Scientists discover hidden world of Hawaii's coral 'twilight zone'

20-year study of deep reefs finds algae meadows and swaths of continuous coral with the highest rate of species found nowhere else in Earth’s seas

Jessica Aldred

Tuesday 4 October 2016 15.39 BST Last modified on Tuesday 4 October 2016 15.40 BST 

The “twilight zone” of Hawaii’s deep coral reefs are home to vast algae meadows and support the highest rates of species found nowhere else in Earth’s seas, scientists have discovered.

A 20-year study of the archipelago’s poorly-explored mesophotic – middle light – coral zone also found the deep-reef habitats are home to many unique and distinct species not found on shallow reefs with vast areas of 100% coral cover.

While much is known about shallow, tropical coral habitats, the richness, diversity and ecological importance of these deep sea ecosystems, found at depths of 30-150 metres, has only recently been understood. 

In one of the largest and most comprehensive studies of its kind, published on Tuesday in the journal PeerJ, the entire 2,590km Hawaiian archipelago was covered over two decades using a combination of submersibles, remote-operated vehicles, drop cameras, data recorders and advanced diving techniques. 

A major focus of the study was to document extensive areas of 100% coral cover at depths of 90m (300 feet) or more off the islands of Maui and Kauai. Vast expanses of continuous coral cover were found extending for tens of square kilometres, dominated by stony, reef-building corals in the genus Leptoseris, a plate-like coral adapted for deepwater environments.

“These are some of the most extensive and densely populated coral reefs in Hawaii,” said Anthony Montgomery, a US Fish and Wildlife biologist and co-author of the study. “It’s amazing to find such rich coral communities down so deep.”

The study identified more than 70 species of macroalgae in extensive meadows that support unique communities of fish and invertebrates. Both corals and algae depend on sunlight for photosynthesis, and the study attributed the existence of many of the deep reef habitats to exceptionally clear water.

Read more