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To study the effects of sublethal doses of mercury on corals, colonies of Porites astreoides (Lamarck) were exposed to nominal concentrations of 0.01, 0.1 and 0.5 mg Hg l -1 using semi-static, chronic bioassays for up to 15 d, with HgCl2 administered by pulses every 3 d (mean con- centration in the water was 0.004, 0.037 and 0.180 mg Hg l -1 , respectively). While total Hg in the corals was directly proportional to Hg exposure, analysis of the different coral compartments (polyps, zooxanthellae and skeleton) showed that zooxanthellae and the skeleton accumulated Hg in direct relation to Hg exposure, but polyp tissue accumulated more Hg at 0.1 than at 0.5 mg Hg l -1 . This sug- gests saturation of Hg only in polyps and/or activation of mechanisms of detoxification. Within a colony, the Hg concentration per unit area of coral surface differed between compartments as follows: zooxanthellae > polyp > skeleton. Colonies exposed to the highest Hg concentration accumulated 1.738 µg Hg cm -2 , 89% of which was found in zooxanthellae, 7% in polyps and 4% in the skeleton. Polyp biomass (dry weight and protein content), zooxanthellae biomass (cell density and protein con- tent), and pigment concentration per unit area of coral surface decreased with Hg exposure. The bio- concentration factor ((Hg) in organism/(Hg) in water) was inversely related to the Hg concentration in water. The capacity of zooxanthellae and the skeleton to concentrate Hg and the decrease in zoo- xanthellae density support the hypothesis that polyps may divert Hg to these 2 coral compartments as a detoxifying mechanism.

Sublethal effects of mercury and its distribution in the coral Porites astreoides