The flatfish belongs exclusively to the order Pleuronectiformes and represents a unique evolutionary adaptation among vertebrates. Unlike rays, which are flattened top to bottom, flatfish are flattened side to side. They lie on the seafloor on what was originally one side of their body. This extreme asymmetry, characterized by having both eyes on one side of the head, allows them to thrive in a benthic existence. The transformation from a typical fish larva to this highly modified adult defines their success in the marine environment.
The Process of Becoming Flat
All flatfish begin life as bilaterally symmetrical larvae, swimming upright in the water column like any other fish. The larval stage has one eye on each side of the head and a vertically oriented body. The shift to the adult form occurs during metamorphosis, a complex biological event that converts the symmetrical fish into an asymmetrical one.
The most visible change is the migration of one eye across the top of the head to the opposite side. This ocular migration requires extensive remodeling of the skull’s skeletal structure. Specifically, the bones of the neurocranium undergo torsion and reabsorption. This rotation of the skull accommodates both eyes moving to the same side of the body, allowing them to point upward when the fish settles on the substrate.
Metamorphosis is largely controlled by endocrine signals, primarily thyroid hormones, similar to metamorphosis in amphibians. As the eye completes its transit, the fish loses its swim bladder, which is necessary for buoyancy. It then adopts a tilted swimming posture before settling fully onto the bottom. The side facing up, the “eyed side,” develops pigmented coloration, while the side resting on the seabed, the “blind side,” loses its pigment and becomes pale or white.
Adapting to Life on the Seabed
The flatfish’s morphology is suited for its bottom-dwelling lifestyle, serving the purposes of defense and predation. Their primary survival tool is their ability to camouflage against the ocean floor. This is achieved using specialized skin cells called chromatophores, which are concentrated on the pigmented eyed side.
Chromatophores contain various pigments, such as black, brown, red, and yellow, which the fish can manipulate. The brain processes visual information from the upward-facing eyes and sends signals to rapidly expand or contract these pigment cells. This control allows the flatfish to instantly match the color and pattern of the surrounding substrate. They often bury themselves partially in the sediment, leaving only their eyes exposed.
This camouflage is central to their feeding strategy as opportunistic ambush predators. Lying motionless, they wait for small fish or crustaceans to pass overhead before launching a quick strike. Their mouth is positioned asymmetrically, optimized for scooping prey directly off the ocean floor. Flatfish can move quickly when necessary, employing whole-body undulations for a fast escape or rapid burial into the substrate.
Diversity Among Flatfish Families
The Pleuronectiformes order contains hundreds of species, categorized based on the direction of ocular migration. This determines whether a flatfish is considered “right-eyed” (dextral) or “left-eyed” (sinistral).
In dextral flatfish, the left eye migrates to the right side of the head, meaning the fish rests on its left side. Examples of dextral species include the Halibut, Flounder, and Soles. Conversely, in sinistral flatfish, the right eye migrates to the left side, and the fish rests on its right side. The Turbot and Brill are well-known examples of sinistral flatfish.
This group exhibits a vast size range. Species range from small sand flounders measuring only a few centimeters to the massive Atlantic Halibut, which can weigh hundreds of kilograms.
It is important to distinguish true flatfish from other flattened marine animals, such as rays and skates. Rays and skates are cartilaginous fish that are flattened from top to bottom (dorso-ventral flattening). This body shape results from an expansion of their pectoral fins, representing a different evolutionary path. Flatfish lie on their side, while rays and skates lie on their belly.