Flounders, a type of flatfish belonging to the order Pleuronectiformes, are masters of disguise in their seafloor habitats. The exact color of a flounder is not fixed, but instead is highly variable and dynamic, shifting to blend in with their immediate surroundings. This ability to instantly alter their appearance allows them to effectively disappear against the ocean bottom. Their impressive mastery of camouflage is an adaptation that helps them both hide from predators and ambush prey.
The Distinct Pigmentation of the Ocular and Blind Sides
A flounder’s coloring is fundamentally asymmetrical, a direct result of its unique flat-bodied, bottom-dwelling lifestyle. The ocular side, which is the upper surface where both eyes are located, is the pigmented side of the fish. This side displays dark, mottled colors like brown, gray, or olive, often speckled with spots or patterns that mimic sand, gravel, or mud. This coloration provides crypsis, or camouflage, when viewed from above by predators like birds or larger fish.
Conversely, the blind side, which is the underside of the fish that rests against the substrate, is consistently pale, typically appearing uniformly white, cream, or off-white. This distinct lack of pigmentation on the lower surface is an evolutionary feature, as this side is rarely exposed to light or viewed by other animals. The asymmetrical coloring is established during their metamorphosis from a symmetrical larval stage to the adult flatfish form. During this transformation, the pigment cells on the blind side slowly diminish, resulting in the non-pigmented appearance.
The Biological Mechanism of Color Adaptation
The flounder’s remarkable ability to change its color and pattern is governed by specialized pigment-containing cells in its skin called chromatophores. These cells, which include melanophores for dark pigments and xanthophores for yellow pigments, are controlled by the nervous system. The process begins when the flounder’s eyes visually assess the surrounding environment, registering the texture, color, and brightness of the substrate.
Sensory input from the eyes is relayed to the brain, which then sends signals through the nervous system to the chromatophores. Within a chromatophore, pigment granules are suspended, and the movement of these granules determines the skin’s visible color. To darken the skin or display a complex pattern, the nervous signal causes the pigment granules to disperse throughout the cell, making the color highly visible.
To lighten the skin, the nervous system signals the pigment to aggregate, or cluster tightly, into the center of the cell. When the pigment is concentrated, the cell appears essentially transparent, allowing the lighter underlying skin or reflective cells, such as iridocytes, to become dominant. This rapid dispersion and aggregation of pigment allows the flounder to shift its appearance, sometimes in just a few seconds, to match a new background.
Environmental Triggers for Camouflage
The primary stimulus for the flounder’s color change mechanism is the visual information received about the seafloor. The fish is highly sensitive to the texture, or substrate type, it is resting on, whether it is fine sand, coarse gravel, mud, or broken coral. The flounder attempts to mimic both the coloration and the spatial scale of the background, which is crucial for effective camouflage against different surfaces.
Experimental studies have demonstrated the extent of this adaptation, showing that some flounders can even successfully mimic highly unnatural patterns, such as a black-and-white checkerboard. This suggests the process is not merely a reaction to color, but a sophisticated response to pattern complexity and light intensity. The capacity to match the background helps the flounder remain stationary while ambushing prey or avoiding detection from predators viewing from above.