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In waters 5 2 m deep, green colonies of reef-building Porites astreoides Lamarck are significantly more abundant than their brown counterparts. To determine whether this distributional pattern reflects differences in the ability of green and brown colonies to tolerate high intensities of ultraviolet-A (320-400 nm) and -B (280-320 nm) radiation, I enhanced UV intensities by transplanting colonies of each color from 6 to 1 m deep. After 104 d, brown P. astreoides exposed to UV radiation at 1 m exhibited algal mitotic indices and linear skeletal extensions that were significantly lower than brown conspecifics shielded from UV light. In contrast, green P. astreoides were unaffected by UV radiation incident at 1 m. These morphspecific differences in UV tolerance corresponded with variation in the quantities of UV light-absorbing mycosporinelike amino acids (MAAs) present in the corals. Both morphs had mycosporine-glycine (A,,, = 3 10 nm), palythine (X,,, = 320 nm), asterina-330 (X,,, = 330 nm), and shinorine (X,,, = 334 nm), but green P. astreoides had significantly greater concentrations of aster-ma-330 than brown colonies both before and after transplantation. Increasing the concentration of a single MAA with a broad absorbance range may represent an effective means of countering high UV intensities, and UV light may be an important abiotic factor structuring the shallow-water distribution of P. astreoides.

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