Walking Catfish (Clarias batrachus)

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Source: eol.org. © CAFS

Sometimes being a relentless survivor can be trouble. That’s often the case with invasive species, which use unique qualities evolved in their native environment to take over other areas that don’t have environmental means to control them. This is true of the incredible walking catfish, our new feature in the Cabinet of Freshwater Curiosities.

Wait. A fish that walks?

Curiouser and curiouser, isn’t it? This bizarre, tenacious animal, originally from southeastern Asia, lives in the stagnant waters and mud of slow-moving rivers, swamps, and ponds, as well as ditches, flooded fields, and rice paddies. However, when it gets the urge to move – for example, if it gets stuck in a temporary pool left after a river flood – it can “walk” across dry land to find a new home. This walking is really more of a wriggle: the fish uses its spiny pectoral fins and twists its body back and forth to waddle awkwardly along the ground. It actually breathes air on these journeys, since it has a special organ that supports its gills, working almost like a lung. Walking catfish can survive out of the water as long as they stay moist – instead of scales, their skin is protected by a layer of mucus. Breathing air also allows them to thrive in “hypoxic” or oxygen-poor water, where other species can’t survive.

So it not only walks, it breathes air and lives in an otherwise deadly environment? Cool!

Well, not totally. The same unique adaptations that make the walking catfish so fascinating also make it a particularly troublesome pest – the Encyclopedia of Life has it on a list of 100 of the world’s worst invasive species. It is found across Southeastern Asia including eastern India, Bangladesh, Myanmar, Thailand, Malaysia, Indonesia, Singapore, Borneo, Laos and the Philippines, although it’s difficult to say how much of this is its original range. In Florida, where it was introduced for aquaculture in the 1960s, it spread into 20 counties within a decade, turning the state’s extensive canal system into an invasion highway. By the 1970s, researchers reported as much as 3,000 pounds per acre of invasive catfish in small Florida ponds, and they have also been found in other states as far apart as California and Connecticut, probably released deliberately or accidentally from aquariums.

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Source: eol.org. Image by: Pam Fuller, USGS

The slimy fish is voracious, eating a smorgasbord of smaller fishes, eggs and larvae, crustaceans and insects, as well as aquatic plants and debris. It can “walk” to isolated pools that are safe from many other species, and is a nightmare for aquaculture, walking from one fish farm to the next and gorging on fish stock, causing millions of dollars of damage. Many countries including the US require a permit to possess the fish, although there are still reports of pet stores selling them. However, controlling them is nearly impossible, since they can easily move to new habitats and can also wriggle into the mud and survive for months without food.

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Source: eol.org. © WoRMS for SMEBD

Ok, so not so cool. Does anyone like the walking catfish?

Although it’s got an undeniably bad reputation as an invasive species, in its native range in Asia it is highly valued by both commercial and subsistence fisherman. The fact that it can survive for so long out of water makes it more attractive because fishermen can easily trade live fish. But it’s a bit of a shocker in Florida when you see a school of thirty fish come up out of the sewer to take a stroll down the street!

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