Groundwater is the physically largest freshwater ecosystem, yet one of the least explored habitats on earth, both because of accessing difficulties and the scarcity of the organisms inhabiting it. Here, we demonstrate how a two-fold approach provides complementary information on the occurrence and diversity of groundwater amphipods. Firstly, we used a citizen science approach in collaboration with municipal water providers who sampled groundwater organisms in their spring catchment boxes over multiple weeks, followed by DNA barcoding. Secondly, we collected four 10 L water samples at each site, in one sampling event, for environmental DNA (eDNA) metabarcoding. We found that citizen science was very effective in describing the distribution and abundance of groundwater amphipods. Although the single time-point of eDNA sampling did not detect as many amphipods, it allowed the assessment of the entire groundwater community, including microorganisms. By combining both methods, we found different amphipod species co-occurring with distinct sequences from the eDNA-metabarcoding dataset, representing mainly micro-eukaryotic species. We also found a distinct correlation between the diversity of amphipods and the overall biodiversity of groundwater organisms detected by eDNA at each site. We thus suggest that these approaches can be used to get a better understanding of subterranean biodiversity.

Subterranean fauna is an important contributor to the global fauna, but it is still understudied and a large part of its taxonomy is not yet resolved. One species complex with unresolved taxonomy is the groundwater amphipod Niphargus ruffoi, endemic to the Alpine chain. Here, we used new samples from across the Alpine arc to review the taxonomic status of the entire clade, including the species N. ruffoi and Niphargus arolaensis. We sequenced four genetic markers from the collected specimens, assessed the phylogenetic position of N. ruffoi within the genus, and studied the structure of this species complex using four molecular species delimitation methods. We tested for recombination using the alignments of the concatenated nuclear rDNA genes. The phylogenetic analyses revealed high support for the monophyly of the studied species complex, defining two lineages (i.e., N. arolaensis and N. ruffoi) within the clade. Molecular species delimitation methods suggested that N. arolaensis is a single species, while N. ruffoi should be considered as a species complex of three (using ITS) to eight (using COI) putative species. Moreover, we found a discrepancy between the different nuclear ribosomal DNA markers, indicating a possible recombination with fragments of 28S DNA of N. ruffoi s. lat. present in the genome of N. arolaensis. For the above-mentioned reasons, the internal phylogenetic structure of N. ruffoi s. lat. could not be fully resolved. Moreover, no clear morphological evidence supported the molecular species delimitation. Consequently, no taxonomic changes were proposed. We postulate that this complex scenario was influenced by Pleistocene climate oscillations with subsequent fragmentation events and secondary contacts, making this an interesting study system to investigate the evolution and biogeography of Alpine clades.

Haplotaxis Hoffmeister, 1843 is a poorly known genus: it is rarely collected because it usually inhabits groundwater, and many species are only known from immature specimens. Even the type species, Haplotaxis gordioides (Hartmann in Oken, 1819) remains poorly defined because of the absence of reference types. Most of the Haplotaxis species have been placed in synonymy with H. gordioides since the end of the 19^th century, a situation that has remained essentially unchanged until now. As a result, the species is supposedly present on all continents, except Antarctica. This observation is all the more questionable as the aquatic subterranean environment is nowadays well known to harbour many species with restricted distribution, due to the strong hydrogeographic isolation and the low dispersal abilities of its inhabitants.
In this study we assessed the hypothesis of H. gordioides as a single species with wide distribution versus a complex of cryptic species with narrow distribution. We used a DNA-barcoding approach based on the COI mitochondrial marker of 46 Haplotaxis specimens collected in Switzerland, mostly as part of a countrywide sampling campaign to study groundwater macroinvertebrates.
Preliminary results suggested that H. gordioides is a complex of at least 6 cryptic species in Switzerland, which has important implications both for the knowledge of the exact identity of the type species and for the synonymy of most of the species described in the 19^th century. However, as it is based on a single-locus approach, this study should be seen as the first step in an integrative taxonomic process that should include additional biological material, the study of complementary markers (especially nuclear), and the morphological study of specimens.

Groundwater is a vast ecosystem harboring a high diversity of specialized taxa. Despite its diversity, groundwater is a still relatively unexplored and threatened ecosystem. Especially the linkage of groundwater with other ecosystems remains largely unknown from the perspective of groundwater fauna. Here, we used citizen science data to get a first baseline knowledge of the occurrence, diversity, and biomass of major macroinvertebrate groups found in shallow groundwater systems of Switzerland. We investigated all organisms collected from the groundwater in 346 spring catchment boxes of municipal drinking water providers. We morphologically identified the organisms on a broad taxonomic level and estimated their biomass and pigmentation using automated image processing. Crustaceans, particularly Niphargus and groundwater isopods, were the most common taxa of obligate groundwater organisms. We also found a surprisingly high number of macroinvertebrates associated with surface and subsurface ecosystems. These taxa might be accidentally entering the groundwater or use it as temporary habitat. In both cases they possibly contribute essential allochthonous energy imports from the surface. We found a positive relationship between the estimated biomass of macroinvertebrates in the samples and the occurrence and abundance of Niphargus. Owing to the widespread occurrences of surface and subsurface macroinvertebrates in our groundwater samples, our study provides evidence for common interactions between groundwater, soil, and surface ecosystems.

Erstmals wurde die Verbreitung und Artenvielfalt der Grundwasserflohkrebse im Einzugsgebiet der Töss, unter Einbezug von Bürgerwissenschaft, räumlich hoch aufgelöst untersucht. Bei der durch die Eawag und die Universität Zürich durchgeführten Studie wurden vier in dieser Region bisher noch nicht nachgewiesene Arten gefunden. Auch konnte gezeigt werden, dass die Daten zur Fundwahrscheinlichkeit, die von Wasserversorgungen erhoben wurden, mit denen von Forschenden vergleichbar sind.