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The limits of osmoregulation : strategies for tolerance and acclimation of 'California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hormorum) to conditions of the Salton Sea Sardella, Brian


The Salton Sea is a large inland lake in southeastern California with salinity currently near 44 g/l that is increasing at a rate of 0.3 g/l annually. Along with salinity, large fluctuations in temperature and dissolved oxygen levels combine to make a very challenging environment that may be responsible for dramatic losses to the Salton Sea's once robust fishery. The dominant species within the fishery is a Mozambique tilapia hybrid (Oreochromis mossambicus x O. urolepis hornorum), which is well known both for euryhalinity and tolerance of extremely high salinities; as such, it provides a unique model for tolerance to hypersaline conditions as well as the interactions of multiple stressors such as those within the Salton Sea. In part one I describe two responses by tilapia to salinities greater than seawater at 25°C. When transfers were conducted below 60 g/l salinity, tilapia maintained osmotic balance without increasing drinking rate, mitochondrial-rich cell (MRC) turnover, or branchial Na⁺,K⁺-ATPase (NKA) activity. With additional increase above 60 g/l, these variables increased in similar fashion to that which has been described in other teleosts during acclimation to elevated salinity. These acclimation responses were defined as response I or response II, with a transition point between the two at 60 g/l. Tilapia exhibiting response I had a reduced whole animal oxygen consumption rate, as well as, liver and brain ATPase activity in proportion to salinity. In part two, I describe how changes in temperature affect the salinity tolerance of this species. Variation in temperature from 25°C to 15 or 35°C resulted in increased plasma osmolality and/or mortality, indicating a combined temperature/salinity stress is more challenging than salinity alone. Using tissue microarrays and laser scanning microscopy, I show that tilapia attempted to respond to the loss of osmotic balance in cold temperatures with MRC hypertrophy and enhanced NKA capacity.

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