Stingrays are a group of cartilaginous fish that belong to the suborder Myliobatoidei within Myliobatiformes, an order of rays. They are characterized by flat, disc-like bodies with a pair of large, wing-like pectoral fins and a whip-like tail. Some species are also equipped with venomous spines on their tails, which play a crucial role in their defense.
They are distributed worldwide except in polar regions. Most species are benthic, spending their time close to the seafloor, where they often bury themselves in sediment and wait for prey to pass by. As carnivores, they feed on a variety of invertebrates, including mollusks, crustaceans, and polychaete worms.
In some countries, such as Malaysia and Singapore, stingrays are cherished as a delicacy.
The size of stingrays varies with species. The largest species, the giant freshwater stingray (Urogymnus polylepis), grows up to 7.2 ft (2.2 m) across and may weigh over 660 lb (300 kg).[1] In contrast, one of the smallest species, the black-tailed antenna ray (Plesiotrygon nana), has a disc-width of about 20 in (52 cm) and weighs only around 27 lb (12.4 kg).[2]
They have a disc-shaped, dorsoventrally flattened body, well-adapted for their benthic lifestyle. This disc is formed by the fusion of the head and large pectoral fins that extend like wings. While the eyes are located on the top of the head, the mouth lies on the ventral side of the body. The dorsal surface of the body often matches the color of the sea floor, helping them camouflage easily.
Their tail is whip-like, extending from the posterior end of the disc. Unlike manta rays, some stingrays (like members of the families Dasyatidae, Urotrygonidae, Urolophidae, and Potamotrygonidae) possess one or more serrated, barbed tail spines. These spines are lined with venom-secreting cells that produce a mucus-like coating, which covers the spine and is released when the sheath is broken.
Just behind the eyes are two respiratory openings called spiracles. Under normal conditions, stingrays draw water in through their mouth, which then passes over the gills for gas exchange. However, when buried in sediment, the mouth is obstructed, so they rely on the spiracles to draw in water for respiration.
Stingrays have specialized electroreceptors called ampullae of Lorenzini that are concentrated around the snout. These sensitive receptors detect weak electric fields produced by the muscle contractions of prey buried under sand.
They also possess a lateral line system made up of sensory hair cells, or neuromasts, that detect water vibrations and pressure changes. Additionally, the nostrils on the ventral side of the body help sense dissolved chemicals, aiding in prey detection.
These fish exhibit a hyostylic jaw suspension, in which the mandibular arch is suspended solely by an articulation with the hyomandibula. Such suspension allows the upper jaw to extend and capture prey. Moreover, the movement of the jaws generates suction to suck the prey from the substrate.
They possess flattened, plate-like teeth made of modified placoid scales. These teeth are arranged in tightly packed rows, providing a continuous grinding surface for processing hard-shelled prey.
Like other cartilaginous fishes, stingrays continuously replace their teeth throughout life. Males of some species, such as the Atlantic stingray (Hypanus sabinus), develop pointed or cuspidate teeth during the breeding season, which they use for grasping females during copulation.[3]
Around 220 species of stingrays are classified into 8 extant families.
Stingrays are thought to have diverged from their closest relatives, the panrays, during the Late Jurassic Period and subsequently diversified throughout the Cretaceous. Fossil evidence of stingray teeth dates back as far as the Early Cretaceous Period.
These fish are the most abundant in tropical and subtropical marine waters worldwide. However, river stingrays and some whiptail stingrays live in freshwater environments.
Although most species occupy the water column near the substrate (demersal), a few, such as the pelagic stingray (Pteroplatytrygon violacea), live in the open ocean.
They are most commonly found in shallow coastal waters, though some species, such as the deepwater stingray (Plesiobatis daviesi), are found at depths of around 900 to 2,230 ft (275 to 680 m).[4]
As carnivorous feeders, stingrays typically feed on benthic invertebrates, such as mollusks (like clams, oysters, snails, and squid), crustaceans (like crabs, shrimp, and lobsters), and worms (typically polychaetes).
Larger species may occasionally prey on small bony fish, including minnows, silversides, and gobies.
Some freshwater species, like the ocellate river stingray (Potamotrygon motoro), have been observed feeding on aquatic insects, including their larvae.[5]
Unlike most fish that rely on their tail fin for propulsion, stingrays swim using their paired pectoral fins. Species with long, narrow, pointed fins generate an oscillatory motion by flapping them up and down, whereas those with short, thick, rounded fins produce undulatory motion through wave-like movements that travel along the entire fin.
Most stingrays are ambush predators, burying themselves in the sediment and waiting stealthily for their prey to approach. They press their pectoral fins against the substrate and raise their heads (a posture called tenting), creating a suction force that pulls the prey underneath the body. On the other hand, some groups, such as round stingrays, use their strong jaws to break open shells and exoskeletons of their prey, spitting out discarded fragments before swallowing soft tissue. Moreover, a few species can also filter plankton through modified gill rakers as they swim.
Although avoidance through camouflage is their first line of defense, when threatened, stingrays with venomous spines can whip their tail upward to deliver a painful sting.
In humans, a stingray’s spine usually causes localized trauma such as pain, swelling, and muscle cramps, but the injuries are rarely fatal, occurring only if a vital organ is punctured. However, they are generally non-aggressive and only react defensively when accidentally stepped on.
On average, stingrays live between 15 and 25 years in the wild.[6] However, for some species, such as the southern stingray (Hypanus americanus), growth models have suggested that females might theoretically reach 30 to 50 years in captivity.[7]
During the breeding season, male stingrays use their ampullae of Lorenzini to detect the weak electrical signals emitted by females. They then follow closely, nudging with the snout and biting the pectoral fins to initiate copulation. Once paired, the male inserts one of his claspers into the female’s cloaca to deliver sperm. Eventually, the eggs are fertilized internally.
Being ovoviviparous, stingrays give birth to live young, with embryos developing inside the uterus of females without a placenta (aplacental viviparity). The embryos initially derive nutrition from a yolk sac, but eventually, when the sac is depleted, the mother provides nourishment through uterine secretions (uterine milk). After 90 to 135 days, the female gives birth to 5 to 13 live, fully developed pups, depending on the species. The juveniles typically disperse shortly, grow in size, and become sexually mature in about 2.5 to 4 years. However, in some species, such as the giant freshwater stingray, the mother may accompany her young till they are about one-third of her size.
These rays are primarily preyed upon by sharks, particularly hammerheads, bull sharks, and tiger sharks. They are occasionally consumed by marine mammals, like killer whales and elephant seals. Some predatory bony fish, such as groupers, snappers, and barracudas, may target juvenile stingrays.
River stingrays fall prey to large predatory fish, like giant catfish, or even crocodilians. Their juveniles are occasionally eaten by wading birds, though such instances are rare.
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