Temporal range: Early Cretaceous–Recent
|Common stingray (female)|
Stingrays are a group of sea rays, which are cartilaginous fish related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes and consist of eight families: Hexatrygonidae (sixgill stingray), Plesiobatidae (deepwater stingray), Urolophidae (stingarees), Urotrygonidae (round rays), Dasyatidae (whiptail stingrays), Potamotrygonidae (river stingrays), Gymnuridae (butterfly rays), and Myliobatidae (eagle rays).
Stingrays are common in coastal tropical and subtropical marine waters throughout the world. Some species, such as Dasyatis thetidis, are found in warmer temperate oceans, and others, such as Plesiobatis daviesi, are found in the deep ocean. The river stingrays, and a number of whiptail stingrays (such as the Niger stingray), 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.
There are about 220 known stingray species organized into 10 families and 29 genera. Stingray species are progressively becoming threatened or vulnerable to extinction, particularly as the 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.
Jaw and teeth
The mouth of the stingray is located on the ventral side of the vertebrate. Stringrays exhibit euhyostyly 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 shedded 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 cusp, 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.
Stingrays can breathe through their spiracles, which are openings just behind their eyes. The respiratory system of stingrays is complicated by having two separate ways to take in water to utilize 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.
During the breeding season, males of various stingray species such as Urolophus halleri, may rely on their ampullae of Lorenzini to sense certain electrical signals given off by mature females before potential copulation. When a male is courting a female, he follows 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".
The stingray uses its paired pectoral fins for moving about. This is in contrast to sharks and most other fishes, which get most of their swimming power from a single caudal (tail) fin. Stingray pectoral fin locomotion can be divided into two categories: undulatory and oscillatory. Stingrays who use undulatory locomotion have shorter thicker fins for slower motile movements in benthic areas. Longer thinner pectoral fins make for faster speeds in oscillation mobility in pelagic zones. Visually distinguishable oscillation has less than one wave going, opposed to undulation having more than one wave at all times.
Feeding behavior and diet
Stingrays employ a wide range of feeding strategies. Some have specialized jaws that allow them to crush hard mollusk shells, whereas others use external mouth structures called cephalic lobes to guide plankton into their oral cavity. Benthic stingrays (those that reside on the sea flooor) are ambush hunters. They wait until prey comes near, then use a strategy called "tenting". With pectoral fins pressed against the substrate, the ray will raise its head, generating a suction force that pulls the prey underneath the body. This form of whole-body suction is analogous to the buccal suction feeding performed by ray-finned fish. Stingrays exhibit a wide range of colors and patterns on their dorsal surface that to help them camouflage with the sandy bottom. Some stingrays can even change color over the course of several days to adjust to new habitats.
Most stingrays feed primarily on mollusks, crustaceans, and occasionally on small fish. Freshwater stingrays in the amazon feed on insects and break down their tough exoskeletons with mammal-like chewing motions. Large pelagic rays like the Manta use ram feeding to consume vast quantities of plankton and have been seen swimming in acrobatic patterns through plankton patches.
Stingrays are not usually aggressive and attack humans only when provoked, such as 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, small 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 fungi. 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.
The venom of the stingray has been relatively unstudied due to the mixture of venomous tissue secretions cells and mucous membrane cell producs that occurs upon secretion from the spinal blade/stinger. Stingrays can have anywhere between 1-3 blades. The spine is covered with the epidermal skin layer. During secretion, the venom punctures through the epidermis and mixes with the mucus to release the venom on its victim. Typically, other venomous organisms create and store their venom in a gland. The stingray is notable in that it stores its venom within tissue cells. The toxins that have been confirmed to be within the venom are cystatins, peroxiredoxin, and galectin. Galectin induces cell death in its victims and cystatins inhibit defense enzymes. In humans, these toxins lead to increased blood flow in the superficial capillaries and cell death. Despite the number of cells and toxins that are within the stingray, there is little relative energy required to produce and store the venom.
The venom is produced and stored in the secretory cells of the vertebral column at the mid-distal region. These secretory cells are housed within the ventrolateral grooves of the spine. The cells of both marine and freshwater stingrays are round and contain a great amount of granule-filled cytoplasm. The cells of marine stingrays are located only within these lateral grooves of the stinger. The cells of freshwater stingray branch out beyond the lateral grooves to cover a larger surface area along the entire blade. Due to this large area and an increased number of proteins within the cells, the venom of freshwater stingrays has a greater toxicity than that of marine stingrays.
Rays are edible, and may be caught as food using fishing lines or spears. Stingray recipes abound throughout the world, with dried forms of the wings being most common. For example, in Malaysia and Singapore, stingray is commonly grilled over charcoal, then served with spicy sambal sauce, or soy sauce. Generally, the most prized parts of the stingray are the wings (flaps is the proper terminology), the "cheek" (the area surrounding the eyes), and the liver. The rest of the ray is considered too rubbery to have any culinary uses.
Stingrays are usually very docile and curious, their usual reaction being to flee any disturbance, but they sometimes brush their fins past any new object they encounter. Nevertheless, certain larger species may be more aggressive and should be approached with caution, as the stingray's defensive reflex (use of its poisoned stinger) may result in serious injury or death.
The skin of the ray is used as an under layer for the cord or leather wrap (known as ito in Japanese) on Japanese swords due to its hard, rough, skin texture that keeps the braided wrap from sliding on the handle during use. They are also used to make exotic shoes, boots, belts, wallets, jackets, and cellphone cases.
Several ethnological sections in museums, such as the British Museum, display arrowheads and spearheads made of stingray stingers, used in Micronesia and elsewhere. Henry de Monfreid stated in his books that before World War II, in the Horn of Africa, whips were made from the tail of big stingrays, and these devices inflicted cruel cuts, so in Aden, the British forbade their use on women and slaves. In former Spanish colonies, a stingray is called raya látigo ("whip ray").
Monfreid also wrote in several places about men of his crew suffering stingray wounds while standing and wading into Red Sea shallows to load or unload smuggled wares: he wrote that to "save the man's life", searing the wound with a red-hot iron was necessary.
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. Permineralized stingray teeth have been found in sedimentary deposits around the world, including fossiliferous outcrops in Morocco.
Spotted eagle rays are widespread along Atlantic coasts
Bull rays are found along European and African coasts
Bat rays are found around the Galápagos Islands and along the US west coast
Ocellate river stingrays are found in South America rivers
Diver interacting with a short-tail stingray
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