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Northern madtom, a venomous catfish species

Collected from a Michigan river, a member of the venomous northern Madtom catfish species rests in an aquarium in an undated photo.

Photograph courtesy Matthew Ross

Maggie Koerth-Baker

for National Geographic News

December 17, 2009

Nature hasn't left the catfish declawed, so to speak.

Some catfish species have been known to be venomous—including a few dangerous enough to kill a human. But scientists knew little about how common venomous catfish are or how the fish produce and deliver their venom.

Turns out, the ability is more widespread than anyone realized—extending to about half of the more than 3,000 known catfish species, according to a new report.

(Related: "All Octopuses Are Venomous, Study Says.")

For the study, biologist Jeremy Wright studied the venom and microscopic tissue structures of 158 catfish species. Based on known relationships among those species, the University of Michigan graduate student concluded that at least 1,250 to 1,625 catfish species are likely venomous.

The explosion in known venomous catfish species, however, doesn't mean you have anything to fear, even if you're an inveterate noodler—especially in North America.

The toxic catfish of North America have relatively mild venom, which in humans would cause about the same amount of pain as a bee sting, Wright said. Some species, including the popular flathead catfish, aren't poisonous at all.

Furthermore, catfish venom is "strictly defensive," Wright said. "They don't use it to hunt."

Spine-Popping, Gland-Ripping Defense

When a catfish feels threatened by a bigger fish, it can pop out the collapsible spines that usually lie close to its sides, making its body wider and harder to swallow.

If the predator bites anyway, the sharp spines cut into its mouth. Meanwhile, pressure on the spines causes them to shift at their bases, ripping the skin over adjacent venom glands. Venom spills out and into the predator's mouth wounds.

In some North American catfish species, venom may have evolved to protect the fish when they're young and at their most vulnerable.

"The venom glands appear to get smaller as the fish gets bigger and bigger," Wright said. "In some of the catfish species that can reach adult sizes of 60 to 80 pounds [27 to 36 kilograms] or larger, there's no evidence of remaining venom glands at all."

Findings published December 4 in the journal BMC Evolutionary Biology.

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