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Freshwater mussels (Unionida) are one of the most imperiled animal groups worldwide, revealing the fastest rates of extinction. Habitat degradation, river pollution and climate change are the primary causes of global decline. However, biological threats for freshwater mussels are still poorly known. Here, we describe a diverse ecological group of leeches (Hirudinea: Glossiphoniidae) inhabiting the mantle cavity of freshwater mussels. So far, examples of mussel-associated leech species are recorded from East Asia, Southeast Asia, India and Nepal, Africa, and North America. This group comprises a dozen glossiphoniid species with a hidden life style inside the mantle cavity of their hosts largely overlooked by researchers. We show that the association with freshwater mussels evolved independently in three leech clades, i.e. Batracobdelloides, Hemiclepsis, and Placobdella, at least since the Miocene. Seven mussel-associated leech species and two additional free-living taxa are described here as new to science.

With the biodiversity crisis continuing unchecked, we need to establish levels and drivers of extinction risk to effectively allocate conservation resources and develop targeted actions. Given that threat appears particularly high in freshwaters, we assessed the extinction risk of 1,500 randomly selected freshwater molluscs using the IUCN Red List Categories and Criteria, as part of the Sampled Red List Index project. We show that close to one third of species in our sample are estimated to be threatened with extinction, with highest levels of threat in the Palearctic, Australasia and Nearctic and among gastropods. Threat levels were higher in lotic than lentic systems. Twenty-seven species were classified as Extinct (eight bivalves and 19 gastropods), mostly from the Nearctic realm and lotic systems. Pollution and the modification of natural systems (e.g. through damming and water abstraction) were the most frequently reported threats to freshwater molluscs, with some regional variation. Given that we found little spatial congruence between species richness patterns of freshwater molluscs and other freshwater taxa, new additional conservation priority areas emerge from our study. We discuss the implications of our findings for freshwater mollusc conservation and important next steps to estimate trends in freshwater mollusc extinction risk over time.

Fourteen species of the family Lymnaeidae were revealed in the samples taken from lowland and mountainous parts of the Ukrainian Transcarpathian. Five species are regionally new: Stagnicola corvus, Radix parapsilia, R. balthica, R. ampla and R. tumida; two regionally rare species (Ladislavella terebra and Lymnaea fragilis) are discovered. The most widespread pond snails appear to be Lymnaea stagnalis and Radix intermedia, the rarest one – L. terebra and R. parapsilia found in single locality each. The lowland part of the Ukrainian Transcarpathian maintains more than 4/5 regional fauna of the Lymnaeidae, though a few species occurred in the mountainous locations collected above the 400 m a.s.l. The check-list of species, with appropriate comments on taxonomy, distribution and some ecological characteristics of the lymnaeids found within the studied area is provided. A list of species of aquatic snails and bivalves co-occurring with the Lymnaeidae in the studied area is provided. Relatively high species diversity of malacofauna in the region combined with rarity of some species allow to consider the Transcarpathian as a specific region apparently representing a separate biogeographic unit. There is a need for a further estimate of the regionally rare species in order to clarify their potential conservation status.