Symbiotic octocorals are important reef inhabitants, from shallow to mesophotic depths. However, information regarding their nutritional ecology along the depth gradient, and in particular the changes in the autotrophic and heterotrophic acquisition of nutrients, is limited, despite the fact that nutrient acquisition is a fundamental process in explaining the distribution of reef organisms. Here, the abundance of three lipid biomarkers, specific for autotrophy and heterotrophy, was investigated in the tissue of four octocoral species and one scleractinian sampled in shallow and upper mesophotic reefs of the oligotrophic Northern Red Sea. Our findings show functional mixotrophy for all dinoflagellate-bearing species, with a significant input of heterotrophic feeding on herbivorous crustacean zooplankton at both depths. The relatively high level of the heterotrophic marker in all symbiotic species in shallow conditions does not corroborate the common idea that corals rely mostly on autotrophy in shallow waters. The increase in heterotrophic capacity with depth was however species-specific, likely related to physiological and morphological characteristics. In addition, octocorals maintained similar concentrations of autotrophic markers with increasing depth, while the scleractinian species exhibited a decrease in these markers with depth. These results are in agreement with previous measurements of photosynthetically-fixed carbon acquisition, suggesting that autotrophy remains stable in Red sea soft corals, whereas it decreases in scleractinian corals. This study highlights the importance of heterotrophy across the euphotic-upper mesophotic depth gradient and brings relevant advances on our understanding of the ecological significance of feeding for reef corals.

中文翻译：

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脂质生物标志物揭示了八爪鱼沿深度梯度的营养可塑性

从浅水层到中度深层，共生八面体是重要的珊瑚礁居民。然而，关于营养物质沿深度梯度的营养生态学信息，特别是营养物质的自养和异养获取的变化，尽管事实是养分获取是解释珊瑚礁生物分布的基本过程，但这一信息是有限的。在这里，研究了在寡营养型北红海的浅层和上层中生珊瑚礁中采样的四种八叶物种和一种巩膜藻组织中的三种脂蛋白生物标记物，它们对自养和异养营养丰富。我们的发现表明，所有含鞭毛鞭毛藻的物种都具有功能性混合营养，在两个深度上均以草食性甲壳类浮游动物的异养为食。在浅水条件下，所有共生物种中异养标记物的含量相对较高，并不证实珊瑚在浅水区主要依赖自养作用这一普遍观念。然而，随着深度的增加，异养菌能力的增加是物种特异性的，可能与生理和形态特征有关。另外，八尾虫的深度随着深度的增加而保持着相似的自养标志物浓度，而鞘翅菌种类则随着深度的增加而减少。这些结果与以前的光合作用固定碳采集的测量结果一致，表明自养在红海软珊瑚中保持稳定，而在巩膜珊瑚中减少。