The question of whether a ray is a type of shark is common, revealing a close evolutionary link between these two distinct groups of sea creatures. Rays and sharks are far closer relatives than either is to a common bony fish like a tuna or cod. Both belong to the same high-level biological category, sharing a common ancestor that lived millions of years ago. Rays are a specialized branch on the same ancient family tree as sharks, having adapted a dramatically different body plan over time.
Shared Ancestry: The Cartilaginous Fish Family
Sharks and rays are members of the class Chondrichthyes, commonly known as the cartilaginous fishes. The defining characteristic of this class is a skeleton composed entirely of cartilage rather than true bone, a feature they share with chimaeras. This lighter, more flexible structure distinguishes them from the majority of fish species that possess bony skeletons.
Both sharks and rays are further grouped into the subclass Elasmobranchii, united by several specific anatomical traits. Elasmobranchs possess five to seven pairs of exposed gill openings that lack a protective bony cover. Their skin is covered in tiny, tooth-like scales called placoid scales, giving their surfaces a sandpaper-like texture. Neither sharks nor rays possess a swim bladder, an organ used by most bony fish for buoyancy control. Instead, they rely on a large, oil-rich liver and the hydrodynamics of their body shape to prevent sinking.
Anatomical Distinction: Key Differences Between Rays and Sharks
Despite their shared ancestry, sharks and rays exhibit profound differences in physical form related to their distinct lifestyles. Sharks generally have a streamlined, torpedo-like body built for active swimming and speed. Rays, by contrast, are dorsoventrally flattened, compressed from top to bottom into a disc-like or kite shape. This flattened shape is an adaptation for their predominantly bottom-dwelling existence on the seafloor.
The arrangement of their gills provides another clear anatomical separation. Sharks have their five to seven gill slits located laterally on the sides of their head, just behind the mouth. Rays, however, have their five pairs of gill slits positioned ventrally, or on the underside of their body. Rays often use a separate opening called a spiracle, located behind the eyes, to actively draw in water while resting on the sediment.
The function and size of their pectoral fins also diverge significantly. In sharks, the pectoral fins are separate from the head and used primarily for steering and providing lift. Rays have dramatically enlarged pectoral fins that are fused to the sides of the head and trunk, creating their distinctive flat disc. These fins function like wings, which rays undulate or flap to propel themselves through the water.
Diverse Forms: Major Groups of Rays and Sharks
The differences in form correspond to a wide variety of species within each group, showing the success of both body plans. The major groups of rays are collectively known as Batoidea, which includes over 600 species. This group encompasses the true rays (Myliobatiformes), such as stingrays, which have a venomous barb on their whip-like tails, and the immense, filter-feeding manta rays. Skates (Rajiformes) are also included, distinguished from true rays by their lack of a stinging spine and their production of rectangular egg cases.
Sharks are classified into multiple orders, reflecting a greater diversity in body shape and ecological roles.
Examples of Shark Orders
The Carcharhiniformes, known as the ground sharks, include species like the tiger shark and hammerhead sharks. The Lamniformes, or mackerel sharks, include apex predators such as the great white shark and the plankton-feeding basking shark. The Orectolobiformes, or carpet sharks, include bottom-dwellers like the nurse shark and wobbegong.