Here’s What Scientists Found in Hawaii’s Mysterious “Twilight Zone”

Deep coral reefs reveal their secrets in a study two decades in the making

Kure Atoll
Kure Atoll, the northernmost reef in the Hawaiian archipelago, hosts mesophotic reefs with the most species unique to a specific location found in any marine ecosystem on Earth. Bishop Museum and NOAA

Hawaii’s brilliant coral reefs are among the world’s most loved and studied. But what’s underneath those vibrant ecosystems? A lot, it turns out—the Hawaii Archipelago’s shallow reefs are just the beginning. Underneath lies a system of deep reefs known to scientists as “the twilight zone”—an area that, as WIRED’s Matt Simon reports, hasn’t been well studied until now. 

In a recent study, published in the journal PeerJ, a team of scientists describe an effort two decades in the making. Their work focused on mesophotic coral ecosystems, or MCEs—reefs that exist in low light areas 100 to 500 feet beneath the ocean surface. These reefs used to be hard to study because they were inaccessible to researchers (hence the name “The Twilight Zone”). As NOAA explains, they start at the point at which conventional scuba diving becomes impossible but are too shallow for many robotic submersibles to explore.

Recently, however, advancing technology has allowed scientists to plumb the depths of MCEs. As Simon reports, divers used rebreathers, which recycle scuba tanks’ helium, allowing them to stay underwater for seven hours. They headed to the reefs along with NOAA submersibles that made it easier to see.

What they observed was magnificent: The scientists not only discovered the largest MCE ever recorded, but a mind-boggling number of species. Forty-three percent of the fish species the team documented were unique to the Hawaiian islands—more than twice the number of distinct species that can be found in shallower reefs above. And in one spot, nearly every single species they found was unique to that region.

The team studied everything from the reefs’ water temperature to light levels, collecting details on its food web and physical structure. The goal was to establish baseline observations to set a foundation for future research. Along the way, they documented some pretty amazing facets of this mostly undiscovered world. For example, they found more than three square miles of uninterrupted deep coral—the largest such patch ever recorded.

With coral reefs all over the world threatened by bleaching and rising ocean temperatures, how might the deep reefs of the Hawaiian Archipelago fare? It’s not clear, say scientists. They did find some evidence that some species common to shallower reefs might use the deep reefs as a refuge, but aren’t sure how true that is for all species. Since MCEs need clear water to survive, they face threats from algae and pollution that affects water quality. And it’s not yet clear how MCEs will respond to global warming or ocean acidification.

“There is still so much of our ocean that is unexplored,” said W. Russell Callender, assistant NOAA administrator for the National Ocean Service, in a release about the study. One thing is sure: Scientists have only scratched the surface, so to speak, of these magnificent reefs.

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