Cottonmouths are semiaquatic, so they’re comfortable both swimming in water (hence their other common name of water moccasin) and basking on land. They are the only venomous snake in the U.S. that spends a lot of time in the water, Live Science previously reported (opens in new tab). Other local names for cottonmouths include black moccasins, gapers, mangrove rattlers, snap jaws, stub-tail snakes, swamp lions, trap jaws, water mambas and water pilots.

Cottonmouths are relatively large, ranging from 2 to 4 feet long (61 to 122 centimeters), according to the Savannah River Ecology Laboratory (opens in new tab). They have thick, muscular bodies covered in ridged scales and blocky heads with large jowls. Their pupils are vertical, similar to cat pupils, and they have dark stripes next to each nostril. Their coloration varies from dark brown or black to olive, banded brown or yellow.

Cottonmouths are often confused with nonvenomous water snake (opens in new tab) species from the genus Nerodia. Cottonmouths and Nerodia species have similar coloring and patterns and are all usually found near water. Even though water snakes are nonvenomous, they can still bite and are often killed by humans out of fear that they are cottonmouths.

There are a few ways you can tell a nonvenomous water snake from a venomous water moccasin, or cottonmouth, according to the University of Florida (opens in new tab). Water snakes are slender compared with cottonmouths, which are thicker and heavier. Water snakes also have longer, thinner tails, and their heads are a similar width to their necks, whereas a cottonmouth’s head is thick, blocky and noticeably wider than the snake’s neck. Water snake pupils are round, not vertical and cat-like like the pupils of cottonmouths. Water snakes also lack the facial pits that are characteristic of pit vipers, such as cottonmouths.

When threatened, nonvenomous water snakes, such as northern water snakes (Nerodia sipedon) and southern water snakes (Nerodia fasciata), often try to appear bigger than they are by flattening their bodies and heads. This flattening makes them look more like cottonmouths. A water snake’s flattened head will look more triangular in shape, but not blocky and thick, like a cottonmouth’s head. A water snake’s head will also still be a similar width to the neck, even when flattened. Trying to kill a snake greatly increases the risk of being bitten by one, according to the University of Florida.

Juvenile cottonmouths have more distinctive bands across their bodies and are lighter brown compared with adult cottonmouths. Juveniles also have bright-yellow tail tips that they use as lure to attract prey. “They undulate the tail tip slowly back and forth to lure prey, such as frogs (opens in new tab), within striking distance,” Viernum said.  The striking patterns present on the juveniles fades with age.