Rays are definitively a type of fish, despite their unique, flattened appearance compared to more common finned species. They are marine animals closely related to sharks and skates, sharing a deep evolutionary history within the aquatic vertebrate family. Rays are classified as true fish because they possess gills for respiration and fins for movement in the water column.
Rays: Members of the Cartilaginous Fish Class
Rays belong to the taxonomic class Chondrichthyes, commonly known as cartilaginous fish. This classification places them alongside sharks and chimaeras, distinguishing them from bony fish (Osteichthyes). The defining feature of this class is that their skeletons are composed entirely of cartilage, a tough and flexible connective tissue, rather than calcified bone.
This cartilaginous structure provides a lighter, more flexible framework. Rays and sharks are further grouped into the subclass Elasmobranchii, sharing characteristics like multiple gill slits. Within this subclass, rays are classified under the Superorder Batoidea, which encompasses over 600 species. Rays are the largest group of all cartilaginous fishes, with an evolutionary history dating back at least 150 million years.
Defining Features of Ray Anatomy
The physical form of a ray is its most distinguishing characteristic, representing an adaptation from the typical fish body plan. Their bodies exhibit extreme dorsal-ventral flattening, meaning they are horizontally compressed into a distinct, disc-like shape. This morphology is effective for their lifestyle, which often involves resting on or burrowing into the seafloor.
A ray’s large, wing-like pectoral fins are fused directly to the sides of their head, forming the expansive disc. Movement is achieved by undulating or flapping these fins, allowing them to glide through the water. Unlike sharks, a ray’s five to six pairs of gill openings are located on the underside (ventral surface) of the body. To prevent inhaling sand while resting, rays breathe by drawing water through two large openings, called spiracles, positioned on the top of their head.
Major Groups of Rays
Diversity within the Superorder Batoidea is significant, with species generally falling into a few major groups. Stingrays, such as the Dasyatidae family, are well-known for their long, whip-like tail featuring one or more venomous, serrated spines used purely for defense. Skates (Order Rajiformes) are similar in appearance but lack the stinging barb and often possess small dorsal fins on their tail.
The largest rays, like the Manta and Devil Rays, are filter feeders that swim in the open ocean. Other groups include the Electric Rays, which possess specialized organs capable of generating a powerful electric current to stun prey or deter predators. Guitarfish and sawfish are also classified as rays, exhibiting a body shape intermediate between that of a typical ray and a shark.