Flatfish are a fascinating group of demersal fish characterized by their distinctly asymmetrical bodies. The terms Fluke and Flounder are often used interchangeably in common language and the seafood market, leading to frequent confusion. While all Fluke are technically a type of Flounder, significant differences in their anatomy, behavior, and environment define them as separate and commercially important species. Understanding these distinctions helps anglers and consumers accurately identify these two flatfish.
The Key Anatomical Difference: Eye Placement
The most reliable method for distinguishing between a Fluke and a Flounder involves observing the orientation of their eyes. This difference results from the metamorphosis flatfish larvae undergo, where one eye migrates across the head as the fish settles onto the seafloor. Fluke (Paralichthys dentatus or Summer Flounder) is designated as a sinistral or “left-eyed” flatfish. When laid flat with the dark, pigmented side facing up, both eyes are found on the left side of the body.
General Flounder species, such as the Winter Flounder, are typically dextral or “right-eyed” fish. When oriented with the pigmented side up, the eyes of a Flounder are located on the right side of the head. This consistent anatomical rule allows for immediate visual identification. The side without eyes is the blind side, which is typically pale white, allowing the fish to lie camouflaged on the bottom.
Jaw Structure and Predatory Habits
The structural difference in the mouth of each species reflects a divergence in their diets and hunting strategies. Fluke is characterized by a relatively large mouth that extends past the eyes, armed with a prominent set of sharp, conical teeth. This structure indicates the Fluke is an active, aggressive predator that often ambushes mobile prey. Their diet is highly piscivorous, consisting primarily of smaller fish, squid, and crustaceans.
In contrast, many true Flounder species, such as the Winter Flounder, possess a much smaller, rounded mouth with smaller, blunter teeth. This structure is designed for a more passive, benthic feeding strategy focused on consuming organisms found within the sediment. Their diet is dominated by bottom-dwelling invertebrates such as marine worms, small mollusks, and crustaceans. Larger Fluke are known to prey upon juvenile Winter Flounder, demonstrating a clear hierarchy between the two species.
Geographical Range and Market Identification
Geographical location provides a practical clue to distinguishing the two fish. The Fluke (Summer Flounder) is most abundant along the Atlantic coast from North Carolina up to Massachusetts. As its name suggests, it is typically found in warmer, inshore waters during the summer months. Flounder species, like the Winter Flounder, have a broader distribution, extending farther north into colder waters from Labrador down to Georgia.
Beyond habitat, physical shape offers a hint: Fluke tend to be more elongated and streamlined, while many Flounder species have a wider, more rounded profile. Fluke can also grow significantly larger. In commercial settings, the term “Flounder” is frequently used as a blanket term for various flatfish, and Fluke is often sold simply as “Flounder.” However, Fluke is generally regarded as a higher-quality fish due to its firm, white flesh and mild, sweet flavor.