Upside Down Catfish
Some species such as the upside down catfish inverted to take advantage of a food supply that was available at the surface.
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Upside-down catfish by Oliver Lucanus.
Q. Although it's been only two years since I first purchased a 10-gallon freshwater fish aquarium as a gift for my son, I'll admit to being hooked on tropical fishkeeping. Testimony to this is the fact that I'm currently maintaining 11 aquariums — up to 55 gallons in size. Recently, I've become fascinated by Synodontis catfish. Why and how do they swim upside-down?
A. Frankly, I've never thought about it, and, of course, I'm tempted to say "because they feel like it." After thinking about this seriously for the first time, I did some research and discovered that at least one other person has thought about this question. The following information is from Willoughby, N. C. 1976. The Buoyancy and Orientation of the Upside-down Catfish of the Genus Synodontis (Pisces: Siluroidei). J Zool London 180:291-314.
According to Dr. Willoughby, there are seven species that can be considered upside-down catfish: Synodontis batensoda, S. nigriventris, S. contractus, S. aterrimus, S. angelicus, S. membranaceus and S. resupinatus. The species most often kept by hobbyists are S. nigriventris and S. contractus (sometimes incorrectly referred to as S. davidii).
He speculates that when food for the fish was scarce on the bottom, some species inverted (swam upside-down) to take advantage of a food supply that was available at the surface. As the fish acquired neutral buoyancy, it would be progressively more difficult to resist the inverting forces. In order to conserve energy, the fish simply "gave in" to inverted swimming. An additional hypothesis suggests that because midwater predators usually attack from below, inverted species are better able to see the approach of predators, enhancing their chances of survival.
Upside-down catfish have different morphology (physical characteristics) than catfish that swim normally. Synodontis have thick head shields, the weight of which is counterbalanced by having a higher body fat content. Body fat aids in the process of inversion. As a result, these fish have a larger adipose fin, which Dr. Willoughby says counteracts the tendency of the body to roll, a problem because of the amount of body fat and where it is located. In addition, upside-down catfish have the same density as the water they swim in, whereas other species are denser than the water. Instead of fighting the forces of rotation and buoyancy, it is easier to swim inverted.