Distribution Of Giant Clam In Coastal Waters Of Sanya And Illumination Adaptability Of Tridacna Crocea

Giant clams,including Tridacna and Hippopus,are a family of bivalve molluscs and inhabit shallow-water coral reef systems in Indo-Pacific tropical waters.There are currently 12 established members of the giant clam family Tridacnidae,in which eight species are distributed in China.As one of the key members in coral reef ecosystems,giant clams have important economic and ecological value.Since 1990s,though the giant clam population in China has been greatly reduced,the distribution of giant clam in the South China Sea,especially in coastal waters of Hainan Island,is still unknown.Furthermore,recent studies have showed that illumination was the key environmental factor to influence the growth and survival of giant clams.However,the mechanism of illumination adaptability of giant clams-zooxanthellae symbiosis remains unknown.In this study,1)the distribution,density and population structure of giant clam at Sanya Coral Reef National Nature Reserve and surrounding sea areas were researched by field cross-sectional survey;2)the metabolism,stable carbon and nitrogen isotope ratios,growth performance and biochemical composition of T.crocea-zooxanthellae symbiosis under different light intensities were researched;3)the photosynthetic performance,oxidative stress and illumination-related genes were investigated in the mantle of giant clam,in order to analyze the illumination adaptability of giant clams.（1）T.noae was the mainly giant clam species distributed in coastal waters of Sanya.the densities of T.noae in Yalong Bay（YB）and Wuzhizhou Island（WI）were2.00×10^-2 ind/m² and 1.92×10^-2 ind/m²,respectively,which were significantly higher than giant clams in Luhuitou Peninsula（LP）and Dongmao-Ximao Island（DXI）（P<0.05）;The depth of T.noae distribution in WI was mainly 2-4 m below mean sea level,while the depth of T.noae distribution in YB was 6-8 m below mean sea level;Four sizes of T.noae（shell length of<5 cm,5-10 cm,10-15 cm,and>15 cm）were found in WI,which meant its population structure was reasonable in this sea area;The favorite substrate of T.noae was the living coral reef,followed by the rock,then the sand;There was a significant correlation in RGB values between T.noae mantle and background colors（P<0.05）.（2）The optimum light intensity of T.crocea（shell length～82.0 mm）was 16,000 lux,in which condition the absorption rates of both ammonia and active phosphate were the highest;After being transferred into light condition,the absorption rates of ammonia and active phosphate were decreased with time,while the oxygen production rates showed the opposite phenomenon;There was significant relationship between assimilation metabolism and light spectra,and blue light was the best,then red light,and at last white light.（3）After 16-week rearing,the stable carbon and nitrogen isotope ratios of T.crocea adductor muscle with different size under different light intensities were detected.Theδ¹³C values of adductor muscle were-19.23‰～-17.94‰,which showed significant relationship with clam size（P<0.05）.Theδ¹⁵N values of adductor muscle were 3.23‰～4.53‰,which showed significant relationship with light intensity（P<0.05）.Theδ¹³C andδ¹⁵N values of symbiotic zooxanthellae separated from T.crocea mantle culrured in5,000,10,000 and 15,000 lux light intensities were-21.09‰/1.90‰,-19.36‰/5.81‰and-18.31‰/6.86‰,respectively.The differences ofδ¹³C between T.crocea and symbiotic zooxanthellae(δ¹³C_h-z)showed significant relationship with light intensity,5,000-lux treatemt>10,000-lux treatemt>15,000-lux treatemt（P<0.05）.（4）Two size classes of T.crocea（small:～50 mm shell length,large:～85 mm shell length）were cultured at three light intensities（5,000,10,000,and 15,000 lux）for 16weeks,to evaluate the effects of light intensity on the survival,growth performance and biochemical composition of different size clams;T.crocea under 15,000 lux light intensity showed a significantly higher growth rate(1.40 mm month^-1and 1.10 mm month^-1 for small and large giant clam groups,respectively)and survival rate（more than95%）.The content of eight amino acids in the mantle was significantly affected by light intensity（P<0.05）,while only one amino acid（glycine）was found in the adductor muscle（P<0.05）.Similarly,there were significant differences in ten mantle amino acids between small and large size groups（P<0.05）,while three amino acids were found in the adductor muscle（P<0.05）.C22:6n3（DHA）was the major polyunsaturated fatty acid（PUFA）in T.crocea.In the adductor muscle,the C22:6n3 profile significantly increased with increasing light intensity and clam size（P<0.05）,while no significant differences of C22:6n3 profiles were found among different light intensities in the mantle.（5）Light/dark alternation plays an important role in growth performance and physiological change of giant clam-zooxanthellae symbiosis in natural ecosystems.The effective photochemical quantum yield of PSII[Y（II）]was high in the early morning,but decreased to a low level at 6 h of light treatment,and recovered slowly thereafter;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;Six paired-end libraries were sequenced in 3 h of light and 12 h of dark treatments.Through the assembling of310,548,208 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 that14,036 genes were up-regulated and 44,361 genes were down-regulated in 3 h of light treatment;The results of quantitative real-time PCR demonstrated that light treatment upregulated 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 increasing photosynthesis（psb A and rbcl）of symbiotic zooxanthellae.In summary,the giant clam population surrounding Hainan Island and its illumination adaptability were studied systematacially for the first time.These results will contribute to the conservation of giant clam resources.