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Underwater Twilight Zone Rariphotic Has So Many Undiscovered, Unique Fish Species

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A never-before explored spot in the ocean was discovered by scientists. This place dubbed as a twilight zone is full of new species of fishes and marine animals.  

A coral reef system in Curacao, a Dutch Caribbean island separated by a deep ocean trench off the mainland of South America, features an entirely new world of biodiversity.

The place called rariphotic is a layer of the ocean between the depths 400 and 1,000 feet where light is limited.

A Whole New World

"It's estimated that 95% of the livable space on our planet is in the ocean, yet only a fraction of that space has been explored," says Carole Baldwin from the Smithsonian National Museum of Natural History and the study's lead author.

The mesophotic coral ecosystems located at depths of 40 to 150 meters have become a refuge for organisms escaping the warm waters and deteriorating reefs due to climate change. Reefs below the mesophotic area are largely unexplored.

Scientists from the Smithsonian Tropical Research Institute and the National Museum of Natural History observed a total of 71 fish species, more than 30 of which are previously unknown species. Most of the fishes found in the rariphotic are much more similar to reef fishes rather than deep-sea fishes.

"About one in every five fish we're finding in the rariphotic of the Caribbean is a new species," says Dr. Ross Robertson, a marine biologist at the Smithsonian Tropical Research Institute and a co-author of the new study documenting the discoveries.

To identify the new species of fish, scientists use modern DNA techniques such as DNA barcoding to determine which species were previously documented and which are entirely new.

Deep Reef Project

Exploration of the low-light reef spot began in 2011 as part of the Deep Reef Observation project that investigates tropical reefs and identifies new species.

So far, the project has discovered more than 50 new fish and invertebrate species. This initiative tracks biological and environmental patterns on shallow to deep reef slopes.

To be able to observe the deep-sea twilight zone, researchers launched expeditions on rariphotic zone using the Curasub, a mini-submarine that can stay submerged in water for up to eight hours at normal pressures.

Water in the rariphotic zone is deep enough for light to penetrate and support the formation of algae that nourishes reef-building corals.

The research also identified a new classification of coral reef faunal zones. Altiphotic is a well-lit zone where coral reefs are abundant. Tropical reef corals and their algae can survive in the mesophotic zone that has medium light. Deep aphotic that has no light and is located 300 meters below.

The full study is published in the journal Nature.

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