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Abstract Subterranean and surface habitats are in stark contrast in several environmental factors. Therefore, adaptation to the subterranean environment typically impedes the (re)colonisation of surface habitats. The genus Niphargus includes amphipod crustaceans that primarily occupy subterranean habitats. All its species show typical adaptations to the subterranean environment. However, some Niphargus species occur in surface‐subterranean ecotones. To understand whether (i) habitat‐based phenotypic divergence is present between the cave and the ecotone species and (ii) similar phenotypes emerge independently in each ecotone, we studied morphological divergence between four cave and four ecotone Niphargus species based on 13 functional morphological traits. To account for different selection acting on the sexes, we included both males (N = 244) and females (N = 222). Nine out of 13 traits showed habitat‐divergence. Traits related to feeding and crawling were shorter, while traits related to oxygenation were larger in ecotone species. Eleven out of 13 traits were sexually dimorphic. Traits related to oxygenation and crawling were larger in females, while the trait related to swimming was larger in males. We found that the extent of sexual dimorphism differs between the habitats in eight traits related to sensing, feeding, oxygenation and crawling. Additionally, we found that in certain traits related to sensing and oxygenation, habitat‐related differences are only present in one sex, but not the other. We conclude that the detected differences between the cave and the ecotone species indicate divergent evolution, where similarities among the different species within habitat type indicate convergent evolution. The high degree of sexual dimorphism paired with differences in sexual dimorphism between the habitats in certain traits suggest that sexual and fecundity selections have comparable effects to environmental selection. Thus, studies of habitat‐dependent adaptations investigating one sex only, or not considering sexual dimorphism, can lead to erroneous conclusions.

Abstract Groundwater represents a vast, but mostly hidden and inaccessible ecosystem. Although often overlooked in freshwater research, groundwater organisms form a significant part of freshwater biodiversity, whereas their functions are crucial in different ecosystem processes. Knowledge on functional traits is generally lacking for most groundwater species worldwide, yet European groundwater amphipods, particularly the family Niphargidae, are an exception. They are well-researched and used as a model system in ecological and evolutionary studies. We focused on this group to assemble a first functional trait dataset dedicated to groundwater species. We gathered data for eight morphological functional traits quantified through 27 measurements for 1123 individuals which represent 180 species and 314 MOTUs. Besides functional trait data, every entry is accompanied with locality information, including habitat type, and DNA sequences if available. The structure of the dataset and data processing information provided along enable wide applicability and extension to other amphipod taxa. When coupled with phylogeny, the dataset may further enhance different aspects of groundwater research, including biodiversity patterns, community assembly processes, and trait evolution.

Abstract Subterranean ecosystems (comprising terrestrial, semi-aquatic, and aquatic components) are increasingly threatened by human activities; however, the current network of surface-protected areas is inadequate to safeguard subterranean biodiversity. Establishing protected areas for subterranean ecosystems is challenging. First, there are technical obstacles in mapping three-dimensional ecosystems with uncertain boundaries. Second, the rarity and endemism of subterranean organisms, combined with a scarcity of taxonomists, delays the accumulation of essential biodiversity knowledge. Third, establishing agreements to preserve subterranean ecosystems requires collaboration among multiple actors with often competing interests. This perspective addresses the challenges of preserving subterranean biodiversity through protected areas. Even in the face of uncertainties, we suggest it is both timely and critical to assess general criteria for subterranean biodiversity protection and implement them based on precautionary principles. To this end, we examine the current status of European protected areas and discuss solutions to improve their coverage of subterranean ecosystems.

We provide an overview of the World Amphipoda Database (WAD), a global species database that is part of the World Register of Marine Species (WoRMS). Launched in 2013, the database contains entries for over 10,500 accepted species names. Edited currently by 31 amphipod taxonomists, following WoRMS priorities, the WAD has at least one editor per major group. All accepted species are checked by the editors, as is the authorship available for all of the names. The higher classification is documented for every species and a type species is recorded for every genus name. This constitutes five of the 13 priorities for completion, set by WoRMS. In 2015, five LifeWatch grants were allocated for WAD activities. These included a general training workshop in 2016, together with data input for the superfamily Lysianassoidea and for a number of non-marine groups. Philanthropy grants in 2019 and 2021 covered more important gaps across the whole group. Further work remains to complete the linking of unaccepted names, original descriptions, and environmental information. Once these tasks are completed, the database will be considered complete for 8 of the 13 priorities, and efforts will continue to input new taxa annually and focus on the remaining priorities, particularly the input of type localities. We give an overview of the current status of the order Amphipoda, providing counts of the number of genera and species within each family belonging to the six suborders currently recognized.

Abstract Climate change affects all ecosystems, but subterranean ecosystems are repeatedly neglected from political and public agendas. Cave habitats are home to unknown and endangered species, with low trait variability and intrinsic vulnerability to recover from human-induced disturbances. We studied the annual variability and cyclicity of temperatures in caves vis-à-vis surface in different climatic areas. We hypothesize that cave temperatures follow the average temperature pattern at the surface for each location with a slight delay in the signal, but we found three different thermal patterns occurring in caves: (1) high positive correlation and a similar thermal pattern to the surface, (2) low correlation and a slight thermal delay of the signal from the surface, and (3) high negative correlation with an extreme delay from the surface. We found daily thermal cycles in some caves, which may potentially control the circadian rhythms of cave organisms. Our results show that caves had lower thermal amplitude than the surface, and that thermal averages within caves approximately correspond to the to the annual average of surface temperature. Caves buffer external temperature and act as refugia for biota in extreme climatic events. Likewise, temperature increases at surface will lead to increment in caves, threatening subterranean biota and ecosystem services.

Understanding phenotype variation is among the central topics in biology. We revise and reanalyze studies of the amphipod genus Niphargus to confront two potential mechanisms driving its phenotype variation, namely, cladogenesis and adaptive evolution. We found evidence for both mechanisms. Reanalysis of a subset of traits using molecular phylogeny showed moderate phylogenetic signal, consistent with the hypothesis that overall phylogenetic variation increases with phylogeny. The phylogenetic signal in Niphargus traits seems to be stronger at the tips of the phylogeny than at basal splits. Indirect evidence suggests that much of the phenotype variation can be attributed to adaptive evolution. Both lines of evidence are consistent with the hypothesis that Niphargus evolved in several adaptive radiations, where theory predicts that most of the phenotype variation evolves early, when ecological niches are vacant. As the niches fill up, the rate of phenotype variation slows down and becomes associated with cladogenetic events. This hypothesis can explain the high level of trait-convergence and unresolved taxonomy above the species level. The main caveats to these hypotheses comprise lack of experimental evidence for trait function and nonquantified heritable component of trait variation. Promising venues towards better understanding of phenotypic variation include studies of ontogenetic variation, functional interactions between traits, and genome–phenotype associations.

Body size is a trait of fundamental ecological and evolutionary importance that is often different between males and females (sexual size dimorphism; SSD). The island rule predicts that small-bodied species tend to evolve larger following a release from interspecific competition and predation in insular environments. According to Rensch’s rule, male body size relative to female body size increases with increasing mean body size. This allometric body size – SSD scaling is explained by male-driven body size evolution. These ecogeographical rules are rarely tested within species, and has not been addressed in a cave–surface context, even though caves represent insular environments (small and isolated with simple communities). By analyzing six cave and nine surface populations of the widespread, primarily surface-dwelling freshwater isopod Asellus aquaticus with male-biased SSD, we tested whether cave populations evolved larger and showed higher SSD than the surface populations. We found extensive between-population variation in body size (maximum divergence being 74%) and SSD (males being 15%–50% larger than females). However, habitat type did not explain the body size and SSD variation and we could not reject isometry in the male–female body size relationship. Hence, we found no support for the island or Rensch’s rules. We conclude that local selective forces stemming from environmental factors other than island vs. mainland or the general surface vs. cave characteristics are responsible for the reported population variation.

Abstract Predator–prey interactions are among the most important biotic interactions shaping ecological communities and driving the evolution of defensive traits. These interactions and their effects on species received little attention in extreme and remote environments, where possibilities for direct observations and experimental manipulation of the animals are limited. In this paper, we study such type of environment, namely caves of the Dinarides (Europe), combining spatial and phylogenetic methods. We focused on several species of Niphargus amphipods living in phreatic lakes, as some of them use the dorsal spines as putative morphological defensive traits. We predicted that these spines represent a defense strategy against the olm (Proteus anguinus), a top predator species in the subterranean waters. We tested for spatial overlap of the olm and Niphargus species and showed that spined species live in closer proximity to and co-occur more frequently with the olm than non-spined species. Modeling of the evolution of the spines onto Niphargus phylogeny implies coevolution of this trait in the presence of olm. We conclude that these spines likely evolved as defensive traits in a predator–prey arms race. Combining multiple analyses, we provide an example for a methodological framework to assess predator–prey interactions when in-situ or laboratory observations are not possible.

Abstract Studying parallel evolution (repeated, independent evolution of similar phenotypes in similar environments) is a powerful tool to understand environment‐dependent selective forces. Surface‐dwelling species that repeatedly and independently colonized caves provide unique models for such studies. The primarily surface‐dwelling Asellus aquaticus species complex is a good candidate to carry out such research, because it colonized several caves in Europe. By comparing 17 functional morphological traits between six cave and nine surface populations of the A. aquaticus species complex, we investigated population divergence in morphology and sexual dimorphism. We found habitat‐dependent population divergence in 10 out of 17 traits, likely reflecting habitat‐driven changes in selection acting on sensory systems, feeding, grooming, and antipredator mechanisms. Sexual dimorphism was present in 15 traits, explained by sexual selection acting on male traits important in male–male agonistic behavior or mate guarding and fecundity selection acting on female traits affecting offspring number and nursing. In eight traits, the degree of sexual dimorphism was habitat dependent. We conclude that cave‐related morphological changes are highly trait‐ and function‐specific and that the strength of sexual/fecundity selection strongly differs between cave and surface habitats. The considerable population variation within habitat type warrants further studies to reveal cave‐specific adaptations besides the parallel patterns.

The Dinaric Karst is a global hotspot for subterranean diversity, with two distinct peaks of species richness in the northwest and southeast, and an area of a lower species richness in the central part. In this article, we present a species list and describe the ecological conditions of the Lukina jama–Trojama cave system, located in the central part of the Dinaric Karst. This cave system is the deepest and one of the most logistically challenging cave systems sampled so far in the Dinaric Karst. Repeated sampling resulted in a list of 45 species, including 25 troglobionts, 3 troglophiles, 16 stygobionts, and 1 stygophile. Most of the recorded species are endemic to the Velebit Mountain, while three species are endemic to the Lukina jama–Trojama cave system. Within the system, species richness peaks in the deepest third of the cave, most likely reflecting the harsh ecological conditions in the upper parts, including ice, cold winds, and occasional waterfalls. Milder and more stable deeper parts of the cave contain a rich subterranean species community, part of which is associated with two very distinct aquatic habitats, the cave hygropetric and the phreatic zone. The newly recognized hotspot of subterranean biodiversity in the central Dinaric Karst, which has emerged between the two known centers of biodiversity, further highlights the species richness in large cave systems, but also challenges the diversity patterns in the Dinaric Karst overall.

There are relatively few known extant adaptive radiations in Europe that predate the Pleistocene. Here, Borko et al. characterize the diversity and diversification of the subterranean amphipod genus Niphargus, showing evidence for a large adaptive radiation associated with massif uplift 15 million years ago.

Knowledge on the diversity and distribution of subterranean organisms is still scattered, even in faunistically relatively well-researched countries such as Switzerland. This is mostly due to the restricted access to these subterranean habitats. Better knowledge on these organisms is needed, because they contribute substantially to overall biodiversity of a region, often contain unique elements of biodiversity, and can potentially be indicative of the ecological status of subterranean ecosystems that are providing important ecosystem services such as drinking water. Past research on subterranean organisms has often used highly specialised sampling techniques and expert knowledge. Here, we show that inclusion of non-professionals can be an alternative and highly promising sampling strategy. We retrieved citizen science-based samples from municipal groundwater wells across Switzerland, mainly from the Swiss Plateau. Opportunistic samples from 313 sites revealed a previously undocumented groundwater fauna including organisms from different major invertebrate groups, with a dominance of crustaceans. Here, we studied amphipods of the genus Niphargus. Among all 363 individuals sampled, we found in total eight nominal species. Two of them, namely N. fontanus and N. kieferi, are reported for Switzerland for the first time. We also found four further phylogenetic lineages that are potentially new species to science. One of them is here formally described as Niphargus arolaensis sp. nov. The description is based on molecular and morphometric data. Our study proves the suitability of citizen science to document subterranean diversity, supports groundwater conservation efforts with data, and raises awareness for the relevance and biodiversity of groundwater amphipods among stakeholders.

Secondary salinization of freshwater is becoming a growing environmental problem. Currently, there is few data available on the effects of salinisation on subterranean crustaceans that are vital for the maintenance of groundwater ecosystem functioning. In this study, the sensitivity of subterranean Niphargus amphipods to NaCl was investigated. We expected that cave-dwelling species would be more sensitive as surface-subterranean boundary species. Eight ecologically different Niphargus species were tested: four live at the boundary between the surface and subterranean ecosystems (N. timavi, N. krameri, N. sphagnicolus, N. spinulifemur), three live in cave streams (N. stygius, N. scopicauda, N. podpecanus), and one species (N. hebereri) lives in anchialine caves and wells. The organisms were exposed to five concentrations of NaCl for 96 h and afterwards the immobility, mortality, and electron transfer system (ETS) activity (a measure for metabolic rate of animals) were evaluated. As expected, the most tolerant species was N. hebereri dwelling in naturally high-salinity habitat. However, contrary to our expectations, the species collected at the surface-subterranean boundary were more sensitive as cave stream species when their immobility and mortality were assessed. Interestingly, the majority of Niphargus tested were more NaCl tolerant as can be deduced from currently available data for subterranean and surface crustaceans. We could not observe a clear trend in ETS activity changes between groups of surface-subterranean boundary and cave streams species after exposure to NaCl stress, but it appears that osmotic stress-induced metabolic rate changes are species-specific. This study shows that amphipods Niphargus can be a valuable subterranean environmental research model and further ecotoxicity research is of interest.

The Palaearctic genus Niphargus is a promising model system to understand subterranean fauna genesis in Europe. The Pannonian Plain (mainly covered by Hungary) in Central Europe, once being the area of the Paratethys, is a key area for Niphargus diversification. However, our knowledge on Hungarian species of Niphargus is primarily based on sporadic taxonomical works from the pre-molecular era. Here, we studied 14 localities, covering the eight valid Hungarian species of Niphargus and including nine previously unstudied populations. Based on sequences of three gene fragments, we reconstructed their phylogeny using maximum likelihood and Bayesian approaches. We found that not all Hungarian species of Niphargus are closely related, and even species sampled at the same localities can belong to different clades. Some Hungarian species form monophyletic clades, while others are nested in various non-Hungarian lineages. The new populations are all genetically distinct from the known species. Our results suggest that the Hungarian Niphargus fauna has originated from seven unrelated clades and its diversity is underestimated due to unknown populations and cryptic species. The detection of genetically distinct species of Niphargus from non-carbonate regions calls for further research efforts. The high diversity and the number of putative new species in the N. tatrensis clade warrants further, high-resolution phylogenetic studies.

Abstract Aim Common species contribute more to species richness patterns (SRPs) than rare species in most studies. Our aim was to test this hypothesis using a novel model system, species living exclusively in subterranean habitats. They consist of mainly rare species (small ranges), only a few of them being common (large ranges), and challenge whether rare species are less important for the development of SRPs in this environment. We separately analyzed aquatic and terrestrial species. Location Western Balkans in southeastern Europe. Methods We assembled two datasets comprising 431 beetle and 145 amphipod species, representing the model groups of subterranean terrestrial and aquatic diversity, respectively. We assessed the importance of rare and common species using the stepwise reconstruction of SRPs and subsequent correlation analyses, corrected also for the cumulative information content of the subsets based on species prevalence. We applied generalized linear regression models to evaluate the importance of rare and common species in forming SRPs. Additionally, we analyzed the contribution of rare and common species in species‐rich cells. Results Patterns of subterranean aquatic and terrestrial species richness overlapped only weakly, with aquatic species having larger ranges than terrestrial ones. Our analyses supported higher importance of common species for forming overall SRPs in both beetles and amphipods. However, in stepwise analysis corrected for information content, results were ambiguous. Common species presented a higher proportion of species than rare species in species‐rich cells. Main Conclusion We have shown that even in habitats with the domination of rare species, it is still common species that drive SRPs. This may be due to an even spatial distribution of rare species or spatial mismatch in hotspots of rare and common species. SRPs of aquatic and terrestrial subterranean organisms overlap very little, so the conservation approaches need to be habitat specific.

Abstract The 15th UN Convention on Biological Diversity (CBD) (COP15) will be held in Kunming, China in October 2021. Historically, CBDs and other multilateral treaties have either alluded to or entirely overlooked the subterranean biome. A multilateral effort to robustly examine, monitor, and incorporate the subterranean biome into future conservation targets will enable the CBD to further improve the ecological effectiveness of protected areas by including groundwater resources, subterranean ecosystem services, and the profoundly endemic subsurface biodiversity. To this end, we proffer a conservation roadmap that embodies five conceptual areas: (1) science gaps and data management needs; (2) anthropogenic stressors; (3) socioeconomic analysis and conflict resolution; (4) environmental education; and (5) national policies and multilateral agreements.

The amphipod genus Niphargus (Amphipoda: Niphargidae Bousfield, 1977) is the most species-rich genus of freshwater amphipods in the World. Species of this genus, which live almost exclusively in subterranean water, offer an interesting model system for basic and applied biodiversity science. Their use, however, is often limited due to the hitherto unresolved taxonomy within the whole genus. As a comprehensive taxonomic revision of the currently >425 Niphargus species is too demanding, it has been suggested that the taxonomy of the genus could be advanced in smaller steps, by reviewing regional faunas, that would eventually integrate into a global revision. In this study, we provide such a revision of Niphargus in Switzerland. First, we molecularly delimited, morphologically diagnosed, and formally described two new species, namely Niphargus luchoffmanni sp. n. and Niphargus tonywhitteni sp. n. Second, we updated and revised a checklist of Niphargus in Switzerland with new findings, and prepared a list of reference sequences for routine molecular identification, available at BOLD and GenBank. All available specimens of 22 known species from the area were morphologically examined, and their morphological variation was compiled in a data file of DEscription Language for TAxonomy, which can be used for automated generation of dichotomous or interactive keys. The data file is freely available at the World Amphipoda Database. Together, the checklist, the library of reference sequences, the DELTA file, but also a list of hitherto unresolved aspects are an important step towards a complete revision of the genus within a well-defined and biogeographically interesting area in Central Europe.

The Postojna-Planina Cave System (PPCS) in central Slovenia is a globally exceptional site of subterranean biodiversity, comprised of many interconnected caves with cumulative passage length exceeding 34 km. Two rivers sink into the caves of the PPCS, called the Pivka and Rak, and join underground into Unica River, which emerges to the surface. The studies of fauna of PPCS began in the 19th century with the first scientific descriptions of specialized cave animals in the world, making it “the cradle of speleobiology”. Currently, the species list of PPCS contains 116 troglobiotic animal species belonging to eight phyla, confirming its status as the richest in the world. Of these, 47 species have been scientifically described from the PPCS, and more than 10 await formal taxonomic descriptions. We expect that further sampling, detailed analyses of less studied taxa, and the use of molecular methods may reveal more species. To keep the cave animals’ checklist in PPCS up-to-date, we have supplemented the printed checklist with an online interface. As the revised checklist is a necessary first step for further activities, we discuss the importance of PPCS in terms of future research and conservation.

Abstract Molecular taxonomy often uncovers cryptic species, reminding us that taxonomic incompleteness is even more severe than previous thought. The importance of cryptic species for conservation is poorly understood. Although some cryptic species may be seriously threatened or otherwise important, they are rarely included in conservation programs as most of them remain undescribed. We analysed the importance of cryptic species in conservation by scrutinizing the South European cryptic complex of the subterranean amphipod Niphargus stygius sensu lato. Using uni- and multilocus delineation methods we show that it consists of 15 parapatric and sympatric species, which we describe using molecular diagnoses. The new species are not mere “taxonomic inflation” as they originate from several distinct branches within the genus and coexist with no evidence of lineage sharing. They are as evolutionarily distinct as average nominal species of the same genus. Ignoring these cryptic species will underestimate the number of subterranean endemics in Slovenia by 12 and in Croatia by four species, although alpha diversity of single caves remains unchanged. The new taxonomy renders national Red Lists largely obsolete, as they list mostly large-ranged species but omit critically endangered single-site endemics. Formal naming of cryptic species is critical for them to be included in conservation policies and faunal listings.

The protection, preservation and restoration of aquatic ecosystems and their functions are of global importance. For European states it became legally binding mainly through the EU-Water Framework Directive (WFD). In order to assess the ecological status of a given water body, aquatic biodiversity data are obtained and compared to a reference water body. The quantified mismatch obtained determines the extent of potential management actions. The current approach to biodiversity assessment is based on morpho-taxonomy. This approach has many drawbacks such as being time consuming, limited in temporal and spatial resolution, and error-prone due to the varying individual taxonomic expertise of the analysts. Novel genomic tools can overcome many of the aforementioned problems and could complement or even replace traditional bioassessment. Yet, a plethora of approaches are independently developed in different institutions, thereby hampering any concerted routine application. The goal of this Action is to nucleate a group of researchers across disciplines with the task to identify gold-standard genomic tools and novel eco-genomic indices for routine application in biodiversity assessments of European fresh- and marine water bodies. Furthermore, DNAqua-Net will provide a platform for training of the next generation of European researchers preparing them for the new technologies. Jointly with water managers, politicians, and other stakeholders, the group will develop a conceptual framework for the standard application of eco-genomic tools as part of legally binding assessments.

A detailed redescription of two endemic, cave-dwelling niphargid species of the Hungarian Mecsek Mts., Niphargus molnari Méhely, 1927 and Niphargus gebhardti Schellenberg, 1934 is given based on newly collected material. Morphology was studied under light microscopy and with scanning electon microscopy. Morphological descriptions are complemented with mitochondrial cytochrome c oxidase subunit I (COI) sequences as barcodes for both species and with notes on their ecology. Using three independent molecular markers we showed that N. gebhardti belongs to the clade distributed between Central and Eastern Europe, whereas phylogenetic relationship of N. molnari to the rest of Niphargus species is not clear. The two species from the Mecsek Mts. are phylogenetically not closely related. Both species need to be treated as vulnerable according to IUCN Red List of Threatened Species.

Niphargus dancaui sp. nov., previously referred to as Niphargus cf. stygius, was sampled from various groundwater sites in and near the town of Mangalia (SE Romania) and described with Movile Cave (a sulfidic, chemoautotrophically based ecosystem) as type locality. A short comparison with Niphargus stygius specimens from Slovenia was made, together with a morphological analysis of interpopulational variability. Males of N. dancaui sp. nov. were relatively large (17 mm), with long antennae, pereiopods and uropod III. Females were slightly smaller, with shorter antennae, pereiopods and uropod III. Interpopulational variability was noticed in the chaetotaxy of the telson lobes and uropod III. N. dancaui sp. nov. is morphologically very close to N. lessiniensis and N. tridentinus, two species present in northern Italy, but distinct genetically from them based on 28S rRNA sequences. Instead, the closest relative of N. dancaui sp. nov. sequenced so far for this marker is N. montanarius, which inhabits a sulfidic cave system in central Italy. The work presented here contributes to our knowledge of groundwater crustacean biodiversity in general and of the systematics of the genus Niphargus in particular.

Recent studies indicate that morphologically cryptic species may be ecologically more different than would be predicted from their morphological similarity and phylogenetic relatedness. However, in biodiversity research it often remains unclear whether cryptic species should be treated as ecologically equivalent, or whether detected differences have ecological significance. In this study, we assessed the ecological equivalence of four morphologically cryptic species of the amphipod genus Niphargus. All species live in a small, isolated area on the Istrian Peninsula in the NW Balkans. The distributional ranges of the species are partially overlapping and all species are living in springs. We reconstructed their ecological niches using morphological traits related to feeding, bioclimatic niche envelope and species' preference for epi-hypogean habitats. The ecological meaning of differences in niches was evaluated using distributional data and co-occurrence frequencies. We show that the species comprise two pairs of sister species. All species differ from each other and the degree of differentiation is not related to phylogenetic relatedness. Moreover, low co-occurrence frequencies in sympatric zones imply present or past interspecific competition. This pattern suggests that species are not differentiated enough to reduce interspecific competition, nor ecologically equivalent to co-exist via neutral dynamics. We tentatively conclude that the question of ecological equivalence relates to the scale of the study: at a fine scale, species' differences may influence dynamics in a local community, whereas at the regional level these species likely play roughly similar ecological roles.

Phenotypically similar species coexisting in extreme environments like sulfidic water are subject to two opposing eco-evolutionary processes: those favoring similarity of environment-specific traits, and those promoting differences of traits related to resource use. The former group of processes includes ecological filtering and convergent or parallel evolution, the latter competitive exclusion, character displacement and divergent evolution. We used a unique eco-evolutionary study system composed of two independent pairs of coexisting amphipod species (genus Niphargus) from the sulfidic caves Movile in Romania and Frasassi in Italy to study the relative contribution and interaction of both processes. We looked at the shape of the multifunctional ventral channel as a trait ostensibly related to oxygenation and sulfide detoxification, and at body size as a resource-related trait. Phylogenetic analysis suggests that the sulfidic caves were colonized separately by ancestors of each species. Species within pairs were more dissimilar in their morphology than expected according to a null model based on regional species pool. This might indicate competitive interactions shaping the morphology of these amphipod species. Moreover, our results suggest that the shape of the ventral channel is not subject to long-term convergent selection or to the process of environmental filtering, and as such probably does not play a role in sulfide tolerance. Nevertheless, the ancestral conditions reconstructed using the comparative method tended to be more similar than null-model expectations. This shift in patterns may reflect a temporal hierarchy of eco-evolutionary processes, in which initial environmental filtering became later on superseded by character displacement or other competition-driven divergent evolutionary processes.

Amphipods are key organisms in many freshwater systems and contribute substantially to the diversity and functioning of macroinvertebrate communities. Furthermore, they are commonly used as bioindicators and for ecotoxicological tests. For many areas, however, diversity and distribution of amphipods is inadequately known, which limits their use in ecological and ecotoxicological studies and handicaps conservation initiatives. We studied the diversity and distribution of amphipods in Switzerland (Central Europe), covering four major drainage basins, an altitudinal gradient of>2,500 m, and various habitats (rivers, streams, lakes and groundwater). We provide the first provisional checklist and detailed information on the distribution and diversity of all amphipod species from Switzerland. In total, we found 29 amphipod species. This includes 16 native and 13 non-native species, one of the latter (Orchestia cavimana) reported here for the first time for Switzerland. The diversity is compared to neighboring countries. We specifically discuss species of the genus Niphargus, which are often receiving less attention. We also found evidence of an even higher level of hidden diversity, and the potential occurrence of further cryptic species. This diversity reflects the biogeographic past of Switzerland, and suggests that amphipods are ideally suited to address questions on endemism and adaptive radiations, post-glaciation re-colonization and invasion dynamics as well as biodiversity-ecosystem functioning relationships in aquatic systems.

In the last decade, several studies have shown that subterranean aquatic habitats harbor cryptic species with restricted geographic ranges, frequently occurring as isolated populations. Previous studies on aquatic subterranean species have implied that habitat heterogeneity can promote speciation and that speciation events can be predicted from species' distributions. We tested the prediction that species distributed across different drainage systems and karst sectors comprise sets of distinct species. Amphipods from the genus Niphargus from 11 caves distributed along the Western Carpathians (Romania) were investigated using three independent molecular markers (COI, H3 and 28S). The results showed that: 1) the studied populations belong to eight different species that derive from two phylogenetically unrelated Niphargus clades; 2) narrow endemic species in fact comprise complexes of morphologically similar species that are indistinguishable without using a molecular approach. The concept of monophyly, concordance between mitochondrial and nuclear DNA, and the value of patristic distances were used as species delimitation criteria. The concept of cryptic species is discussed within the framework of the present work and the contribution of these species to regional biodiversity is also addressed.

Originally described by Meštrov in 1962, hypotelminorheic habitats are superficial subterranean drainages, typically less than a meter or so in depth, that emerge at small seepage springs. These are persistent wet spots, typically with blackened leaves in small depressions. There may be no flow during dry periods, but the underlying clay retains water above. They share the landscape with other small bodies of water (močila in Slovenian), not necessarily connected with groundwater. Hypotelminorheic habitats (mezišča in Slovenian) usually harbor a fauna dominated by species adapted to subterranean life, characteristically without eyes or pigment. The basic chemistry and hydrology of the habitat is described as are the basic faunal elements. The habitat is placed in a more general context by reviewing how species invade the habitat, their morphology, and their possible connection to deeper subterranean habitats.