Flounder are common marine fish found in various oceanic environments worldwide, from tropical to temperate coastal waters and estuaries. They inhabit the seafloor, often near docks, bridges, and coral reefs. While they begin life with eyes on opposite sides, one eye migrates as they mature, resulting in both eyes positioned on one side of their flattened body. This allows them to lie flat on the ocean floor while maintaining a wide upward field of vision.
Key Predators of Flounder
Flounder face predation from a variety of marine animals throughout their lives, with specific threats changing as they grow from vulnerable larvae to larger adults. From the earliest stages, larval and juvenile flounder are particularly susceptible due to their small size and less developed camouflage, often residing higher in the water column where they are more accessible.
Smaller predatory fish, such as striped bass, bluefish, and even other summer flounder, pose a significant threat to young flounder. Invertebrates like crabs and shrimp also opportunistically feed on larval and juvenile flounder. As flounder mature, their size and bottom-dwelling habits offer increased protection, but they remain a food source for a broad range of larger marine predators.
Adult flounder are preyed upon by various large fish species. These include sharks, such as tiger sharks, bull sharks, and great hammerheads. Rays, particularly large stingrays, are also known to feed on these bottom-dwelling fish. Other substantial fish predators include Atlantic cod, spiny dogfish, groupers, moray eels, and monkfish.
Marine mammals also prey on flounder. Orcas and other toothed whales consume flounder, as do harbor seals and Steller sea lions. These marine mammals often hunt in the same coastal and offshore waters where flounder reside.
Various seabirds also target flounder, especially the smaller larval and juvenile stages that are more easily accessible in shallow waters. Species like terns, gulls, and common loons are known to prey on flounder. Ospreys, which primarily consume fish, are also effective predators, diving into the water to snatch their prey.
How Flounder Avoid Predation
Flounder have adaptations and behaviors to avoid predators. Their flattened body shape is a primary characteristic, enabling them to lie almost motionless against the sandy or muddy seafloor. This low profile makes them less visible to predators swimming above.
A key avoidance strategy is their camouflage, an ability to blend with their surroundings. Flounder can rapidly change their skin color and pattern to match the substrate they are resting on, whether it is sand, gravel, or even a checkerboard pattern. This color change is facilitated by specialized pigment-containing cells called chromatophores, which adjust the fish’s appearance.
Flounder also utilize countershading, where their dark topside blends with the deeper water when viewed from above, and their lighter underside blends with the sunlit surface when seen from below. This dual-purpose coloration enhances their concealment in the water column. Furthermore, they often bury themselves partially or entirely in sand or mud, leaving only their independently moving eyes exposed to scan for threats or prey. This burrowing behavior provides a physical barrier and further enhances their camouflage, offering a safe haven from potential predators.