A fish is a vertebrate, meaning it possesses a backbone, which is a definitive feature distinguishing it from invertebrates. This classification places all fish, from the smallest minnow to the largest shark, within the subphylum Vertebrata of the phylum Chordata. This distinction is based entirely on the presence of an internal skeletal rod that develops into a complex spine. All of the approximately 34,000 species of fish worldwide share this fundamental structural characteristic, confirming their place among the vertebrates.
Defining the Two Major Groups
The entire Animal Kingdom is broadly split into two primary groups based on internal structure. The largest of these groups is the invertebrates, which comprise nearly 97% of all animal species on Earth. These animals, which include insects, worms, and mollusks, are defined by the complete absence of a vertebral column or backbone. Instead of an internal skeleton, many invertebrates rely on other structural supports, such as an external shell called an exoskeleton or a hydrostatic skeleton supported by water pressure.
The second group is the vertebrates, which include mammals, birds, reptiles, amphibians, and fish. These animals are defined by the presence of a vertebral column, or spine, which is a segmented series of stiff elements called vertebrae. This internal skeleton, or endoskeleton, provides the main support structure for the body and protects the central nervous system. The development of this internal skeleton allowed vertebrates to grow larger and develop more complex body plans than most invertebrates.
The Definitive Answer: Where Fish Belong
All fish are firmly classified as vertebrates because they possess a true vertebral column that runs the length of their body. This spine is made up of individual, articulating vertebrae that support their muscular structure and protect the spinal cord. In the majority of fish, known as bony fish, this skeletal structure is composed of hard, mineralized bone.
The spine of a bony fish is divided into two main sections: the trunk vertebrae and the caudal vertebrae of the tail. The vertebrae are typically concave at both ends, a shape known as amphicoelous, which allows for necessary range of motion.
Even cartilaginous fish, such as sharks and rays, are considered vertebrates. While their skeleton is made of cartilage instead of bone, they still possess a vertebral column that fulfills the same supportive and protective functions. In these animals, the vertebrae are formed from two cartilaginous tubes that enclose the spinal cord and the remnant of the notochord.
Common Aquatic Confusion
The misconception that a fish might be an invertebrate likely stems from confusion between fish and other aquatic organisms. Many animals that share the water with fish are, in fact, invertebrates. This includes crustaceans like crabs, shrimp, and lobsters, which rely on a hard exoskeleton for support. Other common aquatic invertebrates include mollusks, such as clams, oysters, and snails, which lack an internal spine. Even soft-bodied animals like jellyfish and sea anemones fall into this category, as they have no internal or external skeleton.