Sharks are classified using taxonomy, a system that organizes all living things into a ranked hierarchy. Sharks belong to the Phylum Chordata. Understanding this classification means moving from the broadest category down to the specific traits that define these ancient marine predators. Their lineage extends back over 400 million years, making them older than most terrestrial vertebrates.
Defining the Chordate Phylum
The Phylum Chordata includes a diverse group of animals, including all vertebrates. Sharks meet the criteria for inclusion based on four specific anatomical features that appear at some stage of their development.
The first feature is the notochord, a flexible, rod-shaped structure providing skeletal support along the body’s length. Although this structure is replaced by a backbone in adult vertebrates, it is a marker of the phylum.
Another element is the dorsal hollow nerve cord, a tube of nervous tissue located above the notochord. This cord develops into the brain and spinal cord, forming the central nervous system.
All chordates also exhibit pharyngeal slits or pouches, which are openings in the throat region. These slits may develop into gill structures, as they do in fish, or be modified for other purposes in land animals.
Finally, a post-anal tail is characteristic of the phylum, representing an extension of the body that continues past the anus. Even in species where this tail is reduced, it is present during the embryonic stage. These four traits—the notochord, the dorsal hollow nerve cord, the pharyngeal slits, and the post-anal tail—establish the shark’s position within the Chordata phylum.
The Specific Class of Cartilaginous Fish
Moving down the taxonomic ladder, sharks are placed in the Class Chondrichthyes, a name derived from Greek meaning “cartilage fish.” This classification is based on their skeleton being composed almost entirely of cartilage, a tissue lighter and more flexible than true bone. The lack of a bony skeleton differentiates them from most other fish species.
Sharks lack a swim bladder, the gas-filled organ used for buoyancy control in most bony fish. To counteract their tendency to sink, they possess a massive liver saturated with low-density oils. This large, oil-filled organ provides hydrostatic lift, allowing them to maintain position in the water column. Many species must swim continuously to avoid sinking entirely.
The skin of sharks is also highly specialized, covered in tiny, tooth-like structures known as placoid scales, or dermal denticles. These structures are made of the same materials as teeth and are arranged to reduce drag, giving the skin a rough, sandpaper-like texture that aids in hydrodynamic efficiency. Unlike bony fish, sharks display five to seven pairs of separate, exposed gill slits on the sides of their head. This arrangement requires them to either swim continuously or actively pump water to maintain respiration.
Close Relatives of Sharks
The Class Chondrichthyes is divided into two major subclasses that contain their closest living relatives. The subclass Elasmobranchii includes the sharks, but also encompasses the rays and skates. Rays and skates share the cartilaginous skeleton and multiple exposed gill slits. They have evolved a flattened body plan adapted for life on the ocean floor.
The other subclass is Holocephali, which contains the chimaeras, often referred to as ghost sharks or ratfish. These deep-sea relatives also possess a cartilaginous skeleton, confirming their shared ancestry with sharks. Chimaeras differ visually, having a single external gill opening on each side, a trait that makes them distinct from their elasmobranch cousins.