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Endolithic algae inhabiting skeletons of living corals appear to be adapted to an extreme environment created by the coral. However, measurements on three coral species from the genus Porites revealed that these corals provide several modes of protection to the algae as well. High concentrations of ultraviolet (UV)-absorbing compounds, mycosporine-like amino acids (MAAs), were found in the tissues of all corals examined, but they were not detected in extracts of the endolithic algae. Coral tissues and skeleton filter 93.98-99.5% of the ambient UV radiation and thus shade the endolithic algae from this potentially damaging radiation. In addition endolithic algae are largely relieved from grazing pressure by herbivorous fish, because only 4% of fish bites on Porites corals resulted in exposed endolithic algae. Thus, the coral skeleton provides a refuge to the endolithic algae from some of the environmental pressures normally experienced by free-living algae on the reef.