Certain small reef fish use wing-like fins to ‘fly’ underwater, allowing them to cruise at speeds equivalent to tuna, a team of Australian and US researchers has found. The design of the fins has drawn the attention of underwater submersible designers and the US Office of Naval Research.  

The wing-like fins, discovered by researchers from the ARC Centre of Excellence for Coral Reef Studies, allow these fish to handle strong water currents with ease. And the research team has also found that if these specialised fish are lost to overfishing, reefs worldwide would suffer.

“Some species of parrotfish, wrasse and surgeonfish have developed wing-like fins which they move in a flapping motion, just like a bird,” says Dr Chris Fulton, a lecturer in the School of Botany and Zoology at The Australian National University. “This allows them to generate high swimming speeds with relatively little energy so they can move easily around their turbulent, wave-swept habitats.

“Reef fish just 10 cm long can cruise all day at an average speed of 3.6 km per hour, which is the equivalent of a typical person swimming at more than 60 km per hour.

“We were surveying fish populations on the Great Barrier Reef in 2000 and realised some of them had these unusual, wing-like fins. When we looked on coral reefs in Tahiti, PNG, and the Caribbean, we found more of the ‘winged’ fishes. On coral reefs all around the world, there are fishes that have evolved the same answer to survival in wave-swept habitats – it’s  a beautiful piece of convergent evolution in action.”

Measuring the speed of the fish in a flow tank and while swimming naturally around the reef, Chris found some species could move at up to 10 body lengths per second. By comparison, Olympic champions reach speeds of just 1.3 body lengths per second, and then only for the brief 22 seconds of the 50-metre freestyle sprint.

Most reef fish have paddle-shaped fins, which they move in a rowing action: a backward power stroke to create thrust, followed by a forward recovery stroke. But thrust is only produced on half the fin strokes – limiting these fish to slow swimming speeds.

“We discovered that the fastest reef fishes have tapered fins, which they tend to flap in a figure-of-eight sweep that creates thrust on every stroke. This flapping motion creates lift forces from water flowing over the fin, just as with air moving over the wing of a bird,” Chris explains.

These underwater fliers have been very successful and colonised coral reef habitats all around the world. “Reefs can be challenging places for a fish. The crashing waves create flow speeds that are equivalent to us experiencing cyclone-force winds on land.”

“Some of these fish play a crucial role in maintaining the health of our reefs by eating the fleshy algae that would otherwise overgrow and smother live corals,” Fulton says. “But they’re at risk. In many areas of the Pacific we are seeing these specialised shallow-living fish disappear due to overfishing. If these specialised swimmers go, they won’t be replaced by slower, deeper living fish. So it is important to plan our marine reserves to protect them.”

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

Now Chris and his collaborators David Bellwood from James Cook University and Peter Wainwright from the University of California, Davis are investigating how and when the fish ‘wings’ evolved.

Their work has taken the interest of the US Office of Naval Research, and may influence the design of future remote control submersibles.

“While we are seeing rapid advancements in submersible design, such as the multi-finned Madeleine recently developed by Vassar College, we are yet to match the speed and efficiency achieved by these wing-finned coral reef fishes,” Dr Fulton says. “We still have much to learn from over 50 million years of reef fish evolution.”