The Incredible Diversity of Fish: How Form Equals Function
Fish are among the most diverse creatures on the planet, with over 30,000 species occupying nearly every aquatic habitat imaginable. This vast distribution has led to the evolution of a stunning variety of behaviors and body shapes, allowing fish to survive in environments ranging from the deep-sea abyss to the shallows of salt marshes. Their shapes vary from thin, ribbon-like bodies to sleek, torpedo forms, with each design perfectly aligned to the fish’s specific lifestyle. Tuna, flounder, and pipefish might all have fins and gills, but each live drastically different lives beneath the waves.
To better understand why fish have such diversity, we can draw an analogy to something more familiar: cars. Minivans and Indy 500 race cars look nothing alike, and for a good reason: they are built to do very different things. Race cars are designed for incredible speeds, while minivans are essential for transporting large families and cargo. A minivan racing in the Indy 500 would be left in the dust, as its squat form is no match for the sleek and aerodynamic shape of a race car. Likewise, a sports car would be a very impractical choice for a family vacation.
The same concept applies to fish. Each fish species has a body shape perfectly suited for its lifestyle: how it moves, where it feeds, and the environment it inhabits. Like cars, fish can be classified into categories based on their individual features.
Tuna and marlin, some of the fastest fish in the sea, rely on a bullet-shaped body or fusiform shape to obtain such fast speeds. Fusiform bodies are streamlined, allowing these animals to glide effortlessly through water while minimizing drag—a force that would otherwise significantly slow them down. As a result, fish with a fusiform shape are often found in open water, where speed and efficiency are key to survival.
The striped marlin (Kajikia audax) exemplifies an ideal fusiform shape. Its elongated body, along with a deeply forked tail, allow it to make vast migrations across the open ocean at incredible speeds of up to 50mph, making it one of the fastest fish in the ocean.
Though fish with a fusiform shape commonly inhabit open oceans, it is not the only form seen in pelagic environments. For example, ocean sunfish (Mola mola) are found in open-ocean settings and have a laterally compressed shape with a shortened tail fin, called a clavus, that helps the fish control the direction of its slow-moving body.
This side-to-side flattening of the body is often referred to as compressiform. Though some pelagic fish like the ocean sunfish might exhibit this body shape, most compressiform fish are found in benthic (bottom) or demersal (close association with the bottom) habitats. Many reef-fish families such as butterflyfish, angelfish, and surgeonfish are perfect examples of the laterally compressed shape.
Compressiform fish are experts in moving with quickness and agility, rather than the sustained speed often found in fusiform fish. Their flattened bodies make them highly maneuverable, giving them the ability to dart in and out of spaces like tight reef crevices with ease. This agility is a crucial defense mechanism, as it helps them avoid predators that rely on speed or stealth to capture prey. The instability created by their compressed shape may seem counterintuitive, but it actually makes their movements more unpredictable and difficult for predators to anticipate. As a result, these fish often live in complex environments that require quick reflexes and agile bodies, like reefs, wrecks, and docks.
Unlike the side-to-side flattening seen in compressiform fish, depressiform fish are flattened from top to bottom, giving them a pancake-like appearance. This shape is particularly advantageous for fish that live on the seafloor, where they remain in close contact with the bottom and benefit from reduced exposure to fast-moving water. In addition to their flattened bodies, many depressiform fish also have enlarged pectoral fins. These larger fins, located on the sides, help with stabilization and in some species can even “grip” the bottom, anchoring the fish in place.
The pancake batfish, a member of the family Ogcocephalidae, is a prime example of a depressiform fish uniquely adapted to life on the ocean floor. Its highly flattened head and body, along with its drab, sandy coloration, allow it to blend seamlessly with the surrounding seafloor. As an ambush predator, the pancake batfish remains motionless, patiently waiting for small invertebrates or fish to pass by, at which point it springs into action to capture its prey. When it needs to move, this relatively still creature “walks” on its enlarged pectoral fins, slowly navigating the bottom rather than swimming.
Like the pancake batfish, flatfishes (Pleuronectiformes), are a group of over 800 species that are characterized by a depressiform body shape that is perfectly adapted for life on the seafloor. This group contains some of the most commercially valuable species, like flounder and halibut. All flatfishes go through a process called metamorphosis as they develop. During this rapid period of change, one eye migrates to the other side of their head, resulting in an asymmetrical body plan with both eyes on the upper side. Like other depressiform fish, they lie on the ocean floor, often burying themselves in sand or mud to avoid predators and hunt for prey. Some flatfish species can further conceal themselves using specialized cells called chromatophores to rapidly change color and blend with their surroundings.
Although depressiform fish often rely on concealment to evade predators and capture prey, globiform fish—those with rounded or globe-shaped figures—take a different approach to defense. Species like pufferfish and trunkfish, which are typically slow-moving and lack the speed to escape predators, instead depend on a variety of adaptations, including camouflage, warning colors, toxins, spikes, and the ability to inflate their bodies to appear larger and deter threats.
Studying how a fish’s body type enables it to perform essential behaviors—what scientists call “functional morphology”— allows researchers to detect similarities between extinct and living organisms. This helps them hypothesize how species that no longer exist may have moved and behaved. With practice and a keen eye, one can often infer a fish’s behavior and lifestyle in part by observing its physical features. And for extinct organisms, often the only clues to understanding their lifestyle are from fossilized physical features.
Take ichthyosaurs as an example. These extinct marine reptiles first appeared during the Triassic period, around 251 million years ago. While scientists can’t travel back in time, they can compare the anatomical features of ichthyosaurs to those of modern-day animals. By studying skeletal remains and preserved soft tissue, researchers have found that later ichthyosaurs shared some features with today’s sharks and dolphins. For example, they had compact, torpedo-shaped bodies and crescent-shaped tails.
The influence of the environment on evolution is striking. Despite sharing a similar body plan, marlins, ichthyosaurs and dolphins are very distantly related and didn’t inherit this body shape from a common ancestor. Instead, these unrelated species all independently evolved a fusiform shape because they face or faced similar environmental challenges. This process, called convergent evolution, explains how different species arrive at similar solutions to solve the same problem. Because scientists believe there are a limited number of ways to “design” fast-swimming aquatic organisms; dolphins, ichthyosaurs, tuna, and pelagic sharks like makos, all evolved this streamlined, fusiform body to move efficiently through open water.
By connecting the features of extinct organisms with similar features found in living organisms, researchers can make educated guesses about what function a body part may have served an ancient organism. Based on advanced ichthyosaur skeletons, paleontologists believe they were likely open water predators.
While the body shapes we’ve discussed from fusiform to globiform are among the most recognizable, they represent only a fraction of the diversity found in nature. This variety is a testament to the remarkable radiation in fish shape as they evolved to inhabit highly diverse aquatic environments, from the dark, deep-sea floor to the vast open oceans.