The order Pleuronectiformes, commonly known as flatfish, represents one of the most unusual body plans among vertebrates. These fish, which include species like flounder, sole, and halibut, are characterized by a compressed, disc-like shape and profound bilateral asymmetry in their adult form. Flatfish are born as typical, upright-swimming fish, but they undergo a dramatic transformation. This metamorphosis results in both eyes migrating to one side of the head, a specialized morphology adapted for life spent resting directly on the ocean floor.
The Adaptive Advantage of a Benthic Lifestyle
The flattened body of a flatfish is an adaptation to the benthic zone, the ecological region at the lowest level of a body of water. Living on the seabed requires the fish to avoid detection by predators while simultaneously searching for prey. Lateral compression allows the fish to press its thin body tightly against the substrate, minimizing its silhouette and making it difficult to spot from above. This unique posture enables a highly effective form of camouflage known as crypsis.
The upper surface, or ocular side, is densely populated with specialized pigment cells called chromatophores. These cells allow the flatfish to rapidly change the color and pattern of its skin to precisely match the surrounding sand, gravel, or mud. This dynamic camouflage is neurally controlled, enabling the fish to instantly adjust its appearance to blend into different backgrounds.
The flat shape also facilitates partial burial, a behavior where the fish rapidly vibrates its fins to kick up sediment. It then settles back down, allowing the disturbed material to lightly cover its body, with only the eyes remaining exposed. This ability to disappear into the environment allows the fish to function effectively as an ambush predator.
The Science of Transformation: Metamorphosis and Eye Migration
The specialized adult form of the flatfish is achieved through a dramatic metamorphosis that begins shortly after hatching. Flatfish hatch as planktonic larvae, which are initially bilaterally symmetrical, swimming vertically with one eye on each side of the head. This larval stage typically lasts weeks to months, depending on the species, during which time the fish feeds and grows.
The initiation of metamorphosis is often triggered by hormonal changes, primarily involving thyroid hormones, which signal the onset of physical restructuring. The defining event is the migration of one eye across the top of the head to the opposite side, a process that completely rearranges the bony structure of the skull. This migration involves the asymmetrical growth and remodeling of specific cranial bones, rather than a simple movement.
Bones in the anterior neurocranium, such as the ethmoid plate and frontal bones, begin to grow unevenly. This differential growth causes the skull to twist, effectively pulling the migrating eye around the head. The eye moves along a developing channel formed by the reshaping bone and cartilage, not through the skull itself.
The direction of this eye migration is genetically determined and defines two major groups of flatfish. In “right-eyed” or dextral species, such as halibut and plaice, the left eye migrates to the right side of the head. Conversely, in “left-eyed” or sinistral species, like turbot and some flounders, the right eye moves to the left side. Once metamorphosis is complete, the larva loses its swim bladder, sinks to the bottom, and permanently adopts its asymmetrical, benthic lifestyle.
Specialized Adult Anatomy and Function
The radical transformation results in a fish with two functionally distinct sides. The ocular side, which faces upward, is pigmented and serves as the primary visual and camouflage surface. The blind side, which rests against the substrate, is generally unpigmented and remains a uniform white or pale color.
With both eyes situated on the same side, the flatfish gains a wide-angle, binocular field of vision directed upward. Their eyes are often protrusible, meaning they can be slightly raised and moved independently. This allows them to function like periscopes, scanning for predators above and prey buried beneath the sediment.
The long dorsal and anal fins, which run nearly the entire circumference of the body, become their primary means of locomotion. Flatfish typically move across the seafloor using a characteristic rippling or undulating motion of these fins. This method of swimming is specialized for short, quick bursts when ambushing prey or evading a threat.
Their feeding apparatus is also adapted to their ambush strategy, with the mouth often positioned slightly to one side. Flatfish primarily consume crustaceans, worms, and small fish. They detect this prey using their specialized vision and lateral line system.