Deceleration of morphological evolution in a cryptic species complex and its link to paleontological stasis

Circumtropical distribution and cryptic species of the meiofaunal enteropneust Meioglossus (Harrimaniidae, Hemichordata)

Hemichordata has always played a central role in evolutionary studies of Chordata due to their close phylogenetic affinity and shared morphological characteristics. Hemichordates had no meiofaunal representatives until the surprising discovery of a microscopic, paedomorphic enteropneust Meioglossus psammophilus (Harrimaniidae, Hemichordata) from the Caribbean in 2012. No additional species have been described since, questioning the broader distribution and significance of this genus. However, being less than a millimeter long and superficially resembling an early juvenile acorn worm, Meioglossus may easily be overlooked in both macrofauna and meiofauna surveys. We here present the discovery of 11 additional populations of Meioglossus from shallow subtropical and tropical coralline sands of the Caribbean Sea, Red Sea, Indian Ocean, and East China Sea. These geographically separated populations show identical morphology but differ genetically. Our phylogenetic reconstructions include four gene markers and support the monophyly of Meioglossus. Species delineation analyses revealed eight new cryptic species, which we herein describe using DNA taxonomy. This study reveals a broad circumtropical distribution, supporting the validity and ecological importance of this enigmatic meiobenthic genus. The high cryptic diversity and apparent morphological stasis of Meioglossus may exemplify a potentially common evolutionary 'dead-end' scenario, where groups with highly miniaturized and simplified body plan lose their ability to diversify morphologically.

Species delimitation with limited sampling: An example from rare trapdoor spider genus Cyclocosmia (Mygalomorphae, Halonoproctidae)

The outcome of species delimitation depends on many factors, including conceptual framework, study design, data availability, methodology employed and subjective decision making. Obtaining sufficient taxon sampling in endangered or rare taxa might be difficult, particularly when non-lethal tissue collection cannot be utilized. The need to avoid overexploitation of the natural populations may thus limit methodological framework available for downstream data analyses and bias the results. We test species boundaries in rare North American trapdoor spider genus Cyclocosmia Ausserer (1871) inhabiting the Southern Coastal Plain biodiversity hotspot with the use of genomic data and two multispecies coalescent model methods. We evaluate the performance of each methodology within a limited sampling framework. To mitigate the risk of species over splitting, common in taxa with highly structured populations, we subsequently implement a species validation step via genealogical diversification index (gdi), which accounts for both genetic isolation and gene flow. We delimited eight geographically restricted lineages within sampled North American Cyclocosmia, suggesting that major river drainages in the region are likely barriers to dispersal. Our results suggest that utilizing BPP in the species discovery step might be a good option for datasets comprising hundreds of loci, but fewer individuals, which may be a common scenario for rare taxa. However, we also show that such results should be validated via gdi, in order to avoid over splitting.

Fossil history of fungus host-specificity: Association of conidia of fossil Asterosporium asterospermum with macro- and microremains of Fagus

Fossil staurosporous conidia almost identical to modern conidia of Asterosporium asterospermum were found from three Central European localities ranging from the Late Oligocene (Germany) to Middle/Late Miocene (Poland). Extant A. asterospermum is strictly host-specific and found only on branches or bark of various Fagus species from Europe, Asia and North America. Conspicuous association of conidia of A. asterospermum with numerous macro- and microremains of Fagus were reported from all the localities where fossil conidia of Asterosporium were found confirming the host-specificity of fossil A. asterospermum to ancient beeches. The host-specific relationship of A. asterospermum and beech was presumably established early in the history of the Fagus genus.

How we study cryptic species and their biological implications: A case study from marine shelled gastropods

Methodological and biological considerations are intertwined when studying cryptic species. A potentially large component of modern biodiversity, the frequency of cryptic species among taxonomic groups is not well documented. The term "cryptic species" is imprecisely used in scientific literature, causing ambiguity when interpreting their evolutionary and ecological significance. This study reviews how cryptic species have been defined, discussing implications for taxonomy and biology, and explores these implications with a case study based on recently published literature on extant shelled marine gastropods. Reviewed gastropods were recorded by species. Records of cryptic gastropods were presented by authors with variable levels of confidence but were difficult to disentangle from inherent biases in the study effort. These complexities notwithstanding, most gastropod species discussed were not cryptic. To the degree that this review's sample represents extinct taxa, the results suggest that a high proportion of shelled marine gastropod species are identifiable for study in the fossil record. Much additional work is needed to provide a more adequate understanding of the relative frequency of cryptic species in shelled marine gastropods, which should start with more explicit definitions and targeted case studies.

Integrative Taxonomy Approach Reveals Cryptic Diversity within the Phoretic Pseudoscorpion Genus Lamprochernes (Pseudoscorpiones: Chernetidae)

Pseudoscorpions represent an ancient, but homogeneous group of arachnids. The genus Lamprochernes comprises several morphologically similar species with wide and overlapping distributions. We implemented an integrative approach combining molecular barcoding (cox1), with cytogenetic and morphological analyses in order to assess species boundaries in European Lamprochernes populations. The results suggest ancient origins of Lamprochernes species accompanied by morphological stasis within the genus. Our integrative approach delimited three nominal Lamprochernes species and one cryptic lineage Lamprochernes abditus sp. nov. Despite its Oligocene origin, L. abditus sp. nov. can be distinguished from its closest relative only by molecular and cytogenetic differences, or alternatively, by a complex multivariate morphometric analysis involving other Lamprochernes species. The population structure and common haplotype sharing across geographically distant populations in most Lamprochernes species suggest that a phoretic manner of dispersal is efficient in this group.

Extremely divergent COI sequences within an amphipod species complex: A possible role for endosymbionts?

Some heritable endosymbionts can affect host mtDNA evolution in various ways. Amphipods host diverse endosymbionts, but whether their mtDNA has been influenced by these endosymbionts has yet to be considered. Here, we investigated the role of endosymbionts (microsporidians and Rickettsia) in explaining highly divergent COI sequences in Paracalliope fluviatilis species complex, the most common freshwater amphipods in New Zealand. We first contrasted phylogeographic patterns using COI, ITS, and 28S sequences. While molecular species delimitation methods based on 28S sequences supported 3-4 potential species (N, C, SA, and SB) among freshwater lineages, COI sequences supported 17-27 putative species reflecting high inter-population divergence. The deep divergence between NC and S lineages (~20%; 28S) and the substitution saturation on the 3rd codon position of COI detected even within one lineage (SA) indicate a very high level of morphological stasis. Interestingly, individuals infected and uninfected by Rickettsia comprised divergent COI lineages in one of four populations tested, suggesting a potential influence of endosymbionts in mtDNA patterns. We propose several plausible explanations for divergent COI lineages, although they would need further testing with multiple lines of evidence. Lastly, due to common morphological stasis and the presence of endosymbionts, phylogeographic patterns of amphipods based on mtDNA should be interpreted with caution.

A DNA barcode library for katydids, cave crickets, and leaf-rolling crickets (Tettigoniidae, Rhaphidophoridae and Gryllacrididae) from Zhejiang Province, China

Barcode libraries are generally assembled with two main objectives in mind: specimen identification and species discovery/delimitation. In this study, the standard COI barcode region was sequenced from 681 specimens belonging to katydids (Tettigoniidae), cave crickets (Rhaphidophoridae), and leaf-rolling crickets (Gryllacrididae) from Zhejiang Province, China. Of these, four COI-5P sequences were excluded from subsequent analyses because they were likely NUMTs (nuclear mitochondrial pseudogenes). The final dataset consisted of 677 barcode sequences representing 90 putative species-level taxa. Automated cluster delineation using the Barcode of Life Data System (BOLD) revealed 118 BINs (Barcodes Index Numbers). Among these 90 species-level taxa, 68 corresponded with morphospecies, while the remaining 22 were identified based on reverse taxonomy using BIN assignment. Thirteen of these morphospecies were represented by a single barcode (so-called singletons), and each of 19 morphospecies were split into more than one BIN. The consensus delimitation scheme yielded 55 Molecular Operational Taxonomic Units (MOTUs). Only four morphospecies (I _max > DNN) failed to be recovered as monophyletic clades (i.e., Elimaeaterminalis, Phyllomimusklapperichi, Sinochloraszechwanensis and Xizicushowardi), so it is speculated that these may be species complexes. Therefore, the diversity of katydids, cave crickets, and leaf-rolling crickets in Zhejiang Province is probably slightly higher than what current taxonomy would suggest.

From 13 to 22 in a second stroke: revisiting the European Eumida sanguinea (Phyllodocidae: Annelida) species complex

Eumida sanguinea is a recognized polychaete species complex which, in previous studies, has been reported to have additional undescribed diversity. We detected nine additional lineages by analysing DNA sequence data (mitochondrial: COI, 16S rRNA and nuclear loci: ITS region and 28S rRNA) of E. sanguinea morphotype populations from a broader sampling effort in European marine waters. Traditional morphological features failed to provide consistent differences or unique characters that could be used to distinguish these Eumida species. However, by complementing DNA data with morphometrics, geographic range, colour and pigmentation patterns, we revealed five new species. Two of these undescribed species derived from the previously signalled Eumida lineages S21 and GB22, which are here named as E. schanderi sp. nov. and E. fenwicki sp. nov., respectively. Three other species are based on newly discovered lineages, namely E. fauchaldi sp. nov., E. langenecki sp. nov. and E. pleijeli sp. nov. From the six new lineages remaining, three are represented by fewer than two well-preserved specimens, which prevented further comprehensive analysis. The last three lineages were only distinct when using mitochondrial markers. Integrative taxonomy is essential to elucidate evolutionary phenomena and eventually allow informed use of species complexes exhibiting stasis in biomonitoring or other ecological studies.

Ramisyllis kingghidorahi n. sp., a new branching annelid from Japan

Among over 20,000 species of Annelida, only two branching species with a highly modified body-pattern are known until now: the Syllidae Syllis ramosa McIntosh, 1879, and Ramisyllis multicaudata Glasby et al. (Zoological Journal of the Linnean Society, 164, 481–497, 2012). Both have unusual ramified bodies with one head and multiple branches and live inside the canals of host sponges. Using an integrative approach (combining morphology, internal anatomy, ecology, phylogeny, genetic divergence, and the complete mitochondrial genome), we describe a new branching species from Japan, Ramisyllis kingghidorahi n. sp., inhabiting an undescribed species of Petrosia (Porifera: Demospongiae) from shallow waters. We compare the new species with its closest relative, R. multicaudata; emend the diagnosis of Ramisyllis; and discuss previous reports of S. ramosa. This study suggests a much higher diversity of branching syllids than currently known. Finally, we discuss possible explanations for the feeding behaviour in the new species in relation to its highly ciliated wall of the digestive tubes (especially at the distal branches and anus), and provide a hypothesis for the evolution of branching body patterns as the result of an adaptation to the host sponge labyrinthic canal system.

The multilevel organismal diversity approach deciphers difficult to distinguish nudibranch species complex

Species identification is a key procedure for broad-scoped ecological, phylogeographic and evolutionary studies. However, to perform a taxonomic study in the molecular era is a complicated task that has many pitfalls. In the present study we use particular examples of common but difficult to distinguish European species within the genus of Polycera (Nudibranchia, Mollusca) to discuss the general issues of the "cryptic species" problem that has broad biological and interdisciplinary importance and can significantly impede ecological, evolutionary, and other biodiversity-related research. The largest dataset of molecular and morphological information for European nudibranchs ever applied encompasses a wide geographical area and shapes a robust framework in this study. Four species are recognized in the species complex, including a new one. It is shown that a lack of appropriate taxonomic analysis led recently to considerable errors in species identity assessment of this complex. Chromatic polymorphism for each species is mapped in a periodic-like framework and combined with statistical analysis of the diagnostic features that considerably facilitates identification of particular species in the complex for biologists and practitioners. The present study evidently shows that "cryptic" and "non-cryptic" components are present within the same species. Therefore, this species complex is well suited for the exploring and testing of general biological problems. One of the main conclusions of this study is that division of biological diversity into "cryptic" and "non-cryptic" components is counterproductive. We propose that the central biological phenomenon of a species can instead be universally designated as multilevel organismal diversity thereby provide a practical set of methods for its investigation.

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.

Interstitial Annelida

Members of the following marine annelid families are found almost exclusively in the interstitial environment and are highly adapted to move between sand grains, relying mostly on ciliary locomotion: Apharyngtidae n. fam., Dinophilidae, Diurodrilidae, Nerillidae, Lobatocerebridae, Parergodrilidae, Polygordiidae, Protodrilidae, Protodriloididae, Psammodrilidae and Saccocirridae. This article provides a review of the evolution, systematics, and diversity of these families, with the exception of Parergodrilidae, which was detailed in the review of Orbiniida by Meca, Zhadan, and Struck within this Special Issue. While several of the discussed families have previously only been known by a few described species, recent surveys inclusive of molecular approaches have increased the number of species, showing that all of the aforementioned families exhibit a high degree of cryptic diversity shadowed by a limited number of recognizable morphological traits. This is a challenge for studies of the evolution, taxonomy, and diversity of interstitial families as well as for their identification and incorporation into ecological surveys. By compiling a comprehensive and updated review on these interstitial families, we hope to promote new studies on their intriguing evolutionary histories, adapted life forms and high and hidden diversity.

Incomplete lineage sorting and ancient admixture, and speciation without morphological change in ghost-worm cryptic species

Morphologically similar species, that is cryptic species, may be similar or quasi-similar owing to the deceleration of morphological evolution and stasis. While the factors underlying the deceleration of morphological evolution or stasis in cryptic species remain unknown, decades of research in the field of paleontology on punctuated equilibrium have originated clear hypotheses. Species are expected to remain morphologically identical in scenarios of shared genetic variation, such as hybridization and incomplete lineage sorting, or in scenarios where bottlenecks reduce genetic variation and constrain the evolution of morphology. Here, focusing on three morphologically similar Stygocapitella species, we employ a whole-genome amplification method (WGA) coupled with double-digestion restriction-site associated DNA sequencing (ddRAD) to reconstruct the evolutionary history of the species complex. We explore population structure, use population-level statistics to determine the degree of connectivity between populations and species, and determine the most likely demographic scenarios which generally reject for recent hybridization. We find that the combination of WGA and ddRAD allowed us to obtain genomic-level data from microscopic eukaryotes (∼1 millimetre) opening up opportunities for those working with population genomics and phylogenomics in such taxa. The three species share genetic variance, likely from incomplete lineage sorting and ancient admixture. We speculate that the degree of shared variation might underlie morphological similarity in the Atlantic species complex.

The Early Branching Group of Orbiniida Sensu Struck et al., 2015: Parergodrilidae and Orbiniidae

This review addresses the state of the art of the systematics and the improvements in the biology, ecology and species diversity of the two annelid taxa Parergodrilidae and Orbiniidae, the early branching group of Orbiniida sensu Struck et al., 2015 according to molecular studies. An effort to identify gaps of knowledge is given to understand the distribution, dispersal and the diversity Parergodrilidae and Orbiniidae hold, as well as to give several directions for future research. Parergodrilidae is a taxon of interstitial annelids constituted by the terrestrial Parergodrilus heideri (monotypic genus up to date), reported throughout Europe but also in Korea and North America, and the genus Stygocapitella, which includes eleven species from the upper shore of sandy beaches distributed along Europe and other regions of the world. Orbiniidae contains more than 200 described species spread over 20 valid genera, varying in size from a few millimeters up to 30 cm, distributed globally and living in a wide variety of soft bottoms. Improving the knowledge on these two sister-taxa is crucial for the understanding of the evolution to interstitial forms by progenesis in Annelida.

Endless forms most stupid, icky, and small: The preponderance of noncharismatic invertebrates as integral to a biologically sound view of life

Big, beautiful organisms are useful for biological education, increasing evolution literacy, and biodiversity conservation. But if educators gloss over the ubiquity of streamlined and miniaturized organisms, they unwittingly leave students and the public vulnerable to the idea that the primary evolutionary plot of every metazoan lineage is "progressive" and "favors" complexity. We show that simple, small, and intriguingly repulsive invertebrate animals provide a counterpoint to misconceptions about evolution. Our examples can be immediately deployed in biology courses and outreach. This context emphasizes that chordates are not the pinnacle of evolution. Rather, in the evolution of animals, miniaturization, trait loss, and lack of perfection are at least as frequent as their opposites. Teaching about invertebrate animals in a "tree thinking" context uproots evolution misconceptions (for students and the public alike), provides a mental scaffold for understanding all animals, and helps to cultivate future ambassadors and experts on these little-known, weird, and fascinating taxa.

Modern cryptic species and crocodylian diversity in the fossil record

Advances in molecular biology and genetics are revealing that many recognized crocodylian species are complexes of two or more cryptic species. These discoveries will have a profound impact on interpretation of the crocodyliform fossil record. Our understanding of ranges of intraspecific variation in modern crocodylian morphology may be based on multiple species and thus express both intraspecific and interspecific variation. This raises questions about our ability to recognize modern species in the fossil record, and it also indicates that specimens from disparate localities or horizons may represent not single widespread species, but multiple related species. Ranges of variation in modern species require a thorough re-evaluation, and we may have to revisit previous perceptions of past crocodyliform diversity, rates of evolution or anagenetic lineages in stratigraphic succession. These challenges will not be unique to those studying crocodyliforms and will require sophisticated approaches to variation among modern and fossil specimens.