Behold the beleaguered water flea, the Daphnia magna.

Down near the bottom of the food chain, these transparent micro-crustaceans are the most tested aquatic invertebrate in the world.

Hopping through the water with rapid strokes of its antennae, a single compound eye observing its freshwater environment, this kidney-shaped "index organism" is used by scientists to detect the toxic presence of everything from pesticides to heavy metals.

A Daphnia magna can spend its entire 30- to 40-day life span serving at the pleasure of humans. Arriving in mail-order startup kits, the water fleas are often the live food choice for aquarium fish, or the filter-feeding vacuums that thanklessly scrub the film of green algae from neglected fish tanks.

Water fleas also must endure the humiliation of the high school science fair.

And as if all that wasn't enough responsibility, these tiny shrimp-like creatures now shoulder the threat of international terrorism.

Daphnia magna are among the tiniest foot soldiers being deployed to protect the nation's drinking water supply from intentional contamination by terrorists, and Altoona is just the latest city to utilize them.

The Altoona City Authority recently purchased technology that uses the highly sensitive water fleas to detect poisonous chemical and biological agents in their 13 reservoirs -- agents such as botulinum toxin, cyanide, VX and ricin.

Daphnia magna can do all this because they are highly sensitive organisms. Their transparent bodies reveal everything inside: a tiny, football-shaped heart that beats faster or slower based on slight changes in water temperature; a long intestine containing the water flea's last meal.

To test a water sample, lab technicians administer a sugar to a colony of days-old Daphnia magna that, when metabolized by healthy water fleas, will glow brightly, "like little light bulbs," said Carlos Murawczyk, president of Kingwood Diagnostics, makers of the technology. Daphnia magna that are impaired by toxic water will not glow.

No other organisms are similarly affected, Mr. Murawczyk said.

The $4,500 IQ-TOX test kit produces a result within an hour, rather than the standard two to three days. Annual maintenance runs around $10,000 a year, said Ken Streilein, chief lab technician for the Altoona City Authority.

"We have 13 reservoirs here; some are fairly remote," said Mr. Streilein. "In this day and age of terror alerts, people do, from time to time, come up and ask us, 'What do you do if you suspect a problem?' "

About 35 U.S. cities use the water flea rapid toxicity testing. New York City was the first to buy it, said Mr. Murawczyk. Boston, San Francisco, Philadelphia, Pittsburgh and the entire island of Puerto Rico have followed suit. Altoona, pop: 47,832, is among the smaller towns to acquire the water flea testing system.

"Fort Smith, Ark., has it," said Mr. Murawczyk. "Now don't ask me where Fort Smith is, because I don't know."

The Altoona City Authority, a separate entity from the municipality, operates 13 reservoirs, seven water treatment plants and 12 water tanks that supply many surrounding municipalities in Blair County, said Mr. Streilein. All the water tanks and treatment plants are fenced off, but the reservoirs are in the open and not patrolled, some alongside heavily used highways and railroad tracks. Besides terrorists dumping toxins in the water, officials believe these locations are vulnerable to chemical spills from tanker trucks and train derailments.

"We feel the possibility is there," said Mr. Streilein.

A few years ago, the federal Environmental Protection Agency and the Department of Homeland Security directed all public water systems to do "vulnerability assessments," said Mr. Murawczyk. Looking for tools beyond the standard fences, sensors, locks and dogs, the agency evaluated many technologies that claimed to detect the threat of "water terrorism" quickly, including Kingwood Diagnostics' IQ-TOX.

The advent of water fleas in protecting the public water supply is simply the new reality of today's environment, said Mr. Murawczyk.

"Do all cities have it? No," he said. "Should they all have it? Yes."

Over the years, the EPA, the Centers for Disease Control and the Department of the Army all have studied chemical and biological threats to water. Experts say it would require very large amounts of a contaminant to threaten the safety of a water system, and that introducing such quantities to a water system without being detected is too impractical and difficult. Mr. Murawczyk is aware of such studies, but argues that such acts are feasible.

The authority already does routine testing for mostly organic compounds in their water supply, Mr. Streilein said, but the IQ-TOX test goes "over and above anything we do."

Baltimore, Tallahassee, Little Rock and Fairfax, Va., all have purchased the water flea testing, said Mr. Murawczyk. More recently, the company approached the Altoona City Authority, and board members approved the purchase, taking money out of its laboratory budget.

"That's really a minimal cost for having the ability -- even if it's just averting one scare, it wouldn't necessarily have to be a real toxic incident -- to alleviate the panic before it started," said Mr. Streilein. "It certainly would pay for itself."