The Gulf of Aqaba is home to three giant clam species with differing ecological niches and levels of photosymbiotic activity. Giant clams grow a two‐layered shell where the outer layer is precipitated in close association with photosymbiont‐bearing siphonal mantle, and the inner layer is grown in association with the light‐starved inner mantle. We collected 39 shells of the three species (the cosmopolitan Tridacna maxima and T. squamosa , as well as the rare endemic T. squamosina ) and measured carbon and oxygen isotope ratios from inner and outer shell layers, to test for differences among species and between the layers of their shells. T. squamosina records higher temperatures of shell formation as determined by oxygen isotope paleothermometry, consistent with its status as an obligately shallow‐dwelling species. However, the known negative fractionation imparted on tissue carbon isotopes by photosymbiotic algae did not produce measurable offsets in the carbonate δ¹³C values of the more symbiotic T. squamosina and T. maxima compared to the more heterotrophic T. squamosa . Across all species, outer shell layers recorded mean growth temperatures 1.8°C higher than corresponding inner layers, which we propose is a function of the high insolation, low albedo microenvironment of the outer mantle, and potentially the activity of the symbionts themselves. Population‐wide isotopic sampling of reef‐dwelling bivalve shells can help constrain the ecological niches of rare taxa and help reconstruct their internal physiology.

中文翻译：

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红海巨蛤之间的种间和壳内稳定同位素变化

亚喀巴湾（Gulf of Aqaba）是三个蛤类物种的家园，它们具有不同的生态位和光共生活性水平。巨型蛤grow生长着两层壳，其中外层与带有光子的虹吸管地幔紧密结合，而内层与光饥饿的内地幔相伴生长。我们收集了这三个物种（世界性的Tridacna maxima和T. squamosa，以及罕见的地方性T. squamosina）的39个壳，并从内壳层和壳层测量了碳和氧同位素比，以测试物种之间以及物种之间的差异。他们的壳层。鳞球菌根据氧同位素古热法测定，其记录的壳形成温度更高，与其作为专性浅居物种的状态一致。然而，施加在组织碳已知负分馏由photosymbiotic藻类同位素在碳酸盐没有产生可测量的偏移δ ^13个的C值越共生T. squamosina和T.最大值相比，更异T.鳞。在所有物种中，记录的外壳层平均生长温度比相应的内层高1.8°C，我们认为这是外壳外层的高日照度，低反照率微环境以及共生体自身活动的函数。居住在礁区的双壳类贝壳的全同位素同位素采样可以帮助限制稀有分类单元的生态位，并有助于重建它们的内部生理学。