Flounder are a diverse and widespread group of fish, known for their unique adaptation to life on the ocean floor. Their flat shape and ability to camouflage allow them to thrive in a variety of environments across the globe. Understanding where these fish live requires examining the specific environmental conditions and life stages that dictate their location.
Defining the Flounder Family
The term “flounder” encompasses various species of flatfish belonging to the order Pleuronectiformes. These bony fish share a modified physical structure that influences their habitat use. Their bodies are laterally compressed, allowing them to rest flush against the seafloor.
A defining adaptation is the migration of one eye during development, resulting in both eyes being situated on the upward-facing side of the head. This change transforms them from bilaterally symmetrical larvae into asymmetrical bottom-dwellers, or benthic organisms. This shift allows the flounder to lie on its side, with the unpigmented “blind side” on the substrate and the eyed side facing the water column for camouflage and scanning for prey.
Global Geographical Distribution
Flounder species are found across the world’s oceans, but their greatest diversity and abundance are concentrated in the temperate and cold waters of the Northern Hemisphere. They are prominent in the North Atlantic Ocean, where species like the Winter Flounder and Summer Flounder (also called fluke) are common along the coasts of North America. The European Flounder is widely distributed along European coasts, ranging from Norway down to the Mediterranean.
In the Pacific Ocean, distinct species occupy similar ecological niches, including the Starry Flounder and the Olive Flounder. While some species inhabit sub-tropical zones, the majority of commercially recognized flounder populations are found in the cooler continental shelf regions of the North Pacific and North Atlantic.
Specific Habitat Preferences
The choice of habitat for flounder is determined by three main environmental factors: the type of seafloor, the water depth, and the level of salinity. Flounder species prefer soft bottoms, such as sand, mud, or silt, which is necessary for camouflage. They often bury themselves partially or completely in the substrate, leaving only their eyes exposed. This behavior protects them from predators and allows them to ambush prey.
Their depth range is wide, spanning from the shallowest coastal environments to the deep sea. Many common species are found on the continental shelf, from the tideline down to depths of about \(100text{ meters}\). Some adult flounder, such as the Summer Flounder, move to the outer continental shelf, sometimes reaching depths of \(150text{ meters}\) during colder months.
Flounder display a tolerance for varying salinity levels, making estuaries and river mouths important habitats. Many species, including the Southern Flounder, frequently enter brackish water to feed and grow during their juvenile stages. The European Flounder (Platichthys flesus) is noteworthy for its osmoregulatory ability, allowing it to travel significant distances up into pure freshwater rivers.
Life Cycle and Habitat Shifts
A flounder’s location changes dramatically over its life cycle, involving migration between different environments. The cycle begins with eggs and newly hatched larvae, which are planktonic and float freely in the water column above offshore spawning grounds. These larvae are symmetrical and do not resemble their adult form, spending weeks drifting with the currents.
As they develop, the larvae undergo metamorphosis, where one eye migrates across the head and the body flattens. Following this transformation, the juvenile fish settle onto the seabed and move toward the coast, migrating into shallow, productive estuarine nursery areas. These low-salinity estuaries provide abundant food and shelter for the young fish to grow rapidly.
Mature adults undertake annual seasonal migrations to maximize feeding and reproductive success. Many species move inshore to shallow coastal waters and estuaries during the warmer spring and summer months to feed. As water temperatures drop in the fall and winter, these adults migrate offshore to deeper waters on the continental shelf to spawn, releasing their eggs into the open ocean.