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.