Eels open and close their mouths to breathe. That constant gaping looks aggressive, but it’s the equivalent of you inhaling and exhaling. Because eels lack the gill covers that most fish use to push water passively, they rely on a more active pumping motion to force oxygen-rich water across their gills. The result is a mouth that never seems to stop moving.
How Eels Breathe Without Gill Covers
Most fish have large, flexible gill covers called opercula that help move water smoothly over the gills with relatively little visible effort. Moray eels have only small, circular gill openings set far back behind the head. This means they can’t rely on the same passive flow system. Instead, they use their entire mouth cavity as a pump.
The process works in two phases. First, the eel opens its mouth wide while keeping its gill openings sealed shut. This expands the space inside the mouth and throat, pulling fresh, oxygen-rich water in. Then the mouth closes, the gill openings relax, and the muscles of the mouth cavity contract, pushing that water backward across the gills and out through the small gill slits. The cycle repeats continuously, creating the rhythmic open-close motion you see whenever you watch an eel.
This system is called buccal pumping (named for the buccal cavity, the inside of the mouth). It’s actually quite efficient. Water flows in one direction only, from mouth to gills, so there’s no mixing of fresh water with already-used water. That keeps oxygen levels at the gill surface as high as possible, which matters because water holds far less oxygen than air does.
Why It Looks Threatening
An eel holding its mouth wide open, exposing rows of sharp teeth, triggers an understandable reaction in divers and aquarium visitors. It looks like a warning or a display of aggression. But the behavior is involuntary and constant. A resting eel gapes at the same steady rhythm whether a diver is nearby or not. Aquariums and marine education groups regularly clarify this point because visitors so frequently misread the signal.
That said, eels can and do bite. The difference is in the context and speed. Breathing is slow, rhythmic, and predictable. A defensive or feeding strike is fast, sudden, and directed at a specific target. If you’re watching an eel and its mouth is opening and closing at a steady pace while the rest of its body stays relaxed, it’s just breathing.
How Morays Actually Feed
Moray eels have a remarkable adaptation for eating that goes well beyond their visible teeth. Hidden deep in the throat sits a second set of jaws, called pharyngeal jaws, armed with their own sharp, backward-curving teeth. When a moray catches prey with its front teeth, these internal jaws launch forward into the mouth, grab the food, and pull it back down the throat.
This two-jaw system solves a problem created by the eel’s narrow body. Most fish swallow prey using suction, rapidly expanding the mouth cavity to create a vacuum that pulls food inward. Eels can’t generate enough suction because their heads and mouths are too narrow. The pharyngeal jaws compensate by physically reaching forward and dragging the prey backward, almost like a second pair of hands inside the throat. The capture bite and the transport bite involve similar head movements, but the swallowing phase takes about twice as long as the initial strike.
Other Reasons Eels Open Their Mouths
While breathing accounts for the vast majority of mouth-opening behavior, a few other situations can change the pattern. During a yawn-like stretch, some eels will hold their mouths open wider and longer than a normal breathing cycle. This may help clear debris from the gills or resettle the jaw after eating.
Eels also gape more noticeably when oxygen levels in the water drop. Warmer water holds less dissolved oxygen, so in heated aquarium tanks or during summer months in the wild, you might see an eel pumping its mouth more rapidly or opening wider to push a greater volume of water across the gills. If you keep a pet eel and notice a significant increase in gaping speed, it’s worth checking your water temperature and oxygen levels.
Why Morays Gape More Than Other Fish
You’ll notice this behavior in moray eels far more than in most other fish species precisely because of their anatomy. Fish with full-sized opercula can ventilate their gills with subtle, almost invisible movements. The opercular flaps do most of the work, and the mouth barely needs to open. Morays, with their reduced gill openings, have to compensate by making the mouth cavity do nearly all the pumping. The mouth opens wider, stays open longer, and cycles more visibly. It’s not that morays breathe more than other fish. They just have to work harder, and more visibly, to achieve the same result.