A comparison of two methods of obtaining densities of zooxanthellae in Acropora millepora

Abstract

Quantification of zooxanthellae densities in tissues of reef-building corals aids in the assessment of the extent and severity of coral bleaching. Various methods are available to quantify zooxanthellae densities; however, a direct comparison of these techniques has yet to be done. Here, we compare estimates of zooxanthellae densities obtained using conventional airbrushing coupled with post-tissue-blasting surface area determination, versus a technique whereby zooxanthellae densities are quantified from a known area (0.25 cm2) of tissue after corals have been fixed and decalcified. Estimates of zooxanthellae densities obtained were correlated across replicate colonies (R2＝0.40, n＝81), and both techniques revealed similar patterns of variation among locations. The main benefit of the decalcification technique was reduced processing time, because the technique eliminates the time-consuming process of tissue blasting and retrospective estimates of surface area. We estimate that decalcification halves the processing time per sample, and produces a more accurate estimate of zooxanthellae density.