Fish Out of Water! The mangrove killifish can live out of the water for several months each year.

The mangrove killifish (Rivulus marmoratus) is quite an odd fish — and not just because it can breed without a mate. Scientists recently discovered that it can live inside trees, out of the water, for several months each year. The mangrove killifish can actually change its biology and metabolism so that it can breathe air and live out of the water.

Normally found living in the mangrove swamps of Latin America, Florida and the Caribbean, this killifish can flop its way to the temporary housing of rotting branches and tree trunks when the water dries up around the mangroves.

Researchers discovered hundreds of these normally aggressive fish packed into the tight quarters of rotting logs. Temporarily suspending their aggression, these fish were lined up mouth to tail down the length of paths carved out by insects.

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The World's Smallest Fish

A group of scientists claim to have discovered the world's smallest fish in an acidic peat swamp in Indonesia. (Photo: Maurice Kottelat, Carnol, Switzerland and Raffles Museum/AP Wide World)

It could not have been easy to see the tiny swimmer, but a group of scientists claim to have discovered the world's tiniest fish. The little creature was found recently in an Indonesian swamp.

For now, the fish is known as Paedocypris progenetica. It has a transparent, or see-through, body. Its head doesn't have a skeleton, leaving its brain vulnerable to its surroundings. The fish, which is female, can grow to be only 7.9 millimeters or (0.31 inches) long, about the size of a large mosquito.

Maurice Kottelat, a fish expert from Sweden, led the team of researchers that made the discovery. Kottelat and his team reported their discovery in the Proceedings of the Royal Society, a British scientific journal.

"You don't wake up in the morning and think 'today we will find the smallest fish in the world,'" said Kottelat in an interview. He and his team were most excited by the possibility of having discovered the world's smallest vertebrate, an animal with a backbone. "What's important is finding a complete vertebrae [backbone] in a body so small," said Kottelat. All fish, reptiles, amphibians, birds, and mammals (including humans) are vertebrates.

A "Little" Competition

It wasn't long after Kottelat's announcement that another researcher challenged the claim with a discovery of his own. Fish expert Ted Pietsch, a researcher at the University of Washington, says that he has discovered a fish that is smaller than Kottelat's discovery.

According to Pietsch, the male Photocorynus spiniceps is a tiny ocean fish measuring only about a quarter inch. That statistic would actually make Pietsch's fish the world's smallest vertebrate. Kottelat's would come in at a close second

When Pietsch got in touch with Kottelat to mention the fish—four of which he keeps in his office—Kottelat did not argue. He kindly gave the title of "world's smallest vertebrate" over to Pietsch's unique fish.

"So our little fish is only the smallest freshwater vertebrate," Kottelat wrote. "Now I wait until the next smallest fish is discovered."