Flounder are fish recognized by their distinctive flat bodies and ability to blend into their surroundings. While their flattened form suggests a life closely tied to the seafloor, understanding whether they inhabit freshwater or saltwater environments reveals a nuanced picture of their existence.
Primarily Saltwater Dwellers
The vast majority of flounder species live exclusively in saltwater environments. They thrive across diverse oceanic settings, from shallow coastal waters and bays to the deeper ocean floor. Their global distribution includes the Atlantic and Pacific Oceans, as well as European waters.
Well-known examples of these marine species include the Summer Flounder, Winter Flounder, Southern Flounder, European Plaice, and Halibut. These species depend on the ocean’s salinity for their survival and reproductive cycles. They settle on soft, muddy, or sandy bottoms, where their flattened shape aids their bottom-dwelling lifestyle.
Flounder in Varied Habitats
While most flounder species are marine, some adapt to environments where freshwater and saltwater mix. These transitional zones, known as brackish waters, include estuaries and river mouths. Species like the Southern Flounder can move between freshwater and saltwater settings. Certain species, such as the Hogchoker, are found in brackish estuaries along the Atlantic coast of North America.
True freshwater flounder species are rare, primarily found in South American river systems like the Amazon, Orinoco, Essequibo, Uruguay, and Parana rivers. Many fish marketed as “freshwater flounder” are often brackish water species, like the Hogchoker, which may not thrive long-term in pure freshwater. Some of these “freshwater flounder” are actually misidentified soles, which are related but distinct flatfish.
Adaptations for Aquatic Life
Flounder possess several unique biological adaptations for their aquatic homes. Their distinctive flat body shape allows them to lie pressed against the seabed. This flattened form aids in camouflage, reducing their profile to both predators and prey. They can rapidly change their skin color and pattern to match the surrounding substrate, whether it is sand, gravel, or mud.
A remarkable developmental change in flounder involves the migration of their eyes. At birth, flounder larvae have an eye on each side of their head. As they mature and adopt a bottom-dwelling lifestyle, one eye gradually migrates to the other side of the head, resulting in both eyes being on the upward-facing side. This eye migration allows them to see effectively while lying flat on the seafloor.
Flounder manage their internal salt and water balance through osmoregulation. Saltwater flounder live in an environment saltier than their internal fluids. To prevent dehydration, they drink seawater and excrete excess salt through specialized cells in their gills, producing concentrated urine. Conversely, freshwater and brackish water flounder live in less saline conditions. They absorb water through their skin and gills, excreting large volumes of dilute urine to remove excess water, and actively absorb salts through their gills to maintain their internal balance. These adaptations enable flounder to inhabit a range of aquatic environments successfully.