(Hildebrand & Schroeder, 1928)
|Range of the southern stingray|
The southern stingray (Dasyatis americana) is a whiptail stingray found in tropical and subtropical waters of the Western Atlantic Ocean from New Jersey to southern Brazil. It has a flat, diamond-shaped disc, with a mud brown, olive, and grey dorsal surface and white underbelly (ventral surface). The barb on its tail is serrated and covered in a venomous mucous, used for self-defense.
The southern stingray is adapted for life on the sea bed. The flattened, diamond-shaped body has sharp corners, making it more angular than the discs of other rays. The top of the body varies between olive brown and green in adults, dark grey in juveniles, whilst the underside is predominantly white. The wing-like pectoral fins are used to propel the stingray across the ocean bottom, whilst the slender tail possesses a long, serrated and venomous spine at the base, used for defence. These spines are not fatal to humans, but are incredibly painful if stepped on. The eyes are situated on top of the head of the southern stingray, along with small openings called spiracles. The location of the spiracles enables the stingray to take in water whilst lying on the seabed, or when partially buried in sediment. Water enters the spiracles and leaves through the gill openings, bypassing the mouth which is on the underside. Female stingrays can grow to a disc width of 150 cm, contrary to the smaller male stingrays that reach maximum size at 67 cm.
Southern stingrays are nocturnal predators, who spray water from their mouths or flap their fins vigorously to disturb the substrate and expose hidden prey. This bottom-dwelling species is often found singly or in pairs, and can reach population densities estimated up to 245 per km2 in certain shallow systems thought to be nursery grounds. Dasyatis americana exhibit wave-like locomotion using their pectoral fins. This wave-like motion is important for Dasyatis americana because it allows them to escape predators, forage efficiently, and generally maneuver quickly. Typically, they travel large distances and their foraging area is very expansive. One study provided observations that Dasyatis americana swim along the tide, because of the greater food availability along tides. Dasyatis americana are able to do this because of their high maneuverability and efficient wave-like locomotion. Dasyatis americana either remain solitary or form groups. Groups of Dasyatis americana are usually observed when they mate, for predator protection or even when they are just resting.
In one study, it was evident that when scientists revealed the contents of the stomach of one Dasyatis americana, they found evidence of a great variety of ingested prey (which represented a variety of phyla and families), such as small fishes, worms and crustaceans. As mentioned earlier in this article, the Dasyatis americana swim with a wave-like motion, thus making it easier for them to maneuver and help explain why their foraging area is so vast. They can be identified as opportunistic feeders and continuous foragers, since they exhibit continuous feeding of multiple organisms throughout the day (this helps to explain the stomach contents revealed in the previously mentioned study).
To avoid predators, Dasyatis americana bury and cover themselves in substrate. Their tails contain venom and are also utilized as protection from predators. Some predators of Dasyatis americana are humans and Hammerhead Sharks.
Roles within their ecosystems
In shallow waters, there is a commensal foraging relationship between Dasyatis americana fish and Phalacrocorax auritus birds in coastal areas generally like the Gulf of Mexico. When foraging, the Dasyatis americana dig through the substrate in search of food; however, this also helps to expose certain other fish hidden in the substrate after which the Phalacrocorax auritus will follow behind the Dasyatis americana and eat.
Dasyatis americana are ovoviviparous. Mother's bodies protect unborn offspring, while they are developing inside their mother's body. The embryos receive nutrients from the yolk sack early in development. Later in development, however, when the yolk sac is absorbed, the embryos obtain nutrients from the histotroph (the mother's uterine milk). After the Dasyatis americana offspring are born, and are outside of the mother's body, parental care ceases. In captivity, gestation lasted 135 to 226 days, after which a litter of two to ten young were born.
There is little knowledge or published evidence about the mating systems of Dasyatis americana. Mating stingrays are rarely encountered in the wild. One study, however, does provide detailed observations of Dasyatis americana mating. This study involves observations of one female mating with two males. The study mentions that the female was chased by the two males, with one of the male's biting (or "catching") the female's fin. After copulation, the male releases the female from his bite on her. In addition, soon after giving birth, Dasyatis americana females have the ability to mate again.
Sexual maturity and nursery type
Geographical location plays a large role in the age of sexual maturity. Observations from studies of breeding behavior (of Dasyatis americana during August at Bimini, Bahamas, and early September in Grand Cayman, Cayman Islands). One study shows that when females were placed in captivity, they were considered mature when they were impregnated (around 5 or 6 years old). In this case, males who were 3 or 4 years old were considered to be mature. There is also a difference in the rate at which the females bear young, depending on whether they are raised in captive natural environments or in natural environments. Females raised in captivity bear offspring twice a year, and females that are raised in the wild bear offspring once a year. In addition, there is a positive correlation between the size of the mother and the number of offspring. There is a difference in nurseries for where the Dasyatis americana offspring are raised: there are primary and secondary nurseries which have a clear distinction. The primary nursery is defined as a habitat where a female Dasyatis americana gives birth to her young. On the other hand, the secondary nursery is a habitat where the juvenile Dasyatis americana are raised to mature adults. Little evidence about locations of and migrations between the primary and secondary nurseries is known. An example of a primary nursery is in Belize, where Dasyatis americana females pay seasonal visits for the purposes of mating and giving birth to offspring. During one study, juvenile Dasyatis americana were caught by scientists at 10 to 20 m depths on rock reef surfaces nearby during the months of May, November and December. This specific location of where these juvenile Dasyatis americana were collected was believed to be a secondary nursery.
Studies of Dasyatis americana have shown that they communicate through pheromone signaling. Males communicate with females before copulating by touching and biting the females. Also, after the female gives birth, she releases pheromones that are most likely believed to be produced in her cloaca; one study reported that the birth of offspring attracted males. As previously mentioned in the article, since a female has the ability to mate soon after giving birth to her offspring, it is plausible that these are sex pheromones. The role of pheromones in communication also make sense since Dasyatis americana have strong senses of smell. They have many Ampullae of Lorenzini, usually heavily concentrated around the head. In addition, this gives them the ability to sense certain electric fields which are emitted from hidden prey. In addition, they have special mechanisms for senses vibrations in the water as well as for hearing.
In many parts of the Caribbean such as Grand Cayman, Cayman Islands and Antigua, the southern stingray swims with divers and snorkelers, and are hand fed on a locations such as Stingray City and the Sandbar. On Turks & Caicos, they can be hand fed at a location called Gibbs Cay. Some have become tame enough to be cradled in visitors' arms and feed with pieces of cut up fish. This docile and food-reward driven behaviour has led to many locals comparing the hand-fed and belly-rubbed stingray to an over fed household canine. There are concerns that this feeding, and the high levels of interaction with humans, may be having some negative impacts on the behaviour and ecology of the stingrays.
This article incorporates text from the ARKive fact-file "Southern stingray" under the Creative Commons Attribution-ShareAlike 3.0 Unported License and the GFDL.
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