UV-ABSORBING SUBSTANCES IN ZOOXANTHELLATE AND AZOOXANTHELLATE CLAMS

The effects of UV-A and UV-B radiation on photosynthesis of zooxanthel lae within the siphonal mantle of the giant clam, Tridacna crocea, and in isolation were studied. While UV-B irradiation (2.4 W m(-2), 20 mi n) completely suppressed photosynthesis of the isolated zooxanthellae, it had little effect on their photosynthetic ability if they were irr adiated while within the siphonal mantle of the host tissue. Chemical analysis of the siphonal mantle of T. crocea showed the presence of si gnificant amounts of mycosporine-like amino acids (MAAs), which absorb UV-A and -B light. However, no MAA was detected in the isolated zooxa nthellae. MAAs were concentrated in the siphonal mantle and kidney tis sues in comparison with other tissues. In the siphonal mantle, MAA con centrations were the highest in the outermost surface layer where most of the zooxanthella cells resided. This indicates that the zooxanthel lae are protected from W radiation by a screen of concentrated MAAs in the host clam. Aside from T. crocea, significant amounts of MAAs were found not only in other zooxanthellate clams, such as T. derasa, Hipp opus hippopus, Colculum cardissa and Fragum unedo, but also in a close ly related azooxanthellate clam, Vasticardium subrugosum. On the other hand, no MAA was detected in any of the zooxanthellae from these zoox anthellate clams. No MAA was detected in the tissues of a deep-sea biv alve, Calyptogena soyoae. Although MAAs seem to block strong UV radiat ion in the shallow-water clam, they are probably not essential for the clam's life in the dark. MAAs in shallow-water clams may be derived f rom food and accumulated in their tissues, especially in the siphonal mantle and kidney.