Fish are a diverse group of aquatic animals. The scientific classification of “fish” is more intricate and encompasses a wide array of biological forms. Biologically, the term does not refer to a single, unified group, but rather a collection of different lineages that share a common aquatic lifestyle and certain physical characteristics.
The Broad Biological Classification of Fish
Fish are aquatic vertebrates, possessing a backbone, placing them within the Kingdom Animalia, Phylum Chordata, and Subphylum Vertebrata. This broad classification means they share a fundamental body plan with other vertebrates, including mammals, birds, reptiles, and amphibians.
A defining feature for most fish is their ability to respire underwater using gills. Another common characteristic is the presence of fins, used for movement, steering, and maintaining balance in their aquatic environment. Most fish are poikilothermic, meaning their body temperature fluctuates with the surrounding water temperature. Their bodies are typically streamlined, which helps them move efficiently through water, though body shapes vary widely.
Diverse Groups Within the Fish Category
The term “fish” is not a single taxonomic class but represents a paraphyletic group, meaning it includes a common ancestor and some, but not all, of its descendants. Traditional classification divides fish into three main extant groups. These groups are as distinct from one another as amphibians are from mammals.
Jawless Fish
Jawless fish, belonging to the group Agnatha, are among the most primitive vertebrates. This group includes lampreys and hagfish, which lack jaws and paired fins. Their mouths are typically circular and adapted for suction or filter feeding. Many hagfish retain a notochord throughout their lives instead of developing a full backbone.
Cartilaginous Fish
Cartilaginous fish, classified under the Class Chondrichthyes, are distinguished by skeletons made entirely of cartilage rather than bone. This group includes sharks, rays, and chimaeras. Unlike bony fish, they typically have multiple visible gill slits and lack a swim bladder, requiring continuous movement to avoid sinking. Their skin is often covered in placoid scales, which are tooth-like.
Bony Fish
The vast majority of existing fish species are bony fish, belonging to the Superclass Osteichthyes. These fish possess skeletons primarily composed of bone and have a protective bony flap, called an operculum, covering their gills, which allows them to breathe without constant movement. Most bony fish also have a swim bladder, an internal gas-filled organ that helps them control their buoyancy.
This superclass is further divided into two major subgroups: ray-finned fish (Class Actinopterygii) and lobe-finned fish (Class Sarcopterygii). Ray-finned fish are the most numerous, encompassing familiar species like salmon, tuna, and goldfish, characterized by fins supported by bony rays. Lobe-finned fish, such as coelacanths and lungfish, have fleshy, muscular fins with bones that are homologous to the limbs of terrestrial vertebrates.
Differentiating Fish from Other Aquatic Animals
Many animals live in water but are not classified as fish.
Marine Mammals
Whales, dolphins, and seals are marine mammals. They are warm-blooded, breathe air using lungs, and give birth to live young that nurse with milk, unlike most fish which are cold-blooded, breathe with gills, and lay eggs. Their bodies are typically covered in fur or blubber, not scales.
Reptiles and Amphibians
Sea turtles are reptiles, despite their aquatic lifestyle. They are cold-blooded and breathe air with lungs, necessitating regular trips to the surface. Sea turtles lay their eggs on land, and their bodies are covered in scales or a shell. Frogs and other amphibians, while often starting life in water with gills, undergo metamorphosis to develop lungs and limbs for terrestrial life. Their skin is typically smooth and permeable, requiring moisture, which differs from fish scales.
Aquatic Invertebrates
Aquatic invertebrates like jellyfish and starfish are also not fish. Jellyfish are cnidarians, lacking a backbone, brain, heart, or gills. They absorb oxygen directly through their body walls and use stinging cells for defense and prey capture. Starfish, or sea stars, are echinoderms, characterized by radial symmetry and a water vascular system for movement, completely lacking the vertebrate characteristics of fish. Crustaceans, such as crabs and lobsters, are arthropods with exoskeletons and jointed legs, fundamentally different from fish.