Flounder are a group of fish recognized by their flat body shape and ability to blend into their surroundings. Unlike most fish, adult flounder have both eyes on the same side of their head, which helps them see upwards as they lie on the seafloor. This unique eye placement develops as they mature, with one eye migrating across the head during their larval stage. Their flattened form and camouflage allow them to hide from predators and ambush prey.
Flounder’s Natural Habitat
The majority of flounder species are marine fish, inhabiting saltwater environments such as oceans, seas, and estuaries worldwide. These fish are demersal, spending most of their lives on or near the bottom. They prefer soft, muddy, or sandy substrates, where they can bury themselves to remain hidden. Flounder are ambush predators, lying motionless on the seafloor and using camouflage to surprise prey like small fish, crustaceans, and worms. While many species are found in shallow coastal waters, some can also be found near structures like docks, bridges, and coral reefs.
Adaptations for Varying Salinity
Fish maintain an internal balance of water and salts through osmoregulation. Marine fish, living in a saltier external environment, constantly tend to lose water and gain salt from their surroundings. To counteract this, they actively drink seawater and excrete excess salts primarily through specialized cells in their gills, while also producing concentrated urine. Conversely, freshwater fish face the challenge of constantly gaining water and losing salts. They address this by not drinking water, absorbing salts through their gills, and producing large volumes of dilute urine to eliminate excess water.
Most fish are stenohaline, tolerating only a narrow range of salinity. However, euryhaline fish can adapt to a wide spectrum of salinities, from freshwater to highly saline conditions. Flounder exhibit this euryhaline ability. They are frequently found in dynamic habitats like estuaries and tide pools, where salinity fluctuates with tides and freshwater inflow. Their flexibility involves altering proteins in their gill tissues to adjust how salts enter or exit their bodies, and hormonal regulation also plays a role.
Flounder Species in Freshwater
While most flounder species are marine, a few can inhabit or tolerate freshwater conditions for portions of their lives. The European Flounder (Platichthys flesus) is a well-known example found in both marine and freshwater environments, and it swims considerable distances up rivers.
Another example is the Hogchoker (Trinectes maculatus), a small flatfish found along the Atlantic and Gulf coasts of North and South America. Although they prefer brackish waters (a mix of fresh and saltwater), Hogchokers adapt to various freshwater habitats. Young Hogchokers, after hatching in more saline areas, often migrate upstream into freshwater sections of estuaries and rivers, returning to the ocean to spawn as adults. This species is sometimes marketed as a “freshwater flounder” in the aquarium trade due to its tolerance for lower salinities. Beyond these, some flounder species, such as Apionichthys nattereri and Hypoclinemus mentalis, are native to freshwater systems in South America, including the Amazon River.