Diversification rates in Tardigrada indicate a decreasing tempo of lineage splitting regardless of reproductive mode

Understanding the dynamics of speciation and extinction events is one of the most interesting subjects in evolutionary biology that relates to all life forms, even the smallest ones. Tardigrades are microscopic invertebrates that attracted public and scientific attention mostly due to their ability to enter into the diapause stage called cryptobiosis and in such stage resist extremely harsh environmental conditions. However, although recent research solved a considerable number of phylogenetic uncertainties and further uncovered physiological mechanisms of cryptobiosis, not much attention is given to the evolutionary forces shaping tardigrade diversity. Here, we investigated the effect of reproductive mode on diversification rates in tardigrades using three groups: macrobiotids, echiniscids and milnesids, which represent low, moderate and high levels of parthenogenesis, respectively. Our results showed a decreasing tempo of diversification events for each of the studied groups without any differences that could be ascribed to reproductive mode. We discussed the observed lack of effect in tardigrades acknowledging deficiencies in available data sets and encouraging further studies to understand whether our results can be considered reliable.

Rough backs: taxonomic value of epicuticular sculpturing in the genus Milnesium Doyère, 1840 (Tardigrada: Apochela)

The phylum Tardigrada comprises ~ 1400 described species that inhabit a wide range of ecosystems throughout the globe. Tardigrades are generally considered taxonomically challenging due to a strongly limited number of taxonomically informative morphological traits and microscopic size. Of all tardigrade groups, the taxonomy of Milnesium Doyère, 1840 is particularly difficult because in comparison with most other eutardigrades, the genus lacks the taxonomically informative egg shell ornamentation and/or placoids in the muscle pharynx. Therefore, any new morphological traits that could be used in species delineation and identification are priceless. In this contribution, we review and evaluate taxonomic value of the dorsal cuticle morphology. Specifically, by means of experimental taxonomy, we demonstrate the first extreme case of ontogenetic variability in dorsal epicuticle sculpturing using a new species from Portugal, Milnesium decorumsp. nov. Furthermore, we verify the existence of dorsal gibbosities in Milnesium reticulatum Pilato, Binda, Lisi, 2002, the only species of the genus allegedly exhibiting these structures. Finally, we provide a diagnostic key to the Milnesium granulatum morphogroup.

Reaching the Monophyly: Re-Evaluation of the Enigmatic Species Tenuibiotus hyperonyx (Maucci, 1983) and the Genus Tenuibiotus (Eutardigrada)

Revisions and redescriptions of taxa described in the past and that are now categorized as insufficiently diagnosed often play a crucial role in making further progress in modern taxonomy in many groups of organisms. Here we revised an enigmatic tardigrade species Tenuibiotus hyperonyx (Maucci, 1983) based on the newly discovered topotypic population from the Italian Alps. We performed an integrative analysis of morphological and genetic data in order to present an upgraded species description and elucidate its phylogenetic position. Our results enabled us to confidently place T.hyperonyx within the family Richtersiusidae, as a member of the genus Diaforobiotus. This change, together with a re-assessment of microphotographs of the Tenuibiotus willardi (Pilato, 1977) and Tenuibiotus bozhkae Pilato, Kiosya, Lisi, Inshina & Biserov, 2011 types, led to the discussion on species composition with narrative taxa amendments for the taxonomic parties involved in the proposed alteration.

The importance of being integrative: a remarkable case of synonymy in the genus Viridiscus (Heterotardigrada: Echiniscidae)

There are two predominant sources of taxonomically useful morphological variability in the diverse tardigrade family Echiniscidae: the internal structure and surface sculpture of the cuticular plates covering the dorsum (sculpturing) and the arrangement and morphology of the trunk appendages (chaetotaxy). However, since the appendages often exhibit intraspecific variation (they can be reduced or can develop asymmetrically), sculpturing has been considered more stable at the species level and descriptions of new echiniscid species based solely on morphology are still being published. Here, we present a case study in which a detailed analysis of the morphology and multiple genetic markers of several species of the genus Viridiscus shows that cuticular sculpture may also exhibit considerable intraspecific variation and lead to false taxonomic conclusions. In a population collected from the eastern Nearctic, in the type locality of the recently described species V. miraviridis, individuals with transitional morphotypes between those reported for V. viridissimus and V. miraviridis were found. Importantly, all morphotypes within the viridissimus-miraviridis spectrum were grouped in a single monospecific clade according to rapidly evolving markers (ITS-1, ITS-2 and COI). Given the morphological and genetic evidence, we establish V. miraviridis as a junior synonym of V. viridissimus. This study explicitly demonstrates that a lack of DNA data associated with morphological descriptions of new taxa jeopardizes the efforts to unclutter tardigrade systematics. Additionally, V. perviridis and V. viridissimus are reported from Lâm Đồng Province in southern Vietnam, which considerably broadens their known geographic ranges.

Echiniscidae in the Mascarenes: the wonders of Mauritius

Many regions of the world remain unexplored in terms of the tardigrade diversity, and the islands of the Indian Ocean are no exception. In this work, we report four species of the family Echiniscidae representing three genera from Mauritius, the second largest island in the Mascarene Archipelago. Two species belong in the genus Echiniscus: Echiniscus perarmatus Murray, 1907, a pantropical species, and one new species: Echiniscus insularissp. nov., one of the smallest members of the spinulosus group and the entire genus, being particularly interesting due to the presence of males and supernumerary teeth-like spicules along the margins of the dorsal plates. The new species most closely resembles Echiniscus tropicalis Binda & Pilato, 1995, for which we present extensive multipopulation data and greatly extend its distribution eastwards towards islands of Southeast Asia. Pseudechiniscus (Meridioniscus) mascarenensissp. nov. is a typical member of the subgenus with elongated (dactyloid) cephalic papillae and the pseudosegmental plate IV’ with reduced posterior projections in males. Finally, a Bryodelphax specimen is also recorded. The assemblage of both presumably endemic and widely distributed tardigrade species in Mauritius fits the recent emerging biogeographic patterns for this group of micrometazoans.

Milnesium (Tardigrada: Apochela) in Japan: The First Confirmed Record of Milnesium tardigradum s.s. and Description of Milnesium pacificum sp. nov

Presently, more than 40 species of the genus Milnesium Doyère, 1840 (Tardigrada: Eutardigrada: Apochela: Milnesiidae) have been described. In Japan, however, almost all records of milnesiid tardigrades should be re-examined with the current criteria on the taxonomy of this genus, except for one species, the recently described Milnesium inceptum Morek, Suzuki, Schill, Georgiev, Yankova, Marley, and Michalczyk, 2019. In this study, we found two species, Milnesium pacificum sp. nov. and Milnesium tardigradum Doyère, 1840, from three southern islands and two cold regions in Japan, respectively. Milnesium pacificum sp. nov., having dorsal sculpturing, exhibits an early positive change in claw configuration. On the other hand, M. tardigradum s.s. from Japan has an early negative claw configuration change, as has been reported in a recent study on the neotype population of this species. We performed DNA barcoding for both species, which indicated that M. pacificum sp. nov. has a close affinity with an undescribed Milnesium species collected from Brazil, and that M. tardigradum from Japan represents the recently described subclade that contains specimens from Poland, Hungary, and Russia. The chromosome numbers were 2n = 14 in M. pacificum sp. nov. and 2n = 10 in M. tardigradum. We detected at least three species of the genus Milnesium present in Japan. Our results advance the investigation of the relationship between phylogenetic position and characteristic morphology as well as expand the known geographic range of M. tardigradum.

Novel integrative data for two Milnesium Doyère, 1840 (Tardigrada: Apochela) species from Central Asia

Tardigrada are a phylum of microscopic animals inhabiting a variety of ecosystems, both aquatic and terrestrial, being recognised for their remarkable abilities to withstand tough environmental conditions. The order Apochela groups exclusively carnivorous species, with the vast majority representing the genus Milnesium Doyère, 1840. Representatives of this genus are characterised by simplified morphology, therefore possessing an extremely limited set of taxonomically meaningful morphological traits. Nevertheless, the taxonomy of Milnesium is mostly based on classical data: observations and measurements in light microscopy with the majority of descriptions lacking integrative data, most importantly DNA barcodes, but also scanning electron microscopy photographs and developmental variability analysis. Hence, re-descriptions that include novel integrative data are urgently needed. In this contribution, we provide new taxonomic data for two species described from Central Asia, Milnesium almatyense (a single population) and Milnesium reductum Tumanov, 2006 (five populations): morphometrics, DNA barcodes, SEM observations and description of developmental variability. As a result, we amend the description of both species and reveal phylogenetic relationships of those species and other sequenced congeners. The integrative data confirm the validity of the two species and include them in the growing set of Milnesium species associated with DNA sequences.

Small is beautiful: the first phylogenetic analysis of Bryodelphax Thulin, 1928 (Heterotardigrada, Echiniscidae)

The phyletic relationships both between and within many of tardigrade genera have been barely studied and they remain obscure. Amongst them is the cosmopolitan Bryodelphax, one of the smallest in terms of body size echiniscid genera. The analysis of newly-found populations and species from the Mediterranean region and from South-East Asia gave us an opportunity to present the first phylogeny of this genus, which showed that phenotypic traits used in classical Bryodelphax taxonomy do not correlate with their phyletic relationships. In contrast, geographic distribution of the analysed species suggests their limited dispersal abilities and seems to be a reliable predictor of phylogenetic affinities within the genus. Moreover, we describe three new species of the genus. Bryodelphax australasiaticussp. nov., by having the ventral plate configuration VII:4-4-2-4-2-2-1, is a new member of the weglarskae group with a wide geographic range extending from the Malay Peninsula through the Malay Archipelago to Australia. Bryodelphax decoratussp. nov. from Central Sulawesi (Celebes) also belongs to the weglarskae group (poorly visible ventral plates VII:4-2-2-4-2-2-1) and is closely related to the recently described Bryodelphax arenosus Gąsiorek, 2018, but is differentiated from the latter by well-developed epicuticular granules on the dorsum. Finally, a new dioecious species, Bryodelphax nigripunctatussp. nov., is described from Mallorca and, by the reduced ventral armature (II/III:2-2-(1)), it resembles Bryodelphax maculatus Gąsiorek et al., 2017. The latter species, known so far only from northern Africa, is recorded from Europe for the first time. A taxonomic key to the genus members is also presented.

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.