Morphological crypsis within a crustacean species complex is driven by within-species phenotypic diversification

Cryptic species complexes-genetically divergent lineages with similar phenotypes may appear cryptic not due to true similarity, but because high within-species morphological variation masks interspecies differences, particularly in heterogeneous environments. This study tests whether such lineages exhibit morphological convergence across similar environmental gradients or context-dependent divergence, challenging their cryptic status. We propose a theoretical framework to assess phenotypic differentiation and apply it using the amphipod complex Gammarus roeselii, which diversified in the Balkan Peninsula before one lineage expanded into Central Europe. We examined five lineages across similar environmental gradients to evaluate their phenotypic divergence. Our findings confirmed their cryptic status. However, phenotypic differentiation along environmental gradients revealed MOTU-specific adaptive trajectories: traits under strong selection (e.g., anti-predator defences) showed convergent responses, while morphological (e.g., body length) and physiological traits (e.g., gill area) diverged, likely due to local selection. Some MOTUs exhibited distinct, partially divergent sexual dimorphism, suggesting varied reproductive or ecological strategies within cryptic lineages. Phenotypic variation was greater within than between MOTUs, reinforcing their cryptic status and revealing strong local adaptive responses. This complex exemplifies how adaptive divergence and biogeographic history jointly shape biodiversity, with morphological stasis despite genetic and ecological differentiation exposing hidden speciation and environmental responses.