Flounders, along with sole and halibut, belong to the order Pleuronectiformes, a group of fish characterized by their flattened bodies. This shape and their habit of lying on the seabed raise a fundamental biological question: why are they flat? The answer lies in one of the most dramatic and complex physical transformations in the animal kingdom. This process, known as metamorphosis, allows the flounder to transition from a typical, free-swimming fish into a specialized bottom-dweller.
The Flounder’s Symmetrical Beginnings
Newly hatched flounders begin life looking like almost any other tiny fish larva, exhibiting perfect bilateral symmetry. These transparent larvae are only a few millimeters long and swim upright in the water column. They possess one eye on each side of their head and drift in the pelagic zone, feeding on plankton.
Their initial existence is far removed from the benthic environment they will eventually inhabit. This symmetrical, upright swimming stage is temporary, lasting only days or weeks depending on the species. It is a necessary stage before the onset of developmental changes that fundamentally reshape their anatomy.
The Process of Metamorphosis
The shift from a symmetrical larva to an asymmetric juvenile is an intricate biological process orchestrated by endocrine signals. Thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4), act as the primary triggers, initiating changes in bone, muscle, and tissue development. These hormones cause an acceleration of development.
The most noticeable change is the migration of one eye across the head to the other side, involving the massive remodeling of the skull. The bones in the cranium twist and grow asymmetrically, particularly in the ethmoid region. Differential growth of bone and muscle on the side opposite the migrating eye pushes the eye into its new location.
As the eye migration progresses, the body undergoes physical flattening and the fish begins to swim with an increasing tilt. The fish also loses the functional need for a swim bladder, the organ that helps most fish maintain neutral buoyancy. This organ is reduced or functionally lost as the flounder prepares for a life spent pressed against the ocean floor.
This physical reorientation is accompanied by a change in pigmentation. The side of the body that will face the substrate, known as the “blind side,” loses its color and becomes a dull white or pale hue. Conversely, the side with both eyes, which faces upward, develops complex patterns and colors that allow for effective camouflage against the seabed.
The Survival Advantage of Asymmetry
The flattened, asymmetrical body plan is an adaptation to the bottom-dwelling, or benthic, lifestyle. With both eyes positioned on the upward-facing side, the fish gains a wide visual field while remaining concealed. This positioning allows the flounder to scan the water above for both predators and prey.
The pigmented, upward-facing side can change its color and pattern to precisely match the surrounding substrate, whether sand, gravel, or mud. This background-matching capability provides exceptional camouflage, making the flounder nearly invisible to animals looking down from above. This is crucial for survival, as the fish is vulnerable to attack from larger fish and marine mammals.
The combination of flatness and camouflage supports the flounder’s primary hunting strategy: ambush predation. The fish lies in wait, often partially buried in the sediment, conserving energy while remaining undetected. When a small fish or crustacean ventures too close, the flounder uses a sudden burst of speed to capture its meal.
Lying flat on the ocean floor also minimizes the energy required for movement and positioning. By settling on the bottom, the fish no longer expends the effort needed to maintain its position in the water column. The transformation represents an optimized design for a life lived on the seafloor.