Giant clam growth and isotope data

The health of reef-building corals has declined due to climate change and
pollution. However, less is known about whether giant clams, reef-dwelling
bivalves with a photosymbiotic partnership similar to that found in
reef-building corals, are also threatened by environmental degradation. To
compare giant clam health against a prehistoric baseline, we collected
fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red
Sea. After calibrating daily/twice-daily growth lines from the outer shell
layer, we determined that modern individuals of all three species
(Tridacna maxima, T. squamosa and T. squamosina) grew faster than Holocene
and Pleistocene specimens. Modern specimens also show median shell organic
δ15N values 4.2‰ lower than fossil specimens, which we propose is most
likely due to increased deposition of isotopically light nitrate aerosols
in the modern era. Nitrate fertilization accelerates growth in cultured
Tridacna, so nitrate aerosol deposition may contribute to faster growth in
modern wild populations. Furthermore, colder winter temperatures and past
summer monsoons may have depressed fossil giant clam growth. Giant clams
can serve as sentinels of reef environmental change, both to determine
their individual health and the health of the reefs they inhabit.

造礁珊瑚（reef-building corals）的健康状况因气候变化与污染持续衰退。目前学界对于与造礁珊瑚具有相似光合共生关系的礁栖双壳类——巨蛤（giant clams）是否同样面临环境退化的威胁，相关研究仍较为匮乏。为对比巨蛤的健康状况与史前基线水平，我们从北红海亚喀巴湾（Gulf of Aqaba）采集了化石与现代砗磲（Tridacna）贝壳。通过对壳外层的每日/每两日生长纹进行校准，我们发现所有3个物种（大砗磲Tridacna maxima、鳞砗磲T. squamosa与砗磲T. squamosina）的现代个体生长速度均快于全新世（Holocene）与更新世（Pleistocene）标本。现代标本的贝壳有机质δ¹⁵N中位值较化石标本低4.2‰，我们推测该现象最可能源于现代时期同位素轻硝酸盐气溶胶沉降量的增加。硝酸盐施肥可促进养殖砗磲的生长，因此硝酸盐气溶胶沉降或可推动现代野生砗磲种群生长加速。此外，更低的冬季气温与历史夏季季风活动，可能曾抑制了化石巨蛤的生长。巨蛤可作为珊瑚礁环境变化的哨兵物种，既可用于评估其自身健康状况，也可用于监测其所栖息珊瑚礁的健康状态。