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Loricifera is a phylum of microscopic animals that inhabit marine environments worldwide. Named after their conspicuous and protective lorica, the phylum was first described from Roscoff (France) in 1983 and, hitherto, it contains only 40 species. Based on data collected from Roscoff during the past four decades, we here describe two new species of Nanaloricus, namely Nanaloricus valdemari sp. nov. and Nanaloricus mathildeae sp. nov., as well as a new genus and species, Scutiloricus hugoi gen. et sp. nov. Adults of N. valdemari sp. nov. are distinguished by a pair of unique cuticular ridges, here referred to as longitudinal stripes, spanning laterally along the anterior two thirds of the dorsal lorical plate. N. mathildeae sp. nov. is characterized by strong sexual dimorphism. Specifically, the branches composing the multiform male clavoscalids are much broader as compared to other Nanaloricus species. The two new Nanaloricus species are both characterized by unique sensory organs associated with the double trichoscalids. The size and exact position of these organs differ between the two species. Adults of Scutiloricus hugoi gen. et sp. nov. are characterized by, among other features, a square lorica composed of six cuticular plates with a total of 14 anterior spikes, of which 12 have transverse cuticular ridges and thus appear fenestrated; laterodorsal flosculi arranged linearly; a posterior lorical region characterized by an anal field with a small anal cone flanked by a pair of spurs. Notably, mature females are characterized by a pair of seminal receptacles, a character not previously reported in Loricifera. We discuss the new findings and compare N. valdemari sp. nov. and N. mathildeae sp.nov. with other species assigned to genus Nanaloricus. The distinguishing features of Scutiloricus hugoi gen. et sp. nov. are discussed from a comparative perspective with the other genera of family Nanaloricidae.

Gastrotricha and Platyhelminthes form a clade called Rouphozoa. Representatives of both taxa are main components of meiofaunal communities, but their role in the trophic ecology of marine and freshwater communities is not sufficiently studied. Traditional collection methods for meiofauna are optimized for Ecdysozoa, and include the use of fixatives or flotation techniques that are unsuitable for the preservation and identification of soft-bodied meiofauna. As a result, rouphozoans are usually underestimated in conventional biodiversity surveys and ecological studies. Here, we give an updated outline of their diversity and taxonomy, with some phylogenetic considerations. We describe successfully tested techniques for their recovery and study, and emphasize current knowledge on the ecology, distribution, and dispersal of freshwater gastrotrichs and microturbellarians. We also discuss the opportunities and pitfalls of (meta)barcoding studies as a means of overcoming the taxonomic impediment. Finally, we discuss the importance of rouphozoans in aquatic ecosystems and provide future research directions to fill in crucial gaps in the biology of these organisms needed for understanding their basic role in the ecology of benthos and their place in the trophic networks linking micro-, meio-, and macrofauna of freshwater ecosystems.

Life is not a beach for those animals that survive in the rough ecological conditions found in marine sandy beaches – and yet, microscopic animals thrive on them. We explore the drivers for meiofaunal diversity in beaches by analysing taxonomic and functional patterns of 348 flatworm communities across 116 reflective beaches in the western Mediterranean, totalling 152 species (61.2% new to science). First, we confirm that species richness does not differ between beach hydrodynamic levels (swash, shoaling and surf) but rather depends on the characteristics of each beach. Second, we demonstrate that species composition across those levels depends on the species traits, in addition to geographical and abiotic factors. Third, we highlight that the species functional space has a lower richness than expected and a lower redundancy in the wave‐exposed swash level compared to the shoaling and subtidal levels, suggesting a trait‐based ecological filtering. Finally, we show that those differences depend on the higher frequency of hydrodynamics‐related traits in the species of the swash level. Our results suggest that the rough hydrodynamic conditions in the swash level favour a unique combination of species traits, which might be linked to ecological speciation in flatworms but also in other interstitial animals.

Documenting the diversity of marine life is challenging because many species are cryptic, small, and rare, and belong to poorly known groups. New sequencing technologies, especially when combined with standardized sampling, promise to make comprehensive biodiversity assessments and monitoring feasible on a large scale. We used this approach to characterize patterns of diversity on oyster reefs across a range of geographic scales comprising a temperate location [Virginia (VA)] and a subtropical location [Florida (FL)]. Eukaryotic organisms that colonized multilayered settlement surfaces (autonomous reef monitoring structures) over a 6-mo period were identified by cytochrome c oxidase subunit I barcoding (>2-mm mobile organisms) and metabarcoding (sessile and smaller mobile organisms). In a total area of ∼15.64 m ² and volume of ∼0.09 m ³, 2,179 operational taxonomic units (OTUs) were recorded from 983,056 sequences. However, only 10.9% could be matched to reference barcodes in public databases, with only 8.2% matching barcodes with both genus and species names. Taxonomic coverage was broad, particularly for animals (22 phyla recorded), but 35.6% of OTUs detected via metabarcoding could not be confidently assigned to a taxonomic group. The smallest size fraction (500 to 106 μm) was the most diverse (more than two-thirds of OTUs). There was little taxonomic overlap between VA and FL, and samples separated by ∼2 m were significantly more similar than samples separated by ∼100 m. Ground-truthing with independent assessments of taxonomic composition indicated that both presence–absence information and relative abundance information are captured by metabarcoding data, suggesting considerable potential for ecological studies and environmental monitoring. Significance High-throughput DNA sequencing methods are revolutionizing our ability to census communities, but most analyses have focused on microbes. Using an environmental DNA sequencing approach based on cytochrome c oxidase subunit 1 primers, we document the enormous diversity and fine-scale geographic structuring of the cryptic animals living on oyster reefs, many of which are rare and very small. Sequence data reflected both the presence and relative abundance of organisms, but only 10.9% of the sequences could be matched to reference barcodes in public databases. These results highlight the enormous numbers of marine animal species that remain genetically unanchored to conventional taxonomy and the importance of standardized, genetically based biodiversity surveys to monitor global change.

The metacommunity concept incorporates spatial dynamics into community ecology, shedding light on how local and regional processes interact in structuring ecological communities, and to which measure they are deterministic or stochastic. We reviewed metacommunity studies on freshwater meiobenthos published since 2004, when the main principles of metacommunity theory were conceptualized. The studies (together 19) were observational, focused mainly on ostracods, and rarely on rotifers and nematodes. In accordance with general expectations, the prevalent structuring force was species sorting. Ostracods showed more dispersal limitations than nematodes and rotifers, and there was very little support for dispersal surplus. We discussed the role of body size, dispersal mode, and attachment to sediment for the meiofauna dispersal. Effects of metacommunity context (habitat connectivity, spatial extent, and environmental heterogeneity), study design (e.g., sample size), and statistical approach could not be sufficiently disentangled due to the low number of studies. Local stochasticity, consistent with neutral theory and patch dynamics, was indicated for taxa with weak specialization and metacommunities in small habitats. Our understanding of meiofaunal metacommunities is only fragmentary and it would highly benefit from direct comparisons of taxa with different species traits and between different spatial scales, and studies incorporating temporal dynamics and hypothesis-driven experiments.

Due to the lack of empirical data, meiofauna are often underestimated as prey for freshwater animals and are commonly regarded as trophic dead ends. Here we present a synthesis of recent evidence showing that meiofauna are significant as prey, not only for many benthic macroinvertebrates (chironomids, shrimps, and flatworms) but also for juveniles of widespread freshwater bottom-feeding fish species (e.g., carps, gudgeons, catfish). In this review, we focus on the following questions: (1) Which groups consume meiofauna? (2) In what amounts are meiofauna ingested? (3) Does predatory feeding behavior influence natural meiofaunal communities? (4) Are meiofauna organisms actively ingested or are they bycatch? To answer these questions, we focused on studies that included gut/feces analyses of potential predators and empirical investigations conducted in the laboratory (e.g., functional response experiments and microcosm studies) and in the field (enclosure/exclosure settings). We were able to demonstrate that meiofauna taxa are consumed in high numbers by a wide range of larger organisms. This predation can significantly shape meiofaunal communities, by reducing the abundance, biomass, and production of certain members of the investigated assemblages. However, in most cases, it remains unclear if there is an active predation of meiofauna or a passive ingestion by unselective feeding.

Never heard of harpacticoids, ostracods, gastrotrichs or microturbellarians? This is no surprise, they are so tiny! Yet these taxa and many others more famous (nematodes, rotifers, or tardigrades) show complex behaviours and extraordinary physiologies that allow them to colonize inland waters worldwide. This exuberant fauna is better known as the meiofauna (or meiobenthos). Meiofaunal organisms have been fascinating study objects for zoologists since the seventeenth century and recent research has demonstrated their intermediate role in benthic food webs. This special issue highlights how meiofauna can help freshwater ecologists to describe and predict species distribution patterns, to assess production of biomass and trait functions relationships, as well as to examine the trophic links between microscopic and macroscopic worlds and to better understand species’ resilience to environmental extremes. Overall, meiofaunal organisms are bridging scales, and as such they deserve better integration to develop more comprehensive concepts and theories in ecology.

Much has been published about different aspects of body-size distribution resting on the assumptions of metabolic scaling, although a number of studies in aquatic ecosystems have questioned its generality. This study considers the effects of individual body-mass and biomass variability on scaling properties of multi-species communities (protists, meio- and macroinvertebrates), and their intrinsic variations in assemblage structure. We examine how size traits within communities are distributed on local and regional scales and assess the potential sources of variation affecting whole ecosystems. Our results, built upon seven river catchment communities including 1204 species, revealed micro-meiofauna-dominated biomass distributions driven by stochastic hydrophysical processes that induce a fractal fluctuation scaling, irrespective of trophic levels, shaping local and regional scaling relations. Fractal-scaling differences are largely generated by the frequency of high flow events that influence the biomass assemblage configurations, which are significantly better represented by the Power Fraction model compared to single statistical random models. We conclude that environmental random variability contributes to the decoupling of total biomass and body mass per site from assemblage size, resulting in scale-invariant body-size traits among assemblages and systems. Generally, these findings emphasize that ignoring small-sized species and, thus, the wide range of body sizes makes accurate ecological model predictions, impossible.

In the benthos of lakes, habitat size and structure as well as oxygen or resource availability can affect species distribution at local scales. By contrast, there is little knowledge about the mechanisms that determine the structure of benthic communities at larger scales. Here, we compiled data from the literature (23 studies monitoring 129 sites from 75 lakes located in central and northern Europe) to search for broad patterns in abundance, biomass, and community structure linked to lake trophic state (oligo-, meso-, or eutrophic), habitat features (hard vs. soft substrates), and water depth (littoral vs. profundal). The benthic meiofauna appeared much more abundant in the periphyton, and biomass was lower in oligotrophic lakes. Focusing on free-living nematodes, community structure differed markedly in hard vs soft substrates. Further, nematodes were especially dominant in profundal zones, where their diversity was significantly influenced by lake trophic state. In profundal zones of eutrophic lakes, nematode assemblages were less diverse and dominated by larger, mostly omnivorous or predacious taxa.

Particulate organic carbon (POC) produced in sea ice is often included in stable isotopic food web studies of polar seas as a single value of particulate organic matter (POM), i.e. ‘ice POM’. During 10 field trips to landfast ice off Alaska’s north coast, we examined the seasonal contribution of sea ice-associated meiofauna to total POM and the trophic structure within the sea ice using bulk carbon and nitrogen stable isotope ratios (δ13C, δ15N). Algal biomass, POC/particulate organic nitrogen and meiofaunal abundances increased after the polar night, and a suite of different metazoan meiofauna contributed seasonally substantially to total ice POC amount. δ13C values of meiofauna generally tracked the seasonal enrichment of δ13C in POC suggesting a trophic relationship, also supported by increasing body mass of meiofauna over the seasons. δ15N of individual meiofaunal taxa varied by at least 1.5 trophic levels. δ13C values of some meiofauna were very close to or below POC values suggesting the use of other carbon sources, perhaps including dissolved organic carbon (DOC) and bacteria. Estimated potential grazing rates, based on generated carbon and nitrogen content of individuals in this study, confirmed earlier generally low estimates of grazing impact of the meiofauna on the ice algal spring bloom, leaving large portions of the produced matter as food for pelagic and benthic organisms. These findings suggest a more complex sea ice-based food web structure that should be more commonly incorporated into food web, conceptual and other models.