Why fish in the deep-sea evolve such strange shapes

The deep sea hides life in strange forms. Far below the light, pressure crushes, and temperatures drop near freezing. Yet fish thrive there, shaped by forces we barely understand.

A new study reveals how their bodies changed over millions of years – and why fish living in different parts of the deep ocean evolved in distinct ways.

Elizabeth Santos from The Ohio State University and her team studied almost 3,000 deep-sea species. They compared body shapes, habitats, and evolutionary histories.

The results showed that deep-sea life is far from uniform. Conditions differ sharply between the seafloor and open water, pushing species toward very different designs.

Different fish shapes underwater

Pelagic fish drift in open water. Benthic ones rest on or near the seabed. This study found more variety among pelagic species, from round anglerfish to long, ribbon-like eels.

Benthic fish looked more alike – most had narrow, tapered bodies built for steady movement near the bottom.

“We found that evolution pushes and pulls fish body shape in different directions depending on whether they’re benthic or pelagic,” said Santos. “We talk about the deep sea as if it is sort of all one thing, when really it is not – it is actually quite diverse.”

That diversity traces back to how species live and feed. Open water offers space but little shelter. The seafloor, on the other hand, offers structure but fewer opportunities to roam.

Evolution carved two very different paths through this vast world of darkness.

Depth shapes fish evolution

The deeper a fish lives, the faster its body changes. Santos and her colleagues discovered that deep-sea species evolve quicker than those in shallower waters. But speed doesn’t always mean variety.

Benthic species, which stay near the bottom, change shape rapidly yet remain similar. Pelagic fish, drifting above, evolve slower but in many more ways.

“Colonization of the deep pelagic seems to be a more typical route for achieving diversity than the benthic,” Santos said.

“In the water column, you see more lineages that are very distantly related from each other that probably colonized that habitat at all different times. And that’s why you get a lot of diversity.”

In open water, new arrivals keep mixing with older residents, creating a patchwork of unrelated body types. On the bottom, evolution happens in place. Over time, one lineage splits into several close relatives with nearly identical forms.

Life in the dark

Beyond 200 meters, sunlight fades. Plants can’t survive, and predators must adapt. The study found that long, thin bodies dominate here. This shape helps conserve energy in a cold, still world.

Deep-demersal fishes – like grenadiers and halosaurs – use undulating movements to travel far with little effort. Their bodies make that possible.

“In the deep water column, you don’t see a lot of big, powerful swimmers because it’s a very different kind of environment. It’s dark,” said Santos.

“And so you tend to see fishes that sit in the water and wait for food to find them. Being pelagic in the deep sea seems to be fine for many different types of body shapes, from the blobby all the away to the skinny.”

Without light, food is scarce. Many bottom-dwellers scavenge, following chemical trails to decaying animals. Others float motionless, conserving energy. Movement costs too much in a place where meals come rarely.

Strange fish shapes emerge

Near the surface, design rules are strict. Fast swimmers stay sleek; reef fish stay flat. In the deep, those rules collapse.

Pelagic species face little pressure to move fast. Over time, this relaxed selection allowed unusual forms to persist.

Some deep-sea fish look like floating balloons. Others resemble ribbons or snakes. Anglerfish carry glowing lures that act as bait. These shapes are strange but effective.

Bioluminescence even influences body design – light organs alter balance and proportion, changing the fish’s silhouette.

The team’s findings suggest that variety in deep-sea fish isn’t random. It grows from freedom from speed, sunlight, and constant motion. When survival depends on waiting, evolution explores every possible form.

New patterns with depth

Depth changes everything. In shallow water, survival demands speed and precision. Far below, endurance matters more than agility.

The researchers found that fish across deep habitats, from bottom-dwellers to open-water drifters, share one key trait: elongation.

The cold, dense water favors streamlined bodies. Yet each habitat rewrites evolution in its own way. Benthic fish evolve quickly but remain conservative in shape.

Pelagic species evolve more slowly but display wild diversity. The balance between rate and variety shifts depending on where a fish calls home.

A living puzzle

Santos and her team also mapped how species moved through time. Some migrated downward along the seafloor. Others entered the deep sea through open water.

Once settled, they adapted to local challenges. This back-and-forth movement produced what Santos calls a “mosaic” of evolutionary paths.

Every shift – upward, downward, or sideways – added new possibilities. The result is an ecosystem where no single rule explains all. Depth and lifestyle work together, shaping fish into a gallery of survivors.

Unknown fish shapes

The deep sea remains one of the few places on Earth untouched by humans. Its vastness hides millions of unknown creatures. Each new study offers a glimpse of how life thrives there.

“The one place that humans have not dominated on this planet is the deep sea – and there is still so much to learn about the mystery of what all is there,” said Santos.

“This paper moves us forward with a recognition that evolution can work really differently depending on where exactly in the deep sea the fishes are.”

Her words capture the essence of discovery. Beneath that silent blue, evolution still experiments. Each shape, each glow, each movement tells the story of survival in Earth’s darkest world.

Co-authors of the study include Sarah Friedman of the NOAA Alaska Fisheries Science Center and Christopher Martinez of the University of California, Irvine.

The study is published in the journal Evolution.

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