Stingrays are not sharks, yet they share a deep evolutionary connection. Both belong to a broader group of fish known for their cartilaginous skeletons. While distinct in appearance and lifestyle, their shared lineage highlights millions of years of adaptation within marine environments.
Understanding Sharks
Sharks are a diverse group of cartilaginous fish characterized by their streamlined, torpedo-shaped bodies, often called fusiform. Their skeletons are composed entirely of cartilage, a flexible and durable tissue, rather than bone. Sharks possess five to seven gill slits on the sides of their heads, through which they extract oxygen from the water.
Their skin is covered in tiny, tooth-like scales called placoid scales, which give it a rough, sandpaper-like texture. These powerful predators also feature multiple rows of sharp, replaceable teeth. Sharks belong to the class Chondrichthyes and are further classified under the subclass Elasmobranchii.
Understanding Stingrays
Stingrays exhibit a distinctly flattened body shape, often described as disc-like. Their large pectoral fins are fused to their heads and bodies, forming broad “wings” for graceful movement through the water. Unlike sharks, stingrays have their five to seven gill slits on their underside, or ventral surface.
Many species possess one or more venomous, saw-edged spines on their whip-like tails, primarily used for defense. Like sharks, stingrays have cartilaginous skeletons and placoid scales. They are also members of the class Chondrichthyes and the subclass Elasmobranchii.
Their Evolutionary Relationship
Sharks and stingrays share a common ancestor, a fact supported by fossil evidence, anatomical similarities, and genetic analysis. Both are members of the class Chondrichthyes, distinguishing them from bony fish by their cartilaginous skeletons. They are further grouped within the subclass Elasmobranchii, sharing fundamental characteristics such as multiple gill slits and dermal denticles.
These shared traits also include the absence of a swim bladder, with both relying on large, oil-rich livers and hydrodynamic lift from their fins for buoyancy. Despite these similarities, their evolutionary paths diverged significantly over millions of years. Sharks evolved into the Selachii division, maintaining their torpedo-shaped bodies and side-mounted gill slits, adapting to active predatory roles in the water column.
Rays and skates, conversely, evolved into the Batomorphi division (also known as Batoidea), developing flattened bodies and shifting their gill slits to their ventral surface. This adaptation facilitated a benthic, or bottom-dwelling, lifestyle, allowing them to camouflage themselves in the seafloor and hunt for prey buried in the sediment. The distinct body plans and gill locations reflect different adaptations to their respective ecological niches, showcasing how a shared lineage can lead to diverse forms.