Finds Raise Debate Over World's Smallest Fish

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."