Many people encounter the diverse array of life in aquatic environments and often ponder the biological classification of fish. Understanding where fish fit within the vast animal kingdom requires a clear grasp of fundamental biological distinctions. Such classifications help scientists categorize organisms based on shared characteristics, revealing evolutionary relationships and physiological commonalities.
Differentiating Vertebrates and Invertebrates
Animals are broadly categorized into two major groups based on the presence or absence of a backbone. Vertebrates possess a vertebral column, or spine. This internal skeletal structure is composed of segmented bones called vertebrae, which encase and protect the spinal cord. Examples of vertebrates include mammals, birds, reptiles, and amphibians, all sharing this defining characteristic at some point in their lives.
In contrast, invertebrates do not possess a vertebral column. This group encompasses the vast majority of animal species, making up over 95% of the animal kingdom. Invertebrates display an immense variety of body plans, ranging from soft-bodied organisms to those with hard external shells. Common examples include insects, worms, jellyfish, snails, and crabs. The key distinction between these two groups lies in the presence or absence of this central bony or cartilaginous support structure.
The Vertebrate Nature of Fish
Fish are classified as vertebrates because all true fish possess a vertebral column, or backbone. This backbone forms the main support structure of their bodies, providing support, protecting the spinal cord, and aiding movement through water. Their skeletal system is either made of bone, as seen in bony fish (Osteichthyes), or cartilage, as in cartilaginous fish (Chondrichthyes) like sharks and rays.
Beyond the backbone, fish share other vertebrate characteristics, including a skull that protects a well-defined brain and a complex nervous system. Fish comprise a diverse group, including jawless fish such as lampreys and hagfish, cartilaginous fish like sharks and skates, and bony fish, which are the largest group. Despite their varied forms and habitats, all these true fish possess the fundamental vertebral structure that places them firmly within the vertebrate subphylum. Note that some aquatic organisms, such as jellyfish and starfish, are often mistakenly referred to as “fish” but are, in fact, invertebrates as they lack a backbone.