Stingrays are not part of the shark family, but they are close relatives that share a deep evolutionary history. Both belong to the same broader biological class: the cartilaginous fish. While they share a common ancestor and many fundamental traits, stingrays and sharks separated millions of years ago, evolving into two distinct groups with major differences in body structure and lifestyle. Stingrays are more accurately classified as a specialized branch on the same evolutionary tree as sharks, not a member of their immediate family.
Shared Features of Cartilaginous Fish
The biological relationship between sharks and rays is defined by their shared classification in the Class Chondrichthyes, a name that translates to “cartilaginous fish.” The defining feature of this class is a skeleton composed entirely of cartilage, which is a flexible material instead of true bone tissue. This shared skeletal structure is reinforced by calcium carbonate granules, but it remains distinct from the bony skeletons found in most other fish.
Both groups also possess specialized placoid scales, which are tiny, tooth-like structures that give their skin a rough, sandpaper-like texture. Furthermore, neither sharks nor rays possess a swim bladder, the gas-filled organ used by most bony fish for buoyancy. Instead, they rely on a large, oil-rich liver and dynamic movement to maintain their position in the water column.
How Sharks and Rays Differ
Despite their shared ancestry, sharks and rays diverged into two distinct superorders: Selachii (sharks) and Batoidea (rays, skates, and sawfish). Sharks maintain a torpedo-shaped, or fusiform, body designed for active swimming and hunting in the water column.
In contrast, rays, including stingrays, are dorsoventrally flattened. This flattened shape is created by the dramatic fusion of the large pectoral fins to the side of the head and trunk.
The placement of their gill slits is another major anatomical distinction. Sharks have five to seven pairs of gill slits located laterally on the sides of their head. Rays, due to their flattened design, have their five pairs of gill openings located ventrally, or on the underside of their body. This anatomical difference also impacts locomotion; sharks propel themselves using a powerful caudal (tail) fin, while rays “fly” through the water by undulating or flapping their greatly enlarged pectoral fins.
Stingrays: A Specialized Branch
Stingrays represent a highly specialized group within the Batoidea superorder, adapted specifically for a benthic, or bottom-dwelling, existence. Their flattened body structure allows them to easily bury themselves under sand or mud, which helps them hide from predators and ambush prey. When they are partially buried, their ventral gill slits can become obstructed, necessitating a different method for breathing.
Stingrays developed spiracles, which are modified respiratory openings located on the top of their head, just behind the eyes. These openings draw clean, oxygenated water in, which is then passed over the gills and expelled through the ventral gill slits.
The feature that gives them their name, the venomous caudal barb, is a defensive spine located on the tail. This serrated spine is a modified placoid scale that is covered in a specialized tissue sheath containing venom. When threatened, the ray whips its tail, driving the barb into the attacker to deliver a painful, defensive sting.