Brazilian scientists have undertaken a study which shows why freshwater stingrays are so much more venomous than their marine counterparts.
While marine stingrays have small numbers of carefully placed venom-secreting cells, freshwater rays have venom producing cells covering the entire stinger, which makes the wounds they inflict much more painful.
Scientists working in the Laboratory of Cellular Biology at Sao Paulo's Instituto Butantan compared the morphology of the "stingers" of a number of Brazilian rays spanning marine and freshwater species and found some striking differences in the structure of their stingers.
Their findings are due to be published shortly in the journal Toxicon.
StingersMost species have between one and three stingers on the end of their tails, which are covered with special epidermal protein cells that secrete a toxic venom.
In marine species, the protein cells are located only around or inside special ventrolateral grooves which run along the sides of the stinger.
However, in freshwater stingrays, there are much greater numbers of protein secretory cells and they're spread over the entire surface of the epidermis, so even a small nick in the skin of the victim could see venom being introduced and a deep wound could see a high dose delivered.
Although stingrays are made up of soft, pliable cartilage, their stingers are mineralised, hard and serrated, which allows them to puncture and tear skin easily.
ReflexWhen stingrays are touched dorsally they can often use a reflex action to rapidly whip the tail upwards which can cause the victim to be pierced by the stinger.
Since rays tend to live buried beneath a layer of sand, they can be inadvertently stepped upon by people bathing or wading in areas where they occur.
The resulting wounds cause intense local pain, swelling, redness and secondary infection by bacteria.
Wounds from the freshwater stingrays are considered the worst of all and are often accompanied by tissue necrosis, which causes the flesh around the wound to be eaten away.
A number of freshwater stingray species are kept in aquaria.
For more information see the paper: Pedroso CM, Jared C, Charvet-Almeida P, Almeida MP, Neto DG, Lira MS, Haddad V, Barbaro KC, Antoniazzi MM (2007) - Toxicon. 2007 Jun 23.