Stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae (sixgill stingray), Plesiobatidae (deep water stingray), Urolophidae (stingarees), Urotrygonidae (round rays), Dasyatidae (whiptail stingrays), Potamotrygonidae (river stingrays), Gymnuridae (butterfly rays), and Myliobatidae (eagle rays). Most stingrays have one or more barbed stings (modified from dermal denticles) on the tail, which are used exclusively in self-defense. The stinger may reach a length of approximately 35 cm (14 in), and its underside has two grooves with venom glands. The stinger is covered with a thin layer of skin, the integumentary sheath, in which the venom is concentrated. A few members of the suborder, such as the manta rays and the porcupine ray, do not have stingers. Stingrays are common in coastal tropical and subtropical marine waters throughout the world, and also includes species found in warmer temperate oceans, such as Dasyatis thetidis, and those found in the deep ocean, such as Plesiobatis daviesi. The river stingrays, and a number of whiptail stingrays (such as the Niger stingray), are restricted to fresh water. Most myliobatoids are demersal, but some, such as the pelagic stingray and the eagle rays, are pelagic. While most stingrays are relatively widespread and not currently threatened, for several species (for example Taeniura meyeni, D. colarensis, D. garouaensis, and D. laosensis), the conservation status is more problematic, leading to them being listed as vulnerable or endangered by IUCN. The status of several other species are poorly known, leading to them being listed as Data Deficient. The flattened bodies of stingrays allow them to effectively conceal themselves in their environment. Stingrays do this by agitating the sand and hiding beneath it. Because their eyes are on top of their bodies and their mouths on the undersides, stingrays cannot see their prey; instead, they use smell and electroreceptors (ampullae of Lorenzini) similar to those of sharks. Stingrays feed primarily on molluscs, crustaceans, and occasionally on small fish. Some stingrays' mouths contain two powerful, shell-crushing plates, while other species only have sucking mouthparts. Stingrays settle on the bottom while feeding, often leaving only their eyes and tail visible. Coral reefs are favorite feeding grounds and are usually shared with sharks during high tide. When a male is courting a female, he will follow her closely, biting at her pectoral disc. He then places one of his two claspers into her valve. Stingrays are ovoviviparous, bearing live young in "litters" of five to 13. The female holds the embryos in the womb without a placenta. Instead, the embryos absorb nutrients from a yolk sac, and after the sac is depleted, the mother provides uterine "milk". At the Sea Life London Aquarium, two female stingrays have delivered seven baby stingrays, although the mothers have not been near a male for two years. "Rays have been known to store sperm and not give birth until they decide the timing is right". Stingrays do not aggressively attack humans, though stings do normally occur if a ray is accidentally stepped on. To avoid stepping on a stingray in shallow water, the water should be waded through with a shuffle. Alternatively, before wading, stones can be thrown into the water to scare stingrays away. Contact with the stinger causes local trauma (from the cut itself), pain, swelling, muscle cramps from the venom, and later may result in infection from bacteria or fungus. The injury is very painful, but seldom life-threatening unless the stinger pierces a vital area. The barb usually breaks off in the wound, and surgery may be required to remove the fragments.