A flounder is a type of flatfish characterized by its compressed and asymmetrical body shape. This unique physical structure distinguishes it from most other vertebrates and is an effective adaptation for life on the ocean floor. Found globally in marine and sometimes brackish waters, its distinct flat form allows it to blend seamlessly with the seafloor. This ability makes the flounder a master of camouflage and an effective ambush predator.
Anatomy of the Adult Flounder
The adult flounder has a body that is laterally compressed, meaning it is flattened from side to side. This disc-like profile enables the fish to lie flush against the substrate, which is essential for its bottom-dwelling existence. Both of the flounder’s eyes are positioned on the upward-facing side of its head, granting it a wide view of the water column above.
The side bearing the eyes is called the “eyed side” and is typically pigmented with complex colors and patterns that allow for impressive camouflage. The side resting on the ocean floor, known as the “blind side,” is entirely unpigmented and usually appears pale white or cream. Flounder can actively change the color and pattern of their eyed side by manipulating specialized pigment-containing cells called chromatophores. This allows them to match the surrounding sand, gravel, or mud within minutes.
How Flounder Become Flat
Flounder do not begin life with their asymmetrical appearance but hatch from eggs as bilaterally symmetrical larvae, resembling typical small fish. In this early stage, they swim upright in the water column with an eye on each side of the head. The transformation into a flatfish is triggered by metamorphosis, which occurs shortly after hatching.
During metamorphosis, one eye begins to migrate across the top of the head to join the other eye on the future eyed side. This migration is facilitated by asymmetrical growth and remodeling of the skull bones. This developmental process is regulated by thyroid hormones, which signal the necessary skeletal changes.
As the eye migration concludes, the fish loses its swim bladder and ceases its vertical swimming habit. The flounder then settles permanently onto the seafloor, lying on its blind side. This shift results in a fish perfectly adapted to its benthic lifestyle, with both eyes oriented upward for spotting prey and avoiding predators.
Habitat and Feeding Behavior
Flounder live in close association with the seabed, occupying coastal and estuarine environments. They prefer habitats with soft substrates, such as sand, mud, or silty bottoms, where their flat bodies and camouflage abilities are most effective. These fish are often found in shallow coastal waters, bays, and tidal creeks, tolerating a range of salinities.
The flounder is a carnivorous ambush predator, relying heavily on camouflage and patience to hunt. It often partially buries itself in the sediment, with only its eyes protruding, waiting for unsuspecting prey to pass overhead. Their diet consists mainly of small bottom-dwelling organisms like crustaceans (shrimp and crabs), marine worms, and juvenile fish. The fish strikes with speed when a meal is within range, using its large mouth to capture the prey.