Aquatic locomotion

Aquatic locomotion refers to the biological process of moving through a liquid medium. This ability is not limited to a single group of animals. Instead, swimming has evolved repeatedly across many unrelated lineages. These include arthropods, fish, molluscs, amphibians, reptiles, birds, and mammals. Whether it is a microscopic bacterium or a massive whale, life has found ways to navigate the water.

At the smallest scale, microswimmers use specialized structures to navigate fluids. Many bacteria use a flagellum, which is a long, tail-like organelle. A molecular motor at the base of the flagellum rotates it to create movement. This motor is powered by the movement of protons through an electrochemical gradient. Other organisms, such as ciliates, use cilia. These are hundreds or thousands of tiny, hair-like structures packed in dense arrays. Cilia move in a coordinated metachronal rhythm, where each hair deforms in a wave-like pattern. This allows organisms like the Paramecium to travel at speeds up to 500 micrometers per second.

Some single-celled organisms move without tails or hairs. They use pseudopodia, which are temporary projections of the cell body. This movement is driven by actin polymerization, where a protein called actin builds up between the cell membrane and the cortex. This creates internal pressure that pushes the membrane outward. As the pseudopod extends, cortical tension pulls the rest of the cell body forward. The direction of this movement is often guided by chemotaxis, where the cell moves toward specific chemical attractions. An example of an organism that uses this method is Naegleria fowleri.

Larger animals often rely on jet propulsion to move. This mechanism involves filling a muscular cavity with water and then forcefully expelling it. In cephalopods like squid, water is drawn into the mantle cavity and pushed out through a tube called the hyponome. Because the animal is squirting mass out of its body, its velocity fluctuates. It accelerates while expelling water and decelerates while taking water back in. While this method is energy-intensive and relatively inefficient, it allows for incredible bursts of speed. This makes cephalopods the fastest marine invertebrates, capable of out-accelerating most fish during a chase.

Jellyfish also utilize a form of jet propulsion, but their design is different. They use a one-way water cavity that creates cycles of continuous propulsion followed by rest. This method is very costly; the Froude efficiency for a jellyfish is only about 0.09. This means they spend a great deal of metabolic energy to move. To help, some medusae use elastic fibers within their muscle layers. After a contraction, the bell vibrates at a resonant frequency to help refill the cavity. This helps manage the work required to move through the water.

Many fish prefer undulation, which is the rhythmic waving of the body. Different species use different parts of their anatomy to create this motion. Eel-shaped fish may undulate their entire bodies in sequences. In contrast, streamlined fish like salmon only undulate the caudal, or tail, portion of their bodies. Some predators, like sharks, use stiff fins to create dynamic lift. Other animals use oscillating limbs. Sea turtles, for example, use forelimbs shaped like high-aspect-ratio wings to fly through the water.

Evolutionary history shows how these methods changed over time. The first free-swimming animals appeared during the Early to Middle Cambrian period. These early organisms, such as the Anomalocaridids, used lateral lobes to swim. As competition increased during the Paleozoic, more efficient swimming became vital for survival. This drove the development of fins and tentacles to maintain steady speeds. While many terrestrial animals can still swim, most apes, including humans, have lost the innate swimming instinct.

Different environments also dictate specific swimming styles. Frogs and toads have evolved webbed feet for a style of propulsion that resembles a wide-spreading squat-jump. They use their muscular hindlimbs to kick rearward, which is efficient for shallow, slow-flowing waters. Even small crustaceans have unique methods. Daphnia swim by beating their antennae, while swimming crabs use modified walking legs. From the microscopic scale of a rotating flagellum to the powerful thrust of a swimming fish, aquatic locomotion is a diverse and essential part of life on Earth.