Mollusc species from the Pontocaspian region - an expert opinion list

Fifth mass extinction event triggered the diversification of the largest family of freshwater gastropods (Caenogastropoda: Truncatelloidea: Hydrobiidae)

The fifth mass extinction event (MEE) at the Cretaceous-Palaeogene (K-Pg) boundary 66 million years ago (Ma) led to massive species loss but also triggered the diversification of higher taxa. Five models have been proposed depending on whether this diversification occurred before, during or after the K-Pg boundary and the rate of species accumulation. While the effects of the K-Pg MEE on vertebrate evolution are relatively well understood, the impact on invertebrates, particularly in freshwater ecosystems, remains controversial. One example is the hyperdiverse Hydrobiidae-the most species-rich family of freshwater gastropods. Whereas some studies place its origin in the Jurassic or even Carboniferous, most fossil records postdate the K-Pg event. We therefore used robustly time-calibrated multi-locus phylogenies of >400 species representing >100 hydrobiid genera to unravel its evolutionary history and patterns of diversification. We found that the family started diversifying shortly after the K-Pg boundary (∼60 Ma; 95% highest posterior density 52-69 Ma). Lineage richness gradually increased to the present and phylogenetic diversity until ∼25 Ma. These findings suggest that diversification was not initially driven by ecological opportunity. Combining the two criteria of timing and rate of diversification, a soft-explosive diversification model of aquatic vertebrates best fits the patterns observed. We also show that most higher hydrobiid taxa (i.e. subfamilies) diversified from the Middle Oligocene to Middle Miocene (i.e. 12-28 Ma). Two of the 15 major clades delimited are described here as new subfamilies (i.e. Bullaregiinae n. subfam. and Pontobelgrandiellinae n. subfam.), whose members are restricted to subterranean waters. Our results are an important contribution to understanding how the fifth MEE has shaped evolution and patterns of biodiversity in continental aquatic systems. Given the high extinction risks faced by many hydrobiids today, they also emphasise the need to study the biodiversity of vulnerable ecosystems.

Population genetic insights into establishment, adaptation, and dispersal of the invasive quagga mussel across perialpine lakes

Human activities have facilitated the invasion of freshwater ecosystems by various organisms. Especially, invasive bivalves such as the quagga mussels, Dreissena bugensis, have the potential to alter ecosystem function as they heavily affect the food web. Quagga mussels occur in high abundance, have a high filtration rate, quickly spread within and between waterbodies via pelagic larvae, and colonize various substrates. They have invaded various waterbodies across the Northern Hemisphere. In Central Europe, they have invaded multiple large and deep perialpine lakes with first recordings in Lake Geneva in 2015 and 2016 in Lake Constance. In the deep perialpine lakes, quagga mussels quickly colonized the littoral zone but are also abundant deeper (>80 m), where they are often thinner and brighter shelled. We analysed 675 quagga mussels using ddRAD sequencing to gain in-depth insights into the genetic population structure of quagga mussels across Central European lakes and across various sites and depth habitats in Lake Constance. We revealed substantial genetic differentiation amongst quagga mussel populations from three unconnected lakes, and all populations showed high genetic diversity and effective population size. In Lake Constance, we detected no genetic differentiation amongst quagga mussels sampled across different sites and depth habitats. We also did not identify any convincing candidate loci evidential for adaptation along a depth gradient and a transplant experiment showed no indications of local adaptation to living in the deep based on investigating growth and survival. Hence, the shallow-water and the deep-water morphotypes seem to be a result of phenotypic plasticity rather than local adaptation to depth. In conclusion, our ddRAD approach revealed insight into the establishment of genetically distinct quagga mussel populations in three perialpine lakes and suggests that phenotypic plasticity and life history traits (broadcast spawner with high fecundity and dispersing pelagic larvae) facilitate the fast spread and colonization of various depth habitats by the quagga mussel.

Multidisciplinary Study of the Rybachya Core in the North Caspian Sea during the Holocene

Mollusk fauna is an important component of the Caspian Sea ecosystem alongside ostracods and diatoms. These faunal proxies are essential indicators of hydrological shifts reflecting global and regional climate changes. Adding lithological, geochemical, and geochronological (radiocarbon) data, we revealed paleogeographic events of different scales recorded in the sequence of the Rybachya core from the North Caspian Sea. Here, we present the reconstruction of Mangyshlak paleovalley sediments during the Holocene multi-stage Neocaspian transgression, reflecting global and regional climate changes varying in scale and direction. The determined age of paleovalley-fill sediments, 8070 ± 110 cal yr BP and 7020 ± 140 cal yr BP, suggests that sedimentation processes with extended warming and humidification started later and lasted longer than was assumed earlier. Biological proxies indicate quasi-cyclic variability and shifts from brackish to freshwater conditions throughout the studied interval. Rybachya core was obtained from the early Khvalynian deposits. The Mangyshlak flow formed the depression and eroded the late Khvalynian deposits, which we did not observe in the core structure. It possibly collapsed into paleodepression and acted as a host material for the freshwater lentic faunal association. During the Holocene, we detected a transition from a tranquil water regime to a more dynamic one during the paleovalley gradual filling, followed by marine conditions typical for the modern Caspian Sea.

A DNA barcode reference library for endemic Ponto-Caspian amphipods

The Ponto-Caspian region is an endemicity hotspot that harbours several crustacean radiations, among which amphipods are the most diverse. These poorly known species are severely threatened in their native range, while at the same time they are invading European inland waters with significant ecological consequences. A proper taxonomic knowledge of this fauna is paramount for its conservation within the native region and monitoring outside of it. Here, we assemble a DNA barcode reference library for nearly 60% of all known Ponto-Caspian amphipod species. We use several methods to define molecular operational taxonomic units (MOTUs), based on two mitochondrial markers (COI and 16S), and assess their congruence with current species-level taxonomy based on morphology. Depending on the method, we find that 54–69% of species had congruent morpho-molecular boundaries. The cases of incongruence resulted from lumping distinct morphospecies into a single MOTU (7–27%), splitting a morphospecies into several MOTUs (4–28%), or both (4–11%). MOTUs defined by distance-based methods without a priori divergence thresholds showed the highest congruence with morphological taxonomy. These results indicate that DNA barcoding is valuable for clarifying the diversity of Ponto-Caspian amphipods, but reveals that extensive work is needed to resolve taxonomic uncertainties. Our study advances the DNA barcode reference library for the European aquatic biota, paving the way towards improved taxonomic knowledge needed to enhance monitoring and conservation efforts.

NEW DATA ON THE DISTRIBUTION OF GASTROPOD <i>THEODOXUS PALLASI</i> LINDHOLM, 1924 (MOLLUSCA, GASTROPODA) IN THE KUIBYSHEV RESERVOIR, RUSSIA

The gastropod Theodoxus pallasi Lindholm, 1924, which is a representative of the Ponto-Caspian fauna complex, has been recorded for the first time in the Volga reach of the Kuibyshev reservoir. Previously, it was indicated only for the Near-dam reach of the reservoir. The data indicate the continuing process of settling by the invader mollusk up the Volga River.

Poor hDNA-Derived NGS Data May Provide Sufficient Phylogenetic Information of Potentially Extinct Taxa

Museum material is an important source of metadata for past and recent biological events. With current sequencing technologies, it is possible to obtain historical DNA (hDNA) from older material and/or endangered species to answer taxonomic, systematic, and biogeographical questions. However, hDNA from museum collections is often highly degraded, making it difficult to assess relationships at or above the species level. We therefore studied two probably extinct gastropod species of the genus Laevicaspia, which were collected ∼140 years ago in the Caspian Sea, to map “standard” mitochondrial and nuclear markers and assess both the sequencing depth and the proportion of ambiguous sites as an indicator for the phylogenetic quality of the NGS data. Our study resulted in the first phylogenetically informative mitochondrial and nuclear markers for L. caspia. Assessment of both sequencing depth (mean coverage) and proportion of ambiguous sites suggests that our assembled consensus sequences are reliable for this species. In contrast, no informative gastropod-specific DNA was obtained for L. conus, likely due to a high degree of tissue digestion and contamination with non-gastropod DNA. Nevertheless, our results show that hDNA may in principle yield high-quality sequences for species-level phylogenetic analyses, which underlines the importance of museum collections as valuable archives of the biological past.

Legal Framework for Pontocaspian Biodiversity Conservation in the Danube Delta (Romania and Ukraine)

The declining biodiversity has upsetting consequences for social and economic development and represents a major concern for humanity. Legal and political framework plays an important role in biodiversity conservation planning, implementation, and coordination of actions. Legal provisions are complex and operate on different levels of governance (from supranational to national), which means that the status of single species or populations may be governed by a set of interacting or even conflicting regulations, with increasing complexity for species that occur across national borders. Romania (EU member state) and Ukraine (non-EU member state) exemplify neighboring countries with different governance systems, which share the same endemic aquatic communities inhabiting the transitional zones between freshwater and marine ecosystems, known regionally as Pontocaspian (PC) biota. These communities include flagship species such as sturgeons and less-known crustaceans and mollusks and are severely threatened as a result of human activities. We assessed the legal basis for the protection of PC biota in the Danube Delta and the effectiveness of current conservation approaches based on a review of legal documents and literature, expert opinion, and practitioner reflections regarding PC biodiversity conservation. We found that PC invertebrate species are not adequately addressed in the current legal documents and that the surrogate approach (where protection of umbrella species results in protection of background species) does not work as there is little overlap between the habitats of sturgeons and PC invertebrate communities. Furthermore, the habitat definitions currently used in legal documents lack the level of detail needed to protect PC habitats that are characterized by specific salinity (brackish) conditions. We finish by sketching out recommendations toward improved legal and political frameworks for effective and efficient conservation of PC invertebrate biota.

Rapid spread of a new alien and potentially invasive species, Clathrocaspia knipowitschii (Makarov, 1938) (Gastropoda: Hydrobiidae), in the Danube River

The distribution and spread of a new alien gastropod species, Clathrocaspia knipowitschii (Makarov, 1938) (Gastropoda: Hydrobiidae: Caspiinae) in the Danube River was examined. First findings of this species for Hungary, Slovakia, Romania, Bulgaria and Serbia are presented. Clathrocaspia knipowitschii was initially found in 2013 in the Iron Gate stretch of the Danube River at the border between Romania and Serbia. In 2019 and 2020, the species was found at several sites in the lower Danube in Romania, Serbia and Bulgaria, and also upstream in the middle Hungarian Danube in high population densities. The species appears to have spread along more than 800 km in six years. This finding together with the available abundance data indicates that C. knipowitschii is potentially an invasive species, but further observations are needed.

Decline of unique Pontocaspian biodiversity in the Black Sea Basin: A review

The unique aquatic Pontocaspian (PC) biota of the Black Sea Basin (BSB) is in decline. The lack of detailed knowledge on the status and trends of species, populations, and communities hampers a thorough risk assessment and precludes effective conservation. This paper reviews PC biodiversity trends in the BSB (Bulgaria, Romania, Moldova, Ukraine, and Russia) using endemic mollusks as a model group. We aim to assess changes in PC habitats, community structure, and species distribution over the past century and to identify direct anthropogenic threats. The presence/absence data of target mollusk species were assembled from literature, reports, and personal observations. Pontocaspian biodiversity trends in the northwestern BSB coastal regions were established by comparing 20th- and 21st-century occurrences. The direct drivers of habitat and biodiversity change were identified and documented. We found that a pronounced decline of PC species and communities is driven by (a) damming of rivers, (b) habitat modifications that disturbed previous natural salinity gradients and settings in the studied area, (c) pollution and eutrophication, (d) invasive alien species, and (e) climate change. Four out of the 10 studied regions, namely, the Danube Delta-Razim Lake system, Dniester Liman, Dnieper-Bug estuary, and Taganrog Bay-Don Delta, contain favorable ecological conditions for PC communities and still host threatened endemic PC mollusk species. Distribution data are incomplete, but the scale of deterioration of PC species and communities is evident from the assembled data, as are major direct threats. Pontocaspian biodiversity in the BSB is profoundly affected by human activities. Standardized observation and collection data as well as precise definition of PC biota and habitats are necessary for targeted conservation actions. This study will help to set the research and policy agenda required to improve data collection to accommodate effective conservation of the unique PC biota.

Dreissenids’ breaking loose: differential attachment as a possible driver of the dominance shift between two invasive mussel species

Ponto-Caspian dreissenids are notorious freshwater invaders. Recently, widespread observations show a dominance shift from the early invader, Dreissena polymorpha, to its successor, Dreissena bugensis. These observations likely reflect congeneric species differences in physiological and behavioural traits. Here, we assessed the mussel attachment strength, attachment rate, and the mode of byssal failure as trait differences that could potentially contribute to dominance shifts. The attachment traits were measured in field and laboratory experiments. Fouling plates were deployed in the Rhine-Meuse river delta and dreissenids were collected and acclimatised in 60 L non-aerated freshwater tanks. Attachment strength was positively correlated with shell size. The attachment strength of D. bugensis was significantly greater compared to slower growing D. polymorpha individuals of a dreissenid field assemblage. This corresponded to the superior byssal thread morphology of D. bugensis (i.e. higher number and two times wider byssal threads). Moreover, our results indicated that byssal threads of D. bugensis are stronger than those of D. polymorpha, as the latter ruptured more often. Additionally, D. bugensis had a significantly lower attachment rate than D. polymorpha. Having a greater attachment strength gives D. bugensis an advantage when it comes to withstanding currents and predators. On the other hand, not being attached allows an individual to actively move around. This would allow them to move away from fast changing unfavourable environmental conditions. These attachment traits indicate competitive benefits for D. bugensis over D. polymorpha, therefore possibly contributing to the dominance shifts.

Social network analysis and the implications for Pontocaspian biodiversity conservation in Romania and Ukraine: A comparative study

Romania and Ukraine share the Black Sea coastline, the Danube Delta and associated habitats, which harbor the endemic, aquatic Pontocaspian biota. Currently, this biota is diminishing both in numbers of species and their abundance because of human activities, and its future persistence strongly depends on the adequacy of conservation measures. Romania and Ukraine have a common responsibility to address the conservation of Pontocaspian biodiversity. The two countries, however have different socio-political and legal conservation frameworks, which may result in differences in the social network structure of stakeholder institutions with different implications for Pontocaspian biodiversity conservation. Here, we study the social network structure of stakeholder organizations involved in conservation of Pontocaspian biodiversity in Romania and the implications of network structure for conservation outcomes. Then we compare the findings from Romania to an earlier similar study from Ukraine. We apply a mix of qualitative and quantitative social network analysis methods to combine the content and context of the interactions with relational measures. We show that Pontocaspian biodiversity plays a minor and mostly incidental role in the inter-organizational interactions in Romania. Furthermore, there is room for improvement in the network structure through e.g. more involvement of governmental and nongovernmental organizations and increased motivation of central stakeholders to initiate conservation actions. Social variables, such as lack of funding, hierarchical, non-inclusive system of conservation governance and continuous institutional reforms in the public sector are consequential for the network relations and structure. Social network of stakeholders in Ukraine is more connected and central stakeholders utilize their favorable positions. However, neither in Ukraine is the Pontocaspian biodiversity a driver of organizational interactions. Consequently, both networks translate into sub-optimal conservation actions and the roads to optimal conservation are different. We end with sketching out conservation implications and recommendations for improved national and cross-border conservation efforts.

The Pleistocene-Holocene aquatic molluscs as indicators of the past ecosystem changes in Transbaikalia (Eastern Siberia, Russia)

Data on the historical change of the Transbaikalian malacofauna in the Neopleistocene and Holocene is presented. Aquatic mollusc shells from archaeological excavations of the ancient settlements dating from the Neolithic period to Medieval and also from a drill hole of the Neopleistocene alluvial deposits were collected. In total eight species of bivalve molluscs from the families Margaritiferidae, Unionidae, Lymnocardiidae, Glycymerididae [marine], and two gastropod species from families Viviparidae and Planorbidae were identified. These species were aged using radiocarbon dating. It was found that the species ranged in age from more than 50.000 to 2.080-1.210 years BP. Five species inhabited the Transbaikal region which are locally extirpated today. Their disjunctive ranges in the past included southern Europe and Western and Eastern Siberia to Transbaikalia and in the east to Far East and Primorye Territory of Russia. A remarkable finding is that of the bivalve genus Monodacna, which was found very far from its native range, the Ponto-Caspian region. The time of existence and extirpation of the thermophilic species of genera Monodacna, Planorbis, Lanceolaria and Amuropaludina corresponds to cycles of the warming and cooling in Pleistocene and Holocene according to regional climate chronological scales. These species can be used as palaeoclimate indicators. Change of the regional malacofaunal species composition is connected with the natural climatochron cycles in the Pleistocene and Holocene resulting in evidence for succession. In the course of this succession, these stenothermal species became extirpated on a regional level, decreasing their global ranges.

A revision of the extant species of Theodoxus (Gastropoda, Neritidae) in Asia, with the description of three new species

Asia contains a high species diversity of the freshwater gastropod genusTheodoxus. Recent molecular and morphological reviews of this diversity have uncovered a number of yet undescribed species while suggesting the urgent revision of several others. Moreover, some of these studies have indicated a number of species previously not recorded for this continent. Despite the advancements, a taxonomic revision and an update on the distribution ofTheodoxusspp. in Asia is still pending. Here, we construct the most robust phylogeny ofTheodoxusup to now and review original descriptions, type material, recent taxonomic revisions, compendia, and species lists to provide a comprehensive checklist of all known extant AsianTheodoxusspp. Our checklist also provides descriptions for three recently discovered and yet undescribed species (Theodoxus gururSands &amp; Glöer,sp. nov.,Theodoxus wesselinghiSands &amp; Glöer,sp. nov., andTheodoxus wilkeiSands &amp; Glöer,sp. nov.), as well as shows the need to synonymise several previously described morphospecies. The present revision recognizes 14 extantTheodoxusspp. for Asia. Some of these species are widespread, while others are endemic to just a single location. Based on the revised and new distribution data, we provide updates and new assessments of species conservation statuses.

Old lake versus young taxa: a comparative phylogeographic perspective on the evolution of Caspian Sea gastropods (Neritidae: Theodoxus)

The Caspian Sea has been a highly dynamic environment throughout the Quaternary and witnessed major oscillations in lake level, which were associated with changes in salinity and habitat availability. Such environmental pressures are considered to drive strong phylogeographic structures in species by forcing populations into suitable refugia. However, little is actually known on the effect of lake-level fluctuations in the Caspian Sea on its aquatic biota. We compared the phylogeographic patterns of the aquatic Neritidae snail genus Theodoxus across the Pontocaspian region with refugial populations in southern Iran. Three gene fragments were used to determine relationships and divergence times between the sampled populations in both groups. A dated phylogeny and statistical haplotype networks were generated in conjunction with the analyses of molecular variance and calculations of isolation by distance using distance-based redundancy analyses. Extended Bayesian skyline plots were constructed to assess demographic history. Compared with the southern Iranian populations, we found little phylogeographic structure for the Pontocaspian Theodoxus group, with more recent diversification, homogeneity of haplotypes across the Pontocaspian region and a relatively stable demographic history since the Middle Pleistocene. Our results argue against a strong influence of Caspian Sea low stands on the population structure post the early Pleistocene, whereas high stands may have increased the dispersal possibilities and homogenization of haplotypes across the Pontocaspian region during this time. However, during the early Pleistocene, a more dramatic low stand in the Caspian Sea, around a million years ago, may have caused the reduction in Theodoxus diversity to a single lineage in the region. In addition, our results provide new insights into Theodoxus taxonomy and outlooks for regional conservation.

Historical faunal exchange between the Pontocaspian Basin and North America

Ecrobia is a genus of small brackish-water mud snails with an amphi-Atlantic distribution. Interestingly, the species occurring in the northwestern Atlantic, Ecrobia truncata, is more closely related to the Pontocaspian taxa, Ecrobia grimmi and Ecrobia maritima, than to the species occurring in the northeastern Atlantic and Mediterranean Sea. At least three colonization scenarios may account for this peculiar biogeographical pattern: (1) a recent human-mediated dispersal, (2) a historical transatlantic interchange, and (3) a historical transpolar interchange. To test these three scenarios, we used five operational criteria-time of species divergence, first appearance in the fossil record, dispersal limitation as well as environmental filtering and biotic interactions along the potential migration routes. Specifically, we inferred a time-calibrated molecular phylogeny for Ecrobia and reconstructed a paleogeographical map of the Arctic Ocean at 2.5 million years ago (Mya). Based on the five operational criteria, scenarios 1 and 2 can likely be rejected. In contrast, all criteria support scenario 3 (historical transpolar interchange). It is therefore suggested that a bird-mediated and/or ocean current-mediated faunal interchange via the Arctic Ocean occurred during the Late Pliocene or Early Pleistocene. This dispersal was likely facilitated by reduced distances between the Eurasian and North American/Greenland landmasses, marine introgressions, and/or a stepping-stone system of brackish-water habitats in northern Siberia, as well as a lack of competition along the migration route. As for the direction of dispersal, the scientific data presented are not conclusive. However, there is clearly more support for the scenario of dispersal from the Pontocaspian Basin to North America than vice versa. This is the first study providing evidence for a natural faunal exchange between the Pontocaspian Basin and North America via the Arctic Ocean.

Differential impact of anthropogenic pressures on Caspian Sea ecoregions

Over the past decades, overall ecological conditions in the Caspian Sea have deteriorated. However, a comprehensive understanding of lake-wide spatial differences in anthropogenic pressures is lacking and the biological consequences of human impacts are poorly understood. This paper therefore aims at assessing the individual and combined effects of critical anthropogenic pressures on the Caspian Sea ecoregions. First, cumulative pressure scores were calculated with a cumulative environmental assessment (CEA) analysis. Then, the individual contribution of anthropogenic pressures was quantified. Finally, ecoregion-specific differences were assessed. The analyses show that both cumulative and individual pressure scores are unevenly distributed across the Caspian Sea. The most important individual pressures are invasive species, chemical pollution and poaching. This uneven distribution of pressure scores across Caspian Sea ecoregions creates new challenges for future conservation strategies, as different ecoregions usually require different conservation measures.