The photosymbiosis of tropical giant clams (subfamily Tridacninae) with unicellular algae (Symbiodiniaceae) restricts their distribution to the sunlit, shallow waters of the euphotic zone where organisms are additionally exposed to potentially damaging levels of solar UV radiation. Metabolic and physiological responses of Red Sea Tridacna maxima clams, including net calcification and primary production, as well as valvometry (i.e., shell gaping behavior) were assessed when exposed to simulated high radiation levels received at 3 and 5 m underwater. The two levels of radiation included exposure treatments to photosynthetically active radiation (PAR; 400–700 nm) alone and to both, PAR and ultraviolet-B radiation (UV-B; 280–315 nm). The valvometry data obtained using flexible magnetic sensors indicated that specimens under PAR + UV-B exposure significantly reduced the proportion of their exposed mantle area, a potential photo-protective mechanism which, however, reduces the overall amount of PAR received by the algal symbionts. Consequently, specimens under PAR + UV-B displayed a slight, although non-significant, reduction in primary production rates but no signs of additional oxidative stress, changes in symbiont densities, chlorophyll content, or levels of mycosporine-like amino acids. Net calcification rates of T. maxima were not affected by exposure to UV-B; however, calcification was positively correlated with incident PAR levels. UV-B exposure changes the valvometry, reducing the exposed mantle area which consequently diminishes the available PAR for the photosymbionts. Still, T. maxima maintains high rates of primary production and net calcification, even under high levels of UV-B. This provides experimental support for a recently described, effective UV-defensive mechanism in Tridacninae, in which the photonic cooperation of the associated algal symbionts and giant clam iridocytes is assumed to establish optimal conditions for the photosynthetic performance of the clams’ symbionts.

中文翻译：

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浅礁中的巨蛤：红海砗磲亚科的抗紫外线机制

热带巨蛤（Tridacninae 亚科）与单细胞藻类（Symbiodiniaceae）的光共生将它们的分布限制在阳光充足的浅水区，在那里生物体额外暴露于具有潜在破坏性的太阳紫外线辐射水平。当暴露于水下 3 米和 5 米处的模拟高辐射水平时，评估了红海砗磲蛤的代谢和生理反应，包括净钙化和初级生产，以及瓣膜测量法（即壳张开行为）。两种辐射水平包括单独暴露于光合有效辐射（PAR；400-700 nm）以及同时暴露于 PAR 和紫外线 B 辐射（UV-B；280-315 nm）。使用柔性磁传感器获得的瓣膜测量数据表明，在 PAR + UV-B 暴露下的样本显着降低了暴露地幔面积的比例，这是一种潜在的光保护机制，然而，减少了藻类共生体接收的 PAR 总量。因此，在 PAR + UV-B 下的标本显示初级生产力略有下降，但不显着，但没有出现额外氧化应激、共生体密度变化、叶绿素含量或类菌孢素氨基酸水平的迹象。T. maxima 的净钙化率不受暴露于 UV-B 的影响；然而，钙化与发生的 PAR 水平呈正相关。UV-B 暴露改变了瓣膜测量，减少了暴露的地幔面积，从而减少了光共生体的可用 PAR。尽管如此，即使在高水平的 UV-B 下，T. maxima 仍能保持较高的初级生产率和净钙化率。这为最近描述的砗磲亚科中一种有效的紫外线防御机制提供了实验支持，其中假设相关藻类共生体和巨蛤虹彩细胞的光子合作为蛤蜊共生体的光合作用性能建立了最佳条件。