The aquatic world teems with diverse life. To understand this variety, scientists use a classification system that groups organisms by shared biological characteristics and evolutionary relationships. This system helps define different life forms, including the diverse group known as fish.
What Defines a Fish
A fish is an aquatic, gill-bearing vertebrate animal that typically possesses fins but lacks limbs with digits. Most fish are cold-blooded, meaning their body temperature fluctuates with their surrounding water. They possess a streamlined body shape for efficient movement. They extract oxygen from water using gills, typically located on the sides of their heads.
Many fish have scales covering their bodies for protection, though this characteristic is not universal. Fins, whether paired or unpaired, are used for propulsion, steering, and maintaining balance. All fish have a backbone, placing them within the subphylum Vertebrata. This internal skeleton provides structural support.
The Main Classes of Fish
Scientific classification divides fish into three main groups: jawless fish, cartilaginous fish, and bony fish. These groups reflect significant evolutionary divergences. Fish are the largest group of vertebrates by species count, with over 33,000 extant species.
Jawless fish (superclass Agnatha) are among the most primitive vertebrates. They lack true jaws, instead possessing a round, sucker-like mouth. They typically lack paired fins and scales, and their skeletons are composed of cartilage. Modern examples include lampreys, which can be parasitic, and hagfish, known for slime production.
Cartilaginous fish (class Chondrichthyes) include sharks, rays, and skates. Their skeleton is made entirely or primarily of cartilage, reinforced by calcium deposits. They possess powerful jaws, and their skin is covered by unique tooth-like placoid scales. Unlike bony fish, cartilaginous fish lack a swim bladder and must actively swim to maintain buoyancy. They typically have five to seven pairs of gill slits that open directly to the exterior.
Bony fish (superclass Osteichthyes) constitute the largest and most diverse group, encompassing over 28,000 species. Their skeletons are predominantly bone tissue. Most bony fish have a swim bladder, an air-filled sac that helps them control buoyancy. They also have a protective bony flap called an operculum covering their gills. This group includes ray-finned fish, the majority of living fish species, and lobe-finned fish, known for their fleshy, muscular fins.
Aquatic Animals Not Classified as Fish
Many aquatic animals, despite living in water and sometimes having “fish” in their common names, are not scientifically classified as fish. This is due to fundamental biological differences from true fish.
Marine mammals, such as whales and dolphins, are often mistaken for fish due to their aquatic habitat and streamlined bodies. However, they are warm-blooded, breathe air using lungs, give birth to live young, and nurse them with milk. Unlike fish that move their tails horizontally, whales propel themselves with vertical tail movements. These characteristics place them within the mammal class.
Jellyfish and starfish are invertebrates, lacking a backbone. Jellyfish belong to the phylum Cnidaria and have gelatinous bodies without gills, fins, or scales. Starfish, or sea stars, are echinoderms characterized by radial symmetry and a unique water vascular system for movement. They also lack gills and scales.
Amphibians, including frogs and salamanders, are not fish, despite often beginning their lives in water. While many amphibians have gills during their larval stage, they typically develop lungs as adults and can respire through their moist skin. Unlike fish, amphibians possess limbs adapted for movement on land and undergo metamorphosis from an aquatic larval form to an adult. Their distinct life cycles and physiological adaptations separate them from fish.