Distribution Patterns at Different Spatial Scales Reveal Reproductive Isolation and Frequent Syntopy among Divergent Lineages of an Amphipod Species Complex in Western Carpathian Streams

Recent discoveries of vast cryptic diversity among various aquatic invertebrates pose a challenge for understanding their ecology and biodiversity patterns, as well as for conservation. We studied the distribution, potential hybridization and habitat preferences of divergent lineages of the hyperdiverse Gammarus fossarum species complex (Crustacea: Amphipoda) in their contact zone in the Western Carpathians (Central Europe), to obtain insights into processes that may influence lineage coexistence. We initially screened over 60 sites to assess the regional distribution, confirming the presence of four lineages in streams of the study area. Two to three lineages were found to be syntopic at almost half the sites. Subsequently, nine such localities were studied in detail for the mesohabitat distribution (fast vs. slow water flow), pairing preferences, and temporal stability of the Gammarus community. A significant fraction of regional spatial variation was explained by the position in the dendritic river network, altitude, stream width, and anthropogenic stress. At the local scale, however, we did not find differences in distribution between mesohabitats with contrasting flow rates. Mixed precopulatory pairs in syntopy were extremely rare, even under highly imbalanced lineage ratios, confirming a strong prezygotic reproductive barrier between the studied lineages. This was congruent with species delimitation analyses using molecular markers, which confirmed the absence of any recent gene flow. The frequent and apparently temporally stable syntopy of these reproductively isolated Gammarus lineages warrants further research on the processes that may facilitate this coexistence, focusing on potential finer differences in their ecology, functional morphology, or biotic interactions.