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Why are fish fish-shaped? - Lauren Sallan


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In tropical seas, flying fish leap out of the water, gliding for up to 200 meters, before dipping back into the sea. In the Indo-Pacific, a hunting sailfish swims up to 110 kilometers per hour. These feats are made possible by a fish’s form—which in most species is a smooth, long body, fins, and a tail. Lauren Sallan explains why these features are so common, and what it reveals about fish.

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“Fish” is the common term for a wide variety of vertebrates without land-dwelling ancestors. “Fishes” (the plural of fish when referring to multiple species) therefore includes everything from clownfish to sharks to jawless lampreys, no matter how distantly related. Vertebrates with fingers, including humans, penguins, and whales, are called tetrapods. All tetrapods are descended from a “fish” ancestor with fins—rather than finger-containing flippers—which existed over 350 million years ago. Because of this ancestry, some fishes are more closely related to humans than they are to other fish groups.

Our closest living fishy relatives are the “living fossil” lungfishes and coelacanths. These are the lobe-finned fishes (Sarcopterygii), which have the equivalent of our arm bones in their pectoral fins. These specific bones are missing in the 33,000 species of ray-finned fishes (Actinopterygii), such as salmon and tuna. However, ray-fins share other bones like ours in our skulls, bodies, and jaws. These shared features mean that ray-finned fishes and lobe-finned fishes (including tetrapods) also descend from a common ancestor with these bones. Therefore, they belong to a larger group (clade) called bony fishes (Osteichthyes).

Sharks and rays, known as cartilaginous fishes (Chondrichthyes), have lost all bones in their skeletons, and are only distantly related to us and all other fishes. Yet, the cartilage jaws of sharks show that they also descend from a jawed common ancestor. Sharks and bony fishes belong to a group known as jawed fishes (Gnathostomes), which distinguishes them from jawless fishes, like lamprey and hagfish.

Despite hundreds of millions of years of separate evolution, the vast majority of “fishes” from all groups, except land-living tetrapods, have retained their essential, streamlined shapes and fins. They have also evolved the same kinds of aquatic forms, including reef fishes and eels, throughout the 500-million-year history of vertebrates. 

The study of the function of animal shape is called functional morphology, or biomechanics. Research on fish biomechanics involves many different scientific tools. Scientists can observe live fishes in flow tanks, which are special tanks that use jets to move water over an animal swimming in place; this creates a water “wind tunnel.” They use light, lasers, and particles to see how fishes can use their fins and bodies to move water for propulsion or braking. Researchers also produce and study from models of fishes. These models range from simple shapes to 3D computer renderings, which can include CT scans or real, working robots.

The results of biomechanics studies are used to understand the diversity of fishes and their interactions in ecosystems. By comparing fossils with living forms, scientists have also been able to reconstruct the abilities of fishes that have been extinct for millions of years. Discoveries from fish biomechanics have been applied to the designs of everything from aircraft to submarines to swimsuits. Indeed, many engineering breakthroughs are made possible by scientific research on living shapes. Evolution is a process that selects the best available solutions to common problems. By understanding how these solutions work, we can use them to our advantage.

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