The term “fluke” causes confusion because it refers to two vastly different organisms: the common marine fish, such as the Summer Flounder (Paralichthys dentatus), and a group of parasitic flatworms belonging to the class Trematoda. The marine fluke is an active predator with a complex diet, while the parasitic fluke is a highly specialized internal parasite.
The Diet of Marine Flounder
The Summer Flounder, or marine fluke, is a voracious, opportunistic predator that primarily inhabits the sandy and muddy bottoms of the Atlantic coast of North America. This fish is an ambush hunter, using its flattened body and ability to change color to bury itself in the substrate, leaving only its eyes exposed.
The diet of the Summer Flounder is highly carnivorous and shifts significantly as the fish increases in size. Smaller juveniles, typically less than 225 millimeters in length, primarily consume small invertebrates. Their meals consist of mysid shrimp, sand shrimp, and various small crustaceans like amphipods and isopods.
As the flounder grows into a medium or large adult, its diet becomes increasingly dominated by bony fishes, shifting the feeding focus from invertebrates to teleosts. Common fish prey include Atlantic silversides, killifish, Northern puffer, and sometimes even other flatfish like the Winter Flounder. Crustaceans, such as decapod shrimp and crabs, remain a part of the diet, along with cephalopods like squid, especially for larger individuals.
Nutrient Acquisition by Adult Parasitic Fluke
Adult parasitic flukes, or trematodes, display a feeding mechanism that is drastically different from the predatory fish, relying entirely on the resources of their definitive vertebrate host. Located within specific host organs, such as the bile ducts, lungs, or intestines, these worms do not hunt but instead consume materials readily available in their environment. Adult flukes possess a muscular pharynx, which they use to ingest host materials, and a blind-ending digestive tract.
The specific food source depends on the fluke’s location within the host body. For instance, liver flukes (Fasciola or Clonorchis) inhabit the bile ducts and feed primarily on bile, epithelial cells lining the duct, and blood. Blood flukes (Schistosoma) reside in blood vessels and directly consume blood, while intestinal flukes feed on intestinal contents, mucus, and tissue debris.
Beyond ingestion, many adult flukes absorb nutrients directly through their specialized outer covering, the tegument. The tegument is adapted to take up simple molecules, such as glucose, from the surrounding host fluids, which is especially important for flukes living in nutrient-rich areas like the intestines or blood.
The Sustenance of Larval Fluke Stages
The various larval stages of the parasitic fluke have highly specialized and distinct nutritional requirements tied to their stage in the life cycle and their host environment. The initial free-swimming stage, the miracidium, hatches from the egg and is non-feeding. This ciliated larva relies solely on stored energy reserves to quickly locate and penetrate the first intermediate host, which is typically an aquatic snail.
Once inside the snail, the miracidium transforms into a sporocyst, which is a sac-like structure that also lacks a mouth and digestive tract. The sporocyst, and the subsequent redia stage that develops inside it, obtain all necessary nourishment by directly absorbing nutrients from the snail’s tissue, primarily the hepatopancreas and gonads. Rediae, unlike sporocysts, often develop a pharynx and can actively ingest the host’s cells or even consume competing larval stages within the snail.
The final larval stage to emerge from the snail is the cercaria, which is a motile, free-swimming form that seeks out the next host. Similar to the miracidium, the cercaria is generally non-feeding and depends on stored energy to power its movement and quickly find its next host, whether it is a second intermediate host (like a fish or crustacean) or the final definitive host.