Biological classification is a system scientists use to organize the vast diversity of life on Earth. This structured grouping helps in understanding relationships between organisms and how they have evolved. It provides a framework for studying the immense variety of species and their roles within ecosystems. This organization is important for grasping the scope of Earth’s biodiversity.
Not a Single Class
The term “fish” commonly describes a wide array of aquatic animals, yet scientifically, they do not belong to a single taxonomic class. Instead, “fish” is a general descriptor for diverse vertebrates that live in water, breathe with gills, and typically have fins. This broad grouping is considered paraphyletic, meaning it includes a common ancestor but excludes some descendants, specifically four-limbed vertebrates (tetrapods) that evolved from within a group of fish. Grouping all “fish” into one class would overlook significant evolutionary divergences.
Cartilaginous and Bony Fish
Two distinct classes commonly called “fish” are Chondrichthyes, or cartilaginous fish, and Osteichthyes, or bony fish. Cartilaginous fish, such as sharks, rays, and chimaeras, have skeletons primarily composed of cartilage rather than bone. Their skin is typically covered with placoid scales, which are tooth-like structures, giving their skin a rough, sandpaper-like texture. They lack a swim bladder, relying instead on a large, oil-filled liver for buoyancy, often requiring constant movement to avoid sinking. They also possess multiple exposed gill slits, rather than a single gill cover.
Bony fish, forming the class Osteichthyes, represent the majority of fish species, with over 32,000 described species. Their defining characteristic is a skeleton made of bone, which provides rigid support. Most bony fish have smooth, overlapping scales. A swim bladder, an air-filled organ, allows them to control buoyancy and maintain depth without continuous swimming. Their gills are protected by a bony flap called an operculum, which aids efficient respiration by pumping water over the gills. Familiar examples include salmon, tuna, clownfish, and seahorses, inhabiting both marine and freshwater environments.
Jawless Fish Lineages
A distinct and ancient group of aquatic vertebrates are the jawless fish, classified under the superclass Agnatha. This group includes living examples like lampreys and hagfish, representing some of the earliest vertebrates. These animals lack true jaws, which differentiates them from cartilaginous and bony fish. They also typically do not possess paired fins, and their bodies are scaleless and eel-shaped.
Instead of a bony or cartilaginous spine, jawless fish retain a notochord, a flexible rod-like structure, for support. Lampreys are often parasitic, using a sucker-like mouth with horny teeth to attach to other fish. Hagfish are primarily scavengers, known for producing large amounts of protective slime. Their unique characteristics highlight an early stage in vertebrate evolution.