Unique Corals Found in Hawaii's "Twilight Zone"

Scientists backed by the US National Ocean and Atmospheric Administration working in the Hawaiian Archipelago are calling some of the deep coral reefs found in the region's so-called oceanic "twilight zone" the most extensive on record, with several large areas of 100% coral cover.

They also found that the deep coral reefs studied have twice as many species that are unique to Hawaii than their shallow-water counterparts.

This extensive study of the Hawaiian deep coral reefs, known as mesophotic coral ecosystems, led to some incredible finds published recently in the scientific journal PeerJ.

These mesophotic coral ecosystems, the deepest of the light-dependent coral reef communities found between 30 and 152 meters (100-500 ft) below the ocean's surface, lie well beyond the limits of conventional scuba diving and are among the most poorly explored marine habitats on Earth.

Scientists used a combination of submersibles, remotely operated vehicles and technical diving to study these difficult-to-reach environments, Eureka Alert reported.

Of the fish species documented on mesophotic reefs, 43% were unique to the Hawaiian Islands, which is more than double the 17% of unique species found on shallow Hawaiian reefs.

At the northern end of the archipelago, in the recently expanded Papahanaumokuakea Marine National Monument, nearly all of the species are unique to the region, the highest level recorded from any marine ecosystem on Earth. These findings could offer further insight into the monument's management.

  Largest Continuous Coral System

In Maui's 'Au'au Channel, scientists discovered the largest uninterrupted mesophotic coral ecosystem ever recorded, extending more than 3 square miles at approximately 48 to 91 meters (160 to 300 ft) deep and including areas of 100% coral cover.

"The waters off Maui present the perfect environment for these mesophotic reefs to exist," said Richard Pyle, Bishop Museum scientist and lead author on the publication.

"The area combines clear water, which allows light to reach the corals; good water flow enhancing food availability; shelter from major north and south swells, and a submerged terrace between the islands at the right depth."

Because of the challenges associated with working at such depths, mesophotic coral ecosystems are less understood and often not considered in coral reef management efforts. Overfishing, pollution, coastal development and climate change threaten coral reef ecosystems worldwide, and increased knowledge of mesophotic coral ecosystems will help characterize the health of coral reefs in general, particularly in the face of increasing stress.

"With coral reefs facing a myriad of threats, these findings are important for understanding, managing and protecting coral-reef habitat and the organisms that live on them," said Kimberly Puglise, an oceanographer with NOAA's National Centers for Coastal Ocean Science.

"Some species studied can live in both shallow and mesophotic reefs, and the species could potentially replenish each other if one population is overexploited."