Unveiling species and genetic diversity in gammarids (Amphipoda, Crustacea) from French Pyrenean streams using molecular tools

Abstract

Amphipods are abundant in freshwater ecosystems, where they play a crucial role in key processes such as organic matter decomposition. However, their diversity and spatial distribution remain poorly known due to the prevalence of cryptic species. This study investigates the genetic diversity of gammarids (Gammaridae) in French Pyrenean streams. We used two mitochondrial markers ( COI and 16S ) to analyze specimens collected from 62 sites across the Pyrenean foothills (from east to west, 5–717 m a.s.l.) to reconstruct their taxonomic status and spatial distribution. Our findings reveal the presence of 10 distinct molecular operational taxonomic units (MOTUs) that serve as potential species proxies based on their high mitochondrial divergence. Among these 10 MOTUs, six belonged to the genus Echinogammarus and four to the genus Gammarus . Six MOTUs could be assigned to known nominal species or candidate lineages, while four other MOTUs exhibited an uncertain taxonomic status, which may reflect regional undersampling or the existence of distinct, potentially new lineages. Most of the MOTUs identified were restricted to specific geographic areas delimited by river basin boundaries, displaying a striking longitudinal distribution across the Pyrenees. In the eastern part, we detected the non-native species Gammarus roeselii , representing its first report in this area and indicating a potential recent invasion. These findings highlight the Pyrenees as a hotspot of overlooked gammarid diversity and provide an essential baseline for regional conservation and biomonitoring.