SCIENTISTS PLUMB AMAZON'S BOTTOM.

Byline: Carol Kaesuk Yoon The New York Times

Since the middle of the last century, the Amazon River and its tributaries have provided passage to many an intrepid adventurer heading into South America's interior rain forests. Despite the fact that scientifically minded explorers have been floating down these waterways for more than 100 years, life in the river itself has remained largely unknown.

A handful of scientists have now begun the difficult work of plumbing the Amazon's depths, casting their nets to unveil the deep-water world below. With each haul, long-hidden inhabitants of the Amazon, the largest tropical river system on Earth, are coming to light, including oddities like transparent catfish and electric fish that subsist solely on the tails of other electric fish.

``We've now gone close to 2,500 miles over the Amazon and its tributaries,'' said John G. Lundberg, an ichthyologist, or fish specialist, at the University of Arizona in Tucson. ``You come up with drastically different kinds of fish.''

Richard Robins, the Maytag professor emeritus of ichthyology at University of Miami, said, ``Lundberg's work has been pioneering.'' Robins helped make the case to persuade the National Science Foundation to finance Lundberg's team when it first proposed fishing the deep rivers. He argued that even though the team would probably lose every net it had trying to catch fish from the ragged bottom of these rushing rivers, if it got just one successful haul, the effort would be worthwhile.

``No one knew what was going on down there,'' Robins said. ``It's a big breakthrough.''

Like the Amazon rain forest, the Amazon River is home to creatures of extreme diversity, helping to make South America the continent with more fish species than any other in the world. So far, the Amazon and its tributaries are estimated to harbor at least 2,000 freshwater fish species, twice the number in the United States, Canada and Mexico.

``It's just an overwhelming diversity,'' said William L. Fink, an ichthyologist at the University of Michigan's Museum of Zoology, ``and how all this came to be is a real interesting question. The deep-water faunas are especially odd and the most challenging to get to. You get into strange and interesting new worlds.''

So far, Lundberg and his collaborators, including Cristina Cox Fernandes of Brazil's National Institute of Research of Amazonia and Naercio Menezes of Brazil's Zoology Museum in Sao Paulo, have amassed 125,000 fish, and their ever-rising species count is at 240. What they have uncovered at the greatest depths is a murky, nearly lightless world inhabited by a proliferation of two kinds of creatures: electric fish and catfish.

``The Amazon water is muddy,'' Fink said. ``It's hard to see just a few feet down, and 20 feet down, it's really black.'' Researchers say that may explain why two sorts of fish ready-made for life without light lurk at the bottom.

Electric fish are able to hunt and navigate without vision, using special organs to generate electric fields around their bodies to sense where things are. Catfish are also electroreceptive, and they have taste buds all over their bodies, allowing other senses to dominate over sight.

The most peculiar among the electric fish found by Lundberg and his colleagues are two species of tail-eaters. When researchers examined the fish, their stomachs were filled entirely with the tails of other electric fish. Lundberg said that while the researchers know the fish eat the tails of other species, they may eat the tails of members their own species as well. The electric fish can rapidly regenerate lost parts, which makes the meal of choice of these species both plentiful and naturally renewable.

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PHOTO A previously unknown electric fish has been found in the Amazon.

The New York Times