For many people, the term “fish” suggests an animal with fins, gills, and a backbone, classifying it as a member of the vast group known as vertebrates. This general understanding holds true for the vast majority of aquatic species; over 33,000 species of fish represent more than half of all living vertebrates on Earth. However, the biological classification of “fish” is complex, leading to a small but significant group of exceptions. These outliers challenge the assumption that all fish possess a vertebral column.
Establishing the Rule The Definition of Vertebrates in Fish
The defining characteristic of a vertebrate is the presence of a vertebral column, a segmented series of stiff elements that protect the spinal cord. In bony fish (like tuna, salmon, and bass), this column is composed of individual vertebrae that replace the flexible notochord present during the embryonic stage. These bony fish account for roughly 96% of all extant fish species.
Even cartilaginous fish, such as sharks and rays, are considered true vertebrates despite having skeletons made of cartilage instead of bone. Their axial skeleton still consists of distinct, segmented vertebrae that enclose the spinal cord, fulfilling the primary anatomical requirement. The vertebral bodies provide the main structural support and exhibit the segmented pattern characteristic of the vertebrate lineage. This majority establishes the biological rule: a fish is a vertebrate with a backbone.
The Primary Exception Jawless Fish
The exceptions lie within the most ancient group of aquatic animals: the jawless fish. This lineage evolved before the development of jaws and a fully segmented spinal column. Instead of a vertebral column, their primary axial support remains the notochord, a flexible, fluid-filled sheath extending the length of the body.
This persistent notochord is a primitive feature that places them at the base of the vertebrate family tree. Although they possess a distinct skull, classifying them as craniates, the lack of a fully developed backbone means their status as true vertebrates is often debated. This ancient group provides a living window into the anatomy of the earliest chordates.
Case Study 1 Hagfish
The hagfish is the most extreme example challenging the definition of a fish with a backbone and is frequently not classified as a true vertebrate. These deep-sea scavengers possess a skull but entirely lack a vertebral column. Their body support comes solely from the notochord, an unsegmented, flexible rod of cartilage.
This notochord allows the hagfish flexibility, enabling them to tie themselves into knots when feeding or escaping predators. There are no elements of cartilage or bone that segment the notochord or protect the spinal cord.
The hagfish’s skeletal structure consists only of the cranium, the notochord, and a few small cartilaginous elements near the head and tail. This minimal framework reflects their status as one of the most primitive living craniates, having diverged before the segmented spinal column was established.
Case Study 2 Lampreys
The lamprey, the other living member of the jawless fish group, represents a slightly more advanced stage in backbone evolution. Unlike the hagfish, lampreys possess small, cartilaginous structures called arcualia that partially surround their persistent notochord.
The arcualia are small, paired segments of cartilage located above the notochord, offering protection to the nerve cord. Since these elements are segmented and positioned similarly to the neural arches of true vertebrae, they are considered rudimentary vertebral elements.
This anatomical difference is why lampreys are often categorized as basal vertebrates, or “proto-vertebrates.” Their simple, incomplete vertebral elements show an intermediate stage between the notochord-only support of the hagfish and the fully developed spinal column of bony fish. The lamprey is considered to have a backbone, though it is primitive, cartilaginous, and incomplete.