Flatfish are a distinctive group of ray-finned fish characterized by their uniquely flattened body shape and the unusual placement of both eyes on one side of their head. Unlike most fish, adult flatfish are laterally compressed. This characteristic morphology allows them to inhabit the seafloor, a behavior central to their survival and feeding strategies.
Diversity of Flatfish
The diverse group of flatfish includes over 800 species across 16 families, including well-known types such as flounders, soles, turbots, and halibuts. These species exhibit variations in their eye placement, with some consistently having both eyes on the left side (sinistral) and others on the right side (dextral). For instance, turbots are typically left-eyed, while halibuts and soles are often right-eyed.
Distinguishing features extend beyond eye placement to include body outline, mouth shape, and size. Soles, for example, tend to be smaller and more elongated with a teardrop shape, while halibuts are among the largest flatfish, capable of reaching lengths of over 2.5 meters and weights exceeding 300 kilograms. Flounders represent a broad category, often characterized by an oval to round body shape and remarkable camouflage abilities. Tonguefishes are another distinct group, noted for their elongated, tongue-like appearance and usually lacking pectoral fins.
The Astonishing Transformation
The flattened form of adult flatfish arises from metamorphosis, a developmental process where a bilaterally symmetrical larva transforms into an asymmetrical adult. Newly hatched flatfish larvae resemble typical fish, swimming upright with one eye on each side of their head. As they mature, one eye begins to migrate across the top of the head to join the other on a single side. This migration is accompanied by a twisting of the skull bones and a reorientation of the mouth.
This unique transformation is influenced by thyroid hormones, which mediate the extensive remodeling of the skull and sensory organs. The eye migration is a gradual process, often taking several months, and it involves asymmetric growth of cranial structures like the ethmoid plate and frontal bones. This flattened body allows flatfish to lie inconspicuously on the seabed, aiding camouflage and ambush predation.
Life on the Seabed
Adult flatfish primarily inhabit the seafloor, adopting a benthic lifestyle across various ocean environments, including sandy bottoms, mud, and occasionally rocky areas. Their flattened morphology allows them to remain pressed against the substrate. They can move across the seabed by undulating their dorsal and anal fins, which fringe their bodies.
A key survival strategy for flatfish is their exceptional ability to camouflage. Many species can rapidly change their skin coloration and patterns to match their immediate surroundings, using specialized pigment cells called chromatophores. This dynamic camouflage makes them virtually invisible to both predators and prey. Flatfish often bury themselves in sand or sediment, leaving only their independently movable eyes exposed, which aids in hiding and ambush hunting.
Flatfish in Ecosystems
Flatfish play a role within marine ecosystems, acting as both predators and prey. As bottom-dwelling carnivores, they primarily feed on crustaceans, other benthic invertebrates, and small fish, often employing an ambush predation strategy.
Flatfish also hold importance for human activities. Many species, including flounders, soles, turbots, and halibuts, are highly valued commercial food fish worldwide. They are prized for their lean, white flesh and delicate flavor, making them popular in various culinary traditions. Fishing for flatfish supports commercial fisheries globally.