The bluegill, pictured above, is a freshwater fish native to, and common in, the eastern United States. They grow, in average, to be four to 12 inches long, and are regularly fried up in a pan for lunch or dinner. If you ask Wikipedia what the bluegill is good for — other than eating, that is — you’ll learn that “bluegills also play an important role in pond and lake management to keep crustacean and insect populations low, as a single bluegill population may eat up to six times its own weight in just one summer.”

The United States Army begs to disagree. To them, the bluegill served a different purpose: a swimming anti-terror device.

In 2006, the Army helped develop a system where sets of eight bluegill, held in captivity, were used as an early warning device against attempts to poison the Washington, D.C., New York, and San Francisco water supplies. The fish are not like bomb sniffing dogs, however — if anything, they are more akin to canaries in the proverbial coal mine. Each group of fish are placed in small, individual cubbies equipped with piped-in water from the water supply, and each of these slots is outfitted with electrodes which monitor the fish’s behavior and movement. If six or more of the octet begin to act erratically, the alarm goes off, alerting authorities so they can investigate the water supply further. National Geographic put together a neat little graphic explaining the process.

Bonus fact : Hungary’s anti-terrorism unit has found a different target than the water supply — namely, Brad Pitt movies. Or, more accurately, one Brad Pitt movie, titled World War Z, and scheduled for release in December of this year. In October of 2011, Hungarian officials seized a shipment of guns intended to be used as props the movie. The weapons were supposed to be disabled, but someone screwed up, and shipped in fully functional automatic weapons accidentally.

From the Archives: Redefining Nemo: The simply strange and un-Disney-like reproductive cycle of Nemo and his fellow clownfish.

Related: A wall-mountable fake bluegill…. for $80, for some strange reason. Maybe it doubles as a home security system?