It’s one of the golden rules of the natural world – birds live in trees, fish live in water.
The trouble is, no one bothered to tell the mangrove killifish.
Scientists have discovered that it spends several months of every year out of the water and living inside trees.
Hidden away inside rotten branches and trunks, the remarkable creatures temporarily alter their biological makeup so they can breathe air.
Biologists studying the killifish say they astonished it can cope for so long out of its natural habitat.
The discovery, along with its ability to breed without a mate, must make the mangrove killifish, Rivulus marmoratus Poey, one of the oddest fish known to man.
Around two inches long, they normally live in muddy pools and the flooded burrows of crabs in the mangrove swamps of Florida, Latin American and Caribbean.
The latest discovery was made by biologists wading through swamps in Belize and Florida who found hundreds of killifish hiding out of the water in the rotting branches and trunks of trees.
The fish had flopped their way to their new homes when their pools of water around the roots of mangroves dried up. Inside the logs, they were lined up end to end along tracks carved out by insects.
Dr Scott Taylor of the Brevard County Environmentally Endangered Lands Programme in Florida admitted the creatures were a little odd.
“They really don’t meet standard behavioural criteria for fish,” he told New Scientist magazine.