| Seagrass link to seahorse upright posture|
|Seahorses evolved their upright posture some 25 million years ago, thanks in part to an expansion of vertical seagrass habitat, Australian researchers have found.|
Associate Professor Luciano Beheregaray of Flinders University and Dr Peter Teske of Macquarie University report their findings in the journal Biology Letters today.
Seahorses are unique fish with a horse-shaped head and a habit of swimming upright.
Beheregaray says it has been hard for scientists to work out when exactly seahorses evolved to swim upright.
This is because there are only two known fossils of seahorses - the oldest dating back to 13 million years - and no link between these and horizontally-swimming fish had been found.
"When you look back in time, you don't see intermediate seahorse-like fish," says Beheregaray.
But, he says, there are fish alive today that look like horizontally-swimming seahorses and these could provide clues as to when seahorses evolved to be upright.
Beheregaray and Teske compared the DNA of seahorses and other species from the same family to find out which was the closest living relative to seahorses.
"The pygmy pipehorses are by far the most seahorse-like fish on earth. They do look like the seahorses, but they swim horizontally," says Beheregaray.
He and Teske used molecular dating techniques, which relies on the accumulation of differences in the DNA between the two species to work out when they diverged.
The researchers used the two existing fossil seahorses to calibrate the rate of evolution of DNA in their molecular clock.
And they discovered that the last common ancestor of seahorses and pygmy pipehorses lived around 25 to 28 million years ago.
Beheregaray says at the time that seahorses arose during the Oligocene epoch coincided with the formation of vast areas of shallow water and expansion of seagrass in Australasia - where Teske has previously showed seahorses first evolved.
Seagrass was the perfect habitat for an upright-swimming seahorse, which could camouflage itself in the vertical seagrass blades, he says.
The horizontal-swimming pygmy pipehorses, by contrast, thrived in large algae on reefs and didn't have the need to evolve the upright posture.
"The two groups split in a period when there were conditions favouring that split," says Beheregaray.
"It's like us. We started walking upright when we moved to the savannahs. On the other hand, the seahorses invaded the new vast areas of seagrass."