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Extensive variation in the morphological anti-predator defense mechanism of Gammarus roeselii Gervais, 1835 (Crustacea:Amphipoda)

Gammarus roeselii is a common European amphipod, characterized by dorsal spines that defend against fish predation. Despite significant variability in spine length and number, the extent of this variation has never been quantified. Furthermore, widespread freshwater Gammarus taxa, including G. roeselii, are diverse species complexes, comprising numerous narrowly-endemic cryptic or overlooked species. As such, we hypothesized that the morphological variation among Central European populations of G. roeselii reflects the presence of multiple unrecognized species. We measured body and spine length from phenotypically-diverse populations and sequenced a fragment of the mitochondrial gene for the cytochrome c oxidase subunit I to link phenotypes and lineages. Morphological analyses revealed extensive variation and indicated 2 groups: morph A bearing 3 blunt, short spines and morph B with 3 to 4 long, pointy spines. Phylogenetic analyses indicated conspecificity of populations and morphs due to unexpectedly-small genetic divergence and haplotype sharing, thus refuting our hypothesis. All studied populations belong to a widespread Central and Western European clade, which is nestled among the Southeast European lineages of the species complex. We conclude that the anti-predator defense of G. roeselii in Central Europe is extremely variable despite little inter-population genetic differentiation. We presume that this variation is linked to predation intensity, reflecting either phenotypic plasticity or local adaptations of this successful colonizer, and could also shed more light into the complex evolution of spines in the Amphipoda.