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Ecology of Stingrays
The scientific classification, family name, for stingrays make up the Dasyatidae. The freshwater stingrays of South America make up the family Potamotrygonidae. Dasyatidae is a family of rays, cartilaginous marine fishes, related to skates and sharks. The subcategories of Dasyatidae are Dasyatis, Himantura, Pastinachus sephen and Taeniura. (Jonna)
There are more than five hundred different species of skates and rays in our oceans. Stingrays and skates are similar species that are closely related to sharks. All three are known as batoids. They are all vertebrates, because they have backbones. Their skeletons are made of cartilage instead of bones. Cartilaginous fish belong to the class of fishes known as Chondrichthyes.
Skates and rays may be related to sharks, but their bodies look like theyÕre from different families. Sharks are shaped to glide like submarines through deep water. Skates and rays are flat so they can skim the ocean floor. Sharks use their tails to steer. Skates and rays use their large fins. There is much controversy as to how much different skates and rays weigh. There is the largest ray, the manta ray, which weighs up to 2,860 pounds; that is about as much as a young elephant. On the small side, the clearnose skate usually weighs only a little over two to four pounds. (Wigley)
Usually skates and rays are peaceful, but if threatened or scared they know how to defend themselves. Electric rays can give large electric shocks. Stingrays have a barbed, stinging tail. Eagle rays have one poisonous bony spine that they can use to pierce predators. Many types of skates have rows of spines on their back that can stab and tear oneÕs skin.
Stingrays are carnivores, eating other animals to survive. Their diet is typically hard- shelled invertebrate such as snails, small fish, mullusks and crustaceans . Many times they hide under the sand; then trap their prey against the ocean floor from above with their bodies. They have flat, hard teeth to crush the shells. (Encyclopedia of Animals)
Skates and rays are native to the Atlantic, Pacific, and Indian Oceans. They can be found in tropical to cold temperate climates, living close to the surface or deep in the oceans. Skates and rays can also live where rivers meet the ocean. One difference between skates and stingrays is that at least one kind of stingray lives in freshwater in the South American rivers.
There are a few more differences between skates and stingrays.
Skates use electricity to recognize each other. All skates have an organ that produces a small amount of electric. Scientists think that different species may have different amounts of electricity and this may be how skates recognize one another. (Wigley) Other relationships that rays have are with remoras or suckerfish, they attached themselves to the ray, get protection and feed on material dropped by the ray.
Skates have small teeth, but stingrays are plate-like. Another difference is that one stingray family, known as freshwater stingrays, exist in lakes and rivers of South America.
Stingrays and skates are very different in the way they give birth. All skates lay eggs, which are leathery sacks very similar to shark egg capsules. These sacks sometimes wash up on beaches and are called mermaidÕs purses.
Stingrays are ovoviviparous, that is when the female carries eggs that hatch at the time of birth. After the stingrays mate; up to twelve eggs hatch inside the mother each year. While they are inside her body they feed off the egg sack and then from the blood vessels. Once they are born they are independent. (Encyclopedia of Animals) Baby rays are born in groups called ÒlittersÓ and have to fend for themselves as soon as they are born.
Skates give birth to fertilized eggs in an egg case. The case is often called a ÒmermaidÕs purse.Ó The mother skate deposits egg cases, which have tendrils that anchor them to seaweed or rocks. The mother does not protect the egg cases during the 6-9 months that the skates develop in their cases. Rays give birth to live young. These young are fully formed and can protect themselves. (Babel) Skates and rays develop slowly and can live as long as 50 years. Many of the species do not breed until after theyÕre eight years old.
Here are two similar incidents, but with two extremely different outcomes. Two men both stabbed in the heart by stingrays, one dies, one survives. Forty-four year old, Steve Irwin, known as the Crocodile Hunter, was filming an underwater video series off the north shore of Australia when he was struck in the heart with the serrated, poisonous barb of a stingray. Irwin immediately pulled the barb out; then died only minutes later. Approximately six weeks later, on October 28, 2006, James Bertakis, an eight-two year old man was near his home in Lighthouse Point, Florida, fishing on his boat, with two friends, when a spotted eagle stingray leaped out of the water and landed in his lap. When he grabbed it, the stingray lashed out with its tail. (Tampabay.com) Bertakis, like Irwin, was stabbed in the heart by a stingrayÕs razor-sharp, poisonous barb, however he survived. It makes you wonder how and why a man who had so much knowledge and experiences with animals would be the one to perish in this situation instead of the older man that had no knowledge or training.
And that is probably exactly why Steve Irwin was the one who died. He knew exactly what happened to him and instinctively pulled out the barb. Doctors believe that Steve Irwin may have had a better chance of survival had he not instantly removed the stingray barb from his chest. (Babel ) James Bertakis did not even know stingrays had barbs and was not even aware of what had happened. He did not realize the stingrayÕs barb had broken off into his heart when he grabbed it and threw the stingray into the boat. He only knew he had pain. Doctors choose to airlift him with the foot-long barb remained lodged in his chest until undergoing surgery to remove it. The barb punctured his lung and pierced his heart. The surgery took six hours.
Stingrays are cousins of sharks, but they are rarely a threat to humans and their small mouths are basically no threat at all. The tail of the stingray is capped with a spear or barb made of the same stuff that makes up shark scales, known as dermal denticles. The barb stiffens when the stingray feels threatened. The tip of a stingray's tail is serrated and has venom that can be deadly to predators. The venom is a mainly protein-based toxin that causes great pain and may also alter heart rate and respiration. Stingrays usually do not attack people. If a stingray is stepped on, it will utilize its spine as a form of defense. The spine is the protection for the ray. A stingray injury is usually painful, but not serious. If not treated properly or right away or the spine broke off in the skin; this could cause infection. The stingray is able to grow another spine if it loses one. (Encyclopedia of Animals)
There is an International Shark Attack File kept by the University of Florida, however there is no scientific record for the number of people injured or killed by stingrays. Experts believe the number of deaths by stingrays is low and the fatal encounters are extremely rare and only occur when the location of the wound affects vital organs.
Scientists have tagged skates and rays to monitor their actions and travels. Research suggests that these fish spend their lives in a small area. Disrupting their habitats may lead to their death, because they will not necessarily move elsewhere. (Wigley)
Stingrays and especially skates have slow growth rates and they mature late, low reproductive rates. As a result skates are vulnerable to overfishing and it appears that skates have been overfished and are suffering reduced population levels in many parts of the world.
A number of ray species are decreasing in population. The white skate and common skate are considered critically endangered. The undulate ray is endangered, the sandy ray is considered vulnerable, which means it may become endangered soon. A number of other types of rays are considered near vulnerable, including the thornback ray, shagreen ray, blonde ray, and smalleyed ray. (Gibson)
References
1. Babel, John S. Reproduction, life history, and ecology of the round stingray. 1967
2. Encyclopedia of Animals, EBSCO publishing. #9500100610.
3. Encyclopedia of Animals, EBSCO publishing. #9500100617.
4. Gibson, Jano, Sydney Morning Herald. Irwin might have survived: surgeon October 20, 2006.
5. Jonna, Ryan and Weinheimer, Monica, Family Dasyatidae, Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Dasyatidae.html
6. Tampabay.com, Man stabbed in chest, heart by stingray barb, The Tampa Tribune. October 20, 2006.
7. Wigley, Susan, NOAA Technical Memorandum NMFS NE 171, June 23,
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