Discovery of a “twilight zone” hidden in the ocean, full of previously unknown fish

Credits: iStock

A team of researchers have reported the existence of a “twilight zone” hidden several hundred meters deep in the ocean, full of previously undocumented fish. You can find the details of the study in Scientific Reports.

A team of marine biologists have coined the term for a newly discovered reef-faunal zone, and named it “the rariphotic”. This area is situated between 130 and 309 meters below the surface, and receives very little sunlight. It can be found just below the Mesophotic reef-faunal zone, which extends from around 40 to 150 meters deep. This new zone may be serving as a refuge for reef fish who are seeking to avoid the rising temperatures of surface waters, or who are trying to escape the deterioration of their natural environment.

Numerous researchers are questioning whether these deeper reef zones, sometimes referred to as “coral reef twilight zones” could serve as refuges for sea life. While the researchers in the Smithsonian Institution in the US are trying to answer this question, it has become clear that scientists have only skimmed the surface in terms of understanding the biodiversity of reef fish.

Endemic fish in the Caribbean rariphotic zone studied by the researchers.
Credits: Scientific Reports/Photograph of A. asperilinguis by Patrick Colin, other images by C. C. Baldwin and D. R. Robertson.

“We estimate that 95% of the habitable space on our planet is found in the ocean”, says Carole Baldwin, curator at the Smithsonian National Natural History Museum and the author of the paper. “However, only a fraction of this space has been explored, which is understandable for the areas thousands of kilometers out at sea and many kilometers deep, but the deep tropical reefs are just below the widely-studied shallow reefs. These are not barren areas”, she follows, “but very diverse ecosystems which deserve to be studied.”

The authors defined this new zone based on observations of around 4,500 fish from 71 species, and following around 80 dives going as far as 309 meters deep. Exploring a deep reef area of just 0.2 kilometers squared, the researchers discovered six new types and around 30 new species of fish and invertebrates.

The majority of these fish not only resemble shallow reef fish, but they also look more like them than actual deep sea fish -which belong to very different evolutionary strains. This new zone could in fact be serving as a refuge for reef fish seeking to escape the rising temperatures of the shallow waters and the deterioration of the coral caused by climate change.