The vast majority of creatures classified as fish are vertebrates, meaning they possess a vertebral column, or backbone. The term “fish,” however, covers an enormous range of aquatic animals, and the structure of their internal support system varies significantly. The vertebral column is a central, segmented axis that provides skeletal support and protects the delicate nervous tissue running through the animal’s body.
Defining the Vertebral Column
The vertebral column is the defining characteristic of a vertebrate animal, including mammals, birds, reptiles, amphibians, and most fish. This structure is a flexible series of interlocking segments called vertebrae. The column’s primary function is to stiffen the body, providing a framework for muscle attachment that enables movement through the water.
Each vertebra consists of a central body, or centrum, which surrounds and often replaces the notochord, a primitive rod-like structure present in all vertebrate embryos. A Y-shaped neural arch extends upward from the centrum, creating a protective tunnel for the spinal cord. In fish, the column is differentiated into two regions: the trunk vertebrae, which often feature ribs for protecting internal organs, and the caudal (tail) vertebrae. Caudal vertebrae feature both a neural arch and a hemal arch to protect the main blood vessels and nerve cord.
The Spines of Bony and Cartilaginous Fish
The composition of the spine distinguishes between the two largest groups of fish: bony fish and cartilaginous fish. Bony fish (class Osteichthyes) have a vertebral column made of true bone tissue. This fully ossified, segmented backbone allows for powerful muscle contractions and the characteristic S-shaped swimming motion seen in most species.
The bony vertebrae are spool-shaped and interlock, creating a structure that is lightweight yet robust. This contributes to the success of this group, which accounts for over 96 percent of all fish species. In contrast, cartilaginous fish (Chondrichthyes), including sharks, rays, and skates, possess a skeleton composed primarily of cartilage. Their vertebral column is composed of two cartilaginous tubes that surround the notochord.
While the cartilaginous spine is lighter and more flexible than bone, it is often strengthened through calcification, though it does not undergo true ossification. The spine provides necessary support and protection for the spinal cord. Its composition remains fundamentally different from the true bone found in bony fish.
Distinctions: Fish That Lack True Spines
A few ancient fish lineages challenge the simple definition of a spine. Jawless fish, such as hagfish and lampreys, represent a more primitive stage of skeletal development. Hagfish do not possess a true, segmented vertebral column; their main supportive structure is a persistent notochord that runs the length of their body.
Lampreys are slightly more developed and possess simple cartilaginous elements called arcualia arranged around the notochord, which are considered rudimentary vertebral structures. Neither of these jawless fish, however, has the fully developed, segmented backbone that characterizes all other fish and vertebrates.
Many creatures commonly referred to as “fish,” like jellyfish and starfish, are not fish at all. These animals are invertebrates, meaning they completely lack any form of vertebral column or internal skeleton. Jellyfish rely on a hydrostatic skeleton, which uses water pressure and a gelatinous layer called mesoglea for support. Starfish belong to the phylum Echinodermata and use a water vascular system for movement, with structural support provided by internal calcium carbonate plates, not a backbone.