How Fish Movement Works: From Swimming to Schooling

Fish movement is a product of millions of years of evolution. The physical properties of water, being much denser than air, present unique challenges that fish have overcome through the development of streamlined bodies and specialized propulsive structures.

Anatomy of Aquatic Propulsion

Along the sides of a fish’s body are W-shaped blocks of muscle tissue called myomeres. These muscles contract and relax in a sequential wave from head to tail. This coordinated action creates an S-shaped undulation that pushes against the water, generating forward thrust. The flexibility of the fish’s spine allows this wave of motion to propagate smoothly along the body.

Fins are the primary appendages for control and propulsion. The caudal fin, or tail fin, provides powerful thrust for forward movement as it sweeps from side to side. Stability is managed by the dorsal and anal fins on the top and bottom of the fish, which prevent rolling and yawing. For steering, braking, and fine-tuned maneuvers, fish rely on their paired pectoral and pelvic fins.

To navigate the vertical water column, most bony fish possess a swim bladder. This internal, gas-filled organ functions as a buoyancy control device. By adjusting the amount of gas within the bladder, a fish can alter its density to match the surrounding water. This allows it to maintain a specific depth with minimal energy expenditure.

Classifying Swimming Styles

The mechanics of propulsion are adapted into distinct swimming styles, classified by which parts of the body generate thrust. Each style represents a trade-off between speed, efficiency, and maneuverability that reflects different evolutionary pressures.

Carangiform swimming, named after the jack family, is a common mode where propulsive waves are confined to the rear half of the body. The tail oscillates rapidly to create thrust, a method used by fast-swimming fish like trout and salmon. A more extreme version is Thunniform swimming, used by tuna and some sharks, where movement is almost exclusively limited to a powerful, crescent-shaped tail, allowing for high-speed, sustained swimming.

Anguilliform swimming involves the entire body undulating in a snake-like wave, which is typical of elongated fish such as eels. In contrast, Labriform locomotion keeps the body rigid while propulsion is generated by the rowing motions of the pectoral fins. This style, used by wrasses and parrotfish, allows for precise, slow movements ideal for navigating intricate reef structures.

Collective Movement Patterns

Many fish exhibit social behaviors that lead to organized group movements. A shoal is any group of fish that stays together for social reasons, while a school is a shoal where individuals swim in a synchronized manner. A group can shift from a loose shoal to a tight school in response to threats or migration needs.

Collective movement provides significant survival advantages, primarily through defense against predators. The “many eyes” effect of a large group improves the chances of spotting danger. Once a threat is detected, the synchronized movements of a school can create a confusing spectacle for a predator, making it difficult to target one fish. This combines with a dilution effect, where the statistical chance of any single fish being captured is lower in a larger group.

Moving in a group also enhances foraging success, as information about food can spread more quickly. Schooling provides hydrodynamic advantages, allowing fish to save energy by swimming in the slipstream of others, a behavior known as drafting that is beneficial during long-distance migrations. The shape of the school often adapts to its purpose, forming a wedge for travel or a circular shape for feeding.

Specialized Forms of Locomotion

Beyond swimming, some fish have evolved remarkable methods of locomotion adapted to specific niches, often by modifying their fins for new purposes.

On the boundary between water and land, the mudskipper uses its muscular pectoral fins to “walk” across mudflats at low tide. This allows it to access resources unavailable to fully aquatic fish. Similarly, some species like the sea robin use modified fin rays to crawl along the seabed, probing for food.

Flying fish use their greatly enlarged pectoral fins to glide over the water’s surface, a strategy used to escape underwater predators. They build up speed underwater and then launch themselves into the air, spreading their fins to catch the wind. In another distinct adaptation, some elongated species like sand lances and certain eels are capable of burrowing through soft sediment, using body undulations to move through sand or mud.