What Divers Should Know About Flatfish

Flatfish inhabit oceans worldwide, found from the Arctic through the tropics to Antarctica, with the Indo-West Pacific hosting the highest species diversity. While most species reside in marine environments, many can also enter brackish or fresh water. These demersal fish are famously recognized by their unique adaptation: a laterally-compressed body designed for lying on one side upon the seafloor, with both eyes migrating to face upwards. Typically encountered at depths between 0 and 500 meters, their flattened form is an immediate identifier.

Observing flatfish requires a keen eye, as their cryptic coloration and habits offer exceptional camouflage against predators. Many species can actively change their pigmentation to match the background, akin to some cephalopods, making them blend seamlessly with the substrate. However, a few small tropical species, like those in genera Aseraggodes, Pardachirus, and Zebrias, are known to be poisonous and may display aposematic traits such as conspicuous eyespots. Flatfish exhibit a vast size range, from the tiny 6.5 cm sand flounder to the immense Atlantic halibut, which can reach up to 4.7 meters long.