Taking the phrase “fish out of water” to new heights, research published in The American Naturalist has shown that amphibious blenny fish are literally leaping out of the ocean and on to dry land in order to escape predators.
Blennies, like their distant cousins the mudskippers, are well known as fish that can survive for considerable periods on land, but until now there had been little investigation into why they leave the water.
Observing the behaviour of three species of blenny, all from the sub-family Blennioidei and found on Rarotonga in the Cook Islands, a team of researchers from the University of New South Wales in Australia was the first to examine the question.
The team discovered that the fish move up and down the shoreline to stay out of reach of predatory fish that move with the tide. The researchers had noticed that some species of blenny emerged from the water to spend various amounts of time in rock pools and exposed rocks.
“An escape from predation has been a classic hypothesis for why animals might move into a new environment, but our study is one of the first to actually confirm that this happens in nature,” explains lead author Terry Ord.
To test the hypothesis, the team conducted surveys of the number of blennies and their predators across both water and land, documenting any attacks that occurred.
They tallied up the number of blennies found in three environments: purely aquatic, intertidal, and terrestrial. They also recorded the numbers of birds found loitering around on land, because they comprised the principal non-aquatic predators.
In a novel move, the researchers fashioned fake blennies out of Plasticine, and then scattered them across the dry land and intertidal habitats. They then observed how often the birds struck the decoys.
The results? At low tide, most of the blennies positioned themselves in the intertidal zone while their aquatic predators were restricted to deeper waters. But as soon as high tide hit, the blennies moved onto dry land, indicating that they were actively avoiding areas of high predation risk.
Observing the fate of the fake blennies, the researchers noted that those in rock pools were attacked by birds more often than those on dry land.
The researchers note that the reverse sea change – from ocean to land – made by these fish to avoid water-bound predators might well be a way of understanding more generally how species evolve to colonise new environments in nature.
“The puzzle of why fish emerge onto land is effectively the same question of what motivates any organism to move into an environment starkly different from its ancestral condition,” they write.