Newly Named "Coral Reef Twilight Zone" Is Full of Unknown Life

Welcome to the rariphotic.

To date, humans have explored less than five percent of the ocean. That’s less surprising when you consider that 95 percent of the livable space on Earth is in the ocean, which itself takes up 70 percent of the planet’s surface. Scientists exploring one small fraction of that livable space reported on Tuesday that they’ve discovered never-before-seen species of life lurking in the depths, as well as an entirely new ocean life zone.

This new zone is newly named the “rariphotic,” and it exists between 130 to 309 meters below the surface. Smithsonian scientists identified it while exploring the reef system alongside Curaçao, a Dutch Caribbean island separated from mainland South America by a deep ocean trench. Using a manned submersible, the research team came across an entirely new world of biodiversity, which includes the 30 fish species that they describe in their study in Scientific Reports.

“About one in every five fish we’re finding in the rariphotic of the Caribbean is a new species,” explained co-author and Smithsonian marine biologist D. Ross Robertson, Ph.D., in a statement released Tuesday. Robertson and his team are associated with the Deep Reef Observation Project, an effort to explore life in understudied deep reefs and monitor the ecosystem changes there.

He says that his favorite is the Haptoclinus dropi, the squiggly-looking fish at the top left in the image below.

Representative Caribbean fishes inhabiting the rariphotic zone off Curaçao.

C.C. Baldwin/D.R. Robertson

While the biodiversity the scientists discovered is astounding, the real focus of the new study was to identify and characterize the rariphotic zone, where these reef fish live. This strange zone exists just below a previously defined reef zone called the mesophotic, which extends from 40 to 150 meters below the surface. They scientists hypothesize that it could be a “coral reef twilight zone” — a refuge where shallow-water organisms can seek relief from warming surface waters and deteriorating coral reefs — but its exact purpose is still unclear.

“Reef ecosystems just below the mesophotic are globally under-explored, and the conventional view based on the few studies that mention them was that mesophotic ecosystems transition directly into those of the deep sea,” explained lead author and curator of fishes at the Smithsonian’s National Museum of Natural History Carole Baldwin, Ph.D., in the statement. “Our study reveals a previously unrecognized zone comprising reef vs. deep-sea fishes that links mesophotic and deep-sea ecosystems.”

Classification of faunal zones based on analysis of fish living in Curaçao.

Federico Cabello, Kevin Bryant

With a team of scuba divers and a manned submersible called Curasub, the scientists discovered the rariphotic while exploring an extremely small section of water — just 0.08 square miles of reef. What they found during their 309-meter dive was an underwater world unlike any zone anyone had ever seen: an assemblage of 4,500 reef fish that were closely related and looked similar to reef fish that live in more shallow regions, rather than other deep-ocean fishes. Finding reef fishes that live well below shallow coral reefs suggested they had stumbled on something entirely new.

The manned submersible’s robot arm, which allowed it to safely collect sea creatures and bring them to the surface for study, was key in proving that some of the fish had never been seen before. DNA analysis of the creatures identified species that were new, while genome-level analysis revealed whether the new fish followed the evolutionary patterns of shallow or deep-reef fish.

With this new knowledge, the scientists will continue to monitor the biological and environmental conditions of the deep reefs within the rariphotic. The greatest global threat to coral reef ecosystems in general is climate change, and since we know so little about the rariphotic, it remains to be seen whether living there will help the fish survive the Earth’s increasingly warming waters.

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