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Go to Brilliant.org/SciShow to check out their Introduction to Neural Networks course. [♪ INTRO]. Fish usually don’t do well without water.

Everything about them, from the way they move to the way they breathe, generally requires being immersed in liquid. But walking catfish are surprisingly at home on land. They can survive for up to 18 hours between waterways!.

And it turns out that part of their secret is being able to follow the scent of a suitable wet habitat. Walking catfish are native to Southeastern Asia—though now, they’re found in lots of other places, including the United States. And that’s, in part, because they’re really good at scooching their long, grey bodies over land, army cadet-style.

But walking between waterways seems like a pretty risky thing for a fish to do. Like, there’s the whole breathing bit, as gills don’t work in air. But walking catfish have that part sorted out — they use special lung-like organs when on land.

Still, they can’t wander forever. They risk drying out if they get lost and can’t find a pool to slip into. So scientists have been wondering exactly how they figure out where to go.

Other fish visually search for reflective surfaces and wiggle towards them. But walking catfish don’t. Instead, according to a 2020 study in the Journal of Fish Biology, they can detect bodies of water by their smell.

The scientists took 150 catfish from ponds in Florida and moved them to tanks in a university greenhouse. After the fish were accustomed to their new home, the researchers placed them in the middle of a kiddie pool to see if they’d wiggle their way towards specific smells. Lo and behold, they were drawn to the alluring scent of a pond, but steered clear of hydrogen sulfide, a pungent compound emitted by stagnant pools that lack oxygen.

Weirdly enough, the researchers think the fish are using their sense of taste to orient themselves rather than their sense of smell. That’s because their whisker-like nasal barbules are covered in taste buds. In fact pretty much their entire body has them!

And the fish also wiggled towards the smell of the amino acid alanine—a molecule they’re known to taste underwater. Now, walking catfish do have nostrils, but the researchers don’t think they’re using them to sniff the air. Their nasal tissues and muscles just aren’t really capable of moving air the way they move water.

These fish do have extra copies of a particular group of smelling genes, though, which some researchers have suggested could be involved in recognizing airborne chemicals. And it may be that, rather than sniffing with their nostrils, they sniff with their “lungs”. But, no one has found smell receptors in the air-breathing organs yet... partly because no one’s looked for them.

So maybe this recent study will push scientists to do just that! Either way, given how many walking catfish are wiggling around where they don’t belong, scientists are eager to figure out exactly how they find their way. Those insights could help wildlife managers get invasive populations in check.

And at the same time, they’d teach us more about how these remarkable fish evolved to wander two realms. These fish are the masters of their own universe—and you can master your universe, too, with a little help from today’s sponsor, Brilliant. Brilliant can help you take your science, engineering, computer science and math skills to the next level—not because you have to, but because want to.

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