Seahorse Evolution Breakthrough

A paper published last month in Biology Letters uses the pygmy pipehorse to explain how seahorses evolved.

Two Australian researchers - a marine biologist from Macquarie University in Sydney and a biological oceanographer from Flinders University in Adelaide - have used DNA testing to explain one of the mysteries of seahorse evolution: when and why did they start swimming upright?

Dr. Peter Teske and Professor Luciano Beheregaray used DNA testing to find the seahorse's closest relative - the pygmy pipehorse. The pygmy pipehorse has a tail like a seahorse - built to cling on to plants instead of use to swim - but it swims horizontally.

The scientists examined the two species' DNA and worked out when they split by using molecular dating - a technique which counts the accumulation of differences in DNA to discover when an evolutionary divergence occurred. Two fossil seahorses were used to calibrate the DNA's rate of evolution. They calculated that the nearest ancestor of both species lived between 25 and 28 million years ago.

This coincides with the Oligocene era when the Australian continental plate and the Eurasian plate collided, pushing up sea floors and creating relatively shallow seas instead of the deep oceans that had been there before. This prompted the rapid growth of savannahs of sea grasses - the preferred habitat of the seahorse.

With the appearance of these new savannahs conditions were favorable for the evolutionary divergence. Sea grass is perfect camouflage for an upright-swimming fish and the seahorse adapted and spread across the world. The pygmy pipehorse, which lives on algae on reefs, stayed in Australasia.

Professor Beheregaray commented: "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 sea grass."

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