Have you ever seen a fish yawn?

By Emeritus Professor William Gladstone 3 October 2024
Reading Time: 5 Minutes Print this page
Scientists have investigated why some underwater species yawn – and the reason is not what you might think.

Photographing a yawning frogfish is a bucket-list photo for most underwater photographers.

It’s easy to see why: The stretched jaws and cavernous mouth look spectacular, bizarre and a little terrifying:

A hairy frogfish, opening its mouth, looking like a yawn.
A striate anglerfish (Antennarius striatus) – also known as a hairy frogfish – seemingly yawns.

Fishes were the first vertebrate animals with jaws, they were also the first to yawn.

Many species of fishes (and some sharks) yawn, and for a variety of reasons – although, unlike us humans, they don’t appear to yawn as they’re settling down to sleep, or as they wake in the morning.

When a frogfish opens its mouth slowly, and as wide as it will go, we call it ‘yawning’ because it resembles something very familiar to us, but scientists who study ‘yawning’ in fish think one of the functions of this movement is to prepare the body for a change from one state to another.

Yawning arouses the brain and body by increasing heart rate, the flow of blood though the brain and the movement of fluid in the spinal cord. It has been observed in many different animal groups other than fishes, including mammals, birds, reptiles and amphibians, but – more than most other animal groups – fishes yawn for a whole range of reasons.

Some fishes and sharks yawn just after they’ve eaten. The stretching of the mouth and extension of the bones appears to be a way for them to re-position their jaws. They also yawn in the presence of cleaner fish as a signal to the cleaner that it’s safe to enter their mouth and eat the remains of the last meal lodged in their teeth.

A goatfish, opening its mouth, looking like a yawn.
A blacksaddle goatfish (Parupeneus spilurus) yawns when a cleaner fish is near, inviting the cleaner to inspect and clean the inside of its mouth.

A male cardinalfish yawns while he is courting a female. His yawns become larger and more frequent as the moment of spawning approaches. He’s preparing for an important moment in their mating cycle. As the female releases her ball of eggs he fertilizes them and, before they can be eaten by any waiting predators, he quickly sucks the eggs into his mouth.

Cardinalfish, and other fishes that also adopt this unusual style of parental care, are known as mouth-brooders. Yawning is way for these fish to prepare their jaws and mouth to open quickly and widely to accommodate the hundreds of tiny eggs that they will incubate until the hatchlings swim away.

A male Sydney cardinalfish, carrying a brood of eggs in its mouth.
A male Sydney cardinalfish (Ostorhinchus limenus) carries a brood of eggs in his mouth.

Fish and sharks also yawn at one another as a display of aggression. A territory holder yawns when an intruder approaches, the yawn often preceding a physical attack. A parent that’s guarding a clutch of eggs will yawn at a predator that approaches too closely. It’s easy to see why: The yawn exposes the teeth and makes the yawner suddenly appear much larger. The aim is to frighten the predator, thereby deterring the threat.

A green moray eel opening its mouth, looking like a yawn, as a show of aggression.
A green moray eel (Gymnothorax prasinus) ‘yawns’ to make it suddenly appear much larger and more threatening.

But what does it signify when there are no other fish or predators nearby and the fish is obviously yawning at the photographer? A sabretooth blenny suddenly yawned at me, exposing its scimitar-like fangs, as I was approaching it for a close-up.

A Port Jackson shark resting on the sea floor yawned at me when I was creeping towards it to take a full-frame portrait. Port Jacksons have an array of different teeth types, from small pointed teeth at the front of the jaws to large crushing teeth at the rear, and their jaws are controlled by large and powerful muscles.

These yawns that have been directed at me, and other divers, probably mean the same thing as when they’re done to another fish: it’s a sign the yawner is feeling threatened or stressed and so it yawns to expose some of its best weapons. The yawn reveals rows of sharp teeth, fangs or an immense mouth and it’s done to make us pause or frighten us into retreating. And, like the yawns of mammals, birds and reptiles, these yawns could also be preparing the fish’s nervous system so it’s ready to act quickly if the fish decides to flee or fight.

While I’m thrilled to photograph that decisive moment when a fish yawns at me, I also recognize it as a signal that my subject is feeling threatened or stressed and so it’s time for me to retreat and swim away. I take care not to damage the environment when I’m taking underwater photos and I’d also like to respect my subjects by not creating unnecessary stress for them.

All photographs courtesy of William Gladstone.

William Gladstone is a marine biologist and Emeritus Professor at ​the University of Technology Sydney’s School of Life Sciences with research and teaching interests in marine conservation biology, fish behavioural ecology, and marine environmental management.