A Peacock Flounder (Bothus mancus) stretches its jaws.
Yawning, which is defined as the involuntary process of opening the mouth, breathing deeply, and filling the lungs with air, is a natural response triggered by sleepiness or fatigue in mammals. However, nothing in this definition can be related to fish, except perhaps the fact that it also occurs involuntarily. Researchers cannot explain why it occurs in humans, so explaining why it occurs in fish is even more challenging.
Many fish do open their mouths very wide for a short period, and the wide opening of their mouth resembles a human yawn. It is possible that this movement is not a yawn at all but a behaviour not understood by humans. However, it is believed to be a territorial display, used to attract a mate or deter aggressors. Many fish species produce sounds through various mechanisms, such as rubbing hard body parts together or vibrating their muscles around the swim bladder. Some fish are even named after the sounds they make, such as grunts, drums, and croakers. However, many of these sounds are completely imperceptible to humans. Soldierfish are among the loudest fish on the reef, but the sounds they produce are completely inaudible to scuba divers. A fish that makes this yawning movement may be producing an inaudible sound directed at mates or rivals.
Another explanation could be that, because fish breathe through gills, the mouth-stretching movement could help open the gill covers in response to low oxygen concentrations in the water. By doing so, the fish increases the flow of water through its mouth and over its gills, resulting in an increased flow of oxygen to the fish's tissues.
Often, images of a yawning fish are a stroke of luck; the combination of being in the right place, at the right time, and having the camera pointed the right way. However, there are often a few small and subtle signs in the fish's expressions that precede a yawn. A vague jiggling of the mouth and a partially opened, moving jaw usually (but not always) indicate that a yawn is imminent. When these signs are spotted, it is good to start filming. It may take another minute or so, but chances are you are about to capture this little understood behaviour on camera.
Once a fish yawns, based on my experience, the fish will often repeat this strange but fascinating display several times. Yawning fish are always among the favorite things to film for underwater videographers or photographers. In underwater videos, I always add a stretching sound to the yawn movement.
These stretching sounds are often recorded from sounds produced by dead materials, such as the rubbing of a rubber balloon, rather than from sounds produced by real animals. The importance here in an underwater video is not what produced the sound used but how it is perceived by the viewers.
Ironically, most sounds produced by animals underwater are so muffled, soft, and even inaudible that an added sound, such as the one used in this clip, contributes to the veracity of the film fragment.
This short underwater videoclip has been filmed in Mauritius 🇲🇺
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