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

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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.