Uptake, distribution and excretion of magnesium inOreochromis mossambicus: dependence on magnesium in diet and water.

The euryhaline Mozambique tilapia (Oreochromis mossambicus) shows a more marked ability to adapt to low magnesium levels in food and water than many other fish species. Nonetheless, the internal distribution of magnesium is altered under low-magnesium conditions. The amount of magnesium in scales, and to a lesser extent the vertebral bone, is reduced and hence serve as magnesium reservoirs. The magnesium concentration of muscle is only marginally reduced by low external magnesium, suggesting that magnesium is partitioned to ensure normal muscle functioning. The water magnesium is of vital importance, as exposure to low-magnesium water markedly deminished the ability of tilapia to adapt to low-magnesium feeding. However, magnesium intake from the water, eithervia the integument or drinking, does not increase in lowmagnesium fed fish, despite an increased opercular chloride cell density. The growth related magnesium accumulation of tilapia under low-magnesium conditions approximates the total intake of the element (from the food and from the water), indicating that magnesium losses are minimized and that the magnesium absorption from the gastrointestinal tract may be highly efficient and very important.