Cryptic species
across environmantal gradients

The emergence of genetic methods has led to the discovery of an increasing number of species that previously could not be distinguished from one another on the basis of morphological characteristics. The family Gammaridae (Crustacea, Amphipoda) is a prime example of a group that has recently seen a high degree of genetic diversity uncovered. However, despite their frequency, cryptic species have so far rarely been considered in ecological and evolutionary work. Thus, especially the important question of ecological differentiation of closely related cryptic species – previously considered as a single morphospecies – is rarely addressed, although this is of key importance for evolutionary ecology and conservation biology. Our research project picks up this question and will examine the apparent paradoxon that we have found phenotypic differentiation even within a genetic line in a recent study (Jourdan et al. 2019 Front. Zool.). The widespread occurrence of cryptic species complex Gammarus roeselii made it an ideal model system to investigate phenotypic responses across repeated environmental gradients raising the question of whether phenotypic divergence (of a cryptic lineage) is present and even stronger within than between genetic lineages. This allows us to integrate quantitative analyses of phenotypic variation within a phylogeographic framework.