Flounder is unequivocally a fish, despite its highly unusual appearance that often leads to confusion. Its flattened body shape and the striking placement of both eyes on a single side are adaptations for a life spent on the seabed, but this unconventional form does not change its fundamental biological classification. This unique morphology is the result of one of the most remarkable developmental transformations in the animal kingdom.
The Definitive Answer: Classification of Flounder
Flounder belongs to the bony fish group, falling within the Class Actinopterygii, commonly known as ray-finned fishes. This class includes the vast majority of all fish species and confirms flounder as a true vertebrate. The species is further categorized into the Order Pleuronectiformes, a group of approximately 800 species collectively referred to as the flatfishes.
Flatfishes possess long, continuous dorsal and anal fins that run nearly the entire length of the body, providing propulsion and stability as they undulate along the ocean floor. Unlike many other bony fishes, adult flatfishes lack a swim bladder, which is consistent with their benthic, or bottom-dwelling, lifestyle.
How Flounder Get Their Unique Shape
The flounder’s distinctive, sideways-lying body is the direct result of a biological process called metamorphosis. Flounder begin their lives as pelagic larvae that are bilaterally symmetrical, swimming upright with one eye on each side of the head. This larval stage is brief, lasting only a few weeks to months.
The transformation is triggered by environmental cues and regulated by thyroid hormones, which initiate significant skeletal and muscular changes in the head. During this period, one eye—either the left or the right, depending on the species—begins to migrate across the top of the cranium toward the other side. This migration causes a dramatic twisting and remodeling of the cranial bones. The result is the complete asymmetry characteristic of the adult flounder.
Key Characteristics and Habitat
The asymmetrical adult flounder is perfectly adapted for its life as a demersal fish. With both eyes positioned on the upward-facing side, the flounder lies flat on the substrate, keeping its blind, unpigmented side against the bottom. This eye placement gives it a wide field of view to watch for both prey and predators while remaining hidden.
Flounder are masters of crypsis, or camouflage. They possess specialized pigment cells called chromatophores that allow them to rapidly change the color and pattern of their ocular side to precisely match the surrounding substrate. This adaptation makes them highly effective ambush predators, as they bury themselves partially in soft, muddy, or sandy bottoms with only their eyes exposed. Their diet consists primarily of small, bottom-dwelling organisms, such as crustaceans, worms, and small fish, which they capture with a quick, explosive strike. Flounder inhabit a wide range of marine and estuarine environments worldwide, often found in shallow coastal waters, bays, and river mouths.