Flounders are a unique group of flatfish that possess a specialized anatomy adapted for life spent on the ocean floor. As demersal species, they inhabit the bottom of marine and estuarine environments, often resting on soft, muddy, or sandy substrates. This bottom-dwelling existence dictates a highly specific dietary strategy and reliance on camouflage. The flounder’s diet is a direct reflection of its habitat, consisting almost exclusively of organisms that live on, in, or immediately above the sediment.
The Flounder’s Primary Menu
Adult and sub-adult flounders are generalist carnivores, consuming a wide variety of benthic fauna associated with the seafloor. Crustaceans form a substantial part of their diet, including small crabs, various species of shrimp, and smaller bottom-dwelling creatures like amphipods. Flounders typically catch this prey by lying in wait and ambushing anything that moves within range.
They also heavily target marine worms, particularly polychaetes and annelids, especially in muddier habitats where these invertebrates are abundant. The flounder’s ability to disturb the sediment allows it to access these organisms buried just below the surface. In areas rich with shellfish, adult flounders consume mollusks, often cropping the siphons of partially buried bivalves.
As the flounder grows, its diet shifts to include larger, more mobile prey, making it a capable piscivore. They actively hunt small fish like sand eels, anchovies, and finger mullet that swim near the bottom. This transition reflects an increase in gape size and overall body mass, enabling them to tackle prey that would have been inaccessible when they were smaller.
Life Stage Influence on Diet
The flounder’s diet transforms dramatically as it matures, a process that begins immediately after hatching. Larval flounders start their lives as tiny, free-swimming organisms in the water column, feeding predominantly on microscopic zooplankton. This pelagic stage lasts for approximately 30 to 60 days.
The most significant change occurs during metamorphosis, when the fish settles to the bottom, and one eye migrates across the head. This physical shift is accompanied by a dietary change from floating zooplankton to much smaller benthic organisms. Juveniles begin by consuming meiobenthic crustacea, such as harpacticoid copepods and small amphipods.
As the juvenile flounder grows, its mouth and digestive system develop, enabling it to handle larger macrofauna. This gradual shift in preference minimizes competition for resources between different size classes of flounders in the same habitat. The size of the prey consumed is directly proportional to the size of the fish, demonstrating an orderly progression in its predatory capacity.
Ambush and Acquisition Tactics
The flounder’s flattened body is its primary tool for its highly effective ambush hunting strategy. This demersal predator uses specialized, cryptic coloration to seamlessly match the sand, mud, or gravel substrate it rests upon. The fish can even partially bury itself, flicking its fins to cover its body with a thin layer of sediment, leaving only its eyes exposed to scan the environment.
This remarkable camouflage allows the flounder to wait motionless for unsuspecting prey to pass by. It often positions itself on the down-current side of structures where fast-moving water delivers food. Both eyes are situated on the upper side of the head and can move independently, providing a full field of vision to track prey without moving the rest of its body. The flounder waits until the prey is within a very short range.
When the moment is right, the flounder executes a rapid, powerful strike, launching itself upward and engulfing the prey in its large, expandable mouth. This rapid acquisition tactic relies on speed and surprise, ensuring that the prey is captured before it can react. The flounder’s jaw structure and strong teeth are well-suited for grasping and holding onto the mobile fish and hard-shelled invertebrates that make up its adult diet.