UV-absorbing substances in zooxanthellate and azooxanthellate clams |
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Authors: | M Ishikura C Kato T Maruyama |
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Institution: | (1) Marine Biotechnology Institute, Shimizu Laboratories, 1900 Sodeshi-cho, Shimizu-shi, Shizuoka 424, Japan, JP;(2) Deep Star Group, Japan Marine Science and Technology Center, Natushima-cho, Yokosuka, Kanagawa 237, Japan, JP |
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Abstract: | The effects of UV-A and UV-B radiation on photosynthesis of zooxanthellae 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 min) completely suppressed photosynthesis of the isolated zooxanthellae, it had little effect on their photosynthetic
ability if they were irradiated while within the siphonal mantle of the host tissue. Chemical analysis of the siphonal mantle
of T. crocea showed the presence of significant amounts of mycosporine-like amino acids (MAAs), which absorb UV-A and -B light. However,
no MAA was detected in the isolated zooxanthellae. MAAs were concentrated in the siphonal mantle and kidney tissues in comparison
with other tissues. In the siphonal mantle, MAA concentrations were the highest in the outermost surface layer where most
of the zooxanthella cells resided. This indicates that the zooxanthellae are protected from UV 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, Hippopus hippopus, Colculum cardissa and Fragum unedo, but also in a closely related azooxanthellate clam, Vasticardium subrugosum. On the other hand, no MAA was detected in any of the zooxanthellae from these zooxanthellate clams. No MAA was detected
in the tissues of a deep-sea bivalve, Calyptogena soyoae. Although MAAs seem to block strong UV radiation 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 from food and accumulated in their tissues, especially in the
siphonal mantle and kidney.
Received: 29 November 1996 / Accepted: 13 January 1997 |
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