When we see a fish swimming belly up, the first thing that comes to mind is its death. However, this rule does not always apply to all fish species. Oceanographers and marine biologists have discovered that some of them can live in this position throughout their lives without experiencing any health problems.
Researchers obtained video footage taken in the Izu-Ogasawara deep-sea trench located in the Pacific Ocean off the coast of Japan. The recording captured a large angelfish swimming belly up. Instead of worrying about the fate of the angelfish, oceanographers recalled that in 1999, other scientists also observed similar behavior, but it was then considered an anomaly.
Modern scientists are not so easily fooled—they became interested in this finding and began to investigate other videos that also featured angelfish. As a result, it was found that in many recordings, angelfish were indeed swimming in an inverted position. Moreover, during this time, they were not engaged in any specific activity—living upside down seemed to be normal for them.
In their article published in the Journal of Fish Biology, researchers report that some species of angelfish (including those seen in the video) have natural lures that hang from their snouts. Sometimes these lures can be four times the size of the fish itself. Scientists suggest that this method of swimming belly up was developed by angelfish for convenience—they find it easier to manipulate the heavy lure in the inverted position. In this case, the lure hangs from the upper lip, which is closest to the seafloor when the fish is upside down.
