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The aim of this study was to determine whether magnesium (Mg 21 ) in seawater is required for egg activation of the black tiger shrimp Penaeus monodon and whether manipulation of Mg 21 levels can be used to delay the process and thereby synchronize egg activation. Female P. monodon broodstock were allowed to spawn in arti¢cial seawater containing Mg 21 at varying levels with respect to the normal (100%) level: 100%, 50%, 20% and 0%. Egg activation occurred normally at 100% Mg 21 , incompletely at 50% and 20% Mg 21 levels and did not occur at all with 0% Mg 21 . The fertilization rate with 100% Mg 21 was observed to be 83%, but fertilization failed to take place in all the other groups. The fertilization rate was restored from 0% to 76% following the 20% Mg 21 level treatment when Mg 21 levels returned to normal (100%) as soon as spawning was completed. This study suggests that the level of Mg 21 in seawater plays a vital role in P. monodon egg activation, and that commencement of this process could be delayed by manipulation of the Mg 21 level during and immediately after spawning.

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