When researchers slid hydras — a tiny polyp that under a microscope looks like a slender jellyfish — into water tainted with minute amounts of pharmaceuticals, their mouths, feet and tentacles stopped growing. While the hydras are minuscule, the implications are grave: Chronic exposure to trace levels of commonly found pharmaceuticals can damage a species at the foundation of a food pyramid.

Tiny zooplankton, another sentinel species, died off in the lab when they were exposed to extremely small amounts of a common drug used to treat humans suffering from internal worms and other digesting parasites.

In a landmark, seven-year study published last year, researchers turned an entire pristine Canadian lake into their laboratory, deliberately dripping the active ingredient in birth-control pills into the water in amounts similar to those found to have contaminated aquatic life, plants and water in nature.

After just seven weeks, male fathead minnows began producing yolk proteins, their gonads shrank, and their behavior was feminized — they fought less, floating passively. They also stopped reproducing, resulting in "ultimately, a near extinction of this species from the lake," said the scientists.

While the Canadian study was prompted by human intervention, similar die-offs have occurred in the wild.

In Pakistan, the entire population of a common vulture virtually disappeared after the birds began eating carcasses of cows that had been treated with an anti-inflammatory drug. Scientists, in a 2004 study, said they determined that the birds' kidneys were failing.

"The death of those vultures — the fact that you could get a complete collapse of a population due to pharmaceuticals in the environment — that was a powerful thing," said Christian Daughton, an EPA researcher. "It was a major ecological catastrophe."

In November, at the annual Society of Environmental Toxicology and Chemistry meeting in Milwaukee, 30 new studies related to pharmaceuticals in the environment were presented — hormones found in the Chicago River; abnormalities in Japanese zebra fish; ibuprofen, gemfibrozil, triclosan and naproxen in the lower Great Lakes.

Many of those studies refer to the heralded research at Lake Mead. There, on a recent morning, Steven Goodbred struggled to hold a large wriggling carp with both hands. On the outside, the carp looked fine, vibrant and strong, but the U.S. Geological Survey scientist assumed the worst.

"Typically we see low levels of sex steroids, limited testicular function, low sperm count, that kind of thing," he said slipping the fish into a holding tank and closing the lid. "We'll have to wait and see about this fellow."

These carp live, eat, reproduce and die at the mouth of what amounts to a 30-mile-long drainage system that starts within the toilets and sinks of the casinos, hotels and homes of Sin City.