Jawless fish, formally classified as Agnatha, represent the most ancient lineage of vertebrates still surviving today. They are defined by the absence of a hinged jaw, a structure present in all other vertebrates (Gnathostomes). Lacking a jaw, their feeding style relies on sucking or rasping rather than biting and chewing. With fossil records dating back over 500 million years, these animals offer a window into the earliest stages of vertebrate evolution. The approximately 120 living species are split into two distinct groups.
Key Anatomical Features
The anatomy of jawless fish fundamentally distinguishes them from their jawed relatives. They possess an eel-like, elongated body that lacks paired fins, relying on body undulations and a dorsal or caudal fin for propulsion. Their skeleton is entirely cartilaginous, lacking the internal bone ossification found in most other vertebrates. The main supportive structure is the persistent notochord, a flexible rod running along their back.
A defining feature is the circular, muscular mouth structure, enabling them to attach to surfaces or prey by suction. This unique oral apparatus gives the living groups their collective name, Cyclostomes, or “round mouths.” Respiration occurs through multiple gill pouches or slits that open directly to the outside, rather than the single, protected gill cover (operculum) seen in bony fish. These gill pouches, numbering seven or more pairs, are supported by branchial arches.
The Modern Jawless Fish
The two surviving groups of jawless fish are the hagfish (Class Myxini) and the lampreys (Class Petromyzontida).
Hagfish
Hagfish are exclusively marine scavengers living in deep-sea environments. They are characterized by nearly blind eyes and use a dental plate for rasping flesh from dead animals. They possess a cartilaginous skull but lack true vertebrae. Hagfish are also isosmotic, meaning their body fluids match the salt concentration of the surrounding seawater.
Lampreys
Lampreys often have a parasitic lifestyle as adults, using their sucker-like mouth, lined with rows of horny teeth, to attach to other fish. Unlike hagfish, lampreys possess rudimentary cartilaginous elements (arcualia) surrounding the notochord, which are considered simple vertebrae. They have well-developed eyes, two dorsal fins, and a complex life cycle. This cycle includes a prolonged filter-feeding larval stage (ammocoete) that lives in freshwater sediments. Lampreys are found in both freshwater and marine environments, but all species migrate to freshwater to spawn.
Evolutionary Significance and Ecological Roles
Jawless fish hold a special place in the history of life, representing the earliest known vertebrates. Fossils like the heavily armored Ostracoderms mark the first appearance of a cranium and complex sense organs in the animal kingdom. Studying their simple systems provides fundamental insights into the evolutionary origin of features like the vertebrate nervous system and the paired fins of Gnathostomes.
Hagfish perform a vital ecological service as deep-sea scavengers, rapidly consuming carcasses that sink to the ocean floor. Their unique defense mechanism involves producing copious amounts of slime when threatened, which can clog the gills of potential predators. Lampreys, particularly parasitic species, sometimes cause significant impact on commercial fisheries, especially when introduced to new areas like the Great Lakes. However, their filter-feeding larvae also contribute to water quality by filtering organic matter, and the adults play a role in nutrient cycling during spawning migrations.