Giant clams Tridacna crocea harbor symbiotic zooxanthellae in tubular systems and display high growth rate when exposed to light. Light/dark alternation plays an important role in growth performance and physiological change of giant clam–zooxanthellae symbiosis in natural ecosystems. In this study, photosynthetic performance, antioxidant enzymes, transcriptome, and expression of eight illumination-related genes were investigated in the mantle of giant clam at different treatment times of 12-h light/12-h dark (15,000 lx). (1) The effective photochemical quantum yield of PSII (Y(II)) was high in the early morning, but significantly decreased to a low level at 6 h of light treatment (P < 0.05), and recovered slightly thereafter during light, then dropped at night; (2) Superoxide dismutase (SOD) and catalase (CAT) activity in the clam mantle increased slightly, and peaked at 6 h of light treatment (201.23 U/mg protein and 20.95 U/mg protein, respectively). This was significantly different (P < 0.05) to the results of SOD and CAT activities, exposed to 12 h of dark treatment; (3) Six paired-end libraries were sequenced in 3 h of light and 12 h of dark treatments. Through the assembling of 310,548,208 of clean reads with lengths of 150 bp, a total of 463,645 unigenes were obtained, which mostly derived from giant clam. Bioinformatics analysis revealed that 14,036 genes were up-regulated and 44,361 genes were down-regulated in 3 h of light treatment; (4) The results of quantitative real-time PCR demonstrated that the expression related to circadian rhythm (Bmal1 and NFIL3), antioxidant enzymes (SOD and CAT) and sodium- and chloride-dependent taurine transporters (SLC6A6) of giant clam, as well as photosynthesis (psbA and rbcl) of symbiotic zooxanthellae peaked at ~ 6 h of light phase, except of psbA at 1 h of light treatment. This study provides information on the light-adaptive mechanisms of giant clam–zooxanthellae symbiosis during daily light and dark cycles.

大c蛤Tri科动物在管状系统中具有共生的黄原虫，暴露于光下时显示出高的生长速率。亮/暗交替在自然生态系统中对巨大的蛤-拟恶藻共生的生长性能和生理变化中起着重要作用。在这项研究中，研究了在12小时光照/ 12小时黑暗（15,000 lx）的不同处理时间下，在大蛤the地幔中的光合性能，抗氧化酶，转录组和八个照明相关基因的表达。（1）PSII（Y（II））的有效光化学量子产率在清晨时很高，但在光处理6 h时显着降低至低水平（P <0.05），此后在光照下略微恢复，然后在晚上下降；（2）蛤幔中的超氧化物歧化酶（SOD）和过氧化氢酶（CAT）活性略有增加，并在光照6 h达到峰值（分别为201.23 U / mg蛋白和20.95 U / mg蛋白）。这有很大的不同（P <0.05）暴露于黑暗处理12 h的SOD和CAT活性结果; （3）在3小时的光照和12小时的黑暗处理中对六个配对末端的文库进行测序。通过组装长度为150 bp的310,548,208个纯净读段，总共获得463,645个单基因，主要来自巨蛤。生物信息学分析表明，光处理3 h​​后有14036个基因上调，有44361个基因下调。（4）实时荧光定量PCR结果表明，其表达与昼夜节律（Bmal1和NFIL3），抗氧化酶（SOD和CAT）以及依赖钠和氯的牛磺酸转运蛋白（SLC6A6）有关。）和共生虫黄藻的光合作用（psbA和rbcl）在光照期约6 h达到峰值，但在光照1 h时psbA除外。这项研究提供了有关在每天的明暗循环中巨型蛤zo-黄恶藻共生的光适应机制的信息。