Flatfish are the fish with both eyes on one side of their head. This group includes flounder, sole, halibut, turbot, plaice, sanddabs, and tonguefish, all belonging to the order Pleuronectiformes. There are over 700 species worldwide, and every single one develops this lopsided anatomy. What makes it even stranger: they aren’t born that way.
How One Eye Migrates Across the Head
Flatfish hatch as normal-looking, symmetrical larvae with one eye on each side. Within weeks, one eye begins physically migrating across the top of the skull to join the other. In turbot, the first signs of asymmetry appear around 15 days after fertilization, when the bones between the eyes start narrowing on one side. By day 20, the skull bones begin actively twisting, and a unique bone found only in flatfish (called the pseudomesial bar) starts forming to support the new arrangement. By day 30, the migrating eye is visible from the opposite side of the body. Around day 57, both eyes are fully settled on the same side.
This isn’t just an eye sliding over skin. The entire skull remodels itself. Frontal bones flex, canal bones twist, and new bone grows to accommodate the repositioned eye. Scientists have proposed two explanations for what drives the movement: either the developing asymmetry in the skull physically pulls the eye across, or tissue growth behind the eye on the blind side pushes it over. The two eyes also don’t adapt at the same pace. The migrating eye and the stationary eye activate different biological systems at different times during the process.
As the eye moves, the young fish also shifts from swimming upright to lying flat on the ocean floor. The underside loses its color and becomes pale, while the top side develops pigmentation and camouflage patterns.
Left-Eyed vs. Right-Eyed Species
Not all flatfish have their eyes on the same side. Some families are “right-eyed,” meaning both eyes end up on the right side of the head when the fish lies flat. Others are “left-eyed,” with both eyes on the left. The terminology can be counterintuitive: a left-eyed fish is one whose right eye migrated to the left side, and vice versa.
Most of the flatfish you’ll encounter at a fish market or on the water are right-eyed. Pacific halibut, Dover sole, English sole, petrale sole, arrowtooth flounder, butter sole, rock sole, and rex sole are all right-eyed species. Pacific sanddabs, by contrast, are left-eyed. Turbot are also typically left-eyed.
Some species break their own rules. The starry flounder belongs to a right-eyed family but frequently shows up left-eyed. One particularly interesting genus, Psettodes, is considered the most primitive living flatfish. Its eye migration is incomplete compared to other species, it retains pigmentation on both sides of its body, and left-eyed and right-eyed individuals appear in roughly equal numbers within the same population. It’s essentially a living snapshot of what earlier flatfish may have looked like.
Why This Anatomy Evolved
Having both eyes on one side makes perfect sense once you picture how these fish live. Flatfish spend their adult lives lying on the ocean floor, pressed against sand or mud. If one eye were trapped underneath, it would be both useless and vulnerable to damage. Ancient flatfish whose eyes sat even slightly closer together had a survival advantage: better vision while lying flat meant better odds of spotting predators and catching prey.
Fossil evidence supports this gradual shift. Two extinct species from the Eocene epoch (roughly 50 million years ago), Amphistium and Heteronectes, are the most primitive flatfish relatives ever found. Their skulls were clearly asymmetrical, like modern flatfish, but their eye migration was incomplete. Post-metamorphic adults still had eyes on opposite sides of the head, just positioned unevenly. These fossils fill a gap that puzzled biologists for over a century, showing that the extreme asymmetry of today’s flatfish evolved step by step rather than appearing all at once.
Modern flatfish have turned this adaptation into a full hunting strategy. They lie camouflaged on the seafloor, nearly invisible, with both eyes swiveling independently to provide close to 360-degree vision from above. When prey passes within range, they launch off the bottom using a concealed pectoral fin and a jet of water expelled through their gills.
Common Flatfish You Might Encounter
Flatfish are found in oceans around the world, from shallow estuaries to deep-sea thermal vents. A few species even live in freshwater rivers. The ones most people recognize fall into a handful of categories:
- Halibut are the largest flatfish. Pacific halibut are almost always right-eyed and can grow to several hundred pounds.
- Flounder is a broad common name covering dozens of species across multiple families, both left-eyed and right-eyed.
- Sole includes species like Dover sole, English sole, and petrale sole, most of which are right-eyed in the Pacific.
- Turbot are prized in European cuisine and are typically left-eyed.
- Sanddabs are small, left-eyed flatfish common along the Pacific coast.
- Plaice are right-eyed and widely fished in the North Atlantic and European waters.
Despite looking so different from other fish, flatfish are closely related to perch-like species. Their bizarre body plan is the result of one of the most dramatic transformations in the vertebrate world, a metamorphosis that restructures the skull, shifts the organs, and turns a tiny symmetric larva into an ambush predator lying in wait on the ocean floor.