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[Vince Keegan]

[Vince Keegan]

01. Nile Crocodile



 Photograph by Jonathan Blair, National Geographic

A year-old Nile crocodile attempts to snap up a frog in the St. Lucia Estuary in South Africa (from the National Geographic book Visions of Earth). Also known as the common crocodile, these large reptiles are distributed across much of Africa, and they have earned their reputation as among the most ferocious, deadly animals on the planet.

Male crocs typically measure from 11.5 to 16 feet long (3.5 to 5 meters), but they have been known to exceed 18 feet (5.5 meters) in length. Individual crocodiles attack anything the same size or smaller than them. They are occasionally known to hunt in packs, in which they can take down animals as large as hippos and rhinos. Nile crocodiles occasionaly prey on human beings, with estimates ranging from several hundred to several thousand deaths a year.

The ancient Egyptians feared and revered Nile crocodiles and worked them into their mystic religions. In modern times, the big animals were heavily hunted for their leather, although recent protections have helped stabilize the population at an estimated 250,000 to 500,000.

[Vince Keegan]

02. Snakehead



Photograph by Jason Edwards, National Geographic

Snakeheads are often feared in the West, where populations of the aggressive fish have occasionally taken root as invasive species. After a fisherman found a Northern snakehead (Channa argus) in a pond in Maryland, it caused a media sensation. Biologists warned that the large freshwater fish could readily become established in North America, where it could wreak havoc on native ecosystems.

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.

[Vince Keegan]

03. Mata Mata



 Photograph by Alessandro Mancini, Alamy

A mata mata (Chelus fimbriatus) is a freshwater turtle that inhabits the Amazon and Orinoco basins in South America. The bizarre turtles are entirely aquatic, although they prefer shallow, stagnant water, where they can easily reach their head out of water to breathe.

The mata mata can grow quite large, up to 33 pounds (15 kilograms). They feed on invertebrates and fish and aren't dangerous to people, despite their appearance.

Mata mata are fairly sensitive to water quality, both in captivity and in the wild, so they can be harmed by pollutants.

[Vince Keegan]

04. 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.

[Vince Keegan]

05. Diving Bell Spider



 Photograph by Gerhard Schulz, Getty Images

The diving bell spider (Argyroneta aquatica) is the only known spider in the world that lives entirely underwater. Like other arachnids, it must breathe air, but it provides its own supply by forming a bubble, which it holds by hairs on its legs and abdomen. The spiders must occasionally return to the surface to replenish their air supply, although some gas exchange happens across the surface of their bubbles, so they don't have to come up very often.

The diving bell spider is found in northern and central Europe and parts of northern Asia. Unusual for spiders, the males are bigger than the females, perhaps because the males are more active hunters.

The spiders can inflict a painful bite that is often accompanied by feverish symptoms. Those who are afraid of spiders may be disturbed to know they may not be safe from them, even in the water.

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