1. Identifying the relationships between morphology and trophic niche is at the core of functional morphology. Low resource diversity and fluxes of organic carbon are expected to constrain trophic specialisation of morphological structures because food resources are too scarce to promote trophic differentiation. However, species from low-productivity habitats often exhibit specialised biological traits such as resistance to starvation and high food-finding abilities, which may in turn release constraints on trophic and morphological differentiation among species.
2. Groundwaters are food resource-limited because of the lack of photosynthetic production and limited inputs of organic carbon from surface ecosystems. We tested for co-variation between morphology and trophic habits in co-occurring Niphargus amphipods from five groundwater caves of the Dinaric Karst, Europe. We predicted that the size of gnathopods—the accessory feeding appendages—would positively co-vary with trophic position: species with larger gnathopods should more easily grab and immobilise prey.
3. We quantified gnathopod size and shape by means of morphometric measurements and assessed isotopic niche, trophic position, and carbon signatures using nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotopes. We tested for correlation between morphological traits and trophic position and δ¹³C signatures while accounting for phylogenetic relationships among species.
4. All co-occurring species differed morphologically in at least one gnathopod measurement and all of them differed in their isotopic niches. As predicted, gnathopod size increased with the increasing trophic position. This co-variation probably reflects differences in detritivorous and predatory habits among species: amphipods with larger gnathopods, hence larger muscle and more powerful grip, could more easily subdue prey. Moreover, we found a significant correlation between gnathopod shape and the normalised δ¹³C values, indicating that shape of the gnathopods may be related to exploitation of different food resources.
5. We show that low-productivity subterranean habitat species can exhibit strong trophic specialisation of morphological structures. Gnathopod size and shape of Niphargus amphipods are functional traits that co-vary with trophic habits. Our findings pave the way for investigating how co-variation of morphological and trophic traits may control energy flow and species’ coexistence at lower bounds of habitat productivity.

中文翻译：

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洞穴片脚类动物揭示了低生产力环境中形态和营养生态位之间的共变

1. 识别形态学和营养生态位之间的关系是功能形态学的核心。低资源多样性和有机碳通量预计会限制形态结构的营养专业化，因为食物资源太稀缺而无法促进营养分化。然而，来自低生产力栖息地的物种往往表现出特殊的生物学特征，例如抗饥饿和高觅食能力，这反过来可能会释放对物种之间营养和形态分化的限制。
2. 由于缺乏光合作用生产和来自地表生态系统的有机碳输入有限，地下水是食物资源有限的。我们测试了来自欧洲 Dinaric Karst 的五个地下水洞穴的共生 Niphargus 片脚类动物的形态和营养习性之间的共变。我们预测，颚足类动物的大小——附属的摄食附属物——将与营养位置呈正相关：具有较大颚足类动物的物种应该更容易抓住和固定猎物。
3. 我们通过形态测量测量量化了颌足类动物的大小和形状，并使用氮 (δ ¹⁵ N) 和碳 (δ ¹³ C) 稳定同位素评估了同位素生态位、营养位置和碳特征。我们测试了形态特征与营养位置和 δ ¹³ C 特征之间的相关性，同时考虑了物种之间的系统发育关系。
4. 所有共生物种在至少一种有蹄类动物的测量中在形态上有所不同，并且所有这些物种的同位素生态位都不同。正如预测的那样，颚足类动物的大小随着营养位置的增加而增加。这种共同变化可能反映了物种间食腐和捕食习惯的差异：具有较大颚足动物的片脚类动物，因此肌肉更大，抓地力更强，可以更容易地制服猎物。此外，我们发现颚足动物的形状与归一化的 δ ¹³ C 值之间存在显着相关性，表明颚足动物的形状可能与不同食物资源的开发有关。
5. 我们表明，低生产力的地下栖息地物种可以表现出形态结构的强大营养专业化。Niphargus 片脚类的Gnathopod 大小和形状是与营养习性共同变化的功能特征。我们的发现为研究形态和营养性状的共变如何控制能量流和物种在栖息地生产力下限的共存铺平了道路。