Stingray
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| Stingrays Fossil range: Late Cretaceous–Recent[1] | ||||||||||||
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 Southern stingray, Dasyatis americana | ||||||||||||
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| Dasyatis |
The stingrays are a family—Dasyatidae—of rays, cartilaginous fishes related to sharks. They are common in coastal tropical and subtropical marine waters throughout the world, but the family also includes species found in warmer temperate oceans such as Dasyatis thetidis, and species entirely restricted to fresh water such as D. laosensis and Himantura chaophraya. With the exception of Pteroplatytrygon violacea, all dasyatids are demersal.[2]
They are named after the barbed stinger (actually a modified dermal denticle) on their tail, which is used exclusively in self-defense. The stinger may reach a length of approximately 35 cm, and its underside has two grooves with venom glands.[3] The stinger is covered with a thin layer of skin, the integumentary sheath, in which the venom is concentrated.[4] Some species have several stingers, and a few, notably Urogymnus asperrimus, lack a sting entirely.[5]
Other types of rays also referred to as "stingrays" are the river stingrays (family Potamotrygonidae), the round stingrays (families Urolophidae and Urotrygonidae), the sixgill stingray (family Hexatrygonidae), and the deepwater stingray (family Plesiobatidae). For clarity, the members of the family Dasyatidae are sometimes called whip-tail stingrays.[6]
While most dasyatids are relatively widespread and not currently threatened, there are several species (for example Taeniura meyeni, D. colarensis, D. garouaensis, and D. laosensis) where 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.[7]
Contents[hide] |
[edit] Behavior
[edit] Feeding
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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 electro-receptors (ampullae of Lorenzini) similar to those of sharks. Stingrays feed primarily on mollusks, 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.
[edit] Reproduction
When a male is courting a female, he will follow her closely, biting at her pectoral disc.
Stingrays are ovoviviparous, bearing live young in "litters" of five to thirteen. 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".[8]
[edit] Stingray injuries
Dasyatids generally do not attack aggressively or even actively defend themselves. When threatened, their primary reaction is to swim away. However, when attacked by predators or stepped on, the stinger in their tail is whipped up. This is normally ineffective against sharks, their main predator.[9]
Depending on the size of the stingray, humans are usually stung in the lower limb region. Stings usually occur when swimmers or divers accidentally step on a stingray,[10] but a human is less likely to be stung by simply brushing against the stinger. Surfers and those who enter waters with large populations of stingrays have learned to slide their feet through the sand rather than taking steps, as the rays detect this and swim away. Stamping hard on the bottom as one treads through murky water will also cause them to swim away. Humans who harass stingrays have been known to be stung elsewhere, sometimes leading to fatalities. The stinger usually breaks off in the wound. This is not fatal to the stingray as it will be regrown at a rate close to that of human fingernails (about 1.25 to 2 cm per month). Contact with the stinger causes local trauma (from the cut itself), pain, swelling, and muscle cramps from the venom, and possible later infection from bacteria.[10] Immediate injuries to humans include, but are not limited to: poisoning, punctures, severed arteries, and sometimes death.[11] Fatal stings are very rare, but can happen,[10] famously including Steve Irwin.[12]
Treatment for stings includes application of hot water (optimum temperature is 45 °C (113 °F), taking care not to cause thermal burns),[10] which helps ease pain by denaturing the complex venom protein, and antibiotics. Immediate injection of a local anesthetic in and around the wound, or a regional nerve blockade, can be helpful, as can the use of parenteral opiates such as intramuscular pethidine.[10] Local anesthetic may bring almost instant relief for several hours. Vinegar and papain are ineffective. Pain normally lasts up to 48 hours, but is most severe in the first 30–60 minutes and may be accompanied by nausea, fatigue, headaches, fever, and chills. All stingray injuries should be medically assessed;[10] the wound must be thoroughly cleaned, and surgical exploration is often required to remove any barb fragments remaining in the wound. Following cleaning, an ultrasound is helpful to confirm removal of all the barb fragments.[13] Not all remnants are radio-opaque; but x-ray radiography imaging may be helpful where ultrasound is not available.[10]
