A Hidden Ocean 'Twilight Zone' Has Been Discovered That's Filled with New Species

The world's oceans cover more than two-thirds of the Earth's surface—but despite decades of scientific research, we still know precious little about them. In fact, estimates suggest humans have explored only five percent of the marine environment, according to figures from the National Oceanic and Atmospheric Administration.

Now, in a new study published in the journal Scientific Reports, researchers have investigated a zone of the ocean in coral reef ecosystems where life is so different from the regions above and below that an entirely new category is required to describe it.

The scientists from the Smithsonian Tropical Research Institute (STRI) and the National Museum of Natural History say this low-light region—which they call the rariphotic—extends between depths of around 130 to 300 meters (427 to 984 feet) covering areas known as "deep reefs" or "coral-reef twilight zones" that scientists know very little about.

For the study, the team conducted multiple expeditions, starting in 2011, using a mini-submarine, known as Curasub, to explore a 0.08-square mile area of coral reef off the coast of Curaçao—a Dutch Caribbean island. They found that the rariphotic was filled to the brim with never before seen marine species: in total, they observed 71 species, 30 of which were new to science.

One of the new fish species discovered in the Rariphotic, Haptoclinus dropi, was named for the Smithsonian's Deep Reef Observation Project. Very little is known about deep reefs which are only observable using submersibles. Carole Baldwin, Smithsonian

"About one in every five fish we're finding in the rariphotic of the Caribbean is a new species," D. Ross Robertson, marine biologist at STRI and a co-author of the study, said in a statement.

Most of the fish observed by the team in the rariphotic were related to shallow reef fish, which was a surprise to the researchers because it was previously thought that shallow reef fish couldn't live below around 130 meters.

Many scientists have speculated that deeper reef regions, such as the rariphotic, may act as a refuge for shallow-water creatures seeking an escape from warming waters and coral deterioration—two processes that are being driven by climate change.

In fact, the team's initial motivation for studying deep-reef ecosystems was the declining health of shallow reefs across the world. But it became clear to the researchers that our knowledge regarding the biodiversity of reefs is still limited.

"It's estimated that 95 percent of the livable space on our planet is in the ocean," Carole Baldwin, curator of fishes at the Smithsonian's National Museum of Natural History and lead author of the study, said in a statement. "Yet only a fraction of that space has been explored. That's understandable for areas that are thousands of miles offshore and miles deep. But tropical deep reefs are just below popular, highly studied shallow reefs—essentially our own backyards."

Baldwin added that tropical deep reefs are highly diverse ecosystems which warrant further study.

"Reef ecosystems just below the mesophotic are globally underexplored, and the conventional view based on the few studies that mention them was that mesophotic ecosystems transition directly into those of the deep sea," Baldwin said. "We hope that by naming the deep-reef rariphotic zone, we'll draw attention to the need to continue to explore deep reefs."

Representative Caribbean fishes inhabiting the rariphotic zone off Curaçao. Haptoclinus dropi (Labrisomidae); Pontinus castor (Scorpaenidae); Anthias asperilinguis (Serranidae); Lipogramma evides (Grammatidae); Serranus notospilus (Serranidae); Polylepion sp. (Labridae). Patrick Colin, C. C. Baldwin and D. R. Robertson.

Based on their research, the scientists presented a new classification of coral reef ecosystems:

  • Altiphotic(high light): The new name for the previously unnamed 0-40 meters (0-131 feet) region—the well-lit zone where reef corals are abundant. It extends as deep as conventional scuba divers normally go.
  • Mesophotic (medium light): 40 meters to as deep as 130 meters (131-427 feet)—the maximum depth at which tropical reef-building corals can survive.
  • Rariphotic (low light): Newly discovered zone extending from 130-300 meters (427-984 feet). Below the reef-building coral zone, and as deep as Curasub can go.
  • Deep aphotic (effectively no light): Below 300 meters (984 feet).

Reefs are crucial to the health of the marine ecosystem which billions of people around the world rely on, but are under increasing threat from rising ocean temperatures and ocean acidification, which result in harmful coral bleaching.

Recent research has also shown that the vast amounts of plastic pollution in the ocean can exacerbate these threats. This is worrying given that there is now so much plastic waste in the ocean that scientists want to study it from space, and this amount will triple within a decade, according to a new U.K. government report.