General Knowledge

Why are fish fish-shaped?

In tropical seas, flying fish leap out of the water, gliding for up to 200 meters using wing-like fins, before dipping back into the sea. In the Indo-Pacific, a hunting sailfish can reach speeds of 110 kilometers per hour. That’s 11 times faster than Olympic swimming champion Michael Phelps.

It can then stick up its spiny dorsal fin like a brake, grinding to a dead halt, mid-swim. Each of these physical feats is made possible by a fish’s form, which in most species is a smooth, elongated body, fins, and a tail. These features are shared across thousands of fish species, each introducing its own variations on the theme to survive in unique habitats. What makes these features so commonplace in fish, and what does it reveal about the more than 33,000 fish species that inhabit earth’s rivers, lakes, and seas?

Fish can be split into two main groups, according to the type of motion they favor. The first is body and caudal fin driven motion, and most fish species, about 85%, fall into this group. Here, the body and tail are the primary propelling forces, with fins mainly playing a stabilizing and steering role.

This configuration suits many open-water species, which need speed, thrust and control for constant, efficient swimming. Eels lie at one extreme of this group. Known as anguilliform swimmers, their entire bodies undulate to generate a wave-like motion. Compared to anguilliform fish, species like salmon and trout, known as subcarangiforms, use about two-thirds of their body mass to generate motion, while carangiform swimmers, such as mackerel, only use about a third. Typically, the less of its mass a fish uses to generate motion, the more streamlined its shape.

At the other end of the spectrum from eels are ostraciiform species like boxfish, and thunniform swimmers like tuna. In these fish, the tails, also known as caudal fins, do the work. A tuna’s tail is attached by tendons to multiple muscles in its body. It powers the body like an engine, forcefully catapulting the bullet-like fish to speeds up to 69 kilometers per hour.

The second major fish group relies on median and paired fin motion, meaning they’re propelled through the water predominantly by their fins. Fins allow fine-tuned movement at slow speeds, so this propulsion is typically found in fish that have to navigate complex habitats. Bottom-dwelling fishes, like rays, fall into this group; using their huge pectoral fins, they can lift themselves swiftly off the sea floor.

That conveniently allows them to inhabit shallow seas without being buffeted about by waves. Similarly, shallow-water flatfish use their entire bodies as one big fin to hoist themselves up off the sand. Ocean sunfish lack tails, so they move around slowly by beating their wing-like median fins instead. Similar movements are shared by many reef species, like the queen angelfish, surgeonfish, and wrasse.

Their focus on fins has taken the demand off their bodies, many of which have consequently evolved into unusual and inventive shapes. There are fishes within both groups that seem to be outliers. But if you look closer, you’ll notice that these common traits are disguised. Seahorses, for instance, don’t appear fish-shaped in any conventional way, yet they use their flexible dorsal fins as makeshift tails. A pufferfish may occasionally look more like a lethal balloon, but if it needs to swim rapidly, it’ll retract its spines.

Handfish look like they have legs, but really these limb-like structures are fins, modified to help them amble across the sea floor. For fish, motion underpins survival, so it’s become a huge evolutionary driver of form. The widespread features of fish have been maintained across tens of thousands of fish species, not to mention other ocean-dwelling animals, like penguins, dolphins, sea slugs, and squids. And that’s precisely because they’ve proven so successful.

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