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.