Bizarre Tropical Fish -Mimics Evolution of Land Animals

"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 fish."
~Scott Taylor, researcher

A new study found that 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.

This strange fish, known as the Mangrove Rivulus, is a type of small tropical killifish. It seeks refuge in shallow pools of water in crab burrows, coconut shells or even beer cans left lying on the shore. The fish can be found in the tropical mangrove swamps of Belize, the United States and Brazil.

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 told Reuters. He said he will 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 still keeps functioning.

There are other fish that can survive out of water, but only briefly. 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 for substantial periods of time, but only in a dormant state, whereas the Mangrove Rivulus is still in an active state.

In fact, there is no other known fish that 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. The very first land animals are thought to have evolved from fish that slowly transitioned from their ocean environment to living on the shores, and beyond.

"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," noted Wright.

Surviving on land is not the only unusual thing about these bizarre fish. There are no females. They are either male or hermaphroditic, and females do not seem to exist within the species at all. They are the only known naturally occurring, self-fertilizing vertebrate. Most of the fish have both testes and ovaries, which they use to essentially clone themselves by laying their own, self-fertilized eggs.

Posted by Rebecca Sato