Stingrays of the family Dasyatidae have expanded pectoral fins that form a circular, oval, or rhomboidal disc. These fins extend forward to the snout, such that the head appears enclosed by the disc. The pectoral disc is no more than 1.3 times as wide as it is long. From the side the ray is relatively flat, and the head is even with the body. The eyes are located on the sides of the top of the head, with the spiracles (respiratory openings) close behind the eyes. Like all rays, they have ventral gill openings. These form five small pairs and the internal gill arches do not have filter plates. Their mouths, which contain fleshy papillae on the floor, are small and located under the end of the snout. Since their mouths are directed downward and often placed against the sand, stingrays use their spiracles rather than their mouths for water intake, and, if the gills are covered with sand, the spiracles are also used for expelling water. Stingrays have small to medium-sized teeth that do not form flat crushing plates. Teeth are arranged in rows, with some members of Potamotrygoninae having over 60 rows of teeth in each jaw, arranged in groups of five. Like other rays, stingrays have a spiral valve in their intestine that increases food absorption, and lack a swim bladder. Along with eagle rays (Myliobatidae), stingrays reportedly have the most complex brains of all elasmobranch fishes.
Their dorsal skin may be smooth, or covered with denticles or thorns. They do not have a dorsal fin. Some also lack a caudal (tail) fin, while in others the caudal fin is reduced to long dorsal and ventral fin folds that may or may not extend to the tip of the tail. The tail is usually longer than the disc and bears one or more long, serrated spines behind the pelvic fins. The spines, which are used only in defense, are modified placoid scales, tipped with barbs. Each spine has grooves on its underside that contain venom-producing soft tissue. Stingrays have been reported to whip their tails with such force that they can drive their spines, which may reach 40 cm long, through the wooden bottom of a boat. The stings are constantly being shed and replaced. Members of the subfamily Dasyatinae range from less than 1 m long to more than 4 m long. In at least one species, Dasyatis centraura, females are reported to be larger than males. Stingrays of the subfamily Potamotrygonidae (river stingrays) tend to be smaller, usually less than 30 cm in diameter and less than 1 m long, although a few may attain 2 m. A unique aspect of river stingrays is their chemical adaptation to fresh water; their blood contains very low concentrations of urea, and their rectal gland (used by fishes for salt secretion) is reduced. Some male river stingrays have more prominent cusps on their teeth than females do. Stingrays tend to have drab coloration, but river stingrays in particular often have various patterns and markings over the brown or gray background.
Other Physical Features: ectothermic ; bilateral symmetry ; venomous
Sexual Dimorphism: sexes alike; female larger