Walking catfish are sometimes literally fish out of water, but they can function quite well as a landlubber 鈥� and are now the first fish confirmed to be able to 鈥渟mell鈥� through air.
Native to South-East Asia, walking catfish (Clarias batrachus) are not content to glide through one body of water their whole lives. They can travel to a new pool by leaving the water and 鈥渨alking鈥� on their fins over land, breathing the air.
Similar amphibious fish聽were assumed to use their vision to navigate on land, says Noah Bressman at Chapman University in California. But walking catfish commonly come ashore at night, and have tiny, underdeveloped eyes.
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Curious to find out if the catfish were sensing chemicals in the air, Bressman and his colleagues caught 150 walking catfish and placed each in an enclosure on land to expose them to a battery of odoriferous substances. The fish flopped away from noxious hydrogen sulphide, but chased down the scent of pond water and alanine 鈥� a compound that induces a strong taste response in submerged catfish.
The findings suggest that walking catfish are the first fish known to use 鈥渃hemoreception鈥� out of water, and Bressman says they may use this sense to orient themselves on their overland travels.
Fish weren鈥檛 thought capable of accomplishing this because their olfactory and taste systems have evolved for use in water. Fish have a difficult time sucking air into their nostrils, for example.
鈥淯nderwater, taste and smell go hand in hand,鈥� says Bressman. Unlike humans, who taste with direct contact but can sense smells from long distances, fish 鈥渃an both taste and smell compounds that are originating from a long distance because of the liquid environment鈥�, he says.
Bressman thinks the walking catfish may be using a form of aerial tasting thanks to the taste buds that blanket their entire body, even on their sensitive whiskers.
Walking catfish are invasive in the south-east US, where they gobble up native fish. Understanding how the catfish make their pond-hopping decisions could help humans anticipate and stop their spread, he says.
Journal of Fish Biology
