How Seahorses Evolved to Swim Standing Up

Seahorses are master mimics that use their cryptic colors and upright posture to blend in with plants. When and why the animals developed these unusual characteristics has been a mystery—until now, scientists say.

Tiny brittle bones, such as seahorses’, usually break down before they can be buried under sediment and preserved as fossils. As a result, precious little evidence remains of seahorse evolution.

So to explore the evolutionary history of seahorses, Peter Teske and Luciano Beheregaray of Macquarie University in Sydney, Australia, compared modern seahorse genes with those of the fishes’ nearest living relatives, pygmy pipehorses.

The scientists discovered that seahorses and pipehorses diverged around 25 million years ago. At that time, tectonic activity where Indian and Pacific Oceans meet created many shallow-water habitats.

Seagrasses, which thrive in shallow water, spread rapidly through these newly formed environments. The researchers think that the ancestors of modern seahorses followed.

Horizontally swimming fish living in seagrass beds don’t blend into the environment well and are easily picked off by predators.

A solution seems to have evolved in seahorse ancestors: the upright body shape, which enables seahorses to seamlessly blend in with surrounding seagrass.

“I found this all a bit surprising,” Teske noted. “When I saw my first specimen of pygmy pipehorse, I thought, This looks so much like a seahorse that it must be a seahorse that has, for some reason, returned to swimming in a horizontal position.

“But the genetics clearly show that this species is in fact ancestral.”

Findings published in the journal Biology Letters.

nationalgeographic.com