Now researchers from Macquarie University have solved this marine mystery. Their explanation was published in May in the journal Biology Letters.

Dr Peter Teske and Associate Professor Luciano Beheregaray found that seahorses evolved their upright position largely because of an expansion of seagrass habitats which occurred in Australasia around 20 to 25 million years ago during the Oligocene period. At that time, vast areas of shallow water allowed for the growth and expansion of seagrass – the habitat favoured by the seahorse.

Because there was no known link between seahorses and horizontally-swimming fish, and only two known fossil sites contain seahorses, scientists had difficulty in coming up with a satisfactory explanation for when seahorses evolved to swimming upright, Dr Teske said.

The answer came when they looked at the DNA of other fish belonging to the same family as the seahorse. The closest relative turned out to be the pygmy pipehorse – similar in looks to a seahorse – but a horizontal swimmer that prefers to live on reefs.

The scientists compared DNA and using molecular dating, were able to work out when seahorses and pygmy pipehorses diverged. They found that their last common ancestor lived around 25 to 28 million years ago – but with the emergence of the seagrass, conditions were favourable for the evolutionary split.

“The only major difference between seahorses and their pygmy pipehorse ancestors therefore is really just the upright posture. And that’s a small evolutionary step that probably did not require many genetic changes,” Teske said.