Fish constantly move their mouths in a rhythmic, continuous motion. This activity is fundamental to their survival in aquatic environments, serving several essential purposes. Understanding these movements provides insight into how fish interact with their surroundings.
Respiration
The primary reason fish consistently move their mouths is for respiration, the process of extracting oxygen from water. Fish employ a specialized mechanism called buccal pumping. They open their mouths to draw water into their oral cavity, simultaneously expanding their buccal and opercular cavities to create negative pressure, pulling water inward.
Once water is inside, the fish closes its mouth and contracts these cavities. This forces water over the gills, which are delicate, feathery structures rich in blood vessels. As water passes over the gill filaments, oxygen diffuses from the water into the fish’s bloodstream, and carbon dioxide is released. The water is then expelled through the opercula, which are bony flaps covering and protecting the gills. This continuous, unidirectional flow ensures a constant supply of oxygen-rich water across the gill surface, which is important given the lower oxygen concentration in water compared to air.
Feeding
Beyond breathing, fish utilize their mouths for various feeding strategies. One common method is suction feeding, where fish rapidly open their mouths and expand their buccal cavity to create a vacuum, drawing prey directly into their mouths. This technique is effective for capturing small, elusive prey and is employed by many bony fish.
Another feeding approach is biting, where fish use their jaws to grasp, tear, or cut pieces from larger food sources. This method is common among predatory fish, which possess specialized teeth for this purpose. Filter feeding represents a different strategy, where fish strain small particles, such as plankton, from the water using modified gill rakers or other oral structures. This can involve either swimming through the water with an open mouth (ram feeding) or using suction to draw in water and then expelling it through the filtering apparatus.
Beyond Breathing and Eating
While respiration and feeding account for most mouth movements, fish also use their mouths for other behaviors. One such behavior is a form of “yawning,” often associated with gill flushing or a response to environmental conditions. Fish might open their mouths wide to force water through their gills, helping to clear debris or improve oxygen uptake, especially if oxygen levels in the water are low. If oxygen levels are too low, fish will often move their mouths more rapidly to extract more oxygen.
Mouth movements can also serve as territorial displays or stress responses. Some fish species will open their mouths wide to appear larger or more intimidating to rivals, a visual tactic to assert dominance or defend territory. Mouth movements can also indicate stress, particularly if they are rapid or accompanied by other unusual behaviors. Additionally, some fish engage in cleaning behaviors, using their mouths to mouth surfaces or even other fish to remove parasites or debris.