The movement of fish through water involves a refined interplay between biological structure and fluid dynamics. Because water is a dense medium, fish have evolved efficient methods to generate speed and control movement. Their ability to propel themselves, steer precisely, and maintain a fixed position allows them to navigate diverse habitats, from rivers to deep oceans.
Generating Forward Thrust
Forward thrust is primarily achieved by the alternating contraction of powerful axial muscles, known as myomeres, that run along the fish’s body. These muscles contract sequentially from the head toward the tail on alternating sides, generating a wave of lateral flexion, or undulation. This rhythmic wave travels down the body, pushing against the surrounding water.
The amplitude of this wave increases toward the rear, culminating in a powerful sweep of the caudal fin (tail fin). This broad surface area pushes water backward, creating a reactive force that propels the fish forward, according to Newton’s third law. For fast-swimming species, the force is concentrated at the tail, which acts like a flexible propeller to achieve high velocities. The streamlined body is held rigid to minimize water resistance, converting muscle energy efficiently into forward momentum.
Steering, Braking, and Stability
While the caudal fin generates forward thrust, other fins provide the finer control needed for navigating a three-dimensional environment. The paired fins—the pectoral fins (behind the gills) and the pelvic fins (on the underside)—function like the control surfaces of an aircraft. By changing their angle, the pectoral fins are used for precise steering and turning.
These paired fins also aid in braking, as they can be rapidly extended to increase drag and slow the fish down. They can also be angled as hydrofoils, generating lift to maintain a stable depth or navigate vertically. The unpaired fins, including the dorsal fin and the anal fin, function primarily as stabilizers. These fins prevent unwanted rotational movements, such as rolling or yawing (slipping sideways during motion).
Controlling Depth and Buoyancy
Maintaining neutral buoyancy is a significant challenge for fish, allowing them to hover at a specific depth without expending energy. Most bony fish achieve this using the swim bladder, a gas-filled organ that acts as a hydrostatic device. Adjusting the gas volume within the bladder allows the fish to precisely match its density to the surrounding water.
Gas regulation is managed internally through the gas gland and the rete mirabile, a network of blood vessels. This system secretes gases, primarily oxygen, from the blood into the bladder to increase buoyancy when the fish descends and pressure increases. Conversely, gas is absorbed back into the bloodstream when the fish needs to ascend or decrease buoyancy. The dorsal positioning of the swim bladder also contributes to lateral stability, helping the fish remain upright.
The Major Swimming Styles
Fish movement can be categorized into distinct swimming modes based on which parts of the body are used for propulsion.
The Anguilliform mode, characteristic of eels, involves an undulatory wave passing down the entire body. This whole-body motion results in slow-to-moderate speeds but provides excellent maneuverability in complex environments.
A more common style is the Carangiform mode, seen in fish like salmon and cod, where the undulation is concentrated in the posterior half of the body. This style is efficient for sustained swimming because the head and front remain rigid to reduce drag.
At the extreme end of this spectrum is the Ostraciiform mode, typical of boxfish, where the body is held completely rigid, and only the caudal fin is oscillated rapidly to generate thrust.