By Paul Rincon
BBC News Online science staff
Tuna and mako sharks have evolved very similar swimming anatomy despite being separated by millions of years of evolution, the journal Nature reports.
Mako sharks are related to the famous great white
The findings provide an exceptional example of "convergent evolution", US and German researchers claim.
The two fish types diverged on separated evolutionary paths about 400 million years ago, yet their bodies have come together on a similar design.
This could be the result of exploiting similar ecological niches, experts say.
"It's a fairly major change in anatomy to have that happen. That is the thing that makes it remarkable to us, that this arose independently in the two fish," Professor Robert Shadwick, of Scripps Institution of Oceanography, US, told BBC News Online.
Most fish move their tails by sending a wave of contraction along the length of the body. But this contraction must be limited in its power because each movement of the tail involves bending much of the fish's body.
Red and white
Tuna can drive a powerful tail movement by making forceful muscle contractions in the centre of their bodies. This is facilitated by a unique anatomical arrangement under the skin.
The complex evolutionary changes that gave rise to this anatomy were thought only to have occurred in tuna.
But when researchers from Scripps studied the swimming of mako sharks - related to the famous great white shark - they made some important findings.
The scientists placed mako sharks on a kind of aquatic treadmill - a tube that circulates water from the tail of the fish to the head - so that they swam in place.
Tuna are high speed predators
They measured the shortening, or contraction, of two different types of muscle in the sharks: red and white.
Fish use white muscle for short bursts of power, but it rapidly fatigues. Active swimming requires red muscle, which is fatigue-resistant.
In tunas and makos, muscle mass is concentrated at the centre of the body and linked to the tail via long tendons. In other fish, the muscle is arranged in blocks along the body attached to the vertebral column, which corresponds with their style of swimming.
When mako sharks swim actively, shortening of red muscle occurs in step with bending of the backbone nearer the tail. Therefore, red muscle at the centre of the body causes movement toward the end of the tail, a pattern of movement seen in tuna.
Professor Ian Johnston, of St Andrews University, UK, said he could think of more striking examples of convergent evolution.
"If you take biological antifreezes, completely unrelated taxonomic and geographical groups have come up with the same molecule, in some cases deriving it from a different starter molecule.
"[Mako sharks and tuna] inhabit tropical, semi-tropical oceans which are effectively like deserts in terms of food supply. So they have to migrate over long distances, swim very efficiently; they're large and reach high speeds.
An aquatic "treadmill" was used in the experiment
"They're supreme predators of the ocean, so they've both evolved from different starting points to that top predator, pelagic niche."
Dr Adam Summers, of the University of California, Irvine, US, speculated that understanding the mechanisms behind the locomotion of tuna and mako sharks could lead to high-speed autonomous underwater vehicles.
"The data from these two high-speed swimmers seem clearly to endorse a solution that puts as much emphasis on the placement of actuators as on the overall shape of the vehicle," he said.