Sperm storage is a widespread reproductive strategy, allowing females to retain viable sperm for extended periods after mating. It facilitates sperm preservation and utilization and can influence postcopulatory sexual selection (PCSS). While sperm storage is well documented in species with both internal and external fertilization, its implications in species with intermediate fertilization strategies, such as spermcasting, remain less explored. In these species, sperm are released into the environment and subsequently taken up by females, who may store it for later use. Some species of tardigrades, a phylum of microscopic animals, have sperm storage organs (SSOs), but the reproductive implications of SSOs remain poorly understood. In this study, we compared reproductive behaviour and success (i.e. mating and egg-laying patterns) between three macrobiotid tardigrade species with SSOs and two without them. We hypothesized that species with SSOs would show more frequent mating, increased egg deposition rates and greater reproductive success, as has been observed in other taxa. Surprisingly, however, no significant differences in mating behaviour or reproductive success were found. However, species with SSOs showed sustained egg-laying activity, suggesting that SSOs may allow optimization of reproductive output over time. The latter could serve as a fertilization insurance mechanism, to cope with the common but extreme fluctuating environmental conditions, such as anhydrobiosis stress. Owing to the low statistical power, these findings should be interpreted cautiously, and further investigations are needed to elucidate the role of SSOs in the reproductive dynamics in this animal group, particularly the potential for PCSS processes. (c) 2026 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

"Sex" is a compound of many different concepts that contribute to a multiplicity of meanings. As a result, "sex" appears in a wide range of scientific research for different purposes, from describing a process of recombination and cell division to systems of categorising individuals. Identifying the meaning of "sex" in any particular context requires understanding the construction of contributing component concepts and their relationship with one another. We offer the metaphor of a labyrinth to describe the process of interpreting the meaning "sex" and uncovering its context-dependent purpose. In this way, the meanings of "sex" related to "sexual reproduction", "sex types", "sex determination", and/or "sexual systems" can be elucidated by identifying how these concepts relate to suppositions about what properties of "recombination, "meiosis", "syngamy", or "gametes" are present and to which aspects of a trait, individual, or lineage they apply. By denoting "sex" and related terminology within quotation marks, we emphasise the contextualism of language in the theory and practise of research and resultant knowledge about the natural world. The structure of a labyrinth reflects the incredible capacity of "sex" to describe the diversity and disparity of eukaryotic reproductive strategies. The analytical framework of the labyrinth highlights that different conceptual versions can arise in the consideration of reproductive trait evolution based on the relationships among concepts. Through a critical evaluation of the concepts that contribute to the labyrinth of "sex" across biology, we gain a more nuanced appreciation for the operation of "sex" within scientific knowledge.

Sorochytrium milnesiophthora is a necrotrophic pathogen of tardigrades and a member of the poorly sampled zoosporic phylum Blastocladiomycota. First described in 1985, it was previously known only from the type locality, the Appalachian Mountains of North Carolina, USA, and had not been documented in over 35 years. We rediscovered S. milnesiophthora in a lichen sample from Finland, cultured the fungus, and sequenced its genome. Ribosomal DNA and phylogenomic analyses placed S. milnesiophthora in the Blastocladiales as sister to the rest of the order. The Sorochytriaceae clade also included environmental sequences from Brazil, Italy, Poland, and Sweden, indicating a wide distribution and substantial unexplored biodiversity of Sorochytrium-like fungi. To our surprise, S. milnesiophthora was found to have an exceptionally large nuclear rDNA ITS1-5.8S-ITS2 (internal transcribed spacer [ITS]) region of approximately 5500 bp, the largest known in Fungi. This extraordinary ITS length may represent an adaptation to desiccation by slowing ribosome biogenesis. Further studies should examine how S. milnesiophthora infection intersects with tardigrade cryptobiosis and whether the fungus can also survive the extreme conditions for which its host is famous. Deposition of S. milnesiophthora in the Collection of Zoosporic Eufungi at University of Michigan ensures its long-term availability for future investigations.

The Alps are recognised as a hotspot of biodiversity and have historically been extensively studied. However, despite the number of studies devoted to the biodiversity of the Alps, large knowledge gaps are still present. The Southwestern Alps are a biodiverse region known for hosting many endemic animals and plants. To increase the faunistic knowledge of the Southwestern Alps, we have produced the first survey of the tardigrades from Monviso. Notably, we sequenced the mitogenome of the rarely found Arctic-alpine Acanthechiniscus victor (Ehrenberg, 1853), which is an important addition to the existing knowledge as it is the second mitogenome to be sequenced for the tardigrade superfamily Echiniscoidea. The findings of this study highlight how continued explorations of the European mountain ecosystem are still needed to fill in the knowledge gaps regarding its biodiversity.

Restoration has become one of the key measures in improving the state of biodiversity. However, restoration outcomes are in many cases not well understood, and species communities have not been monitored long enough to determine whether they can or cannot recover completely to the pristine reference state. So far, it is particularly poorly known how the communities of microscopic fauna vary within and between pristine, drained and restored peatland habitats. We collected 270 moss samples from a replicated restoration experimental set up to study whether tardigrade communities differ between pristine, drained and restored pine mire forests. In addition, we estimated the associations between tardigrade genera and the moss type they live in. We found a weak pattern of tardigrade occupancy probability being lower in drained and restored sites than in pristine sites. Furthermore, some tardigrade genera were less likely to occur at the drained and restored (11 to 16 years after restoration) sites when compared to pristine sites, although no significant differences in community compositions between treatments were found. We found notable within site variation, which is indicative of high patchiness in tardigrade distribution. We also found strong associations between some of the moss types and tardigrade occurrences. Therefore, tardigrade occurrence seems to be more linked to microhabitat changes, such as mosses, than treatment of the sites. Although the differences between treatments were small, our results show that drained and restored pine mire forests may provide less suitable habitat for tardigrades than the pristine sites do. Favorable habitat conditions for tardigrades in these ecosystems are likely to arise from combinations of large- (e.g., hydrology) and small-scale (moss type) environmental variables that are both affected by drainage and restoration.

Organisms face physiological stress from environmental conditions, impacting survival, maintenance, and reproductive strategies. Cryptobiosis, a reversible state of metabolic inactivity that enables organisms to survive extreme conditions such as desiccation (anhydrobiosis), is known to have physiological costs, but its effects on mate choice and reproductive success remain unclear. Post-anhydrobiotic individuals might prioritize survival over reproduction (in the short term), thus making them less attractive to potential mates. We investigated how anhydrobiosis influences mate choice and reproductive success in the tardigrade Macrobiotus annewintersae. If cryptobiosis has deleterious effects on body condition, we predicted that post-anhydrobiotic individuals would exhibit delayed mate choice, be unfavoured when competing with control individuals, and suffer short-term mating opportunity costs (males) and/or fecundity costs (females). We also expected males to prioritize sperm motility recovery when presented with a mating opportunity. Post-anhydrobiotic individuals showed no delay in mate choice and were not unfavoured when competing with control individuals. However, post-anhydrobiotic males did not accelerate sperm motility recovery when paired with females, and most post-anhydrobiotic females did not lay eggs after mating opportunities. This study highlights the reproductive costs of anhydrobiosis, emphasizing the role of individual physiological state and timing of sexual reproduction in response to extreme environmental stress.

The tardigrade genus Richtersius Pilato & Binda, 1989 has been considered monotypic for more than 30 years since its establishment and is frequently used in experimental studies on physiological adaptations to stress. Only recently, integrative taxonomy has allowed us to disentangle and describe different but similar species. In this study, we provide a taxonomic reanalysis of the genus Richtersius with an integrative description of two new species based on light and scanning electron microscopy as well as DNA sequencing of four markers (18S rDNA, 28S rDNA, ITS-2, and COI). Richtersius nicolai sp. nov. and Richtersius ingemari sp. nov. are distinguished from congeneric species based on a combination of pore density in newborn’s dorsal cuticle, egg diameters, placoid sizes and reproductive modes. This reanalysis of the genus Richtersius will facilitate the future descriptions of new species and provides a solid taxonomic background for the identification of the species used in experimental research.

Tardigrades are well-known for their ability to tolerate extreme environmental conditions such as heat, drought and lack of oxygen by undergoing cryptobiosis. The molecular responses to stress have been studied in detail, but the physiological and morphogenetic changes during cryptobiosis are less understood. We developed new live high-resolution fluorescence microscopy protocols to visualize the tardigrade response to lack of oxygen - anoxybiosis. High-resolution time-lapse imaging enabled analysis of cellular morphology and tracking of cell movements during anoxybiosis. These analyses revealed considerable changes in morphology, composition and movement of storage cells. Our observations and new imaging protocols can be used to study morphological and cellular response to stress in tardigrades.

Routine histochemical techniques are capable of producing vast amount of information from diverse sample types, but these techniques are limited in their ability to generate 3D information. Autofluorescence imaging can be used to analyse samples in 3D but it suffers from weak/low signal intensities. Here, we describe a simple chemical treatment with glutaraldehyde to enhance autofluorescence for 3D fluorescence imaging and to generate detailed morphological images on whole-mount samples. This methodology is straightforward and cost-effective to implement, suitable for a wide range of organisms and sample types. Furthermore, it can be readily integrated with standard confocal and fluorescence microscopes for analysis. This approach has the potential to facilitate the analysis of biological 3D structures and research in developmental biology, including studies on model and non-model organisms.

An important, but generally overlooked source in intraspecific variability of characters used in tardigrades taxonomy is sexual dimorphism. Dioecious tardigrades species with unisexual or bisexual populations are common, but external sexual dimorphism is rarely observed. The discovery and integrative analysis of a Diploechiniscus species population with high sexual dimorphism from Calvert Island (Canada) has allowed its description as a new species. Diploechiniscus dimorphus n. sp. is characterized by a different chaetotaxy in males and females, and by enlarged cephalic sensory appendages in the males. The discovery of this new species shows that it is important to consider sexual dimorphism as a source of intraspecific variability in tardigrade taxonomy due to its implications on the use of traits commonly used to delineate species such as chaetotaxy. Lastly, the finding of individuals of Diploechiniscus horningi ( Schuster and Grigarick, 1971 ), a species previously synonymized with Diploechiniscus oihonnae ( Richters, 1903 ), allows us to prove that it is genetically a separate species and to reinstate it.

Despite the great diversity of parental care types found in amphibians, studies linking them to post-copulatory sexually selected traits are scarce, presumably due to a lack of data. Valencia-Aguilar et al. used fieldwork and museum collections to show that paternal care appears to trade-off with testes size in glass frogs.

Sexual dimorphism is widespread among animals, with diverse patterns and proposed explanations observed across the Tree of Life. Here we present the first formal analysis of the patterns of sexual dimorphism in body size and cephalic sensory appendages across 40 species (from 10 genera) of armoured tardigrades (Echiniscidae). Phylogenetic signal was found for body size traits and the cephalic papilla relative size, indicating that the association between these traits between the sexes has high evolutionary persistence. The Echiniscidae body size dimorphism is generally female-biased, which would be in accordance with the fecundity hypothesis. No strong evidence of allometric patterns of body size sexual dimorphism was found. In contrast, some of the cephalic appendages show male-biased sexual dimorphism, particularly those that, by being more innervated, are thought to function as chemodetection organs used by males during mate search. The latter is consistent with the sexual selection hypothesis. As the first systematic quantification and analysis of the patterns of sexual dimorphism in the phylum Tardigrada, this study provides important insights into their ecology and evolution, such as corroborating the suggestion that cephalic appendages evolved for mate searching.

Tardigrades (Tardigrada) are a phylum of micrometazoans found in all biomes on Earth, but their ecology and habitat preferences remain vastly understudied. Boreal peatlands include a diversity of habitat types and high structural heterogeneity that represents an interesting system to study some of the poorly known habitat preferences of tardigrades. Here, we investigate for the first time tardigrade communities in peatland mosses and the latter's potential associations with key environmental variables. We collected 116 moss samples from 13 sites representing different peatland types and management histories. We found that tardigrades are common and diverse in boreal peatlands, as tardigrades were present in 72% of the collected samples and we identified 14 tardigrade genera. Tardigrade abundance seemed to increase alongside the increasing tree basal area and the density was higher in the microtopographic level further from the water table level, that is, hummocks (mean 117/moss gram) than in lawns/hollows (mean 84/moss gram). Furthermore, the highest tardigrade density was found in the moss taxa that are associated with forested peatland types (i.e., feather mosses) (321 mean/moss gram). Finally, we found interesting patterns regarding tardigrade functional diversity, as carnivorous tardigrades were found only in peatlands with tree basal area > 20 m(2) and mostly in hummocks. Our study demonstrates that the habitat heterogeneity of peatlands (e.g., variation in moisture and vegetation cover) represents an interesting system to study tardigrade ecology and habitat preferences. However, since we found variation in tardigrade abundance and communities across peatland types and microhabitats within peatlands, our results highlight that such studies should be conducted with numerous replicate samples and a systematic study design that properly addresses the habitat heterogeneity between and within different peatland types.

Moss samples were collected from trees and rocks in Haida Gwaii, British Columbia, Canada, and examined for the presence of tardigrades. Specimens from 24 taxa were found in 17 out of the 22 examined samples. New species records for British Columbia are provided and undescribed Grevenius and Crenubiotus species were found; a division in four morphogroups of Grevenius, based on number and presence of placoids, is provided to aid in the future identification. In addition, three specimens of a new species belonging to a potential new undescribed Diploechiniscus species were identified. The finding of Macrobiotus occidentalis occidentalis also provides the occasion to transfer the latter one to the genus Diaforobiotus, for which a new dichotomous key for the identification of its species is given and to redefine the family Richtersiusidae. The DNA sequences of selected taxa are also provided. The high number of tardigrade species collected from a relatively low number of samples highlight how still unexplored is tardigrade diversity, particularly in still-largely insular island systems like Haida Gwaii.

Antarctica is considered one of the most inhospitable places to life due to its low temperatures and lack of liquid water. However, meiofauna taxa such as tardigrades not only thrive, but also show remarkable biodiversity in this habitat. Tardigrades are a phylum of small animals (less than 1 mm in length) mainly known for their ability to withstand freezing and desiccation, which are common stressors in Antarctica. The tardigrade genus Mesobiotus is widely distributed and abundant in Antarctica, particularly in the Antarctic Peninsula, where it is been hypothesized it was present even before the continent froze. Different Mesobiotus species have been recorded from the Antarctic Peninsula, but most are based on DNA data and lack assignment to described species. We used integrative taxonomy methods (DNA and morphology combined) on a novel population of Mesobiotus aradasi from Antarctic Peninsula. The latter allowed us to link previous molecular and morphological records of this species and to build a comprehensive map of its distribution. We show that this species is present across almost all the Antarctic Peninsula, whereas there are no records of it for any other Antarctica region. This study highlights the importance of integrating molecular and morphological methodologies and their complementarity when working with biodiversity and distribution data. The M. aradasi distribution highlights its endemicity in the Antarctic Peninsula and calls for attention to possible future threats to this species.

Tardigrades have been recorded from a variety of habitats including mosses, lichens, leaflitter, streams, and marine sediments; however, reports from rock pools are still scarce. Rock pools across the world are known to host diverse invertebrate communities and endemisms are common. We provide the description by integrative taxonomy of Mesobiotus huecoensis sp. nov., found in the sediment of an ephemeral rock pool in Box Canyon Recreational Area in New Mexico (USA), and placed it in the M. montanus morphogroup based on the presence of eggs with hemispherical processes. This new species has elongated claws, particularly on the fourth pair of legs. Elongated claws are typical of freshwater tardigrades, and could represent an adaptation that allows the new species to better move in the substrate when the rock pool is fully inundated. We also provide information on the sperm morphology and mating behaviour of this new species. The finding of this new species highlights the importance of ephemeral rock pools for the discovery of new taxa and the need for their study and conservation.

Global warming effects in temperate and polar regions include higher average temperatures and a decrease in snow cover, which together lead to an increase in the number of freeze-thaw cycles (FTC). These changes could affect the fitness of both terrestrial and aquatic species. In this study, we tested how tardigrades, ubiquitous microscopic invertebrates, face FTC. Tardigrades are amongst the most resistant animals to unfavorable conditions, including long and deep freezing periods, and are an emerging model group for invertebrate ecology and evolution. We used 12 populations of tardigrades, representing different families within order Parachela, inhabiting different ecosystems (glaciers, snow, terrestrial, aquatic), found in various substrates (mosses, sediments in lakes, cryoconite on glaciers, and snow), and originating from different latitudes and altitudes. We estimated the number of cycles required to kill 50% of individuals and tested for its association with ecological characteristics of the natural habitat (e.g., number of months with predicted FTC), while accounting for phylogeny. The most resistant tardigrades to FTC were the ones from mountain areas and glaciers. The estimated number of cycles required to kill 50% of individuals was the highest for mountainous species inhabiting rock pools and cryoconite holes on glaciers (30 and 14 FTC, respectively). Tardigrades from lowlands were the most sensitive to changes, with 50% of individuals dying after three FTC, while lacustrine and subtropical tardigrades required only one FTC to reach 50% mortality. Our study shows that the response to recurrent freezing stress is taxon dependent and related to the local environmental conditions. The predicted increase of FTC cycles will negatively impact tardigrade populations. Considering the abundance and various trophic roles of tardigrades, reduction in population sizes or the disappearance of some fragile species could affect the functioning of both aquatic and terrestrial ecosystems. Tardigrades are candidate indicators of how freeze-thaw cycles impact ubiquitous microscopic metazoans with similar physiological capabilities.

The tardigrade genus Acutuncus has been long thought to be an Antarctic endemism, well adapted to this harsh environment. The Antarctic endemicity of Acutuncus was recently dispelled with the description of Acutuncus mariae Zawierucha, 2020 found in the Svalbard archipelago. The integrated analyses on two newly found Acutuncus populations from UK and Italy, and a population of Acutuncus antarcticus found close to its type locality allowed us to expand the climatic and geographic range of the genus Acutuncus. These findings also allowed us to re-evaluate the morphological diagnoses of Acutuncus and accommodate it in the newly proposed monotypic family Acutuncidae fam. nov. Two new Acutuncus species morpho-groups are instituted based on eggs morphology: one (Acutuncus antarcticus morphogroup) including the Antarctic Acutuncus taxa characterized by eggs with long pillars within the chorion and eggs laid freely to the environment, the other (Acutuncus mariae morphogroup) including the European species, characterized by eggs with short pillars within the chorion and eggs laid in the exuvium. Finally, we describe two new Acutuncus species from Europe: Acutuncus mecnuffi sp. nov. and Acutuncus giovanniniae sp. nov.

In studies on micrometazoans, sample storage and processing methods are mostly decided based on sample quality (e.g., substrate type and moisture level), and the choice of methods may affect the reliability of the data. However, these methods are poorly studied and rarely reported in detail. Our aim was to determine the methodological compromise between efficiency and reliability required for large-scale quantitative meiofaunal ecological studies. Specifically, we tested whether storage duration (necessary for large number of samples) affects the density or community composition of tardigrades in moss samples. We focus on a largely unexplored limnoterrestrial ecosystem - boreal peatlands, where moss moisture levels are naturally variable across different microhabitats and moss species. We collected seven moss samples from a peatland in Central Finland, kept them in a refrigerator and extracted tardigrades using the Baermann wet funnel at 1, 24, 48 and 96 h post sampling. We found a significant decrease in tardigrade density (32 % on average), but no changes in community composition, after the first 24 h of storage. Based on these results, we recommend that samples collected from wet limnoterrestrial habitats should be processed within 24 h to ensure accuracy and comparability of large-scale quantitative data on tardigrade ecology.