Candiru
Photograph by Max Gibbs, Getty Images
Perhaps the most feared freshwater species of all, candiru are parasitic catfish in the family Trichomycteridae. These small, narrow fish normally feed on the gills of larger fish in the Amazon. However, over the past few centuries there have been reports of these creatures lodging in the urethras of men and women.
Some native peoples along the Amazon have described means of preventing such a disturbing infection, which is said to be hard to remedy, even with surgery. Protection measures are said to include tough clothing, tying off extremities with ligatures, and avoiding urination around rivers. It was long believed that urine attracted the candiru, although a recent study showed that the animals displayed no interest in it.
There are very few documented cases of candiru attacks on human beings, and a much discussed incident from 1997 has been met with considerable skepticism. Further, with an average width of a quarter of an inch and average length of three inches, the fish are a little large to enter a urethra. Still, the prospect remains frightening enough to keep many people out of tropical waters, even far away from the candiru's range.
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Snakehead
The voracious top-level predators can reach a length of three feet (one meter). They prey on invertebrates, frogs, and smaller fish, though they are known to attack anything moving when they are breeding.
Snakeheads can breathe air and can survive out of water for up to four days. They can survive much longer periods of drought by burrowing into the mud. Several species of the fish are native to much of Asia, where they are commonly caught and are prized for the dinner table. They are also frequently kept as aquarium fish and are noted for their aggressive behavior.
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Giant Catfish
Photograph by John Robertson, Alamy
Divers work with a model European catfish in the Great Lake at Ostersund in Sweden. Large catfish live in many rivers throughout the world, where they are important scavengers.The largest on record is the Mekong giant catfish, which has reached recorded sizes up to 10.5 feet (3.2 meters) and 660 pounds (300 kilograms). Once distributed across several countries in Southeast Asia, the Mekong giant catfish is now critically endangered, thanks to habitat disruption. Not much is known about the world's biggest freshwater fish, although conservation efforts are underway.
Big catfish are rarely considered dangerous to people. The Mekong species can live to be more than 60 years old.
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Electric Eel
Photograph from Bettman Archive/Corbis
Electra the electric eel (Electrophorus electricus) demonstrates her shocking power at Ford Motor Company's "Cycle of Production" exhibit at the 1939 New York World's Fair. Despite their name, electric eels are actually a type of knifefish and are more closely related to catfish than they are to true eels. These unusual fish inhabit waterways in the Amazon and Orinoco basins of South America, where they hunt prey and defend themselves by producing powerful bursts of electricity.Electric eels gulp air from the surface in order to breathe. Thanks to specialized internal organs, they can produce pulses of electricity greater than 500 volts, with a current greater than one amp. That's enough to kill an adult human being.
Electric eels mostly hunt invertebrates, though adults also consume fish and small mammals. They only attack human beings if they are disturbed. They tend to live in murky, stagnate waters. Scientists have long been fascinated by the species and have probed its impressive electrical abilities.
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Tiger Fish
Photograph by Chris Johns, National Geographic
Widely distributed across much of Africa, tiger fish are fierce predators with large, razor-sharp teeth. They often hunt in packs and occasionally eat large animals. Attacks on human beings are rare but not unheard of.The two largest species are the goliath tiger fish (Hydrocynus goliath) and the Hydrocynus vittatus, which is commonly called the tiger fish. Both are prized as game fish. The goliath tiger fish, which can reach sizes up to 110 pounds (50 kg), is found in the Congo River and Lake Tanganyika. The tiger fish can weigh up to 33 pounds (15 kg) and is found in the Zambezi River system.
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Piranha
Photograph by Martin Shields, Alamy
Notorious for their sharp teeth and voracious appetites, piranhas inhabit several of the major river basins in South America. These omnivorous fish are known for their taste for meat, although attacks on human beings are quite rare, despite breathless accounts from early explorers.In a historic visit to Brazil, Theodore Roosevelt famously saw a group of piranhas shredding pieces of a cow carcass in seconds. His dramatic account would color popular imagination for years, even though it was based on a manipulated spectacle in which fishermen blocked off a group of the fish and starved them beforehand.
Still, piranhas are important scavengers and predators in their native rivers, and they often resort to cannibalism if food gets scarce. It's true that local fishermen occasionally have scars from close encounters with them.
It's unknown how many species of piranhas exist, with estimates ranging from 30 to 60. —Brian Clark Howard
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