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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.

Soon after hatching from their eggs, it turns out, males of this species use their oversize eyes and nostrils to find a female. They bite into her flesh and literally fuse with her body, creating what is essentially a single, hermaphroditic creature. The male never eats again, getting its nutrition directly from the female's blood, its role reduced to that of a mere sperm provider.

"They're rare as all get out, and they're almost always dead when they come up" in nets, Pietsch said. But at 6.2 millimeters, he asserted, that male is the smallest fish ever found. And as far as he is concerned, the contest ended last Tuesday when he got the call from Guinness World Records.

But Britz and co-worker Maurice Kottelat bristled at Pietsch's claim.

"We are talking about the smallest fish species ," said Kottelat, a researcher with the National University of Singapore and president of the European Ichthyological Society. "If the female was also small then you could discuss this. But you cannot go by the mature size of just one sex."

Britz goes further, ridiculing Pietsch's males as little more than appendages to the female.

"That's not true," Pietsch countered, sounding a little offended. "It's got a heart and gills. It is not a degenerate castoff."

Which brings us to Schindleria brevipinguis , or the "stout infantfish," another finalist in the marathon of miniatures.

H.J. Walker of the Scripps Institution of Oceanography in La Jolla, Calif., discovered that species in 2004 along with William Watson of the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center there. Walker notes that when the International Game Fish Association gives out its coveted world records, they are not for length, but for weight or mass. And as best as he and Watson can tell, no adult fish weighs less than Schindleria .

That's because, although they're a tad longer than the others, they're a whole lot skinnier.

"Mass really ought to come into it, rather than just plain length," Watson said, noting that one male Schindleria that looked to be (but was not proved to be) sexually mature was just 0.7 milligrams, and an 8.4 millimeter female that was definitely mature has an estimated mass of just 1.5 milligrams or so.

Britz and Pietsch concede that Schindleria may well be the world's lightest adult fish (the anglerfish male can't be weighed, because it is attached to its mate), but they're still haggling over the relevance of that compared with length.

All agree, however, on the importance of learning more about why some fish have evolved to be so small -- and on the need to protect the environments where they live.

Smallness can be a benefit to fish that live in nutrient-poor environs -- such as the deepest ocean depths, where food is sparse -- and offers a novel way to hide from predators.

"One way to avoid getting eaten is to be big, but being very small can work, too," Watson said.

But little else is known about the benefits of being tiny -- much less whether today's miniatures are the descendants of similarly small fish or are the products of gradual shrinkage over time. Comparative studies with other living and fossilized species are difficult because the structural characteristics scientists turn to -- including the way that various skull bones fit together and the patterns of tiny "lateral line" canals in those bones -- are too small to see or are entirely absent in these mini-fish.

Meanwhile, the Southeast Asian peat bogs that are home to Paedocypris are being converted into oil palm and pineapple plantations and shrimp farms, Kottelat said. And Australia's Great Barrier Reef, where Schindleria lives, is threatened by disease and rising water levels from climate change.

"No doubt there's a lot more out there we don't know about yet," Watson said, "and it's sad to think we might not find out before those habitats are gone."

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