In the Congo Basin in Africa, a species of catfish that naturally swims upside-down has puzzled humans for hundreds of years. Over 4000 years ago in ancient Egypt, carvings of upside-down catfish appeared on the walls of royal tombs, and people at that time wore pendants in the shape of upside-down catfish, believed to have protective properties against drowning.
This fish is known as the “Upside-down catfish.” Within the Synodontis genus, there are seven species that swim upside-down.
The Upside-down catfish are now mainly bred in aquariums and can survive up to 15 years in the appropriate environment. Depending on the species, they spend almost their entire lives in an upside-down position. Their body structure is identical to fish that swim normally. They can grow up to 20 inches in length.
One theory suggests that their upside-down swimming is for enhancing feeding efficiency. These fish often nibble on the surface of submerged branches and driftwood, and swimming upside-down makes it easier for them to access these areas. When they swim upside-down to feed from the water upwards, they can easily catch prey such as insect larvae near the water surface. Additionally, it is easier for them to access the oxygen-rich thin layer of water near the surface, which helps them breathe.
Upside-down catfish live in environments with low light and dense vegetation, such as swampy areas. Therefore, their ability to swim upside-down is crucial for their survival when the water source lacks oxygen.
Professor Lauren Chapman from McGill University in Canada has been studying how fish in African rivers cope with oxygen-deprived environments. She says, “Upside-down catfish exhibit a range of adaptive features that give them a survival advantage near the water surface.”
In one experiment, she found that upside-down catfish are more adept at breathing near the water surface than catfish swimming normally, as the latter have to exert more effort to achieve the same result.
Chapman states that it is currently unclear whether the upside-down swimming behavior of these fish is due to species evolution in oxygen-deprived environments or influenced by other factors. However, the oxygen content in the water indeed affects many wild fish, leading to changes in gill size and reduced egg production.
Upside-down catfish can actually swim in an upright position, and they often switch back to the “normal” posture when foraging at the bottom of the water.
A research team led by Ken Ohnishi at Nara Medical University in Japan found that these fish tend to swim upside-down when near objects or at the bottom of the water, possibly to nibble on food under the surface of objects. They rarely swim in the middle layer of water but rather near the bottom or surface.
Research by Robert Blake, a former biology professor at the University of British Columbia, shows that for fish, as long as the water depth is sufficient, the efficiency of upside-down and upright swimming is the same. However, near the water surface, the situation changes. Whether it is fish or Olympic swimmers, they are affected by “wave drag.” Fish generate less wave drag when swimming upside-down near the water’s surface.
This article drew references from the website “Amusing Planet” and reports from KQED Television in the San Francisco Bay Area.