While many aquatic creatures inhabit the world’s waters, not all are scientifically categorized as fish. The term “fish” refers to a diverse array of vertebrate animals that share specific biological traits, distinguishing them from other marine and freshwater organisms. These characteristics provide insight into their unique adaptations for an aquatic existence.
Essential Biological Traits
Fish are aquatic vertebrate animals, possessing an internal backbone or spinal column. This vertebral column provides structural support and protects the spinal cord.
Fish primarily rely on gills to extract oxygen directly from water. Water enters through the fish’s mouth and passes over specialized gill filaments, which contain extensive networks of capillaries. Oxygen diffuses across these membranes into the bloodstream, while carbon dioxide is expelled. This process often utilizes a highly efficient countercurrent exchange system, maximizing oxygen absorption.
Fins serve as appendages for movement, balance, and steering in their aquatic environment. Paired fins, such as pectoral and pelvic fins, aid in stability, braking, and maneuvering. Unpaired fins, including the dorsal, caudal (tail), and anal fins, provide stability, prevent rolling, and generate propulsion. The caudal fin is the main propeller, enabling forward movement.
Most fish bodies are covered with protective scales, small plates formed from the dermal layer of the skin. These scales offer a physical barrier against injury, infection, and predators, while contributing to the fish’s streamlined shape to reduce drag. Various types of scales exist, such as placoid (sharks), ganoid (gars), and cycloid or ctenoid (most bony fish), each adapted to the fish’s lifestyle.
Fish are predominantly ectothermic, or “cold-blooded,” meaning their body temperature largely depends on the surrounding water. Unlike endothermic animals, fish rely on external environmental temperatures. This adaptation allows them to conserve metabolic energy, directing resources towards growth and reproduction. While most fish are ectothermic, some active swimmers, like tuna and the opah, can maintain a higher core body temperature through metabolic heat.
Fish spend their entire life cycle in water, from hatching to adulthood. Their physical and physiological characteristics are adapted to aquatic conditions, making water their permanent environment.
The Diverse World of Fish
The biological definition of fish encompasses a variety of species, classified into major groups that share the core characteristics of aquatic vertebrates with gills and fins. This diversity reflects millions of years of evolution, allowing fish to colonize nearly every aquatic environment. Over 33,000 extant species of fish exist, making them the largest group of vertebrates.
One ancient group is the jawless fish, or Agnatha, including lampreys and hagfish. These primitive fish lack jaws, paired fins, and scales, possessing a cylindrical body. They respire using gills located in pouches, demonstrating the foundational gill-breathing trait.
Cartilaginous fish (Chondrichthyes) have skeletons made entirely of cartilage. This group includes sharks, rays, and chimaeras. These fish possess movable jaws and placoid scales, which give their skin a sandpaper-like texture. They rely on gills for respiration but often need to keep moving to ensure water flows over their gills.
The largest and most diverse group are bony fish (Osteichthyes), comprising approximately 96% of all fish species. Their skeletons are composed of bone, and most species have an operculum, a bony flap that covers and protects their gills, allowing them to pump water over them without continuous movement. This group includes ray-finned and lobe-finned fish, exhibiting an array of shapes, sizes, and adaptations, while maintaining fundamental fish characteristics.
Animals Often Mistaken for Fish
Many aquatic animals are commonly, yet incorrectly, referred to as “fish.” These animals, despite living in water, lack one or more essential biological traits that define true fish.
Marine mammals, such as whales, dolphins, and seals, are frequently mistaken for fish due to their aquatic habitats and streamlined bodies. However, these animals are warm-blooded (endothermic) and breathe air using lungs, unlike ectothermic fish that rely on gills. They also possess hair and give birth to live young that feed on milk.
Amphibians, including frogs and salamanders, inhabit aquatic environments, particularly during their early life stages. While many have gills as larvae, they typically undergo metamorphosis, developing lungs and often living a semi-aquatic or terrestrial adult life. They are not exclusively aquatic and lack the fins and scales characteristic of most fish.
Invertebrates like jellyfish, starfish, crabs, and squid are entirely aquatic but differ from fish. Their most significant distinction is the lack of a vertebral column. These organisms have diverse body plans, respiratory systems (e.g., jellyfish respire through their skin, crabs use gills, squid have specialized gills), and modes of locomotion that do not involve fins like fish. They also lack the bony or placoid scales found on fish.