STINGRAYS STAY OUT OF A PICKLE

Abstract

In the stingray world, Himantura signifer is a bit of a Cinderella compared to its well-known Amazonian cousin, Potamotrygon motoro. Living in the fresh waters of the Batang Hari river of Sumatra, Himantura passes the seasons moving up and down the river, but unlike its South American relative, it occasionally ventures far down stream, into brackish delta waters; an environment better suited to its ancient ancestors. How Himantura handles the tricky transition from fresh to salt water wasn't clear, so Yuen K. Ip and an international team of coworkers put the Asian elasmobranch through its osmoregulatory paces, to find out how it stays out of a pickle in salt water(p. 2931). Knowing that stingrays, which live in the open sea, maintain high levels of urea, Ip and his team monitored the Asian stingray's urea levels as they gently raised the salt concentration. Even in fresh water, the stingray had higher urea levels than other freshwater species, and as the team raised the salt levels,the stingray matched the rise by increasing the amount of urea in its blood. They also monitored the amount of urea and ammonia that the animals excreted as the salt concentration increased, and found both fell. Instead of dumping ammonia, they were using it to synthesise extra urea, adding it to the urea they also retained in their bodies to survive in the brackish water. But even though the fish cut their urea losses, they were never able to retain enough of the osmolyte to rejoin their ancestors out at sea.