Jawless fish (Agnatha) represent an ancient lineage of vertebrates that first appeared over 500 million years ago, predating the evolution of jaws in other fish. Their lack of a hinged jaw profoundly shapes their unique feeding strategies. The two primary surviving groups are the hagfish and the lampreys. These eel-like creatures occupy distinct ecological niches in marine and freshwater environments, using specialized non-jaw-based mechanisms for feeding.
Hagfish: Deep-Sea Scavenging and Detritivory
Hagfish (Myxini) are primary scavengers found in cold, deep-sea marine environments. Their diet consists of soft-bodied invertebrates, marine snow, and large animal carcasses that sink to the ocean floor. They possess an extremely low metabolism, enabling them to survive for months between large meals.
When a carcass is detected, hagfish congregate and enter the body through an opening or by burrowing into the soft tissue. Their feeding apparatus includes two rows of keratinous dental plates on a tongue-like projection used for grasping and tearing flesh, allowing them to efficiently consume carrion from the inside out.
To gain leverage, hagfish employ a knotting behavior. They loop their flexible bodies into a knot, press it against the carcass, and slide the knot toward their head to pull the food away. This provides the mechanical advantage their jawless mouths lack. They also absorb dissolved organic matter directly through their skin and gills, maximizing nutrient intake.
Lampreys: A Dual-Stage Diet
The feeding habits of lampreys (Petromyzontida) differ dramatically between their larval and adult life stages. The larval stage, known as the ammocoete, is non-parasitic and can last from two to over ten years. Ammocoetes live buried in the soft sediment of freshwater streams and rivers, acting as filter feeders.
These larvae draw in water through their mouths, filtering out microscopic organisms, algae, bacteria, and organic detritus. Organic detritus makes up the majority of their diet, sometimes over 97%, with algae and bacteria forming the remainder. This microphagous diet supports their long larval phase.
After metamorphosis, many species of adult lampreys become external parasites, though some species are non-feeding. The parasitic adults migrate to larger bodies of water, such as lakes or the ocean, and attach to bony fish using a muscular oral disc that functions as a powerful suction cup. Their mouth is lined with rows of sharp, keratinized teeth, and a rasping tongue structure is used to bore through the host’s scales and skin. Once attached, they consume the host’s blood and body fluids, often injecting an anticoagulant to maintain the flow of nourishment.
Ecological Role of Jawless Fish
The specialized diets of jawless fish have significant consequences for their ecosystems. Hagfish are deep-sea recyclers, preventing the accumulation of large carcasses on the seafloor. Their scavenging activities redistribute nutrients and carbon back into the deep-sea food web, supporting other benthic life.
Lampreys also contribute to nutrient cycling, particularly in freshwater systems, where their filter-feeding larvae improve water quality by processing organic matter in stream sediments. The parasitic adult lampreys act as regulators of fish populations in their native environments by preying on larger fish. However, when species like the sea lamprey are introduced to new areas, their predatory behavior can severely impact commercial and sport fisheries.