Ray fish belong to the superorder Batoidea, an ancient lineage of marine animals that includes skates, stingrays, and electric rays. They are part of the class Chondrichthyes, making them relatives of sharks and chimaeras because their skeletons are primarily made of cartilage instead of bone. This group of cartilaginous fish, known scientifically as batoids, has evolved a unique, flattened body plan well-suited for life in diverse marine and even some freshwater environments. With over 600 species, rays represent the most diverse group within the cartilaginous fish class.
Defining Physical Characteristics
The most distinguishing feature of a ray is its highly flattened, disc-like body shape. This results from the enormous expansion and fusion of its pectoral fins to the sides of its head. This disc can be kite-shaped, rounded, or triangular, giving the animal a manta-like appearance as it “flies” through the water.
Rays have a specific arrangement of sensory and respiratory organs that reflects their flattened form. Their eyes and the two large openings called spiracles are located on the dorsal, or topside, of the disc. The spiracles draw in water for respiration, which is important for species that spend much of their time buried or resting on the seabed.
In contrast, the mouth and five pairs of gill slits are positioned on the ventral, or underside, of the body. This placement allows rays to feed on bottom-dwelling prey and breathe without taking in sediment through their mouths. Unlike many bony fish, rays lack a swim bladder, meaning they must either constantly swim or rest directly on the substrate to maintain their position.
Classification of Major Ray Groups
The superorder Batoidea encompasses a wide variety of forms, but the most common distinction is between stingrays and skates. Stingrays belong to the order Myliobatiformes and are characterized by a long, whip-like tail that typically bears one or more serrated, venomous spines used solely for defense. Reproduction in stingrays is generally viviparous, meaning they give birth to live, fully formed young.
Skates, classified in the order Rajiformes, lack the venomous spine found on stingrays. Their tails are generally thicker and shorter, sometimes featuring small dorsal fins near the tip. Skates are oviparous, reproducing by laying eggs encased in a tough, leathery shell often called a “mermaid’s purse,” which anchors to the seabed.
Other notable groups include the Mobulidae family, which contains the massive manta and devil rays, often referred to as pelagic filter feeders. These species are known for their enormous wingspans and have specialized cephalic fins that funnel plankton into their mouths. Electric rays (Torpediniformes) possess specialized electric organs within their pectoral fin discs that can generate a powerful shock for stunning prey and self-defense.
Ecological Roles and Habits
Rays occupy diverse habitats, ranging from the intertidal zone to abyssal depths, with most species inhabiting the continental shelves. Many rays, such as stingrays and skates, are benthic, meaning they are bottom-dwelling animals adapted for life on the seafloor. Other groups, like manta and eagle rays, are pelagic, soaring through the open ocean water column.
Rays exhibit varied feeding strategies, although all known species are carnivorous. Many benthic rays are ambush predators that use their electroreceptors, the ampullae of Lorenzini, to detect the faint electrical fields generated by prey buried in the sediment. They often consume invertebrates like mollusks and crustaceans, using powerful, plate-like teeth to crush hard shells.
Larger rays, like the pelagic manta, are filter feeders, using specialized gill rakers to strain vast quantities of plankton from the water as they swim. Movement varies by species; many rays move by rhythmically undulating the edges of their flexible pectoral fins, while others flap their fins like wings to propel themselves forward.