Drivers of cryptic species coexistence in stream ecosystems – a case study using gammarid amphipods from Sicily

在过去的几十年里，基于DNA的生物评估发现，由于神秘物种的存在，物种多样性被大大低估了。由于隐蔽物种在形态上无法区分，隐蔽物种的真实数量及其分布和生态特征在很大程度上仍不清楚。目前的证据支持，神秘物种通常不是共生的，而是生活在不同的地区，或者如果在一个地区共生，它们很少在句法中被发现。考虑到共存通常是由不同物种之间的生态位差异促成的，这一发现是意料之中的。由于隐蔽物种在系统发育上非常相似，人们经常认为它们类似于“生态克隆”，因此从竞争排斥理论的角度来看，它们可能不会共存。有趣的是，很少有研究报道共存的隐秘物种。一个突出的例子是在西西里岛的一个河流流域出现了六个神秘的伽马利亚端足类分类群(Echinogammarus Sicilianus)。这一发现是出乎意料的，并提出了一个问题：共存是稳定的还是不稳定的，以及如何维持这种共存。群落生态学理论提出了允许物种稳定共存的不同机制：i)资源分配，ii)空间或时间回避策略，iii)依赖密度的生物机制，如平衡群落的原基选择。在我们的项目中，我们将以西西里岛神秘物种共存的例外情况为例，研究不同的神秘物种是如何在局部尺度上和随着时间的推移而划分的。为了回答这个问题，我们将在一年的时间里对河网中现有的溪流微生境进行采样。我们将使用基于DNA的物种分配方法来鉴定所有标本。随后，我们将使用DNA元编码来研究每个地点和微生境中不同物种的肠道内容物和微生物群，以分析这些神秘物种之间是否存在饮食专门化。通过同位素分析，我们将支持关于营养专门化的数据。在最后一步，我们将研究微孢子虫和棘头虫寄生虫在单个隐蔽物种中的流行率和多样性。我们将测试流行率和寄主专一性是否随着时间的推移而变化，以测试通过寄生虫对隐蔽物种的密度依赖控制。最后，所有数据将与土地利用数据一起用于检验关于非生物和生物因素对物种存在和共存的影响的相互竞争的假设。由此，我们将得出神秘物种的共存是稳定的还是不稳定的。虽然迄今只报告了一起这类神秘物种共存的案例，但该项目的结果将为如何结合生态学中不同的方法论和概念办法来分析和理解淡水生境中神秘物种共存的驱动因素提供一个蓝图。

Over the past decades, DNA-based bioassessments have uncovered that species diversity has been greatly underestimated due to the presence of cryptic species. Because cryptic species are morphologically undistinguishable, the true number of cryptic species as well as their distribution and ecological characteristics remain largely unknown. Present evidence supports that cryptic species typically do not co-occur but rather live in different regions or if co-occurring in a region, they are rarely found in syntopy. This finding is expected given that coexistence is typically enabled by niche differences between different species. As cryptic species are phylogenetically very similar, it is often assumed that they resemble 'ecological clones' and thus may not coexist in view of competition exclusion theory. Interestingly, few studies have reported coexisting cryptic species. One prominent example is the occurrence of six cryptic species of a gammaridean amphipod taxon (Echinogammarus sicilianus) in a river basin in Sicily. This finding is unexpected and raises the question whether co-existence is stable or unstable, and how it is maintained. Different mechanisms have been proposed by community ecological theory which allow stable coexistence of species: i) resource partitioning, ii) spatial or temporal avoidance strategies, and iii) density-dependent biotic mechanisms such as apostatic selection that balance communities. In our project, we will use the exceptional case of cryptic species co-occurrence in Sicily as an example to study how different cryptic species are partitioned at the local scale and over time. To answer this question, we will sample the available stream microhabitats in the river network over the course of one year. We will use DNA-based species assignment methods to identify all specimens. Subsequently, we will study the gut contents and microbiome of the different species per site and microhabitat using DNA metabarcoding to analyse whether there is a dietary specialisation among the cryptic species. With isotopic analyses we will back up the data on trophic specialisation. In a last step we will study the prevalence and diversity of microsporidian and acanthocephalan parasites in the individual cryptic species. We will test whether prevalence and host specificity changes over time in order to test the density-dependent control of cryptic species through parasites. Finally, all data will be used together with land-use data to test competing hypotheses on the impact of abiotic and biotic factors on species presence and coexistence. From this we will derive whether coexistence of cryptic species is stable or unstable. While only a single such case of cryptic species co-occurring has been reported so far, the results from this project will provide a blueprint on how to combine different methodological and conceptual approaches in ecology to analyse and understand drivers of cryptic species coexistence in freshwater habitats.