You might suppose this catfish is sick, or just confused. But swimming belly-up actually helps it camouflage and breathe better than its right-side-up cousins.
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Normally, an upside-down fish in your tank is bad news. As in, it’s time for a new goldfish.
That’s because most fish have an internal air sac called a “swim bladder” that allows them to control their buoyancy and orientation. They fill the bladder with air when they want to rise, and deflate it when they want to sink. Fish without swim bladders, like sharks, have to swim constantly to keep from dropping to the bottom.
If an aquarium fish is listing to one side or flops over on its back, it often means it has swim bladder disease, a potentially life-threatening condition usually brought on parasites, overfeeding, or high nitrate levels in the water.
But for a few remarkable fish, being upside-down means everything is great.
In fact, seven species of catfish native to Central Africa live most of their lives upended. These topsy-turvy swimmers are anatomically identical to their right-side up cousins, despite having such an unusual orientation.
People’s fascination with the odd alignment of these fish goes back centuries. Studies of these quizzical fish have found a number of reasons why swimming upside down makes a lot of sense.
In an upside-down position, fish produce a lot less wave drag. That means upside-down catfish do a better job feeding on insect larvae at the waterline than their right-side up counterparts, who have to return to deeper water to rest.
There’s something else at the surface that’s even more important to a fish’s survival than food: oxygen. The gas essential to life readily dissolves from the air into the water, where it becomes concentrated in a thin layer at the waterline — right where the upside-down catfish’s mouth and gills are perfectly positioned to get it.
Scientists estimate that upside-down catfishes have been working out their survival strategy for as long at 35 million years. Besides their breathing and feeding behavior, the blotched upside-down catfish from the Congo Basin has also evolved a dark patch on its underside to make it harder to see against dark water.
That coloration is remarkable because it’s the opposite of most sea creatures, which tend to be darker on top and lighter on the bottom, a common adaptation called “countershading” that offsets the effects of sunlight.
The blotched upside-down catfish’s “reverse” countershading has earned it the scientific name negriventris, which means black-bellied.
— How many kinds of fish swim upside down?
A total of seven species in Africa swim that way. Upside-down swimming may have evolved independent in a few of the species – and at least one more time in a catfish from Asia.
— How do fish stay upright?
They have an air-filled swim bladder on the inside that that they can fill or deflate to maintain balance or to move up or down in the water column.
— What are the benefits of swimming upside down?
Upside down, a fish swims more efficiently at the waterline, where there’s more oxygen and better access to some prey.
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The California Academy of Sciences has upside-down catfish in its aquarium collection: https://www.calacademy.org/exhibits/steinhart-aquarium
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