The common understanding of “fish” often includes almost any organism living in the water, leading to the misapplication of the term to a wide variety of aquatic life. Scientifically, the classification of a fish is highly precise, rooted in biological and anatomical criteria that define it as an aquatic vertebrate. These characteristics separate true fish from other water-dwelling animals. This article clarifies the biological attributes used by scientists to define this diverse group and distinguish them from animals that merely share their habitat.
Core Biological Attributes
To be classified as a true fish, an organism must possess several non-negotiable biological attributes. The primary requirement is that it is a vertebrate, meaning the animal must have a backbone or spinal column. The presence of this bony or cartilaginous internal skeleton is a fundamental trait that immediately excludes all invertebrates, such as squid or crabs, from being considered fish.
A defining physiological trait of fish is the use of gills for respiration, allowing them to extract dissolved oxygen directly from the surrounding water. Water is pulled through the mouth and pumped over the gill filaments, facilitating the efficient exchange of gases. Fish are characterized by their appendages, which take the form of fins rather than limbs with digits. These fins are supported by bony or cartilaginous rays and are used for propulsion, steering, and maintaining balance.
True fish are generally ectothermic, meaning their internal body temperature fluctuates with the external environment, classifying them as cold-blooded. This trait is effective for life in water, which is a stable thermal environment, minimizing the energy required to regulate body heat. However, some large, active swimmers, such as certain species of tuna and sharks, demonstrate regional endothermy, allowing them to maintain higher core temperatures in specific muscles for enhanced performance.
The Three Scientific Classes of Fish
The organisms that satisfy the core biological definition are traditionally grouped into three major extant classes, reflecting their distinct skeletal structures and evolutionary lineages. The largest and most diverse group is the Osteichthyes, or bony fish, representing approximately half of all living vertebrates. Bony fish have a skeleton primarily composed of bone and possess a specialized structure called an operculum, which is a protective bony flap covering the gills. This operculum allows them to draw water over their gills without the need for constant forward motion, a technique known as buccal pumping.
A second distinctive feature of most bony fish is the presence of a swim bladder, an internal gas-filled organ that helps them regulate buoyancy and maintain a specific depth in the water column. The second major class is the Chondrichthyes, or cartilaginous fish, which includes sharks, rays, and chimaeras. Unlike their bony counterparts, these species possess an entire skeleton made of cartilage, a flexible tissue that is significantly lighter than bone. Cartilaginous fish lack a swim bladder and instead rely on large, oil-filled livers and dynamic lift generated by their pectoral fins to prevent sinking.
Chondrichthyes also differ in their respiratory anatomy, as they do not have an operculum, instead featuring multiple separate gill slits that are exposed directly to the outside environment. The third and most evolutionarily primitive group is the Agnatha, or jawless fish, which includes lampreys and hagfish. These species are characterized by the absence of true jaws and paired fins, featuring a simple, cartilaginous skeleton and a notochord for support. Extant agnathans represent the most basal line of vertebrates.
Common Aquatic Animals That Do Not Qualify
Many aquatic organisms are colloquially referred to as “fish,” but they fail to meet the biological criteria established for true fish, often leading to common scientific misconceptions. Marine mammals, such as whales and dolphins, are frequently mistaken for fish due to their streamlined, aquatic body shape, but they are fundamentally different. They are endothermic, meaning they are warm-blooded and maintain a constant internal body temperature. Furthermore, they breathe air using lungs and possess mammary glands, placing them firmly within the class Mammalia.
Another large group of excluded animals are the invertebrates, which fail the most fundamental test of fish classification: the requirement of a vertebral column. Jellyfish, starfish, and squid are all invertebrates belonging to different phyla entirely, meaning they lack any form of internal bony or cartilaginous skeleton. Their gelatinous bodies and primitive nervous systems are completely distinct from the complex organ systems of vertebrates.
Amphibians and reptiles that inhabit water, like sea turtles or salamanders, are also not fish because they possess limbs with digits and breathe using lungs, even if they spend a significant portion of their lives submerged. Sea turtles, as reptiles, must surface to breathe air, unlike fish which rely on gills. These animals belong to the tetrapod lineage, meaning they are four-limbed vertebrates, which is the scientific designation that excludes them from the definition of a fish.