Cover photo: Crew members of the research vessel, the L/M Admirante Guimar es, repair a winch while navigating the Amazon.

Cover background and above: Amazon electric fish, Sternarchohynchus oxyhynchus.

During their long trek on the Amazon, researchers pause at the INPA research institute in Manaus, Brazil.

An Amazon electric fish.

Researchers use trawl nets to pull Amazon fish into the boat. With this method, they discovered new species of fish that have startling adaptations.

The fully-equipped L/M Almirante Guimaraes research vessel.

Today, at last, a handful of intrepid explorers -- biologists funded by NSF's Biotic Surveys and Inventories Program -- have begun to seek out these myriad creatures. What they are finding is a world of fish species long kept secret, a mystery obscured by the river's formidably fast and deep currents. With each cast of their nets, the biologists have brought more and more of the river's bizarre inhabitants to light, including such oddities as electric fish with strange culinary tastes, and transparent catfish.

"We've now gone close to 2,500 miles over the Amazon and its tributaries," says John G. Lundberg, a fish specialist, or ichthyologist, at the University of Arizona in Tucson. "These things are spectacular. You come up with drastically different kinds of fish. The more people look, the more interesting things they'll find."

Ichthyologist Richard Robins, formerly of the University of Miami, and currently of the University of Kansas, called Lundberg's work pioneering, saying, "No one knew what was going on down there. It's a big deal -- a big breakthrough."

The work is crucial, adds Meredith Lane, director of NSF's Biotic Surveys and Inventories Program, because until the various species in any given habitat are known, no other questions of interest can be sensibly asked.

According to Lane, "The first fundamental question must be: What species exist in the ecological system to be studied? There is no way of knowing unless people go out and look."

Under the Amazon

The Amazon River, whose many tributaries spread like a finely branching root system across the northern half of South America, is a perfect place to begin looking. For, like the creatures of the surrounding rainforests, the fish of the Amazon are extremely diverse. In fact, it is because of the Amazon's abundance that South America is the continent with the most species of freshwater fish in the world. South America is home to at least 2,000 species, twice as many as in the United States, Canada and Mexico combined.

So far, Lundberg's team, which includes Brazilian collaborators Cristina Cox Fernandes of Brazil's National Institute of Research of Amazonia and Naercio Menezes of Brazil's University of Sao Paulo's Zoology Museum, has amassed 125,000 specimens across a total of 240 species. What these species are teaching researchers is that beneath the reflective surface of the river, there is another whole world of fish, including some that are decidedly odd.

It is no surprise that these two types of fish are the main attractions of the river's underworld; both are ready-made for life without light. Electric fish can hunt and navigate in complete darkness using an electric field that they generate around their bodies to sense where things are located in space. Catfish are also electroreceptive, plus they have taste buds all over their bodies. These features combine to allow them to function quite well without the aid of sight.

Yet even within the normal parameters of life-without-light, some species stand out.

For example, Lundberg and his colleagues found two species of electric fish that eat only the tails of other electric fish. While it seems like a strange diet, it turns out to be a good choice. Electric fish can quickly regenerate parts that have been munched off -- like a tail -- making this dining treat both plentiful and naturally renewable.

Researchers also found an array of eyeless, or nearly eyeless, catfish and electric fish. Among these blind swimmers, the scientists netted a minuscule transparent catfish, a female, inside of which were eggs. Measuring in at just one-third of an inch long, she comes close to the world record for smallest fish at sexual maturity. Yet this tiny, blind, transparent fish is not entirely vulnerable. She swims armored, bearing heavy plates on her sides.

The most curious of all, however, may be the fish bearing nobs and other "doodads" for which researchers have yet to find a function. Living in a small stretch of the Rio Negro, a Brazilian tributary of the Amazon, is an electric fish that sports a bizarre and unique tongue-like projection just above its chin. "It's not a tooth. It's a soft organ," says Lundberg, explaining that nothing like it has been seen on any electric fish before.

Worth the Risk

Researchers on the expedition quickly discovered that there is a reason why the fish in the depths of the river have remained a mystery. Getting them out can be an extremely difficult undertaking.

Robins says that when the work was first proposed, he had argued that NSF should fund Lundberg's team, even though they would probably lose every net they had trying to catch fish at the ragged bottom of these rushing rivers. With so little known, even if they had only one successful haul, he says, it would have been worth the effort. And just as predicted, the team has lost plenty of equipment to the quickly moving waters.

In order to catch the fish, the researchers throw in a weighted net with a mesh fine enough to capture the tiniest of fish. The nets careen out of sight into water passing by at rates of six or more feet per second. The researchers then tow the net and retrieve it from the bottom, which can lie anywhere from 30 to 150 feet below the surface. Meanwhile, the nets are prey to the rocks and dead trees spiking the river bottom and the branches racing by at every depth.

These waters, so hard on nets, are more dangerous still for the people involved. Though getting into the river to look at the heart of the Amazon might seem tempting, it remains strictly off limits. "That far from shore you'd be swept away downstream," Lundberg says. "There's no way a person could survive that."