According to a new paper published in the journal Animal Behaviour, the Pacific leaping blenny (Alticus arnoldorum) – a unique fish that lives on land and can leap large distances – uses camouflage to avoid attacks by predators such as birds, lizards and crabs.

The Pacific leaping blenny is a 4 to 8-cm-long tropical fish found in reefs in Samoa and the Marianas, Society, and Cook Islands, in the western and southern Pacific Ocean. It remains on land all its adult life but has to stay moist to be able to breathe through its gills and skin.

Pacific leaping blennies move quickly over complex rocky surfaces using a unique tail-twisting behavior combined with expanded pectoral and tail fins that let them cling to almost any firm surface. To reach higher ground in a hurry, they can also twist their bodies and flick their tails to leap many times their own body length.

Australian biologists from the University of New South Wales studied Pacific leaping blennies in their natural habitat on the tropical island of Guam.

“This terrestrial fish spends all of its adult life living on the rocks in the splash zone, hopping around defending its territory, feeding and courting mates. They offer a unique opportunity to discover in a living animal how the transition from water to the land has taken place,” said study senior author Dr Terry Ord.

Dr Ord with co-author Courtney Morgans measured the color of 5 different populations of the fish around the island and compared this with the color of the rocks they lived on.

“They were virtually identical in each case. The fish’s body color is camouflaged to match the rocks, presumably so they aren’t obvious to predators,” Dr Ord explained.

To see if background matching reduced predation, the scientists created realistic-looking models of blennies out of plasticine.

“We put lots of these model blennies on the rocks where the fish live, as well as on an adjacent beach where their body color against the sand made them much more conspicuous to predators,” Dr Ord said.

“After several days we collected the models and recorded how often birds, lizards and crabs had attacked them from the marks in the plasticine. We found the models on the sand were attacked far more frequently than those on the rocks. This means the fish are uniquely camouflaged to their rocky environments and this helps them avoid being eaten by land predators.”

The team then studied the body color of closely related species of fish, some of which lived in the water and some of which were amphibious, sharing their time between land and sea.

“These species provide an evolutionary snapshot of each stage of the land invasion by fish,” Dr Ord said.

The similarities in color between these species and the land-dwelling fish suggest the ancestors of the land-dwelling fish already had a coloration that matched the rocky shoreline before they moved out of the water, which would have made it easier for them to survive in their new habitat.

______

Bibliographic information: Courtney L. Morgans & Terry J. Ord. 2013. Natural selection in novel environments: predation selects for background matching in the body colour of a land fish. Animal Behaviour, vol. 86, no. 6, pp. 1241–1249; doi: 10.1016/j.anbehav.2013.09.027