Flatfish

Lore & Background

Flatfish are found in oceans worldwide, from the Arctic to Antarctica, with species diversity centered in the Indo-West Pacific. Most species inhabit depths between 0 and 500 m, though a few have been recorded deeper than 1,500 m. None have been confirmed from abyssal or hadal zones; a reported observation from the Mariana Trench has been questioned by ichthyologists. Among deepwater species is Symphurus thermophilus, a tonguefish that congregates around sulfur ponds at hydrothermal vents—no other flatfish is known from such ecosystems. Many species enter brackish or fresh water, and some soles and tonguefish are entirely restricted to fresh water. The most obvious characteristic is asymmetry, with both eyes on the same side of the head. In some families, eyes are usually on the right side (dextral), in others on the left (sinistral). The primitive spiny turbots include equal numbers of right- and left-sided individuals and are generally less asymmetrical. Flatfish also have protrusible eyes and an extension of the dorsal fin onto the head. The upper side is pigmented, often for camouflage, and some species can change pigmentation using chromatophores. The blind side is usually colorless or very pale. Some flatfish have aposematic traits such as eyespots, and several small tropical species are poisonous. Juveniles of Soleichthys maculosus mimic toxic flatworms. Flatfish lay eggs that hatch into symmetrical larvae with a swim bladder. Through the influence of thyroid hormones, they metamorphose: one eye migrates across the top of the head, the swim bladder and spines are lost, and the fish sinks to the bottom, lying on its blind side. Hybrids are well known in flatfishes, with the Pleuronectidae having the largest number of reported hybrids of marine fishes. Two famous intergeneric hybrids are between European plaice and European flounder in the Baltic Sea, and between English sole and starry flounder in Puget Sound.

Reader's Guide

Flatfish are significant as a dramatic example of evolutionary adaptation. Their unique asymmetry—both eyes on one side—has long been cited in discussions of natural selection. Fossil evidence from genera such as Amphistium and Heteronectes, dating to about 50 million years ago, shows intermediate forms with one eye closer to the top of the head but still on each side, supporting gradual evolution rather than sudden saltation. This resolved a long-standing enigma, as earlier researchers had suggested that a partially migrated eye would be maladaptive. Flatfish are also economically important: many species are targeted by fisheries worldwide. Their cryptic coloration and ability to change pigmentation make them a model for camouflage studies. The presence of hybrids, especially among Pleuronectidae, provides insight into speciation and genetic exchange in marine environments. The discovery of a hydrothermal vent-associated tonguefish expands understanding of flatfish ecological range, while the absence of confirmed records from abyssal zones highlights limits to their depth distribution.

Did You Know?

• Flatfish are the only vertebrates that undergo a loss of symmetry, with one eye migrating to the other side during metamorphosis.
• The primitive genus Psettodes may develop into right-facing or left-facing individuals, while most flatfish families are consistently one-sided.
• A reported flatfish from the Mariana Trench at nearly 11 km depth has been questioned by ichthyologists and is not recognized as valid.
• The tonguefish Symphurus thermophilus is the only flatfish known from hydrothermal vent ecosystems.