Agnathans represent a group of aquatic animals that stand as the most ancient and primitive surviving lineage of vertebrates. The name Agnatha is derived from the Greek words “a-” (without) and “gnathos” (jaw), describing their defining anatomical feature. This group consists of fish-like creatures that lack the hinged jaw characteristic of all other vertebrates. Today, Agnathans are a small but significant group, offering a direct window into the early stages of vertebrate evolution.
Defining Characteristics of Jawless Vertebrates
The primary feature differentiating Agnathans from jawed vertebrates (Gnathostomes) is the absence of a hinged jaw structure. Their internal support is composed of a cartilaginous skeleton, which is generally simpler than the skeletons found in later fish. The notochord, a flexible rod of tissue, remains present throughout their adult life, providing the main axial support.
Agnathans also lack paired appendages, such as the pectoral and pelvic fins. Their movement is primarily eel-like and relies on body undulations. Gas exchange occurs through a series of pouch-like gill structures that open to the exterior through pores or slits. Furthermore, these jawless vertebrates possess only a single median nostril, contrasting with the paired nostrils found in Gnathostomes.
The Modern Agnathan Lineages: Lampreys and Hagfish
The two surviving groups of Agnathans, collectively known as Cyclostomes, are the hagfish (Myxini) and the lampreys (Petromyzontidae). Hagfish are exclusively marine scavengers that inhabit deep-sea environments, typically burrowing into the sediment during the day. They produce copious amounts of a fibrous, sticky slime when threatened, which can rapidly expand in water to deter predators.
Hagfish feed on dead organisms using a dental plate and employ a distinctive “knotting” behavior to gain leverage for tearing food or scraping off defensive slime. Their development is direct, meaning the young hatch as miniature versions of the adults without a larval stage. Lampreys, conversely, have a more complex life cycle, featuring a prolonged larval stage called the ammocoete.
The ammocoete is a blind, filter-feeding organism that lives buried in the silt of freshwater streams for several years. The larva then undergoes a metamorphosis into the adult form, which for many species is parasitic. Parasitic lampreys migrate to the ocean or large lakes, attaching to other fish via a sucker-like mouth to feed on blood and body fluids. Anadromous species return to freshwater to reproduce, building nests and dying shortly after spawning.
Position in the Tree of Life
Agnathans occupy a profound position in the history of life, representing the earliest lineage to diverge from the main vertebrate stock. They are considered the sister group to all Gnathostomes (jawed vertebrates). The oldest fossil evidence of jawless fish dates back over 500 million years, placing them at the beginning of the vertebrate timeline.
Extinct jawless fish, informally referred to as Ostracoderms, were heavily armored with bony plates and scales, a feature not present in modern Agnathans. Research suggests that many armored Ostracoderms may be more closely related to jawed vertebrates than to modern lampreys and hagfish. This complexity indicates that the evolution of the jawless state was a diverse period of experimentation that ultimately set the stage for the rise of all subsequent vertebrates.