Variation in anhydrobiotic survival of two eutardigrade morphospecies: a story of cryptic species and their dispersal

Homology of the head sensory structures between Heterotardigrada and Eutardigrada supported in a new species of water bear (Ramazzottiidae: Ramazzottius)

Phylum Tardigrada is represented by microscopic eight-legged panarthropods that inhabit terrestrial and marine environments. Although tardigrades are emerging model animals for areas of research including physiology, evolutionary biology, and astrobiology, knowledge of their external morphology remains insufficient. For instance, homologies between marine and terrestrial relatives largely remain unexplored. In the present study we provide detailed pictures of the head sensory organs in a new tardigrade, Ramazzottius groenlandensis sp. nov. Specimens were collected from a mixed moss and lichen sample on Ella Island, East Greenland. The new species differs from congeneric species in the presence of polygonal sculpturing on the dorsal cuticle, which is accentuated in the posterior region of the body, a lateral papilla on leg IV, and distinctive egg morphology. A Bayesian phylogenetic analysis (18S rRNA + 28S rRNA + COI) places the new species within the genus Ramazzottius with high confidence. Interestingly, the new species shows a full set of well-developed cephalic organs, which correspond to all sensory fields found in eutardigrades. Details on the full set of head organs were present only for heterotardigrades. The surface of these organs is covered with small pores, which presumably play a sensory role. This discovery suggests the homology of head sensory structures between heterotardigrades and eutardigrades, implying that the distinctive arrangement and positioning of sensory organs on the head is a plesiomorphic feature of tardigrades. Moreover, we find that the Ramazzottius oberhaeuseri morphotype forms a morphogroup, not a monophyletic species complex.

How long can tardigrades survive in the anhydrobiotic state? A search for tardigrade anhydrobiosis patterns

Anhydrobiosis is a desiccation tolerance that denotes the ability to survive almost complete dehydration without sustaining damage. The knowledge on the survival capacity of various tardigrade species in anhydrobiosis is still very limited. Our research compares anhydrobiotic capacities of four tardigrade species from different genera, i.e. Echiniscus testudo, Paramacrobiotus experimentalis, Pseudohexapodibius degenerans and Macrobiotus pseudohufelandi, whose feeding behavior and occupied habitats are different. Additionally, in the case of Ech. testudo, we analyzed two populations: one urban and one from a natural habitat. The observed tardigrade species displayed clear differences in their anhydrobiotic capacity, which appear to be determined by the habitat rather than nutritional behavior of species sharing the same habitat type. The results also indicate that the longer the state of anhydrobiosis lasts, the more time the animals need to return to activity.

Two new tardigrade genera from New Zealand's Southern Alp glaciers display morphological stasis and parallel evolution

Tardigrada is an invertebrate phylum that often constitutes a dominant micrometazoan group on glaciers worldwide. We investigated tardigrades residing in surface ice above the equilibrium line altitude (ELA) on three temperate glaciers of New Zealand's Southern Alps. Morphological, morphometric and multilocus DNA analyses (CO1, 18S rRNA, 28S rRNA, ITS-2) revealed two new genera comprising four species, of which two are formally described here: Kopakaius gen. nov. nicolae sp. nov. and Kararehius gen. nov. gregorii sp. nov. The former is represented by three genetically distinct phyletic lineages akin to species. According to CO1, Kopakaius gen. nov. nicolae sp. nov. inhabits Whataroa Glacier only while the remaining two Kopakaius species occur on Fox and Franz Joseph Glaciers, suggesting low dispersal capabilities. Although morphological characteristics of the new genera could indicate affinity with the subfamily Itaquasconinae, phylogenetic analysis placed them confidently in the subfamily Diphasconinae. Kopakaius gen. nov. lack placoids in the pharynx similar with some Itaquasconinae, whereas dark pigmentation and claw shape aligns them with the glacier-obligate genus, Cryobiotus (subfamily Hypsibiinae), which is an example of parallel evolution. The second genus, Kararehius gen nov. could be classified as Adropion-like (subfamily Itaquasconinae), but differs greatly by genetics (placed in the subfamily Diphasconinae) as well as morphology (e.g., lack of septulum), exemplify deep stasis in Hypsibiidae. Our results suggest that glacier fragmentation during the Pleistocene triggered tardigrade speciation, making it a suitable model for studies on allopatric divergence in glacier meiofauna.

An integrative description of a new Richtersius species from Greece (Tardigrada: Eutardigrada: Richtersiusidae)

In this paper, we describe a new tardigrade species, Richtersius tertius sp. n., from Greece. The description is based on morphological and morphometric analysis using light and scanning electron microscopy as well as genetic analysis based on four molecular markers (DNA sequences of three nuclear, i.e., 18S rRNA, 28S rRNA, ITS-2 and one mitochondrial COI fragment). Morphological and morphometric differences, together with genetic comparisons, provide independent verifications of Richtersius tertius sp. n. as a species new to science. Phenotypically, the new taxon differs from Richtersius coronifer (Richters, 1903) and Richtersius ziemowiti Kayastha, Berdi, Miaduchowska, Gawlak, Łukasiewicz, Gołdyn, Jędrzejewski et Kaczmarek, 2020 mainly by the morphology and size of cuticular pores, present only in hatchlings (first instars), as well as some morphometric characters. The results presented herein contribute further to the recognition of the morphological variability and biodiversity within Richtersius, with Richtersius tertius sp. n. being the third species formally described within the genus.

Molecular Phylogenetics, Speciation, and Long Distance Dispersal in Tardigrade Evolution: A case study of the genusMilnesium

Microorganisms (sensu lato, i.e., including micrometazoans) are thought to have cosmopolitan geographic distributions due to their theoretically unlimited dispersal capabilities, a consequence of their tiny size, population dynamics, and resistant forms. However, several molecular studies of microorganisms have identified biogeographic patterns indicating cryptic speciation and/or weak species definitions. Using a multi-locus approach with the genus Milnesium (Tardigrada), we aimed to determine the genetic structure of populations worldwide and the effects of long distance dispersal (LDD) on genetic connectivity and relationships across the six continents. Our results on this micrometazoan's genetic structure and LDD at global and micro-local scales indicate contrasting patterns not easily explained by a unique or simple phenomenon. Overall, we report three key findings: (i) confirmation of long distance dispersal for tardigrades, (ii) populations with globally-shared or endemic micro-local haplotypes, and (iii) a supported genetic structure instead of the homogeneous genetic distribution hypothesized for microorganisms with LDD capabilities. Moreover, incongruences between our morphological and molecular results suggest that species delimitation within the genus Milnesium could be problematic due to homoplasy. Duality found for Milnesium populations at the global scale, namely, a molecular phylogenetic structure mixed with widely distributed haplotypes (but without any apparent biogeographic structure), is similar to patterns observed for some unicellular, prokaryotic and eukaryotic, microorganisms. Factors influencing these patterns are discussed within an evolutionary framework.

Integrative taxonomy resolves species identities within the Macrobiotus pallarii complex (Eutardigrada: Macrobiotidae)

The taxonomy of many groups of meiofauna is challenging due to their low number of diagnostic morphological characters and their small body size. Therefore, with the advent of molecular techniques that provide a new source of traits, many cryptic species have started to be discovered. Tardigrades are not an exception, and many once thought to be cosmopolitan taxa are being found to be complexes of phenotypically similar species. Macrobiotus pallarii Maucci, 1954 was originally described in South Italy and has been subsequently recorded in Europe, America, and Asia. This allegedly wide geographic range suggests that multiple species may be hidden under this name. Moreover, recently, genetic evidence to support this was put forward, and the Macrobiotus pallarii complex has been proposed to accommodate putative species related to M. pallarii. Here, we describe three new pseudocryptic species based on populations that would have been all classified as Macrobiotus pallarii if molecular methods were not employed. Using an integrative taxonomy approach, we analyzed animals and eggs from the topotypic population of Macrobiotus pallarii, together with four other populations of the complex. We recovered four distinct phylogenetic lineages that, despite the overlap of morphometric traits, can be separated phenotypically by subtle but discrete morphological characters. One lineage corresponds to Macrobiotus pallarii, whereas the other three are newly described as Macrobiotus margoae Stec, Vecchi & Bartels, sp. nov. from the USA, Macrobiotus ripperi Stec, Vecchi & Michalczyk, sp. nov. from Poland and Finland, and Macrobiotus pseudopallarii Stec, Vecchi & Michalczyk, sp. nov. from Montenegro. To facilitate species identification, we provide a dichotomous key for species of the M. pallarii complex. Delimitation of these pseudocryptic taxa highlights the need for an integrative approach to uncover the phylum's diversity in full.

A new redescription of Richtersius coronifer, supported by transcriptome, provides resources for describing concealed species diversity within the monotypic genus Richtersius (Eutardigrada)

Richtersius coronifer, the nominal species for the family Richtersiidae and a popular laboratory model, exemplifies a common problem in modern tardigrade taxonomy. Despite undeniable progress in the field, many old and incomplete descriptions of taxa hinder both species delimitation and the estimation of species diversity and distribution. Although for over a century this species has been recorded throughout the world, recent research indicates that records to date are likely to represent a species complex rather than a single cosmopolitan species. However, in order to recognise and name species diversity within the complex, an integrative redescription of the nominal species is first needed. Here, we describe an R. coronifer population collected from Spitsbergen, i.e., one of the two localities mentioned in the original description, with detailed morphological and morphometric data associated with standard DNA sequences of four standard genetic markers (18S rRNA, 28S rRNA, ITS-2, and COI) and supported by transcriptome sequencing. We propose replacement of the neotype designated in 1981 by Maucci and Ramazzotti, as it is impossible to verify whether the existing neotype is conspecific with specimens studied by Richters in 1903 and 1904. Finally, using newly obtained cytochrome c oxidase subunit I (COI) sequences of populations from Spitsbergen, Italy, Poland, and Greece together with sequences deposited in GenBank (China, Greenland, Italy, Mongolia), we performed genetic species delimitation, which indicated seven distinct potential species within the genus Richtersius, in addition to the nominal taxon. This study marks a starting point for further research on the taxonomy of and species diversity within the genus. Moreover, this work has the potential to be the first tardigrade redescription to provide both genetic barcodes and a transcriptome of the species in question.

Echiniscus virginicus complex: the first case of pseudocryptic allopatry and pantropical distribution in tardigrades

Mainly because of the problems with species delineation, the biogeography of microscopic organisms is notoriously difficult to elucidate. In this contribution, variable nuclear and mitochondrial DNA markers were sequenced from individual specimens representing the Echiniscus virginicus complex that are morphologically indistinguishable under light microscopy (five populations from the temperate Eastern Nearctic and 13 populations from the subtropical and tropical zone). A range of methods was used to dissect components of variability within the complex (Bayesian inference, haplotype networks, Poisson tree processes, automatic barcode gap discovery delineations, principal components analysis and ANOVA). We found deep divergence between the temperate Eastern Nearctic E. virginicus and pantropical Echiniscus lineatus in all three genetic markers. In contrast, intraspecific genetic variation was very low, regardless of the geographical distance between the populations. Moreover, for the first time, statistical predictions of tardigrade geographical distributions were modelled. The factor determining the allopatric geographical ranges of deceptively similar species analysed in this study is most likely to be the type of climate. Our study shows that widespread tardigrade species exist, and both geographical distribution modelling and the genetic structure of populations of the pantropical E. lineatus suggest wind-mediated (aeolian) passive long-distance dispersal.

An integrated study of the biodiversity within the Pseudechiniscus suillus–facettalis group (Heterotardigrada: Echiniscidae)

Pseudechiniscus is the second most species-rich genus in Heterotardigrada and in the family Echiniscidae. However, previous studies have pointed out polyphyly and heterogeneity in this taxon. The recent erection of the genus Acanthechiniscus was another step in making Pseudechiniscus monophyletic, but species identification is still problematic. The present investigation aims at clarifying biodiversity and taxonomy of Pseudechiniscus taxa, with a special focus on species pertaining to the so-called ‘suillus–facettalis group’, by using an integrated approach of morphological and molecular investigations. The analysis of sequences from specimens sampled in Europe and Asia confirms the monophyly of the genus Pseudechiniscus. Inside the genus, two main evolutionary lineages are recognizable: the P. novaezeelandiae lineage and the P. suillus–facettalis group lineage. Inside the P. suillus–facettalis group, COI molecular data points out a very high variability between sampled localities, but in some cases also among specimens sampled in the same locality (up to 33.3% p-distance). The integrated approach to the study of Pseudechiniscus allows confirmation of its monophyly and highlights the relationships in the taxon, pointing to its global distribution.

High diversity in species, reproductive modes and distribution within the Paramacrobiotus richtersi complex (Eutardigrada, Macrobiotidae)

For many years, Paramacrobiotus richtersi was reported to consist of populations with different chromosome numbers and reproductive modes. To clarify the relationships among different populations, the type locality of the species (Clare Island, Ireland) and several Italian localities were sampled. Populations were investigated with an integrated approach, using morphological (LM, CLSM, SEM), morphometric, karyological, and molecular (18S rRNA, cox1 genes) data. Paramacrobiotus richtersi was redescribed and a neotype designed from the Irish bisexual population. Animals of all populations had very similar qualitative and quantitative characters, apart from the absence of males and the presence of triploidy in some of them, whereas some differences were recorded in the egg shell. All populations examined had the same 18S haplotype, while 21 haplotypes were found in the cox1 gene. In four cases, those qualitative characters were correlated with clear molecular (cox1) differences (genetic distance 14.6-21.8%). The integrative approach, which considered the morphological differences in the eggs, the reproductive biology and the wide genetic distances among putative species, led to the description of four new species (Paramacrobiotus arduus sp. n., Paramacrobiotus celsus sp. n., Paramacrobiotus depressus sp. n., Paramacrobiotus spatialis sp. n.) and two Unconfirmed Candidate Species (UCS) within the P. richtersi complex. Paramacrobiotus fairbanksi, the only ascertained parthenogenetic, triploid species, was redescribed and showed a wide distribution (Italy, Spain, Poland, Alaska), while the amphimictic species showed limited distributions. The difference in distribution between apomictic and amphimictic populations can be explained by the difference in the dispersal potentials associated with these two types of reproduction.

Long-distance passive dispersal in microscopic aquatic animals

Given their dormancy capability (long-term resistant stages) and their ability to colonise and reproduce, microscopic aquatic animals have been suggested having cosmopolitan distribution. Their dormant stages may be continuously moved by mobile elements through the entire planet to any suitable habitat, preventing the formation of biogeographical patterns. In this review, I will go through the evidence we have on the most common microscopic aquatic animals, namely nematodes, rotifers, and tardigrades, for each of the assumptions allowing long-distance dispersal (dormancy, viability, and reproduction) and all the evidence we have for transportation, directly from surveys of dispersing stages, and indirectly from the outcome of successful dispersal in biogeographical and phylogeographical studies. The current knowledge reveals biogeographical patterns also for microscopic organisms, with species-specific differences in ecological features that make some taxa indeed cosmopolitan with the potential for long-distance dispersal, but others with restricted geographic distributions.

Genetic diversity and biogeography of the south polar water bear Acutuncus antarcticus (Eutardigrada : Hypsibiidae) – evidence that it is a truly pan-Antarctic species

Antarctica is an ice-dominated continent and all its terrestrial and freshwater habitats are fragmented, which leads to genetic divergence and, eventually, speciation. Acutuncus antarcticus is the most common Antarctic tardigrade and its cryptobiotic capabilities, small size and parthenogenetic reproduction present a high potential for dispersal and colonisation. Morphological (light and electron microscopy, karyology) and molecular (18S rRNA and cytochrome c oxidase subunit I (COI) genes) analyses on seven populations of A. antarcticus elucidated the genetic diversity and distribution of this species. All analysed populations were morphologically indistinguishable and made up of diploid females. All specimens presented the same 18S rRNA sequence. In contrast, COI analysis showed higher variability, with most Victoria Land populations presenting up to five different haplotypes. Genetic distances between Victoria Land specimens and those found elsewhere in Antarctica were low, while distances between Dronning Maud Land and specimens from elsewhere were high. Our analyses show that A. antarcticus can still be considered a pan-Antarctic species, although the moderately high genetic diversity within Victoria Land indicates the potential for speciation events. Regions of Victoria Land are considered to have been possible refugia during the last glacial maximum and a current biodiversity hotspot, which the populations of A. antarcticus mirror with a higher diversity than in other regions of Antarctica.

Anhydrobiosis in tardigrades--the last decade

The current state of knowledge about anhydrobiosis in tardigrades is presented. In response to adverse environmental conditions tardigrades arrest their metabolic activity and after complete dehydration enter the so-called "tun" state. In this ametabolic state they are able to tolerate exposure to various chemical and physical extremes. These micrometazoans have evolved various kinds of morphological, physiological and molecular adaptations to reduce the effects of desiccation. In this review we address behavioral adaptation, morphological features and molecules which determine the anhydrobiotic survival. The influence of the time spent in anhydrobiotic state on the lifespan and DNA and the role of the antioxidant defense system are also considered. Finally we summarize recent input from the "omics" sciences.

Survival of freezing by hydrated tardigrades inhabiting terrestrial and freshwater habitats

The seasonality and unpredictability of environmental conditions at high altitudes and latitudes govern the life cycle patterns of organisms, giving rise to stresses that cause death or development of specific adaptations. Ice formation is a major variable affecting the survival of both freshwater fauna and fauna inhabiting lichens, mosses and leaf litter. Tardigrades occupy a wide range of niches in marine, freshwater and terrestrial environments. The highest number of species is found in terrestrial habitats thanks to their ability to enter anhydrobiosis and cryobiosis. The cryobiotic ability of tardigrade species from polar regions is well known. Consequently, we focused our research on the ability to survive freezing in the active hydrated state using seven tardigrade species differing in phylogenetic position and collected at various altitudes and from different habitats in a temperate area. Specimens were cooled at different cooling rates (from 0.31° C min(-1) to 3.26° C min(-1)). Even though the final survival and the time required by animals to recover to active life were both inversely related to the cooling rate, highly significant interspecific differences were found. Species survival ability ranged from excellent to none. Species living in xeric habitats withstood freezing better than those living in hygrophilous habitats, while true limnic species did not exhibit any cryobiotic ability. The ability to withstand freezing seems linked to the anhydrobiotic ability. The differences in cryptobiotic performance among tardigrade species seem more influenced by selective pressures linked to local adaptation to habitat characteristics than by phylogenetic relationships.

Myoanatomy of the marine tardigrade Halobiotus crispae (Eutardigrada: Hypsibiidae)

The muscular architecture of Halobiotus crispae (Eutardigrada: Hypsibiidae) was examined by means of fluorescent-coupled phalloidin in combination with confocal laser scanning microscopy and computer-aided three-dimensional reconstruction, in addition to light microscopy (Nomarski), scanning electron microscopy, and transmission electron microscopy (TEM). The somatic musculature of H. crispae is composed of structurally independent muscle fibers, which can be divided into a dorsal, ventral, dorsoventral, and a lateral musculature. Moreover, a distinct leg musculature is found. The number and arrangement of muscles differ in each leg. Noticeably, the fourth leg contains much fewer muscles when compared with the other legs. Buccopharyngeal musculature (myoepithelial muscles), intestinal musculature, and cloacal musculature comprise the animal's visceral musculature. TEM of stylet and leg musculature revealed ultrastructural similarities between these two muscle groups. Furthermore, microtubules are found in the epidermal cells of both leg and stylet muscle attachments. This would indicate that the stylet and stylet glands are homologues to the claw and claw glands, respectively. When comparing with previously published data on both heterotardigrade and eutardigrade species, it becomes obvious that eutardigrades possess very similar numbers and arrangement of muscles, yet differ in a number of significant details of their myoanatomy. This study establishes a morphological framework for the use of muscular architecture in elucidating tardigrade phylogeny.

DNA barcoding in Tardigrada: the first case study on Macrobiotus macrocalix Bertolani & Rebecchi 1993 (Eutardigrada, Macrobiotidae)

Morphological and molecular studies on a tardigrade species have been carried out to verify the possibility of using a DNA barcoding approach for species identification in this phylum. Macrobiotus macrocalix Bertolani & Rebecchi, 1993 was chosen as the test species since it belongs to a group of species in which the taxonomy is quite problematic. Animals and eggs belonging to three Italian and one Swedish populations have been investigated. Both morphological and molecular analyses show that all the populations belong to the same species. The low genetic distances recorded among the studied populations (0.3-1.0%) and the high genetic distance (15.9-16.3%) between these populations and a closely related species confirm the possibility of identifying a specimen of this species by its cytochrome oxidase subunit I sequence. Data from other authors support our results indicating that DNA barcoding can be applied to tardigrades. With our protocols, we have obtained voucher specimens that enable us to show a correspondence between morphology and molecular data.