Video of a leaping, legless fish called Alticus arnoldorum, or the Pacific leaping blenny, hopping from rock to rock with a tail-twirling technique.

You’ve heard of flying fish, but did you know about the legless leaping fish, which spends its life on land? Scientists studying these strange, rock-dwelling creatures called blennies have found that they can actually camouflage themselves against the rocks on which they live.

The findings, described in the journal Animal Behaviour, provide some insight into the factors behind the evolution of a water creature into a land animal.

There are many species of blenny, and they’re largely water-bound – but the Pacific leaping blenny, Alticus arnoldorum, spends its life on rocks in the splash zones along the shores of Guam. It ranges from roughly 1.5 inches to 3 inches long and leaps by twisting its tail. Although it lives on land, it sticks to the splash zones along the shoreline because it has to stay moist enough to breathe through its skin and gills.

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Such land-dwelling blennies are fascinating species to study, because they offer a window onto the transition between land and sea. This is of interest to biologists, given that all terrestrial vertebrate animals – humans included – are thought to have descended from a lineage of fish that made moved from water to land.

“You could say we’re hairy fish,” said study coauthor Terry Ord, an evolutionary ecologist at the University of New South Wales in Australia.

Scientists have study these gilled, legless creatures to see how a fish out of water might actually end up thriving in this new environment.

“How do you deal with breathing on land, or feeding on land, or exposure to new predators?” Ord asked.

Ord and his student, lead author Courtney Morgans, wanted to answer the last part of that question. In water, they're fair game for larger fish. And on land, no longer protected by a layer of water, blennies have to deal with a different set of predators: birds, lizards and land crabs. So the researchers first looked at five different blenny populations around the ocean and compared their body coloration to the rocks they lived on. In each case, the fish and rocks matched – a sign that they were probably using their greenish-brown bodies and their mottled patterning as camouflage.

To test this idea, the researchers molded plasticine – essentially play-dough for grown-ups, but more brown and less fun – by hand to create hundreds of fake model blennies. They put some in their native, rocky habitats, and others on nearby beaches where their body color stood out against the bright sand – potentially making them easy targets.

Sure enough, after they collected the models, the sand-sitters were riddled with far more V-shaped gouges from birds, tooth marks from lizards and blunt-scissor snips from land crabs than their rock-dwelling brethren were.

Then they studied other closely related fish, some of which were water dwellers and others that were amphibious. They found that the swimmers shared very similar colors with the landlubber fish – which means that perhaps the blennies already had the right coloration to survive on land before they ever set fin on it. If the creatures hadn’t, they might have quickly gotten picked off before they could establish themselves on this new territory. 

“They’re evolutionary snapshots of the transition from water onto land,” Ord said of these closely related species. “And so we can study the challenges in each of those steps that these animals have to face.”

Not just any fish can survive this sort of transition, Ord said – they have to have the right set of characteristics, shaped by their previous environment, to help them evade a whole new set of dangers. Next he plans to study why a fish leaves water for land at all.

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