Diversity and Biogeography of Crustaceans in Continental Waters

Continental freshwater ecosystems occupy only a small fraction of the Earth’s surface, yet they sustain an exceptionally high share of global biodiversity and are among the most rapidly degrading habitats [1]. Crustaceans, ranging from microscopic copepods and cladocerans to large-bodied decapods, represent key components of these systems. They function as primary consumers, predators, and bioturbators, while also serving as sensitive indicators of environmental changes [2,3,4]. Their biogeographic patterns reflect the combined imprints of ancient geology, Pleistocene climate fluctuations, and contemporary anthropogenic pressures [5,6]. This Special Issue of Diversity, “Diversity and Biogeography of Crustaceans in Continental Waters”, assembles eight original research papers that collectively cover a broad geographic range—from the arctic islands of the Russian Far East to lakes in northwestern Italy—and approach crustacean diversity from multiple, complementary perspectives, including taxonomy and species description, historical biogeography, community ecology along environmental gradients, biological invasions, and ecotoxicology.

The contributions can be grouped into four overarching themes, discussed in turn below: (i) historical biogeography, relict lineages, and phylogeography; (ii) environmental gradients as fundamental structuring forces; (iii) biological invasions and faunal homogenization; and (iv) emerging pollutants and the use of freshwater crustaceans as model organisms in ecotoxicological research.

Three papers address how Pleistocene and earlier events have shaped the present-day distribution of continental crustaceans. Marin and Palatov describe Niphargus rostovi sp. nov. (Amphipoda: Niphargidae) from a spring in Rostov-on-Don, representing the first stygobiotic representative of this genus recorded from the lowland sector of the northern Black/Azov Sea region. The new species is among the smallest known within the genus Niphargus and belongs to the “carpathicus” complex, with COI-based estimates placing its divergence from the closest Greek congener at more than 10 million years ago. The occurrence of three stygobiotic crustacean species in the same spring—together with the crangonyctids Diasynurella kiwi and Pontonyx donensis—strongly suggests that the lower Don region constitutes a previously unrecognized glacial refugium, extending known Western Palaearctic refugia beyond the classic mountain-sheltered areas. The biogeographic role of pre-Pleistocene relict endemics is examined at a continental scale by Krolenko et al., who analyzed 442 qualitative samples of planktonic Cladocera collected along a latitudinal transect from Khabarovsk to Jeju Island (South Korea). They identified 101 species grouped into four faunistic complexes. Benthic and littoral habitats display a clear latitudinal replacement of boreal taxa by tropical ones, whereas the open-water plankton is dominated by relict endemics and lacks a comparable mixing. The authors propose that these pre-Pleistocene endemics occupied planktonic niches prior to the arrival of boreal and tropical colonizers, thereby constraining subsequent northward and southward range expansions. This study underscores how biotic interactions and colonization history can preserve biogeographic boundaries across regions. A third phylogeographic perspective is provided by Sukhikh et al., who combine morphological and molecular analyses of the Ponto-Caspian calanoid copepod Eurytemora velox across populations from Europe, the Southern Urals, and Ob Bay in Western Siberia. Although nuclear markers reveal little differentiation, mitochondrial COI data indicate that Ural and Siberian populations form a distinct lineage that diverged from European populations in the Middle Pleistocene. The species itself likely originated during the Palaeogene–Neogene. These findings complicate the conventional scenario of a recent west-to-east invasion into Siberia and instead suggest long-term persistence in local refugia, paralleling the refugial patterns inferred by Marin and Palatov for subterranean stygobiotic fauna.

Two papers investigate how abiotic gradients, primarily salinity, structure crustacean and broader hydrobiont assemblages in distinct geographic contexts. Novichkova et al. present the first comprehensive survey of freshwater invertebrates from lakes and streams of the Shantar Islands (Sea of Okhotsk) and adjacent mainland habitats. The study documents contrasting responses of three major organismal groups along the salinity gradient of the brackish Lake Bolshoe. Zooplankton attain their highest diversity at the freshwater-brackish transition zone; meiobenthic diversity peaks in the most saline area due to penetration by marine species; and the dominance structure of macrozoobenthos shifts sharply at the critical salinity threshold of 7‰, where amphibiotic insects yield dominance to amphipods. Groundwater habitats on small Mediterranean islands provide an extreme case of the salinity–diversity relationship. Di Lorenzo et al. report a clear segregation of crustacean assemblages along a conductivity gradient: stygobiotic Copepoda and Syncarida occurred exclusively in low- and intermediate-salinity bores, while euryhaline, non-stygobiotic taxa prevailed at higher conductivity, and the most saline bore was entirely devoid of them. Stygobiotic species on Pianosa are regarded as “living fossils”, characterized by relict origins, long life cycles, and limited dispersal ability so that their local extinction effectively erases unique populations. The study emphasizes that salinity acts as a primary driver of crustacean distribution in the island’s groundwater ecosystems.

Two additional papers examine the impacts of non-indigenous crustaceans and other alien taxa on native communities. Aubakirova et al. describe the contemporary zooplankton assemblage of the Middle Caspian Sea, which is now heavily dominated by invasive species comprising more than 60% of total species richness in the study area. During the research (2020–2021), only three native species were recorded, of which only Podonevadne camptonyx ranked among the dominant taxa. The dynamics of zooplankton composition and abundance appear to be driven both by salinity fluctuations and biotic interactions, including grazing pressure from the invasive ctenophore Mnemiopsis leidyi. At an earlier stage of invasion dynamics, Boggero et al. report the first systematic survey of non-native crayfish in lowland lakes of Piedmont, northwestern Italy. Three North American crayfish species were recorded. The results indicate that alien species spread slowly into isolated water bodies, particularly where crayfish predators persist and poaching pressure exists. The authors argue that monitoring ongoing invasions provides a critical basis for assessing the transformation of freshwater ecosystems under global climate warming.

Extending the invasion theme into the realm of pollution, Huang et al. investigate the fate of microplastics in the digestive tract of red swamp crayfish under controlled laboratory conditions. They trace the dynamics of microplastic accumulation within crustacean tissues and examine how digestion modifies particle integrity. The study provides valuable insights into the ecological risks associated with this emerging pollutant and lays the groundwork for future research on the bioaccumulation and trophic transfer of microplastics through aquatic food webs.

Collectively, the eight papers assembled in this Special Issue, despite their diversity of scope and methodology, converge on a unifying message: continental water bodies harbor crustacean diversity that is ancient in origin, highly sensitive to environmental change, and increasingly threatened by human activity. The studies underscore that the distribution of crustacean fauna in inland waters still bears the imprint of past environmental conditions that are no longer evident or easily predictable. Molecular approaches therefore play a crucial role in reconstructing historical dispersal routes and identifying refugia of preglacial fauna. At the same time, the present-day distributions of crustacean species remain highly dynamic against the backdrop of accelerating climate change. The spread of invasive species, fueled by human actions, continues to drive irreversible shifts in native assemblages. Yet the research represented here captures only a small part of the expanding field of crustacean studies in continental waters. This Special Issue seeks to highlight the enduring importance of investigating both individual species and the broader ecological processes shaping crustacean planktonic and benthic assemblages worldwide.