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First published online March 9, 2004
Journal of Experimental Biology 207, 1399-1413 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00895
Physiological, biochemical and morphological indicators of osmoregulatory stress in `California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) exposed to hypersaline water
1 Department of Zoology, University of British Columbia, 6270 University
Boulevard, Vancouver, BC V6T 1Z4 Canada
2 Department of Biology, San Diego State University, 5500 Campanile Road,
San Diego, CA 92182 USA
3 Department of Biology, University of San Diego, 5998 Alcala Park, San
Diego CA, 92110 USA
* Author for correspondence (e-mail: sardella{at}zoology.ubc.ca)
Accepted 23 January 2004
The salinity tolerance of the `California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum), a current inhabitant of the hypersaline Salton Sea in California, USA, was investigated to identify osmoregulatory stress indicators for possible use in developing a model of salinity tolerance. Seawater-acclimated (35 g l–1) tilapia hybrids were exposed to salinities from 35–95 g l–1, using gradual and direct transfer protocols, and physiological (plasma osmolality, [Na+], [Cl–], oxygen consumption, drinking rate, hematocrit, mean cell hemoglobin concentration, and muscle water content), biochemical (Na+, K+-ATPase) and morphological (number of mature, accessory, immature and apoptotic chloride cells) indicators of osmoregulatory stress were measured. Tilapia tolerated salinities ranging from 35 g l–1 to 65 g l–1 with little or no change in osmoregulatory status; however, in fish exposed to 75–95 g l–1 salinity, plasma osmolality, [Na+], [Cl–], Na+, K+-ATPase, and the number of apoptotic chloride cells, all showed increases. The increase in apoptotic chloride cells at salinities greater than 55 g l–1, prior to changes in physiological and biochemical parameters, indicates that it may be the most sensitive indicator of osmoregulatory stress. Oxygen consumption decreased with salinity, indicating a reduction in activity level at high salinity. Finally, `California' Mozambique tilapia have a salinity tolerance similar to that of pure Mozambique tilapia; however, cellular necrosis at 95 g l–1 indicates they may be unable to withstand extreme salinities for extended periods of time.
Key words: `California' Mozambique tilapia, Oreochromis mossambicus x O. urolepis hornorum, osmoregulatory stress, Salton Sea, chloride cell, salinity challenge
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