Forked Tails Allow 'Twilight Zone' Fish To Swim Silently Past Deep-Water Predators

Reef fish with forked tails may be able to dive deeper below water, increasing their chances of long-term survival, researchers from the ARC Centre of Excellence in Coral Reef Studies reveal in a new study. Understanding fish depth ranges could ultimately lead to improved conservation methods.

"We found that the 'caudal fin aspect ratio', which measures the shape of the fishes tail, is the best predictor of which fish can live in a range of deep and shallow reefs," Dr. Tom Bridge, lead author from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said in a news release. "In other words, fishes with more forked tails are significantly more likely to be found in both shallow and deep habitats than species with more rounded tails."

To date, researchers are unsure exactly why fish with "more forked tails" are able to dive deeper; in other words, why such a tail provides an advantage. Perhaps a forked tail allows fish to swim silently and remain off predator radar. 

"The capacity for 'stealth swimming' is particularly important in deeper habitats, where light irradiance and wave energy are low and species rely on sensing changes in water pressure to capture prey and avoid predators," Dr. Bridge added.

When most people think of coral reefs, they think of shallow, sun-lit waters; however, there are "twilight zone" reefs located between 50 and 150 meters underwater that support diverse species. Forked fins may come in handy there.

While deep, pitch-black, cold waters present a series of threats, venturing into the unknown depths of the "twilight zone" has its benefits, too. Researchers suggest deep-diving species may be able to evade natural disasters such as cyclones and coral bleaching, as well as population declines and possible extinction.

"Identifying which species can occur over a broad depth range is important for understanding which fish are more vulnerable to local population declines and extinction, particularly from disturbances such as cyclones and coral bleaching events," Dr. Osmar Luiz, study co-author from Macquarie University, concluded. 

The study was recently published in the journal Proceedings of the Royal Society B.

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