Stingrays are a group of rays, a type of cartilaginous fish. They are classified in the suborder Myliobatoidei of the order Myliobatiformes and consist of eleven families: Hexatrygonidae (sixgill stingrays), Gymnuridae (butterfly rays), Plesiobatidae (deepwater stingrays), Urolophidae (round stingrays), Myliobatidae (eagle rays), Aetobatidae (pelagic eagle rays), Mobulidae (manta and devil rays), Rhinopteridae (cownose rays), Dasyatidae (whiptail stingrays), Potamotrygonidae (river stingrays), and Urotrygonidae (American round stingrays). There are about 220 known stingray species organized into 29 genera. Stingrays are common in coastal tropical and subtropical marine waters throughout the world. Some species, such as the thorntail stingray (Dasyatis thetidis), are found in warmer temperate oceans and others, such as the deepwater stingray (Plesiobatis daviesi), are found in the deep ocean. The river stingrays and a number of whiptail stingrays (such as the Niger stingray (Fontitrygon garouaensis)) are restricted to fresh water. Most myliobatoids are demersal (inhabiting the next-to-lowest zone in the water column), but some, such as the pelagic stingray and the eagle rays, are pelagic. Stingray species are progressively becoming threatened or vulnerable to extinction, particularly as a consequence of unregulated fishing. As of 2013, 45 species have been listed as vulnerable or endangered by the IUCN. The status of some other species is poorly known, leading to their being listed as data deficient.
Evolution
Stingrays diverged from their closest relatives, the panrays, during the Late Jurassic period, and diversified over the course of the Cretaceous into the different extant families today. The earliest stingrays appear to have been benthic, with the ancestors of the eagle rays becoming pelagic during the early Late Cretaceous.
Fossils
Permineralized stingray teeth have been found in sedimentary deposits around the world as far back as the Early Cretaceous. The oldest known stingray taxon is "Dasyatis" speetonensis from the Hauterivian of England, whose teeth most closely resemble that of the extant sixgill stingray (Hexatrygon). Although stingray teeth are rare on sea bottoms compared to the similar shark teeth, scuba divers searching for the latter do encounter the teeth of stingrays.
Full-body stingray fossils are very rare but are known from certain lagerstätten that preserve soft-bodied animals. The extinct Cyclobatis of the Cretaceous of Lebanon is thought to be a skate that had convergently evolved a highly stingray-like body plan, although its exact taxonomic placement is still uncertain. True stingray fossils become more common in the Eocene, with the extinct freshwater stingrays Heliobatis and Asterotrygon known from the Green River Formation. A diversity of stingray fossils is known from the Eocene Monte Bolca formation from Italy, including the early stingaree Arechia, as well as Dasyomyliobatis, which is thought to represent a transitional form between stingrays and eagle rays, and the highly unusual Lessiniabatis, which had an extremely short and slender tail with no sting.
Anatomy
Jaw and teeth The mouth of the stingray is located on the ventral side of the vertebrate. Stingrays exhibit hyostylic jaw suspension, which means that the mandibular arch is only suspended by an articulation with the hyomandibula. This type of suspensions allows for the upper jaw to have high mobility and protrude outward. The teeth are modified placoid scales that are regularly shed and replaced. In general, the teeth have a root implanted within the connective tissue and a visible portion of the tooth, is large and flat, allowing them to crush the bodies of hard shelled prey. Male stingrays display sexual dimorphism by developing cusps, or pointed ends, to some of their teeth. During mating season, some stingray species fully change their tooth morphology which then returns to baseline during non-mating seasons.
Spiracles Spiracles are small openings that allow some fish and amphibians to breathe. Stingray spiracles are openings just behind its eyes. The respiratory system of stingrays is complicated by having two separate ways to take in water to use the oxygen. Most of the time stingrays take in water using their mouth and then send the water through the gills for gas exchange. This is efficient, but the mouth cannot be used when hunting because the stingrays bury themselves in the ocean sediment and wait for prey to swim by. So the stingray switches to using its spiracles. With the spiracles, they can draw water free from sediment directly into their gills for gas exchange. These alternate ventilation organs are less efficient than the mouth, since spiracles are unable to pull the same volume of water. However, it is enough when the stingray is quietly waiting to ambush its prey. The flattened bodies of stingrays allow them to effectively conceal themselves in their environments. 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 after capture; instead, they use smell and electroreceptors (ampullae of Lorenzini) similar to those of sharks. Stingrays settle on the bottom while feeding, often leaving only their eyes and tails visible. Coral reefs are favorite feeding grounds and are usually shared with sharks during high tide.
Behavior
Reproduction
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