A tropical fish that lives in mangrove swamps across the Americas can survive out of water for months at a time, similar to how animals adapted to land millions of years ago, a new study shows.
The Mangrove Rivulus, a type of small tropical killifish, seeks refuge in shallow pools of water in crab burrows, coconut shells or even old beer cans in the tropical mangrove swamps of Belize, Brazil and the United States.
When their habitat dries up, they live on the land in logs, said Scott Taylor, a researcher at the Brevard County Environmentally Endangered Lands Program in central Florida.
The fish, whose scientific name is Rivulus marmoratus, can grow as large as three inches. They group together in logs hollowed out by insects and breathe air through their skin instead of their gills until they can find water again.
The scientific breakthrough came after a trip to Belize.
"We kicked over a log and the fish just came tumbling out," Taylor said. He plans to publish his study on the fish in The American Naturalist journal early next year.
In lab tests, Taylor said he found the fish can survive for up to 66 days out of water without eating, and their metabolism keeps functioning.
Some other fish can survive briefly out of water. The walking catfish found in Southeast Asia can wriggle over land for hours at a time, while lungfish found in Australia, Africa and South America can survive out of water, but only in a dormant state.
No other known fish can be out of water as long as the Mangrove Rivulus and remain active, according to Patricia Wright, a biologist at Canada's University of Guelph.
"They can survive for weeks without really dropping their metabolic rate. They remain relatively responsive and active for weeks in air," she said.
The fish may hold clues to how animals evolved over time.
"These animals live in an environment that is similar to conditions that existed millions of year ago, when animals began making the transition from water onto land," she added.
Surviving on land is not the only unusual behavior exhibited by the fish. They have both testes and ovaries and essentially clone themselves by laying their own, already fertilized eggs.
"This is probably the coolest fish around, not only do they have a very bizarre sex life, but they really don't meet standard behavioral criteria for fishes," said Taylor in a summary of his paper.