Aquatic locomotion

Many things move through the water.

Small bugs use tiny hairs to push. Some tiny bugs even use long tails to spin and move.

Larger animals have more ways to swim. Squid use a jet to move. They suck in water and squirt it out. This pushes them forward.

Fish move by wiggling their bodies. Some fish use strong fins to move.

Turtles move their flippers like wings. They look like they are flying!

Frogs use their back legs to kick. This helps them jump through the water. It is a very good way to move.

Swimming helps animals find food. It also helps them stay safe. Many different creatures have learned to swim.

Many living things move through water. This is called aquatic locomotion.

Tiny life forms use many ways to move. Some use flagella. These are long tails that spin like a motor. Others use cilia. These are tiny hairs that move in waves.

Some single cells move by stretching their bodies. They push out parts called pseudopodia. This happens when pressure builds up inside the cell.

Larger animals have different ways to swim. Squid and jellyfish use jet propulsion. They fill a body cavity with water. Then they squirt the water out to move. This is a fast way to move. It helps them catch food or hide.

Fish often move by wiggling their bodies. This is called undulation. Some fish use fins to help them move. Sea turtles move flippers like wings. They look like they are flying through the sea.

Frogs use their strong back legs. They kick to push through the water. This works like a jump in the water.

Moving through water is a special skill called aquatic locomotion. Many different living things have learned how to swim in their own ways. This ability has appeared many times in the history of life. Tiny single-celled organisms use small parts to push themselves through liquid. Larger animals like fish, birds, and even mammals use much bigger movements.

Small swimmers use very tiny tools to move. Some bacteria have a flagellum, which is a long tail that spins like a motor. They use protons to make this tail rotate. Other tiny life forms use cilia, which are hundreds of tiny hairs. These hairs move in waves to push the organism forward. Some cells even use pseudopodia, or temporary body projections, to crawl. This happens when pressure builds up inside the cell membrane.

History shows us that swimming has been around for a very long time. The first free-swimming animals appeared during the Early to Middle Cambrian period. These early swimmers were mostly related to arthropods. They were called Anomalocaridids and used side lobes to swim. Later, cephalopods became active swimmers known as nekton. Many animals that live on land can still swim today. However, most apes, including humans, have lost the instinct to swim.

Many large animals use jet propulsion to move quickly. Animals like squid fill a muscular cavity with water. They then squirt the water out through a tube called a hyponome. This pushes the animal in the opposite direction. This method can be very tiring and uses a lot of energy. It is not as efficient as the way a fish swims.

Jellyfish also use a version of this to move through the sea.

Other animals use different body shapes to glide through the water. Fish often use undulation, which means they wiggle their bodies in waves. Some fish, like salmon, only wiggle the back part of their bodies. Sea turtles use flippers that look like wings to fly through the water.

Frogs use their strong back legs to kick, much like a jump. This is a very efficient way to move in shallow water. Even tiny crustaceans like Daphnia use their antennae to swim.

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.