of tropical coral life is larger and more resilient than the well-publicized bleaching crisis suggests. It also means a reef the world forgot about is worth protecting now.

Knowledge takeaway: Mesophotic coral reefs grow at 30 to 150 metres depth on a low-light energy budget; the Benin reef was first flagged in 1963-64 and assumed dead for 60 years; healthy deep reefs could act as a climate refuge for shallow tropical coral.