EFFECT OF ENVIRONMENTAL CALCIUM LEVELS ON CALCIUM-UPTAKE IN TILAPIA LARVAE (OREOCHROMIS-MOSSAMBICUS)

Effects of environmental calcium concentrations on the survival, growt h, body calcium content and calcium uptake kinetics in developing tila pia (Oreochromis mossambicus) larvae were studied. Fertilized eggs wer e incubated in high- and low-calcium artificial freshwater (0.88-0.96 mmol l(-1) vs. 0.02-0.03 mmol l(-1) CaCl2 or CaSO4) until 3 days after hatching. Tilapia larvae showed similar hatching rates and wet weight s in either high- or low-calcium medium, indicating neither the develo pment nor the growth in tilapia larvae was affected by the environment al calcium levels. The body calcium content in low-calcium groups was about 90-95% that of high-calcium groups, No matter what calcium sourc e was used (CaCl2 or CaSO4), acclimation to low calcium medium caused a stimulation of calcium uptake (measured in 0.2 mmol l(-1) calcium), i.e., 1.2-1.3 fold higher than that of high calcium groups. This enhan ced calcium uptake capacity was characterized by a 50% decrease in K-m and a 25% increase in J(max). Effect of different calcium salts on ca lcium influx was significant only in low calcium level, i.e., calcium influx in low-CaCl2 group higher than that in low-CaSO4 group. These r esults suggest that tilapia larvae are able to modulate their calcium uptake mechanism to maintain normal body calcium content and growth in environments with different levels of calcium.