MAGNESIUM OR MAGNESIUM TO CALCIUM RATIO?  EFFECTS OF DIFFERENT LEVELS IN LOW SALINITY WATER ON GROWTH AND SURVIVAL OF PACIFIC WHITE SHRIMP Litopenaeus vannamei

Magnesium (Mg²⁺) is the most abundant intracellular divalent cation with a central role in many cellular processes such as activation of a large number of enzymes, hormonal signaling, protein synthesis, cell division, etc., while calcium (Ca²⁺) is generally the most abundant cation found within the body of crustaceans. These elements also comprise a major portion of their exoskeleton, and greatly affect molting frequency, hemolymph osmolality and carapace mineralization. Scientists argue that shrimp uptake of Mg²⁺ may be influenced by the concentration of Ca²⁺ and vise-versa due to their chemical similarities by having the same electron configuration in the outermost shell. Inland low salinity aquifers such as those of west Alabama possess concentrations of Mg²⁺ and Ca²⁺ that vary widely. This culture water is quite deficient in Mg²⁺ while excess levels of Ca²⁺ have been recorded on most farms (Mg:Ca ratios of 0.5 - 1.0 or lower are quite common).

The present study was conducted as an incomplete 3x3 factorial design with three Mg²⁺ (101, 56, 28 mg/L) and Ca²⁺ (35, 105, 210 mg/L) levels at 3 g/L salinity (while keeping  the remaining  ionic concentrations constant) to determine the effect of different Mg²⁺ and Ca²⁺ combinations on the growth, survival and physiology of shrimp. However, due to a limitation of the research system, the treatment with the lowest Mg²⁺ and highest Ca²⁺ (28 and 210 mg/L, respectively) was dropped. A low-cost salt mixture (LCSM) was used (comprised of Na, K, Ca and Mg concentrations of 298, 9, 17, and 39 mg/L, respectively in a 1-g/L solution, which is closely comparable to the major cations in 1-g/L dilute seawater) to formulate waters with different Mg²⁺ and Ca²⁺ levels. Three growth trials (initial weight of 0.15 ± 0.02, 0.30 ± 0.02, 0.90 ± 0.02 g, respectively) were carried out in 150 L polyethylene tanks (stocking density = 20 shrimp/tank), each equipped with a miniature fluidized bed bio-filter. Shrimp were fed four times per day using a standard ration for 21-days in each trial and growth performance, survival, hemolymph osmolality, osmoregulatory capacity, ionic concentration in hemolymph and whole body of shrimp were measured at the end of each experiment. Results indicate treatments with 101 mg/L Mg²⁺ had significantly higher growth and survival irrespective to the level of Ca²⁺ in water, indicating no significant interaction between Ca²⁺ and Mg²⁺ or no significant effect of Ca²⁺ on growth and survival of shrimp.