Creatures of the Amazon
Fresh Water Stingrays

By Mike Collis

Most people think of stingrays as fish of the ocean but here in the Amazon there are numerous species of freshwater stingrays. Some other major rivers around the world have ray populations but most freshwater ray species are found in Amazonia. New species of stingrays are being discovered all the time. In 2010 two new stingray species were found on the Nanay River not far from Iquitos. There were perfectly round with a diameter of about 5 feet. They were named pancake stingrays because of their round shape. Stingrays can be found all along the Amazon and its tributaries, from Peru and Colombia in the West to the mouth of the Amazon in North Eastern Brazil. Some ray species are also found in other tropical South American rivers with no direct connection to the Amazon. Stingrays are a very ancient species, tracking their evolutionary history as far back as 300 million years. The most commonly found stingrays in South American rivers are Potamotrygon. It is thought that all of these Amazon rays are closely related to Pacific Marine Rays. Their isolation would have occurred when the Andes Mountains rose up about 15 million years ago, blocking the westward flow of the river to the Pacific Ocean as it was then and forcing it to flow east all the way to the Atlantic. This trapped many rays in the new system. This isolation and the Amazon's tropical climate and seasonal changes in water levels created ideal circumstances and great pressure for evolutionary changes, as represented by the huge variety of stingrays found in this one river system today. Even individual species that are found along the whole river, such as Motoros and Histrix, are polymorphic, each displaying their own wide range of colours and patterns as habitat and diet change subtly between regions.

All stingrays have their own special habits including feeding likes and dislikes. Stingrays can be found in lakes, fast moving shallow streams and big slow moving rivers. They can grow to an enormous size. Amazon stingrays of 250lbs and more are common. Some people claim that they have seen specimens of over 1,000lbs. Stingrays like shallow water and whenever local people go swimming in the rivers they always shuffle their feet when moving around in shallow water. This disturbance usually startles the ray and it swims harmlessly away. If you tread on a ray, it will strike with its tail which has a rudimentary stinger with venom that would make the strongest of people wither and cry. Stingrays are so-called for their serrated, dagger-like stinger located on the top of and lying flat towards the tip of the tail. Usually sheathed in a thin layer of skin and not always easy to see, this effective defence weapon is made of a protein complex, and is accompanied by a nasty venom usually released when the skin sheath of the stinger is broke open. If the stinger cuts you it can cause large blisters and an intense burning, throbbing pain. The immediate treatment for this is to immerse the wound in water as hot as can be tolerated, which helps to neutralize the toxins. This is not always possible, especially on the river so the traditional first aid treatment is to urinate on the wound which works because urine is both hot, sterile and slightly acidic. With treatment, the sting and swelling goes down. If you are lucky there will only be a small spot of necrosis that will go away in a few weeks time. Some ray populations are booming to the point where they are considered a pest on the sandy beaches of the Amazon. People are employed on popular beaches in Peru and Brazil to keep them clear of the rays, which tend to gather during the day in shoals and bask just under the sand in shallow water, just the place where bathers walk.

Gender identification is easy. Just like their relatives the sharks and marine rays, male freshwater stingrays have claspers, one on either side of the base of the tail and slightly underneath. In sexually mature males these are easily seen from above and with immature young males they can be seen from their underside. Females do not have claspers. When mature enough to breed rays reproduce by internal fertilization and give birth to live young after a gestation period of approximately 3 months. 'Pups' are usually born in litters of from 2 to 8, depending upon the age and size of their parents.

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