Magnesium is involved in a variety of physiological processes in marine animals and is known to be deleterious in both excess and deficiency. The effects of magnesium concentration ranging from 700 mg/L (low), 1344 mg/L (control), and 2000 mg/L (high) on size and pulse rate in upside‐down jellyfish (Cassiopea andromeda) medusae were examined in two separate 28‐day trials. Exposure to low magnesium resulted in significantly (p < .05) higher pulse rates and decreased bell diameter and also produced oral arm degradation. Exposure to high magnesium resulted in significantly (p < .05) lower pulse rates and decreased bell diameter as well as oral arm cupping. In both low and high magnesium, almost all specimens changed color from pale blue on Day 1, to brown by Day 28, suggesting a loss of zooxanthellae. The decrease in bell diameter and color change was more pronounced and occurred more rapidly in low magnesium. The results of both trials demonstrate the deleterious effects of high and low magnesium on C. andromeda and emphasize the importance of monitoring magnesium concentration to maintain healthy display animals in public aquaria.

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