A fish is generally defined as an aquatic vertebrate animal that possesses gills throughout its life and has limbs, if any, in the shape of fins. Early biological classification systems divided all living fishes into three major traditional categories based primarily on observable physical characteristics. These groupings represented distinct evolutionary steps and structural blueprints, allowing for a clear understanding of fish anatomy and lineage.
Agnatha: The Jawless Fishes
This ancient group represents the most primitive living vertebrates, characterized by the complete absence of jaws (Superclass Agnatha). Their mouths are circular, funnel-like structures used for sucking or rasping, often featuring keratinous “teeth.” Modern representatives, such as lampreys and hagfish, lack paired fins, possessing only median fins.
Their internal skeleton is entirely cartilaginous, and they retain a persistent notochord instead of a full vertebral column. Hagfish are marine scavengers that secrete protective mucus when disturbed. Lampreys are often parasitic, attaching to other fish to feed on blood.
Chondrichthyes: The Cartilaginous Fishes
Members of the Class Chondrichthyes, including sharks, rays, and skates, are defined by skeletons made entirely of cartilage rather than true bone. This cartilage is often calcified for strength but lacks true bony tissue. Their bodies are covered in minute, tooth-like structures called placoid scales, which give their skin a rough texture and help reduce drag.
Unlike most fishes, they lack a swim bladder for buoyancy control. To compensate, many species must swim continuously or use large, oil-filled livers to maintain neutral buoyancy. They possess multiple separate gill slits (typically five to seven pairs) that open directly to the exterior.
They lack the protective bony gill cover (operculum) found in other fish groups. This necessitates a constant flow of water over the gills, often achieved through forward movement known as ram ventilation.
Osteichthyes: The Bony Fishes
The Class Osteichthyes, or bony fishes, are the most diverse group of vertebrates on Earth, encompassing over 96% of all living fish species. Their skeleton is composed primarily of true bone. They possess a protective bony operculum that covers their gills, allowing them to breathe without needing to swim constantly by pumping water over the gills.
A significant adaptation is the gas-filled swim bladder, which allows the fish to precisely regulate buoyancy and maintain depth without expending energy. Osteichthyes is traditionally split into the lobe-finned fishes (Sarcopterygii) and the ray-finned fishes (Actinopterygii). Ray-finned fishes, such as tuna and salmon, feature fins supported by slender bony rays.
Lobe-finned fishes, though less numerous, have fleshy, paired fins containing robust central bones and muscles, resembling the structure of a tetrapod limb. This group includes coelacanths and lungfish. Their scales can be cycloid, ctenoid, or ganoid, offering various levels of protection and flexibility.
Why This Traditional Grouping is Outdated
Modern biological classification relies on cladistics, grouping organisms based on shared ancestry. A valid evolutionary group must be monophyletic, including a common ancestor and all its descendants. The traditional Superclass Pisces (Agnatha, Chondrichthyes, and Osteichthyes) is considered an artificial, paraphyletic grouping.
This is because the “fish” category excludes a significant group of descendants that evolved directly from the bony fish lineage: the Tetrapods. Land vertebrates—amphibians, reptiles, birds, and mammals—all trace their ancestry back to the lobe-finned fish (Sarcopterygii), a subgroup of Osteichthyes.
Under a strictly monophyletic system, if bony fish are included, their descendants (tetrapods) must also be included. Since the term “fish” excludes these land-dwelling vertebrates, it fails to represent a complete branch of the evolutionary tree.