All Images Courtesy Richard Pyle.

We tend to think of coral reefs as luminous, undersea jungles that pepper the shallow, scuba-friendly tropics. But deeper down, in a region about as bright as Pluto on a sunny day, there lie vast reef ecosystems unknown to science.

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Richard Pyle, a zoologist at the Bishop Museum, has spent the last twenty years surveying one of these landscapes—the so-called “twilight zone” coral beds that encircle the Hawaiian archipelago at depths of 30 to 150 meters (100 to 500 feet), where light is perpetually dim. His extensive findings are summarized today in the journal Peer J, in the most comprehensive study of low-light coral reefs ever conducted. And they reveal a world far stranger and more complex than we imagined.

Pisces V submersible on the seafloor.

Using a range of high-tech gadgets and vehicles—ROVs and submersibles, drop-down cameras and environmental sensors—Pyle and his colleagues have documented vast, poorly-lit areas of complete coral cover, spanning tens of square kilometers at depths of 90 meters (300 feet) or more off the islands of Maui and Kaua’i. Interspersed alongside these reefs are thick green “meadows,” home to dozens of species of never-before-described algae. Both corals and algae require sunlight for photosynthesis, leading Pyle to suspect they can only exist here due to the exceptional clarity of Hawaii’s waters.

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One of the most intriguing discoveries to come out of Pyle’s survey is that mesophytic reefs are hotbeds for endemic fish—species found nowhere else on Earth. While only 17 percent of fish species in Hawaii’s shallow reefs are unique to the archipelago, that number jumps to over 50 percent when you plunge below 70 meters.

Algae beds like this one play a critical role in deep coral reef ecosystems.

“The extent of fish endemism on these deep coral reefs, particularly in the Northwestern Hawaiian Islands, is astonishing,” study co-author Randall Kosaki said in a statement. “We were able to document the highest rates of endemism of any marine environment on Earth.”

One theory is that these reefs represent a refuge, where numerous lineages have have survived for millions of years even as ice ages rework Earths’ surface over and over.

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Perhaps most importantly, the study highlights highlights just how little we know about life in Earth’s dark biosphere—which is a problem, because we can’t easily protect what we don’t understand. Then again, a couple of enormous new marine national monuments is a great start.

Swimming along a mesophytic coral bed.
Pisces V submersible on the seafloor.
A scientist places an acrylic dome over the coral to be used in the staining experiment.
A scientist explores a mesophytic reef.
A mesophytic reef filled abuzz with fish.
Two humpback whales encountered during the exploration.
A scientist and submersible collect samples and explore the seafloor.
A scientist rises to the surface after completing decompression.

[Peer J]