Leaping land fish avoids predators by blending in

Life for a fish out of water can’t be easy, but the Pacific leaping blenny makes it work. These legless land-dwelling fish (Alticus arnoldorum), just four to eight centimeters long, live on rocks at the ocean’s edge on the shores of places like Guam and Samoa. Despite the lack of limbs, blennies can leap, a move made possible with a twist of their tails and a push off the rock, scientists discovered in 2010. The fish can flit about quickly, but that doesn’t explain how they avoid getting eaten by the birds, land crabs and lizards that find the blenny to be a tasty meal.

Courtney Morgans and Terry Ord of the University of New South Wales in Australia suspected that leaping blennies were making use of camouflage. While all blennies are decorated in earth tones, there are subtle color differences across populations of the fish.

The researchers began by determining how closely five populations of Pacific leaping blenny in Guam matched the rocks on which they lived. In all five groups, the color of the fish was virtually indistinguishable from their rocky backgrounds. Morgans and Ord then set up a predation experiment to test whether the critters that eat blennies are really fooled by the camouflage. They made plasticine models of the fish and set them out on rocks and on the beach. The fake fish on the beach were much more likely to be attacked; they had more than twice as many puncture marks and missing body parts.

“We can say … that cryptic body coloration … does provide an effective means of minimizing predation,” the researchers write in the December Animal Behaviour, “but it is also a strategy that is highly habitat specific.” That realization led the researchers to wonder whether the camouflage strategy was one that evolved after the fish first moved onto land or if the coloring was already there and helped the transition out of the water.

It’s certainly possible that the Pacific leaping blenny started out as, say, a bright blue fish when it became a land dweller. A few advantageous mutations may have occurred, creating the muddled stone coloring seen today. Those camouflaged fish would have had a survival advantage, and over time all the bright blue blennies would have died out, eaten by birds or lizards and unable to pass their distinct coloring to the later generations. That’s how we often think of evolution.

But it’s probably not the case with the leaping blenny, say the researchers. They collected examples of 12 species of fish from Guam — seven that live only in the water and five that are amphibious — that are closely related to Pacific leaping blennies. Nearly all of those species are similar in color and brightness to the Pacific leaping blenny. The blenny’s marine ancestor would probably have blended right into this group, Morgans and Ord say. And that rocklike coloring would have aided its transition to land, giving the species time to adapt to that drier environment without too many individuals getting eaten.

Pacific leaping blennies live in the south and west Pacific. Blennies like those shown here in Guam evade predators by blending in with their habitats.  Video: terryjord/YouTube for the Ord Lab