The term “fluke” is confusing because it refers to two completely different organisms: a popular marine fish and a type of parasitic flatworm. This ambiguity leads to a common search query about whether a fluke is a fish or a worm. The distinction is fundamental, resting on vertebrate versus invertebrate classification. Understanding the specific scientific identity of each resolves the confusion.
The Marine Fluke: A Clear Classification
The animal commonly referred to as a fluke is a fish belonging to the flatfish group. In North America, this marine fluke is most often the Summer Flounder (Paralichthys dentatus). This species is a ray-finned fish, classified in the order Pleuronectiformes, the taxonomic grouping for all flatfish. Its identity as a fish is confirmed by its vertebrate structure, gills for respiration, and fins for propulsion.
Flatfish are demersal organisms, meaning they live on or near the sea bottom. The Summer Flounder is a predatory fish found primarily in the Atlantic Ocean. It actively hunts prey such as shrimp, squid, and smaller fish. It is highly valued in commercial and recreational fishing, making its common name widely recognized.
Unique Features of the Flatfish Family
The flatfish body plan is an adaptation that transforms the organism into a flattened, bottom-dwelling specialist. The fluke begins life symmetrically as a larva, swimming upright with an eye on each side of the head and possessing a swim bladder. As the fish matures, a process of metamorphosis begins, driven by physiological changes.
During this transformation, one eye migrates across the top of the head to the opposite side. The skull twists to accommodate both eyes on one side of the body. The fish loses its swim bladder, causing it to sink and settle onto the ocean floor.
The fish lies on its “blind side,” which lacks color, while the side with both eyes faces upward. This upward-facing side exhibits cryptic coloration, providing camouflage against the substrate. This asymmetry allows the adult fluke to rest on the seabed with both eyes oriented upward, giving it an unobstructed view of predators and prey.
The side resting on the sediment, known as the abocular side, often loses its pigmentation and remains pale white. The eye migration facilitates an ambush predatory lifestyle. The flatfish often burrows slightly into the sand, using its specialized body shape to wait for prey.
Addressing the Other Meaning: Parasitic Flukes
The second biological entity called a fluke belongs to the Class Trematoda, which are parasitic flatworms. These trematodes are invertebrates, classifying them as worms. They are part of the phylum Platyhelminthes, a group of simple, unsegmented animals with flattened bodies.
Parasitic flukes are much smaller than the fish, ranging from a few millimeters to several centimeters in length. They are defined by a life cycle that requires a host. Common types include blood flukes, liver flukes, and lung flukes, named for the organ system they inhabit.
Many species, such as the liver fluke Fasciola hepatica, require a complex life cycle involving an intermediate host, typically a freshwater snail. The eggs are passed in the feces of the definitive host and hatch into a free-swimming larva called a miracidium. The miracidium penetrates the snail, develops through several stages, and emerges as a cercaria. This cercaria can then infect the final host directly or by encysting on aquatic plants.