Certain fish exhibit a unique behavior: swimming on their side. This orientation is a key adaptation for survival in the aquatic realm.
Meet the Flatfish
Flatfish, belonging to the order Pleuronectiformes, are known for swimming on their side. This diverse group of approximately 680 species has distinctly flattened, oval-shaped bodies. Unlike most fish, adult flatfish are asymmetrical, with both eyes positioned on one side of their head. They primarily dwell on the ocean floor.
Familiar examples include flounder, sole, and halibut. These fish occupy various marine environments, from shallow coastal waters to deeper ocean habitats, often preferring sandy or muddy bottoms. Their flattened bodies allow them to lie flat against the substrate, which is central to their survival and feeding.
The Purpose Behind Sideways Swimming
The flattened body of flatfish offers advantages for survival in their bottom-dwelling environment. This morphology is suited for camouflage, allowing them to blend seamlessly with the seafloor. Specialized pigment cells in their skin, called chromatophores, enable them to rapidly change color and pattern to match their surroundings, making them nearly invisible to both predators and prey.
Their flattened shape also facilitates an effective ambush predation strategy. Flatfish can lie motionless, often partially buried in sand or sediment, with only their eyes protruding. From this concealed position, they wait for unsuspecting prey, such as small fish, crustaceans, or marine invertebrates, before launching a swift attack. This “sit-and-wait” approach conserves energy while maximizing hunting efficiency.
This lifestyle also aids in evading predators. By camouflaging themselves, flatfish become difficult for larger predators, such as sharks and rays, to detect. The ability to quickly bury themselves in the substrate provides physical protection, making them less vulnerable.
From Upright to Flat
The transformation of flatfish from a typical fish shape to their distinctive flattened form is a developmental process called metamorphosis. Newly hatched flatfish larvae are initially symmetrical, resembling other fish with an eye on each side of their head. They swim upright in the water column during this early stage.
As they mature, a profound change occurs in their cranial structure. One eye begins to migrate across the top of the head, eventually coming to rest next to the other eye on the opposite side of the skull. This eye migration is accompanied by significant remodeling of the skull bones and muscles, leading to the adult asymmetrical appearance. The side of the body where both eyes end up becomes the “ocular” or upward-facing side, while the side without eyes becomes the “blind” or downward-facing side.
Once this metamorphosis is complete, the flatfish settles to the ocean floor, adopting its permanent sideways orientation. The upward-facing side typically develops coloration and patterns that match the seabed, while the blind side remains unpigmented and often white. This unique anatomical arrangement is essential for their bottom-dwelling, camouflaged existence.