What is driving them out of the water?
More than 350 million years ago, a fish emerged out of the confines of its aquatic habitat and crawled onto land for the first time. And that fish started a trend.
According to a study published in the journal Evolution, fish have since made the transition from water to land numerous times.
“I think intuitively it would seem that a ‘fish out of water’ is an unusual thing. The cliché says it all, really,” study lead author Terry Ord from the University of New South Wales in Australia tells The Science Explorer. “But it is striking about how often it occurs.”
By compiling a list of all living fish reported to be active on land — 130 species from 33 families — and examining their evolutionary relationships, the researchers discovered that in most cases, this behavior evolved independently in the different families.
Venturing out of the water was always believed to be a rare occurrence for fish because it is fraught with challenges; landlubbers are faced with the fundamental tasks of devising new ways to breathe, move, and reproduce on land. But one of the biggest stumbling blocks that these fish encounter on their terrestrial stomping grounds is the risk of drying out.
The researchers noticed that fish that spent time on land tended to have more cylindrical bodies. Because rounder shapes have relatively lower surface areas, a fish on land with a tube-like body would have less skin directly exposed to the dry air than a similarly-sized flat fish.
“In the case of these fish, being round sausage-shaped fish rather than streamlined objects gives them more of an edge against desiccation out of water,” Ord explains.
A rounder shape might also assist bottom-dwelling fish in moving around the seafloor with ease, which would predispose them to crawling on land.
One particular group of intertidal fish, called blennies, were found to have made the transition to land at least three, and as many and seven, different times. When in the water, these fish spend a lot of time treading on the seafloor, and the detritus on which they feed is also available on land. Both of these traits might well have facilitated the blennies’ transition to land.
Why these fish would choose to leave the water is still a mystery, though Ord offers some ideas.
“The intertidal zone is a pretty dangerous environment because of predation,” he says. “There are a lot of nasty predators hunting in the intertidal, so spending time out of the water would be beneficial for escaping those enemies too.
He adds, “The action of the waves up against the rocks also creates lots of potential nest holes that blennies might take advantage of that are less in abundant under water.”
These fish also often encounter the problem of getting stuck in shallow rock pools, which may heat up and leach their oxygen in low tide, Ord explains. Fish may therefore actively strand themselves out of the water to avoid suffocating.
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