Coral reefs in Hawaii’s oceanic twilight zone, where light still penetrates and photosynthesis occurs, are abundant and host a wide variety of life, a new study shows.

A paper published Tuesday in the journal PeerJ revealed that some of these ecosystems off the Hawaiian archipelago, particularly an area off Maui, are the most extensive deep-water reefs ever recorded.

The ecosystems, found in waters from 100 to 500 feet deep, host more than twice the amount of unique Hawaiian fish species as their shallow-water counterparts, and they are much more extensive than previously known.

“What is unique about this study is how vast and dense the coral cover is,” Richard Pyle, a Bishop Museum researcher and lead author of the publication, said.

Researchers surveyed much of the twilight zone, technically known as the mesophotic coral ecosystem, around the state and found a few hot spots that were particularly productive.

In a channel off Maui, the team said they found the largest uninterrupted coral ecosystem ever recorded, measuring more than 3 square miles with some areas showing 100 percent coral cover.

Previously, similar reefs in the Gulf of Mexico and off Okinawa, Japan, had been observed, but in much smaller colonies and at shallower depths.

“Although there was a bit of a hint that corals could survive … down at those depths, these reefs off Maui were far and away much more dramatic both because they were deeper and they had higher coral cover percentage,” Pyle said. They covered a “vastly larger” area.

The Maui corals were in an area that combines clear water, plentiful food and shelter from major swells.

While some coral lives in much deeper water, this discovery represents the deepest reef-building species that rely on sun-fueled algae to survive.

The difficulty of exploring depths beyond 100 feet means a large amount of knowledge about coral reefs is derived from those in more shallow water, said Randall Kosaki, deputy superintendent for the Papa- hanaumokuakea Marine National Monument at the National Oceanic and Atmospheric Administration.

“So the purpose of this study really was to characterize the other 80 percent of their depth range and see what’s down there,” he said.

The team also discovered an unusually high number of endemic fish species living in some areas, particularly in the Northwestern Hawaiian Islands. There the team found that nearly 100 percent of the fish species were unique to the region — the highest level of endemism ever recorded in any marine ecosystem.

It is “absolutely off the scale globally,” Kosaki said. “They are very significant contributions to global biodiversity.”

The team did not document any coral bleaching at those depths, but heat from spiking surface temperatures can penetrate deep into the ocean and cause harm to the fragile habitats.

“There is a time sensitivity to this exploration because due to climate change and other factors, we’re at risk of losing species before we even know they exist,” Kosaki said.

The study was conducted over the past 20 years by 16 scientists from NOAA, the University of Hawaii, Bishop Museum and state agencies.

The researchers contend that the largely unexplored ecosystem requires further understanding and protection.

One theory that they are exploring is something called the “deep reef refuge” hypothesis.

“If shallow coral reefs are more vulnerable to threats from, say, runoff or overfishing or whatever, then down deep these reefs could potentially serve as refuges for those species,” Pyle said.

But that assumes that the shallow reefs are more vulnerable, and it would only apply to the species that could survive at both depths, he added.

“We’re not really sure we can jump to that assumption,” Pyle said.

“They are incredibly unique. With the higher rates of endemism, they are incredibly rich,” Pyle said. “We should value them every bit as much as we value the shallow reefs.”