Finds Raise Debate Over World's Smallest Fish

By Rick Weiss
Washington Post Staff Writer
Monday, February 13, 2006

Mirror, mirror on the wall . . . who's the smallest of them all?

That's the big question for researchers these days after a spate of discoveries of extremely tiny fish.

The jostling over whose fish is the world's smallest reveals a mix of collegiality and competitiveness. The scientists involved claim to be friends. This isn't a race to the bottom, they say. This is about biology, evolution and a wider appreciation of the spectacular menagerie that lives on our fabulously biodiverse planet.

But in the next breath these scientists can't help belittling -- at least a little -- one another's claims. When one confided in an interview last week that Guinness World Records was poised to anoint his fish as the smallest, he could not conceal his sense of triumph.

After hearing that news from a reporter, another scientist couldn't help but voice dismay.

"We sent Guinness an application almost two years ago and never heard back from them," he said with a hint of irritation. "I'm going to have to follow up on this."

Key to the debate is the meaning of the word "small." Does it mean shortest? Skinniest? Most lightweight? And can a species be the smallest if the males are minuscule but the females are relatively obese, as is the case in some species?

"It all depends on which question you ask," said Ralf Britz, an ichthyologist at London's Natural History Museum, who contributed to a recent scientific report that triggered the spat.

It began three weeks ago with a report in a British journal, the Proceedings of the Royal Society B, submitted by Britz and three others. They described a newly discovered freshwater fish, named Paedocypris, that they said was "the world's smallest vertebrate."

Paedocypris adults, captured in fine mesh nets on the Indonesian island of Sumatra, are as small as 7.9 millimeters long, or one-third of an inch. They live in peat bogs -- shallow bodies of water the color of black tea -- where they feed on even smaller, spineless animals called zooplankton. The rest of their time is spent hiding under leaves and bits of debris to keep from being eaten by bigger fish -- which would be every other kind of fish on the planet.

Or almost every other fish, said Ted Pietsch, whose tail fins went up when he read the British report. An ichthyologist at the University of Washington, Pietsch published a report in the journal Ichthyological Research last fall describing a fish just 6.2 millimeters long, or one-quarter inch.

That male specimen of the deep-sea anglerfish Photocorynus spiniceps was pulled from a depth of more than 1,300 meters (4,300 feet) in the Philippine Sea, and it might have escaped attention but for one thing: It was attached to a female that was almost eight times the male's length.

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