‘Twilight zone’ fish swim silently with forked tails

An international team of researchers has identified a way to predict which reef fish can live across a greater range of depths, from shallow depths to the mesophotic or “twilight zone,” increasing their chances of surviving natural disasters such as cyclones and coral bleaching.
Study lead author, Dr Tom Bridge from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, says the research, published in the journal Proceedings of the Royal Society B, found that tail shape can help predict if a fish is likely to exist across a range of water depths.

“We thought that the ability to see in deep, dark waters would influence which fish could live in both shallow and deep habitats; however, we found that the ‘caudal fin aspect ratio,’ which measures the shape of the fish’s tail, is the best predictor of which fish can live in both sun-drenched shallows and the ‘twilight zone’,” Dr Bridge says.
“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.”
Dr Bridge says it’s not known exactly why this is the case, though it’s suspected that the forked tail allows fish to swim more silently.
“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.”
Coral reefs are typically thought to occur in shallow, sun-lit waters, but new technology is revealing that reefs in the ocean’s ‘twilight zone’—50–150 m deep—support diverse and unique communities.
However, conditions on these deep reefs can be challenging for coral reef fishes, with low light, high pressure, and low temperatures.
Study co-author, Dr Osmar Luiz from Macquarie University says species that can survive in the twilight zone may be less susceptible to population declines and 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.”
The researchers say the next step is to understand exactly what it is about the forked tails that provides fishes with such an advantage in deeper water.
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