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First published online January 19, 2006
Journal of Experimental Biology 209, 484-492 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02002
Exposure to brackish water, upon feeding, leads to enhanced conservation of nitrogen and increased urea synthesis and retention in the Asian freshwater stingray Himantura signifer
1 Natural Sciences and Science Education, National Institute of Education,
Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic
of Singapore
2 Department of Biological Sciences, National University of Singapore, 10
Kent Ridge Road, Singapore 117543, Republic of Singapore
* Author for correspondence (e-mail: sfchew{at}nie.edu.sg)
Accepted 16 November 2005
The white-edge freshwater whip ray Himantura signifer is
ammonotelic in freshwater, but retains the capacities of urea synthesis and
ureosmotic osmoregulation to survive in brackish water. The first objective of
this study was to examine whether exposure to brackish water would lead to
increases in food intake, and/or conservation of nitrogen in H.
signifer upon daily feeding. Results obtained showed that a progressive
increase in ambient salinity, from 1
to 15 over a 10-day
period, did not lead to an increase in daily food intake. However, there were
significant reductions in daily rates of ammonia and urea excretion in H.
signifer during salinity changes, especially between day 5 (in 10
water) and day 10 (in 15 water) when compared to those of the control
kept in 1 water. Consequently, there was a significant decrease in the
percentage of nitrogen (N) from the food being excreted as nitrogenous waste
(ammonia-N+urea-N) during this period. On day 10, the tissue urea contents in
fish exposed to 15 water were significantly greater than those of fish
kept in 1 water, and the excess urea-N accumulated in the former fish
could totally account for the cumulative deficit in excretion of
urea-N+ammonia-N during the 10-day period. Thus, it can be concluded that
H. signifer is N-limited, and conserved more N from food when exposed
to brackish water. The conserved N was converted to urea, which was retained
in tissues for osmoregulation. The second objective of this study was to
elucidate whether the retention of the capacity of N conservation in H.
signifer would lead to an accumulation of urea in fish exposed to not
only 15 water, but also 1 water, upon feeding. For fish
pre-acclimated to 1 water or 15 water for 10 days and then
fasted for 48 h, the rate of ammonia excretion in fish exposed to 15
water was consistently lower than that of fish exposed to 1 water,
throughout the 36-h post-feeding period. In addition, the hourly rate of urea
excretion in the former was significantly lower than that of the latter
between hours 12 and 36. There were postprandial increases in ammonia contents
in the muscle, liver, stomach, intestine, brain and plasma of fish kept in
1 water; but postprandial increases in ammonia occurred only in the
liver and brain of fish exposed to 15 water, and the magnitudes of
increases in the latter were smaller than those in the former. Indeed,
postprandial increases in tissue urea contents occurred in both groups of
fish, but the greatest increase in urea content was observed in the muscle of
fish exposed to 15 water. Taken together, these results indicate that
H. signifer in freshwater could be confronted with postprandial
osmotic stress because of its capacity of conserving N and increasing urea
synthesis upon feeding.
Key words: ammonia, feeding, stingray, Himantura signifer, nitrogen metabolism, osmoregulation, urea
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