Watching a goldfish, tetra, or angelfish swim around in an aquarium can be very hypnotic. They seem to move fluidly, effortlessly, as though they were born to do just that. And they were! Swimming is the core of a fish’s existence, and to understand how fish swim is to understand the majority of their physiology, and why they have lived for centuries in the habitats they do.
How fish swim in water
Let’s start with what we see and know about a fish- nearly all of them have scales. Scales vary in size, texture, color, placement, and general makeup. It’s easy to see that scales are protective and act as armor for a fish, but scales also help out with how fish swim. They make the fish’s body aerodynamic and enable a smooth glide through the water. They are also quite flexible, which is useful for bending and weaving through water. While they can be of service, scales don’t make the fish an excellent swimmer, as not every fish wears scales on their body. Most catfish, for example, have no scales, though some varieties have other types of armor.
The next fish feature that’s easy to point out are the fins. The caudal fin, located at the tail end of the fish, is it’s true ace in the hole for cutting through water. The shape and size of these fins vary from species to species. Examples of caudal fin shapes include forked, lunate (which has a sweeping dip in the center, like a phase of the moon), concave, rounded, rhombic, truncate (square), and double truncated (can look like a subtle letter ‘w’). The shape of the caudal fin has a direct impact on how fast a fish can swim, and the dexterity with which they avoid and redirect their bodies. For example, fish with very long, flashy caudal fins are slower swimmers, and therefore would be ideal prey in nature’s waters. The long, flashy caudal fin you see in pet stores is not actually a product of natural evolution, but human selection. These caudal fins are very attractive to people when considering domestic fish as an ambient or decorative display item, especially in tropical fish. They are quite beautiful to behold, but detrimental when it comes to out-swimming another fish, the potential predator. Fish with shorter, forked and lunate caudal fins are among the fastest swimmers. The sailfish, thought to be earth’s fastest-swimming fish, can travel at speeds topping 70 mph. It owes some of that to its’ starkly forked caudal fin.
Other fins help a fish to swim also, and these vary from species to species even more than caudal fins. Dorsal, pectoral, anal, and pelvic fins all aid in stability and propulsion in the water. Dorsal fins and anal fins are particularly important for stability, so the fish does not just roll over in the water. Previously, much research indicated that caudal fins were the only fins responsible for propulsion. But in the last few years, experiments have introduced data supporting the hypothesis that dorsal and anal fins do actually aid fish in swimming, not just in staying upright. These tests seem to show that dorsal and anal fins assist, meaning that the speed and dexterity of a fish swimming in the water depends on the interplay of a few of its’ fins, not just the caudal.
If fins are like the propellors of a helicopter, muscles are the engine. Muscles are the real key topic in learning how fish swim. The fins, made of cartilage, are not connected to the spine or bones of a fish, but rather, they are directly connected to the muscles. Proportionally, a fish’s body has more muscle than any other living thing with a spine. These muscles move in a wave-like formation, bending sinuously and quickly to move them through the water. The undulations run from head to caudal fin, reacting with the movement of the waters they swim in. There are generally two types of muscle in a fish- red and white. The red muscle runs straight along the body and is found directly under the skin. The white, which is usually the thicker, denser muscle, is intricately shaped like a helix. The white muscle is good for shorter, massive bursts of energy while swimming. These muscles can be especially concentrated for action at the back end of the fish, turning that caudal fin into the propeller.
Other parts of fish anatomy aid in swimming. Some fish have an organ called a swim bladder that helps maintain depth in the water without utilizing the muscle and fins as much, therefore reserving energy. Then there is the vertebrae of the fish. Imagine if you had no bones in your shoulders, hips, pelvis, or limbs. You would rely on the action of your spine for movement, and chances are you could get quite a bit of movement out of it. The skeletal structure is responsible for holding the whole picture together, from muscles to fins. Fish either have a vertebrae made of bone, or of cartilage. The flexible spine moves with the muscle contractions, making those wave-like motions you can visually detect when watching a fish swim in the water. The longer the fish, the more visible the spinal action is.
So as you can see, there are many biologically intelligent factors that come together to make a fish the natural swimmer that it is. As with all features explored when learning about fish, all are rarely true for any fish. Fish are a very diverse group of animal life with many species present in all types of water, all over the world. As every ecosystem depends on biodiversity to enrich resources, expand knowledge, and keep life and nature stable, fish are an amazing asset to the planet. Their presence and ability to propagate in every aquatic environment provides ecological balance and maintains water quality. The next time you are lucky enough to see a fish swimming in a lake, pond, or ocean, contemplate how nature has orchestrated everything so harmoniously.