Eels hiding in shady underwater caves seem to adapt to the dark. This is because some of them were born blind in one eye and without skin over the organ.

What you need to know:

The newly described bean-eyed moray eel (Uropterygius cyamommatus) is the first known eel species to inhabit anquialine caves, caves carved into volcanic or limestone rock connected to the ocean and whose water levels fluctuate with the tides.

The study on this eel species was published in the scientific journal Raffles Bulletin of Zoology at the end of March.

Eel study

During expeditions to Christmas Island in Australia and Panglao Island in the Philippines, scientists found two specimens that had no visible left eyes, suggesting that the eels may have adapted to the dark environment by going blind, one eye at a time.

Only two specimens from Christmas Island have a reduced left eye and we cannot know if this is natural or if they only damaged their eyes after birth. But their eye proportions are the smallest we’ve seen in moray eels, so we speculate that may be the result of adaptation to the aphotic or low-light environment.

Wen-Chien Huang, PhD candidate in marine biotechnology at National Sun Yat-sen University in Taiwan and lead author of the research, in an interview with Live Science

Cave explorers first caught this species of eel on the island of Panglao in 2001. And several specimens are housed at the Lee Kong Chian Museum of Natural History in Singapore. But until now no one had recognized them as their own kind.

How the study was done

For the new study, Huang and his colleagues analyzed nine samples collected between 2001 and 2011. Two of them had “a reduced left eye embedded in the skin,” with no apparent change in the underlying bone structure.

The researchers think they have captured evolution in action and that, in the absence of light, the skin that invades the eels’ eyes could save them from the high energy cost associated with vision.

It is not uncommon for fish that live in caves to go completely blind. And many of the nearly 300 species of fish that live in caves have been there.

A species closely related to the eel studied in the research, the low-backed moray eel (U. oligospondylus), has similarly reduced eyes and hides in the shadows among wave-broken rocks, where it relies on its sense of smell to locate prey.

Scientists aren’t yet sure why skin is growing on the eels’ eyes and whether this possible adaptation to cave habitat is spreading among the population.

Due to the low number of preserved specimens, the researchers did not perform genetic testing and other molecular tests to answer these questions, Huang explained. “These are questions that interest us, but they can only be answered when more fresh specimens become available,” she added.

Trajectory of these eels

Researchers caught the most recent specimens of these eels more than a decade ago in traps and preserved them in alcohol.

It is unclear why or when this eel species retreated to the dark depths of the cave. But the authors of the new study suspect it may be related to their voracious appetites.

I think one of the reasons they went to inhabit the caves is the food source, as there are abundant crustaceans inside the caves.

Wen-Chien Huang, lead author of the research, in an interview with Live Science

Scientists who caught them reported that the “gluttonous” eels greedily devoured the bait they used to lure them.

With information from living science

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Source: Olhar Digital