Chris Fulton, Australian National University

Fish first began crawling onto dry land about 400 million years ago, kicking off an evolutionary chain of events that led to humans. But their reasons for exiting the sea have been uncertain.

To look for clues, Terry Ord at the University of New South Wales in Australia and his colleagues have been studying several species of blenny fish or ‘blennies’ at Rarotonga, the largest of the Cook Islands.

At low tide, blennies are commonly found swimming in rock pools around the edges of the island. But when high tide moves in, they climb up to dry land and shuffle around the rocks until the tide retreats.


The researchers found that this is most likely to avoid predators that swim in with the rising tide – mainly bigger fish like flounders and lionfish.

To test what would happen if blennies did not have an escape plan, they made plasticine models and submerged them in the sea. The blenny mimics ended up with puncture wounds, bite marks and chunks missing.

Island life

Of course, there are still dangers for blennies on land – the researchers observed the occasional bird attack – but predation risk on land is about a third that of underwater.

What’s more, moving onto land has additional benefits for blennies. Holes in the rocks provide sheltered nests for laying eggs, and they can maintain their diet of algae and bacteria by nibbling them off rocks.

In fact, several species of blenny fish at Rarotonga and elsewhere have already made the full transition to land-dwelling species. They continue to breathe with their gills, but have developed stronger tail fins that allow them to jump from rock to rock.

Other fish families have also crossed the land-water interface over the last 400 million years – about 33 families in total.

Ord believes that many evolutionary processes – not just the transition of fish to land – have been driven by the need to escape predators. It is often assumed that animals move homes to find new sources of food, but in many cases, escaping predators is a stronger impetus, he says.

“If you never look over the fence, you’ll never know that the grass is greener,” he says. “However, if you’re forced to the other side to escape something, you may then realise it has additional benefits and want to stay there and adapt.”

Mike Lee at Flinders University in Adelaide, Australia, agrees that this escape instinct is a powerful driver. “The convergent evolution of terrestriality in blennies on oceanic islands – which are relatively free of land predators – is definitely strong evidence that they are climbing out of the water to escape being eaten,” he says.

However, the jury is still out on whether predator escape is a more compelling reason to relocate than the desire to find more food, says Lee.

“A lot of aquatic reptiles and mammals seem to be attracted to food underwater, including iguanas, sea snakes and sea otters,” he says. “They’d rather eat well, even if it means they are more likely to be eaten.”

Journal reference: The American Naturalist, DOI: 10.1086/691155

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