The Caribbean enigma: the presence of unusual cryptic diversity in intertidal mites (Arachnida, Acari, Oribatida)

Seven new species from Eocene Baltic amber reveal surprising diversity and suggest possible speciation scenarios in the relictual family Collohmanniidae (Acari: Oribatida)

This study provides a comprehensive morphological analysis of ten fossil specimens from the genus Collohmannia, found in Eocene Baltic amber, with nine representing different species. Collohmanniidae, a unique monogeneric family of oribatid mites are among the largest oribatids. They feed on leaf litter and present significant sexual dimorphism and courtship behavior, which includes the transfer of nuptial food from male to female. Extant Collohmannia species inhabit isolated mountain forest areas. A fragmentation hypothesis has been proposed to account for their highly disjunctive distribution, suggesting that climate change divided a large forest area previously occupied by a parent species, leading to the allopatric evolution of several species of Collohmannia. However, with the discovery of a significant number of fossil species in Eocene Baltic amber we propose an alternate hypothesis with the Baltic amber forest serving as an arena for speciation of Collohmanniidae. According to it, the driving force of speciation was dietary and was based on gut microbiome specificity, reinforced by the female choice during the courtship ritual. In this paper we describe seven new fossil species of Collohmannia (C. sellnicki sp. nov., C. albertii sp. nov., C. nortoni sp. nov., C. groehni sp. nov., C. kerneggeri sp. nov., C. weiterschani sp. nov., C. clavata sp. nov.), re-describe C. schusteri based on study of the holotype, synonymize Embolacarus with Collohmannia, propose and describe the neotype of C. pergrata comb. nov., and provide a key to all described Collohmanniidae. Together with the hypotheses of speciation we propose verification experiments to be performed on extant Collohmannia mites.

The Caribbean intertidal mite Alismobates inexpectatus (Acari, Oribatida), an unexpected case of cryptic diversity?

Molecular genetic analyses of Caribbean populations of the supposedly widespread intertidal oribatid mite Alismobates inexpectatus revealed the existence of a cryptic species. The new species, Alismobates piratus sp. n., shows considerable COI and 18S rRNA gene sequence divergences and although morphometric analyses indicate considerable variation between the taxa, no distinguishing morphological feature could be detected. The extreme intertidal environment is suggested to be responsible for the observed morphological stasis of the two species and vicariance is supposed to be responsible for their speciation. Alismobates piratus sp. n. was found on Hispaniola, Guadeloupe, Barbados and Curaçao indicating a predominant distribution on the Greater and Lesser Antilles, whereas the occurrence of A. inexpectatus is primarily restricted to Central America, the northern Caribbean and the Greater Antilles. Haplotype network analyses indicate distinct geographic structuring and the absence of recent gene flow among the Antillean A. piratus sp. n. populations. Central American and Antillean populations of A. inexpectatus show similar patterns but populations from Bermuda and the Bahamas are characterized by a common origin and subsequent expansion. Genetic landscape analysis demonstrates that vast stretches of open ocean, like the Caribbean Basin and the Western Atlantic, act as rather effective barriers, whereas the continuous continental coastline of Central and North America may facilitate dispersal. Genetic data also indicates that the Gulf Stream plays an important role for the biogeography of intertidal oribatid mites as it may be responsible for the strong link between Central and North American populations as well as for the colonization of Bermuda.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13127-023-00624-9.

A closer look reveals hidden diversity in the intertidal Caribbean Fortuyniidae (Acari, Oribatida)

A molecular genetic and morphometric investigation revealed the supposedly widespread Caribbean and Western Atlantic intertidal oribatid mite species Fortuynia atlantica to comprise at least two different species. Although there are no distinct morphological differences separating these taxa, COI and 18S sequence divergence data, as well as different species delimitation analyses, clearly identify the two species. Fortuynia atlantica is distributed in the northern Caribbean and the Western Atlantic and the new Fortuynia antillea sp. nov. is presently endemic to Barbados. Vicariance is supposed to be responsible for their genetic diversification and stabilizing selection caused by the extreme intertidal environment is suggested to be the reason for the found morphological stasis. The genetic structure of Fortuynia atlantica indicates that Bermudian populations are derived from the northern Caribbean and thus support the theory of dispersal by drifting on the Gulf Stream. Haplotype network data suggest that Bermudian and Bahamian populations were largely shaped by colonization, expansion and extinction events caused by dramatic sea level changes during the Pleistocene. A preliminary phylogenetic analysis based on 18S gene sequences indicates that the globally distributed genus Fortuynia may be a monophyletic group, whereas Caribbean and Western Atlantic members are distinctly separated from the Indo-Pacific and Western Pacific species.

Like parent, like child - Ontogenetic development of claws of intertidal arthropods (Acari, Oribatida) from different ecological niches

The shape of claws of adult air-breathing intertidal oribatid mites shows a strong correlation with ecology. As nothing is known about juvenile characteristics of this trait, the ontogenetic development of these attachment devices was studied for the first time with comprehensive geometric morphometric methods. In nine investigated species, claws of immature stages can be classified into the same ecological categories as adults, i.e. juveniles of several rock-dwelling species already show higher and stronger curved claw shapes with smaller claw angles, while juvenile instars of the single investigated mangrove-dwelling species possess lower and less curved claws with wider claw angles. For each species, claw curvature is almost static during the complete development and claw length grows directly proportional with increasing body size. Developmental changes in body size and weight are thus mainly compensated by a simple relative growth in size. Despite the finding that claw shapes of early developmental stages are already classifiable into ecological categories, their shapes also change slightly during development to become more 'rock', 'mix' or 'mangrove'-like, respectively. The present results demonstrate that ecology is also the most important factor in shaping the claws of immatures and that attachment in the intertidal environment is vital for the survival of each mobile stage.

Hidden biodiversity in microarthropods (Acari, Oribatida, Eremaeoidea, Caleremaeus)

A challenge for taxonomists all over the world and across all taxonomic groups is recognizing and delimiting species, and cryptic species are even more challenging. However, an accurate identification is fundamental for all biological studies from ecology to conversation biology. We used a multidisciplinary approach including genetics as well as morphological and ecological data to assess if an easily recognizable, widely distributed and euryoecious mite taxon represents one and the same species. According to phylogenetic (based on mitochondrial and nuclear genes) and species delimitation analyses, five distinct putative species were detected and supported by high genetic distances. These genetic lineages correlate well with ecological data, and each species could be associated to its own (micro)habitat. Subsequently, slight morphological differences were found and provide additional evidence that five different species occur in Central and Southern Europe. The minuteness and the characteristic habitus of Caleremaeus monilipes tempted to neglect potential higher species diversity. This problem might concern several other "well-known" euryoecious microarthropods. Five new species of the genus Caleremaeus are described, namely Caleremaeus mentobellus sp. nov., C. lignophilus sp. nov., C. alpinus sp. nov., C. elevatus sp. nov., and C. hispanicus sp. nov. Additionally, a morphological evaluation of C. monilipes is presented.

A taxonomist's nightmare - Cryptic diversity in Caribbean intertidal arthropods (Arachnida, Acari, Oribatida)

There has been a long controversy about what defines a species and how to delimitate them which resulted in the existence of more than two dozen different species concepts. Recent research on so-called "cryptic species" heated up this debate as some scientists argue that these cryptic species are only a result of incompatible species concepts. While this may be true, we should keep in mind that all concepts are nothing more than human constructs and that the phenomenon of high phenotypic similarity despite reproductive isolation is real. To investigate and understand this phenomenon it is important to classify and name cryptic species as it allows to communicate them with other fields of science that use Linnaean binomials. To provide a common framework for the description of cryptic species, we propose a possible protocol of how to formally name and describe these taxa in practice. The most important point of this protocol is to explain which species concept was used to delimitate the cryptic taxon. As a model, we present the case of the allegedly widespread Caribbean intertidal mite Thalassozetes barbara, which in fact consists of seven phenotypically very similar but genetically distinct species. All species are island or short-range endemics with poor dispersal abilities that have evolved in geographic isolation. Stabilizing selection caused by the extreme conditions of the intertidal environment is suggested to be responsible for the morphological stasis of this cryptic species complex.

Long-term stasis in acariform mites provides evidence for morphologically stable evolution: Molecular vs. morphological differentiation in Linopodes (Acariformes; Prostigmata)

Molecular species delimitation, usually by COI DNA barcoding, shows that cryptic speciation is a common phenomenon observed in most animal phyla. Cryptic species have frequently been observed among all major taxa of mites. The mites of the eupodoid genus Linopodes are cosmopolitan in distribution and are most often found in soil-related habitats. Currently, the genus consists of 22 morphologically similar species, which, in practice, are indistinguishable on the basis of their morphological features. The diagnostic issue of the Linopodes species may be caused by the poor delineation of the species, which need taxonomic revision, or the low morphological variability among cryptic species. In this paper, we present the results of molecular species delimitation carried out using sampled Linopodes populations and the level of morphological inter/intraspecific variation within defined groups. We compared COI, 18S and 28S sequence data together with morphological characters. The molecular delimitation revealed seven well-defined species of Linopodes based on DNA sequences. A well-supported phylogenetic tree revealed the same seven species, while morphological analysis showed negligible phenotypic differentiation among the species revealed. We demonstrate that mites can undergo changes in their DNA accompanied by morphological stasis lasting at least 80 MY.

Distinct phylogeographic patterns in populations of two oribatid mite species from the genus Pantelozetes (Acari, Oribatida, Thyrisomidae) in Central Europe

Oribatid mites are important decomposers of dead organic matter in soils across the world. Their origin dates back at least 380 Mya. Multiple severe climatic changes during Late Pliocene and Pleistocene shaped the migration patterns of these organisms and should be reflected in the genetic variability of their current populations. In this study, we examined the genetic diversity and phylogeographic structure as well as the evolutionary history of populations of two ecologically different oribatid mite species. Pantelozetes cavaticus is a troglophile oribatid mite known mainly from Central European caves, whereas Pantelozetes paolii is a common surface eurytopic species with Holarctic distribution. We used two molecular markers-mitochondrial cytochrome c oxidase subunit I (COI) and the nuclear D3 region of the 28S rDNA gene-to reveal phylogenetic relationships between contemporary populations. Whereas the D3 region showed minimal or no variability within populations, COI appeared to be a relevant marker for population studies. Phylogeographic analysis based on COI detected two lineages of P. cavaticus ('Czech' and 'Slovak'), which separated during the Late Pliocene (2.9 Mya) and revealed the existence of one new species. In contrast, three identified genetic lineages of P. paolii (radiation time 2.9 and 1.2 Mya, respectively) uncovered in this study were found to coexist in the distant sampling localities, suggesting a connection between populations even over long distances.

Testing for phylogenetic signal in claws suggests great influence of ecology on Caribbean intertidal arthropods (Acari, Oribatida)

Claws are common biological attachment devices that can be found in a wide variety of animal groups. Their curvature and size are supposed to be parameters related to ecological aspects. Mites, known as very small arthropods, occupy a wide range of ecological niches and are a perfect model system to investigate correlations of claw morphology with ecology. There is only one study regarding this question in littoral mites but the phylogenetic impact, which plays an important role in the evolution of morphological traits, was not tested. We investigated claw shapes of different Caribbean populations of five species showing different substrate/habitat preferences. We used geometric morphometrics to quantify claw shape and tested for phylogenetic signal within this morphological trait. Even in closely related populations, we found clear claw shapes for hard versus soft substrate, confirming previous findings. Surprisingly, we found no phylogenetic signal within the trait, which demonstrates that ecology (different surfaces and substrates) has acted as one of the primary selective forces in the diversification of claw shapes. Considering that the basic claw design may be the same in the majority of arthropods, our results have important implications for further investigations of claw morphology and its ecological relevance within this phylum.

Patterns of intraspecific morphological variability in soil mites reflect their dispersal ability

The ability to disperse is one of the most important factors influencing the biogeography of species and speciation processes. Highly mobile species have been shown to lack geographic population structures, whereas less mobile species show genetically strongly subdivided populations which are expected to also display at least subtle phenotypic differences. Geometric morphometric methods (GMM) were now used to analyze morphological differences between European populations of a presumed non-phoretic, little mobile mite species in comparison to a highly mobile, phoretic species. The non-phoretic species Scutacarus carinthiacus showed a phenotypic population structure, whereas the phoretic species S. acarorum displayed homogeneity. These different patterns most probably can be explained by different levels of gene flow due to different dispersal abilities of the two species. GMM proved to be a sensitive tool that is especially recommendable for the analysis of (old) museum material and/or specimens in microscopic slides, which are not suitable for molecular genetic analysis.

Unexpected diversity in the host-generalist oribatid mite Paraleius leontonychus (Oribatida, Scheloribatidae) phoretic on Palearctic bark beetles

Bark beetles are feared as pests in forestry but they also support a large number of other taxa that exploit the beetles and their galleries. Among arthropods, mites are the largest taxon associated with bark beetles. Many of these mites are phoretic and often involved in complex interactions with the beetles and other organisms. Within the oribatid mite family Scheloribatidae, only two of the three nominal species of Paraleius have been frequently found in galleries of bark beetles and on the beetles themselves. One of the species, P. leontonychus, has a wide distribution range spanning over three ecozones of the world and is believed to be a host generalist, reported from numerous bark beetle and tree species. In the present study, phylogenetic analyses of one mitochondrial and two nuclear genes identified six well supported, fairly divergent clades within P. leontonychus which we consider to represent distinct species based on molecular species delimitation methods and largely congruent clustering in mitochondrial and nuclear gene trees. These species do not tend to be strictly host specific and might occur syntopically. Moreover, mito-nuclear discordance indicates a case of past hybridization/introgression among distinct Paraleius species, the first case of interspecific hybridization reported in mites other than ticks.

Revisiting the Evolution of Arboreal Life in Oribatid Mites

Though mostly soil dwelling, oribatid mites are found in all kind of habitats, with several species exclusively living on trees. Using previously published DNA sequences and eco-morphological data available from the literature, we inferred the number of transitions between soil dwelling to a truly arboreal lifestyle in oribatid mites and the shape evolution of a particular morphological structure of a sense organ (bothridial seta (= sensillus) of a trichobothrium), the shape of which was previously reported to be associated with an arboreal lifestyle. Our data suggest that a truly arboreal lifestyle evolved several times independently in oribatid mites, but much less often than previously proposed in the past. Even though all truly arboreal species indeed seem to possess a capitate sensillus, this character is not exclusive for arboreal taxa. Nonetheless, since all truly arboreal species do have a capitate sensillus, this might be considered an important (pre-)adaptation to a life on trees. We further provide guidelines on how the term “arboreal” should be applied in future mite research and emphasize the importance of exact microhabitat characterization, as this will greatly facilitate comparisons across studies.