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First published online July 23, 2003
The osmotic response of the Asian freshwater stingray (Himantura signifer) to increased salinity: a comparison with marine (Taeniura lymma) and Amazonian freshwater (Potamotrygon motoro) stingrays
1 Department of Biological Science, National University of Singapore, Kent
Ridge, Singapore 117543, Republic of Singapore
2 Natural Sciences, National Institute of Education, Nanyang Technological
University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore
3 Department of Zoology, University of Guelph, Guelph, Ontario, Canada, NIG
2W1
* Author for correspondence (e-mail: dbsipyk{at}nus.edu.sg)
Accepted 19 May 2003
The white-edge freshwater whip ray Himantura signifer can survive
in freshwater (0.7
) indefinitely or in brackish water (20) for
at least two weeks in the laboratory. In freshwater, the blood plasma was
maintained hyperosmotic to that of the external medium. There was
approximately 44 mmol l-1 of urea in the plasma, with the rest of
the osmolality made up mainly by Na+ and Cl-. In
freshwater, it was not completely ureotelic, excreting up to 45% of its
nitrogenous waste as urea. Unlike the South American freshwater stingray
Potamotrygon motoro, H. signifer has a functional ornithine-urea
cycle (OUC) in the liver, with hepatic carbamoylphosphate synthetase III (CPS
III) and glutamine synthetase (GS) activities lower than those of the marine
blue-spotted fan tail ray Taeniura lymma. More importantly, the
stomach of H. signifer also possesses a functional OUC, the capacity
(based on CPS III activity) of which was approximately 70% that in the liver.
When H. signifer was exposed to a progressive increase in salinity
through an 8-day period, there was a continuous decrease in the rate of
ammonia excretion. In 20 water, urea levels in the muscle, brain and
plasma increased significantly. In the plasma, osmolality increased to 571
mosmol kg-1, in which urea contributed 83 mmol l-1.
Approximately 59% of the excess urea accumulated in the tissues of the
specimens exposed to 20 water was equivalent to the deficit in ammonia
excretion through the 8-day period, indicating that an increase in the rate of
urea synthesis de novo at higher salinities would have occurred.
Indeed, there was an induction in the activity of CPS III in both the liver
and stomach, and activities of GS, ornithine transcarbamoylase and arginase in
the liver. Furthermore, there was a significant decrease in the rate of urea
excretion during passage through 5, 10 and 15 water.
Although the local T. lymma in full-strength sea water (30)
had a much greater plasma urea concentration (380 mmol l-1), its
urea excretion rate (4.7 µmol day-1 g-1) was
comparable with that of H. signifier in 20 water. Therefore,
H. signifer appears to have reduced its capacity to retain urea in
order to survive in the freshwater environment and, consequently, it could not
survive well in full-strength seawater.
Key words: ammonia, amino acid, elasmobranch, freshwater stingray, Himantura signifer, ornithine-urea cycle, osmoregulation, Potamotrygon motoro, stingray, Taeniura lymma, urea, urea transporter
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