Sharks and rays are closely related, sharing a deep evolutionary history that places them within the same major biological group. Their similarities are rooted in a shared anatomical blueprint that distinguishes them from most other fish species. Although they look dramatically different today, their ancient ancestors were the same. Understanding this connection requires looking beyond their appearance to the foundational biological class they both inhabit.
Defining the Cartilaginous Fish
The scientific classification that unites sharks and rays is the class Chondrichthyes, derived from Greek meaning “cartilage fish.” This group represents an ancient line of jawed vertebrates whose skeletons are primarily composed of cartilage, a tissue lighter and more flexible than true bone. This cartilaginous structure, which is never fully ossified, is the defining characteristic of the class.
The class Chondrichthyes is organized into two main subclasses: Elasmobranchii and Holocephali. Elasmobranchii contains all modern sharks, rays, skates, and sawfish. Holocephali consists of the chimaeras, or “ghost sharks,” which represent a distinct evolutionary branch.
This classification separates them from bony fish (class Osteichthyes). The Elasmobranchii subclass is further divided into two divisions: Selachii (sharks) and Batomorphi (rays, skates, and their relatives). Biologically, the difference between a shark and a ray is less significant than the difference between a shark and a tuna.
Shared Features That Prove the Link
The common ancestry of sharks and rays is evident in multiple shared anatomical and physiological features. Both groups possess placoid scales, a unique body covering structurally homologous to teeth. These tiny, tooth-like structures, also known as dermal denticles, cover the skin and help reduce drag, providing a hydrodynamic advantage.
Another shared trait is the Ampullae of Lorenzini, a specialized electroreception system. This network consists of small, jelly-filled pores concentrated around the head and snout. The Ampullae of Lorenzini allow both sharks and rays to detect the faint electrical fields generated by potential prey, even when hidden from sight.
Both Elasmobranch groups lack a swim bladder, the gas-filled organ used by most bony fish for buoyancy. Instead, they rely on a liver rich in low-density oils, primarily squalene, to aid in flotation. This shared adaptation necessitates continuous movement for many shark species and explains why rays often rest on the seafloor.
How Rays and Sharks Became Different
Despite their shared heritage, sharks and rays diverged evolutionarily to exploit different ecological niches, resulting in distinct body plans. Most sharks retained the torpedo-like, or fusiform, structure optimized for swimming through the water column. Rays, however, evolved a dorsoventrally flattened shape, perfectly suited for a life spent on the ocean floor, or benthic zone.
This shift to a benthic lifestyle led to changes in gill placement and fin structure. Sharks typically have five to seven gill slits located laterally, requiring constant movement to push water over their gills. Rays moved these gill openings to the ventral (underside) of the body, and their large pectoral fins fused with the head, creating the characteristic disc shape.
For respiration, most rays draw water in through spiracles, a pair of openings located on the top of the head. This prevents them from inhaling substrate while resting on the bottom, an adaptation reduced or absent in most active sharks. This evolutionary separation, which began around the Jurassic period, allowed the Batomorphi to colonize bottom-dwelling habitats successfully.