The native distribution of Chaetogammarus trichiatus extends across the Ponto-Caspian region, including the lagoons and estuarine habitats of the northwestern Black Sea (Rewicz et al. 2016; Copilaș-Ciocianu et al. 2023). C. trichiatus was first reported as an alien species in Germany in 1996 (Weinzierl et al. 1997) and has since spread throughout the Danube basin (Boets et al. 2012) and several catchments in northeastern France (Labat et al. 2011). In Switzerland, the species has been recorded in the Rhine basin since 2003 and, more recently, in the Aare basin in 2025 (Info fauna 2026). It has also been reported from the upper Dnieper River in Belarus (Lipinskaya et al. 2018) and the Oder catchment in Central Europe (Copilaș-Ciocianu et al. 2023).

In Lake Geneva, non-native crustaceans now constitute almost the entire amphipod fauna. Native amphipods, Gammarus pulex (Linnaeus, 1758) and Gammarus fossarum Koch, 1836, are progressively displaced by Dikerogammarus villosus Sowinsky, 1894 (Bollache 2004; Lods-Crozet and Reymond 2006) and are only rarely found in surveys. The limited sampling effort conducted along the French shoreline during the last two decades has reduced the likelihood of detecting newly established alien amphipods, whereas regular monitoring has continued on the Swiss shore (i.e. CIPEL monitoring).

In 2024, samplings conducted along the French littoral of Lake Geneva led to the first detection of the Ponto-Caspian amphipod C. trichiatus in the lake. Here, we report the first record of C. trichiatus in Lake Geneva, describe the morphological characters that distinguish it from other amphipods occurring in the lake, and confirm its identification using mitochondrial COI DNA barcoding. We also characterize the habitat in which the species was collected and discuss its possible introduction pathway, as well as the implications of this finding for interpreting previous surveys and guiding future monitoring in Lake Geneva.

On 12 August 2024, C. trichiatus was detected for the first time in Lake Geneva at the Evian-les-Bains site. Species identification of the two specimens was carried out by the authors using morphological examination and was further validated by DNA barcoding based on the mitochondrial cytochrome c oxidase subunit I (COI) gene.

The native distribution region of C. trichiatus includes the Black Sea, the Azov Sea, their deltas as well as the coastal lakes of Bulgaria and Romania (Cărăuşu et al. 1955; Mordukhaj-Boltovskoj 1969). Chaetogammarus trichiatus is characterized by a relatively short life cycle and generation time, brood protection, and an omnivorous feeding strategy (Schmidt-Kloiber and Hering 2026). The species exhibits high ecological plasticity, being able to tolerate a broad range of environmental conditions, including wide salinity gradients (bij de Vaate et al. 2002). Outside its native range, C. trichiatus has successfully colonized several European river systems, including the Upper Danube and the Lower Rhine, probably via the Main–Danube Canal (Holdich and Pöckl 2007). Despite its ecological plasticity, the species is only infrequently recorded in lake environments (but see Rewicz et al. 2016) and is generally considered to be more closely associated with large rivers (Labat et al. 2011; Info fauna 2026) where it occupies a variety of habitats, ranging from artificial riprap structures to more natural environments such as gravel banks, mud, roots, and stony substrates (Borza 2009).

In the Rhone River Basin, records from the lower and middle French Rhone River suggest an ongoing expansion, with occurrences reported downstream of Lyon (France) at Péage-de-Roussillon, Baix-Logis-Neuf, Donzère-Mondragon and Montélimar between 2017 and 2022 in the main river channel and two active secondary channels (Forcellini pers. comm.). This indicates a slow and progressive upstream colonization of the Rhone River basin prior to its detection in Lake Geneva. Nevertheless, because the species has not been recorded in the French Upper Rhone, it is unlikely that its arrival in Lake Geneva originated from upstream dispersal through the French Rhone corridor. This hypothesis is further supported by the absence of records from the littoral zone of the western basin (i.e. small lake) of Lake Geneva, despite regular monitoring efforts since more than ten years. According to Gallardo and Aldridge (2013), proximity to ports is a key predictor of the distribution of several Ponto-Caspian invasive species, including C. trichiatus. Consequently, human-mediated transport appears to be the most plausible explanation for this finding.

In newly colonized ecosystems, C. trichiatus frequently co-occurs with other Ponto-Caspian amphipods, including Chaetogammarus ischnus, Dikerogammarus villosus, Dikerogammarus haemobaphes and Chelicorophium robustum, as documented in the Rhine River (Labat et al. 2011). The present finding suggests that C. trichiatus can coexist with related crustacean species already present in Lake Geneva as e.g. D. villosus and Chelicorophium curvispinum. However, its establishment may also lead to future changes in community structure (Borza 2009). Competitive interactions are likely to occur within the bryophyte mats covering riprap and cobble habitats colonized by C. trichiatus, potentially affecting populations of C. curvispinum and Dikerogammarus spp., as well as other bryophyte-associated aquatic macroinvertebrates such as insect larvae and small gastropods. Nevertheless, C. curvispinum and Dikerogammarus spp. also inhabit deeper soft substrates and macrophytes. They generally occur there in lower numbers, but these habitats may provide alternative refuges and thereby limit the impact of competition with C. trichiatus.

Beyond Lake Geneva and the Rhone River, the present record raises the possibility of further colonization of other peri-alpine and mountain lakes. Lake Geneva is surrounded by numerous lakes supporting recreational boating, with watercraft occasionally transferred between water bodies. If introduced, C. trichiatus may find suitable habitats along bryophyte-covered rocky shores that are common in alpine lakes. This may be of particular concern for alpine lakes supporting glacial relict populations of native amphipods, such as Gammarus lacustris (Alther et al. 2017), where the introduction of non-native species could threaten these conservation-important populations.

The present finding has practical implications for the interpretation of previous surveys and future monitoring in Lake Geneva. Small individuals of C. trichiatus may easily be mistaken for Gammarus spp. when examined only in lateral view, particularly during routine sorting of lake samples. Careful examination of the third uropod is therefore essential to distinguish Chaetogammarus from Gammarus, while the dense recurved setae on this appendage allows C. trichiatus to be separated from C. ischnus. Re-examination of preserved material could help determine whether C. trichiatus had been present in Lake Geneva before its first detection in 2024. In addition, targeted monitoring at sites located near the major ports of Lake Geneva may be particularly valuable for the early detection of newly introduced exotic species. The identification key provided here should facilitate the routine recognition of the species during macroinvertebrate surveys and reduce the risk of future misidentifications.