Flatfish

Some fish live on the sea floor.

They lie on their side. This helps them hide in the sand.

When they are young, they look like normal fish. Then, one eye moves to the top of their head.

This lets them see everything above them.

Many fish have these names. You might know the flounder or the sole. Some are even called halibut.

These fish can be very small. Others can grow to be very large. They live in oceans all over the world.

They use colors to hide from hungry animals.

It is hard to see them on the bottom!

Flatfish are a special group of fish. They spend their lives on the ocean floor. Most fish have one eye on each side. Flatfish are different. As they grow, one eye moves across their head. This is called metamorphosis, which is a big change in body shape.

This move lets both eyes face upward. The fish then lies on its side. This helps them hide in the sand.

Some flatfish have eyes on the left side. Others have eyes on the right. They use colors to hide from predators. This is called camouflage. Some even use chromatophores, which are tiny parts that change color.

This helps them match the ground.

Flatfish live in oceans everywhere. They live from the cold Arctic to the warm tropics. Most stay in shallow water. Some live in very deep water. A tonguefish even lives near hot vents on the seafloor. Some species are very small. Others, like the halibut, grow very large.

They can weigh a lot!

Flatfish are a unique group of ray-finned fish. They are famous for how they live on the ocean floor. Most fish swim upright with one eye on each side. Flatfish are different because they lie on their sides. This means their bodies are laterally compressed, or flattened side-to-side.

This shape helps them stay close to the seabed. They use this position to hide from animals that might eat them. This clever way of living is a special adaptation for the seafloor.

How does a fish end up with both eyes on one side? It starts when the fish is a tiny larva. At first, these larvae look like normal, symmetrical fish. They swim in the water instead of staying on the bottom. As they grow, they go through metamorphosis, which is a big change in body shape.

During this time, one eye migrates across the top of the head. This move leaves the fish blind on one side. The fish also loses its swim bladder, which is a part that helps it float. Finally, it sinks to the bottom to live its adult life.

Scientists have studied how these fish changed over a very long time. For a long time, people thought this change happened all at once. They thought a fish with only one eye on top would not survive. But fossil evidence has changed what we know.

Fossils like Amphistium, from about 50 million years ago, show a middle step. These ancient fish had heads that were not quite as lopsided as modern ones. Paleontologist Matt Friedman found that this change happened gradually. It was a slow process of natural selection over many years.

Flatfish can be found in oceans all over the world. They live in the cold Arctic and the warm tropics. Most stay in shallow water, but some live very deep. A tonguefish called Symphurus thermophilus even lives near hot sulfur vents.

These fish come in many different sizes. The tiny sand flounder is only about 2 inches long. On the other hand, the Atlantic halibut can grow very large. Some of these big fish can weigh hundreds of pounds. There are many common names for them, like sole, turbot, and plaice.

Many things in nature use tricks to stay safe. Flatfish use camouflage to blend into the sand. They have special parts called chromatophores that help them change color. This is a lot like how an octopus hides. Some flatfish even use bright eyespots to scare others away. A few small tropical species are even poisonous to stay safe. Just like a person might wear a disguise, these fish use their bodies to disappear. This helps them survive in a busy ocean.

Flatfish are a specialized group of ray-finned fish belonging to the suborder Pleuronectoidei. They are best known for their unique, asymmetrical bodies. Most fish swim upright with one eye on each side of their head. Flatfish, however, lie on one side of their laterally compressed bodies upon the seafloor. This means their bodies are flattened from side to side. In their adult form, both eyes are located on the side of the head facing upward. The other side of the head is called the "blind side" because it lies against the substrate.

The transformation from a normal fish to a flatfish is a complex process called metamorphosis. Flatfish begin their lives as larvae that look like typical, symmetrical fish. These larvae are elongated and possess a swim bladder to help them float. They swim in the water column as ichthyoplankton rather than staying on the bottom. During metamorphosis, thyroid hormones trigger a massive change in their body structure. One eye actually migrates across the top of the head to the other side. The fish also loses its swim bladder and its protective spines. Finally, the fish sinks to the bottom to begin its life as a bottom-dweller.

There are different types of flatfish based on which side their eyes are on. Some species are dextral, meaning they are right-eyed flatfish with eyes on the right side. Others are sinistral, or left-eyed flatfish, with eyes on the left side. For example, the Pleuronectidae family lies on its left side with eyes on the right. The Paralichthyidae family lies on its right side with eyes on the left. Some primitive genera, such as Psettodes, are more flexible. They can develop into either right-facing or left-facing individuals. This variation shows how diverse the group has become.

Flatfish use several biological tools to survive on the seafloor. Most rely on cryptic coloration, which is a form of camouflage to hide from predators. Some species possess chromatophores, which are special cells that allow them to change their skin color to match the background. This is a similar ability seen in many cephalopods, like octopuses. Other species use aposematic traits, such as bright eyespots, to warn predators. A few small tropical species are even poisonous. Some young fish, like the juvenile Soleichthys maculosus, even mimic the color and swimming patterns of toxic flatworms to stay safe.

Scientists have long debated how these fish evolved such a strange shape. For many years, researchers thought the change happened through saltation, or a sudden jump. They believed a fish with only one eye on top would be at a disadvantage. However, fossil evidence has provided a different story. Fossils of the genus Amphistium, dating to about 50 million years ago, show a middle step. These ancient fish were less asymmetrical than modern flatfish. They still had one eye on each side, but one eye was positioned closer to the top of the head.

This discovery helped paleontologist Matt Friedman conclude that evolution happened gradually. He suggests the change was driven by natural selection rather than a sudden mutation. Primitive flatfish may have propped their heads up above the sand to see better. This would have allowed them to survive even with a partially migrated eye. This gradual process allowed them to transition from swimming in the water to living on the seabed. The fossil record now shows a clearer path from typical fish to the highly specialized flatfish we see today.

Flatfish are found in oceans all over the world, from the Arctic to Antarctica. They are most diverse in the Indo-West Pacific region. Most species live in shallow waters, but some can be found at great depths. A tonguefish called Symphurus thermophilus is unique because it lives near hydrothermal vents. These are areas on the seafloor where hot sulfur emerges. Flatfish also show incredible variety in size. The tiny sand flounder is only about 2 inches long and weighs very little. In contrast, the Atlantic halibut can grow to massive sizes and weigh hundreds of pounds.

Because they are so widespread, many different names are used for them. Common names include sole, turbot, plaice, halibut, and flounder. However, these names do not always match scientific relationships. For instance, the three species commonly called "halibut" are actually part of the right-eye flounder family. The spiny turbots are also not closely related to "true" turbots. Instead, they occupy a basal position on the phylogenetic tree, meaning they are more closely related to the earliest flatfish. This shows that even common names can hide the true complexity of nature.