Comparison of ventral organ development across Pycnogonida (Arthropoda, Chelicerata) provides evidence for a plesiomorphic mode of late neurogenesis in sea spiders and myriapods

Epimorphic development in tropical shallow-water Nymphonidae (Arthropoda: Pycnogonida) revealed by fluorescence imaging

BACKGROUND

Extant lineages of sea spiders (Pycnogonida) exhibit different types of development. Most commonly, pycnogonids hatch as a minute, feeding protonymphon larva with subsequent anamorphic development. However, especially in cold water habitats at higher latitudes and in the deep sea, some taxa have large, lecithotrophic larvae, or even undergo extended embryonic development with significantly advanced postlarval hatching stages. Similar biogeographic trends are observed in other marine invertebrates, often referred to as "Thorson's rule".

RESULTS

To expand our knowledge on the developmental diversity in the most speciose pycnogonid genus Nymphon, we studied the developmental stages of the two tropical representatives N. floridanum and N. micronesicum., We compared classical scanning electron microscopy with fluorescence-based approaches to determine which imaging strategy is better suited for the ethanol-fixed material available. Both species show epimorphic development and hatch as an advanced, lecithotrophic postlarval instar possessing the anlagen of all body segments. Leg pairs 1-3 show a considerable degree of differentiation at hatching, but their proximal regions remain coiled and hidden under the cuticle of the hatching instar. The adult palp and oviger are not anteceded by three-articled larval limbs, but differentiate directly from non-articulated limb buds during postembryonic development.

CONCLUSIONS

Fluorescence imaging yielded more reliable morphological data than classical scanning electron microscopy, being the method of choice for maximal information gain from rare and fragile sea spider samples fixed in high-percentage ethanol. The discovery of epimorphic development with lecithotrophic postlarval instars in two small Nymphon species from tropical shallow-water habitats challenges the notion that this developmental pathway represents an exclusive cold-water adaptation in Nymphonidae. Instead, close phylogenetic affinities to the likewise more direct-developing Callipallenidae hint at a common evolutionary origin of this trait in the clade Nymphonoidea (Callipallenidae + Nymphonidae). The lack of functional palpal and ovigeral larval limbs in callipallenids and postlarval hatchers among nymphonids may be a derived character of Nymphonoidea. To further test this hypothesis, a stable and well-resolved phylogenetic backbone for Nymphonoidea is key.

The sea spider Pycnogonum litorale overturns the paradigm of the absence of axial regeneration in molting animals

Regenerative abilities and their evolution in the different animal lineages have fascinated generations of biologists. While some taxa are capable of restoring entire individuals from small body fragments, others can regrow only specific structures or lack structural regeneration completely. In contrast to many other protostomes, including the segmented annelids, molting animals (Ecdysozoa) are commonly considered incapable of primary body axis regeneration, which has been hypothesized to be linked to the evolution of their protective cuticular exoskeleton. This holds also for the extraordinarily diverse, segmented arthropods. Contradicting this long-standing paradigm, we here show that immatures of the sea spider Pycnogonum litorale reestablish the posterior body pole after transverse amputation and can regrow almost complete segments and the terminal body region, including the hindgut, anus, and musculature. Depending on the amputation level, normal phenotypes or hypomeric six-legged forms develop. Remarkably, also the hypomeric animals regain reproductive functionality by ectopic formation of gonoducts and gonopores. The discovery of such complex regenerative patterns in an extant arthropod challenges the hitherto widely assumed evolutionary loss of axial regeneration during ecdysozoan evolution. Rather, the branching of sea spiders at the base of Chelicerata and their likely ancestral anamorphic development suggests that the arthropod stem species may have featured similar regenerative capabilities. Accordingly, our results provide an incentive for renewed comparative regeneration studies across ecdysozoans, with the aim to resolve whether this trait was potentially even inherited from the protostome ancestor.

A microCT-based atlas of the central nervous system and midgut in sea spiders (Pycnogonida) sheds first light on evolutionary trends at the family level

Background

Pycnogonida (sea spiders) is the sister group of all other extant chelicerates (spiders, scorpions and relatives) and thus represents an important taxon to inform early chelicerate evolution. Notably, phylogenetic analyses have challenged traditional hypotheses on the relationships of the major pycnogonid lineages (families), indicating external morphological traits previously used to deduce inter-familial affinities to be highly homoplastic. This erodes some of the support for phylogenetic information content in external morphology and calls for the study of additional data classes to test and underpin in-group relationships advocated in molecular analyses. In this regard, pycnogonid internal anatomy remains largely unexplored and taxon coverage in the studies available is limited.

Results

Based on micro-computed X-ray tomography and 3D reconstruction, we created a comprehensive atlas of in-situ representations of the central nervous system and midgut layout in all pycnogonid families. Beyond that, immunolabeling for tubulin and synapsin was used to reveal selected details of ganglionic architecture. The ventral nerve cord consistently features an array of separate ganglia, but some lineages exhibit extended composite ganglia, due to neuromere fusion. Further, inter-ganglionic distances and ganglion positions relative to segment borders vary, with an anterior shift in several families. Intersegmental nerves target longitudinal muscles and are lacking if the latter are reduced. Across families, the midgut displays linear leg diverticula. In Pycnogonidae, however, complex multi-branching diverticula occur, which may be evolutionarily correlated with a reduction of the heart.

Conclusions

Several gross neuroanatomical features are linked to external morphology, including intersegmental nerve reduction in concert with trunk segment fusion, or antero-posterior ganglion shifts in partial correlation to trunk elongation/compaction. Mapping on a recent phylogenomic phylogeny shows disjunct distributions of these traits. Other characters show no such dependency and help to underpin closer affinities in sub-branches of the pycnogonid tree, as exemplified by the tripartite subesophageal ganglion of Pycnogonidae and Rhynchothoracidae. Building on this gross anatomical atlas, future studies should now aim to leverage the full potential of neuroanatomy for phylogenetic interrogation by deciphering pycnogonid nervous system architecture in more detail, given that pioneering work on neuron subsets revealed complex character sets with unequivocal homologies across some families.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00459-8.

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

BACKGROUND

Phylogenomic studies over the past two decades have consolidated the major branches of the arthropod tree of life. However, especially within the Chelicerata (spiders, scorpions, and kin), interrelationships of the constituent taxa remain controversial. While sea spiders (Pycnogonida) are firmly established as sister group of all other extant representatives (Euchelicerata), euchelicerate phylogeny itself is still contested. One key issue concerns the marine horseshoe crabs (Xiphosura), which recent studies recover either as sister group of terrestrial Arachnida or nested within the latter, with significant impact on postulated terrestrialization scenarios and long-standing paradigms of ancestral chelicerate traits. In potential support of a nested placement, previous neuroanatomical studies highlighted similarities in the visual pathway of xiphosurans and some arachnopulmonates (scorpions, whip scorpions, whip spiders). However, contradictory descriptions of the pycnogonid visual system hamper outgroup comparison and thus character polarization.

RESULTS

To advance the understanding of the pycnogonid brain and its sense organs with the aim of elucidating chelicerate visual system evolution, a wide range of families were studied using a combination of micro-computed X-ray tomography, histology, dye tracing, and immunolabeling of tubulin, the neuropil marker synapsin, and several neuroactive substances (including histamine, serotonin, tyrosine hydroxylase, and orcokinin). Contrary to previous descriptions, the visual system displays a serial layout with only one first-order visual neuropil connected to a bilayered arcuate body by catecholaminergic interneurons. Fluorescent dye tracing reveals a previously reported second visual neuropil as the target of axons from the lateral sense organ instead of the eyes.

CONCLUSIONS

Ground pattern reconstruction reveals remarkable neuroanatomical stasis in the pycnogonid visual system since the Ordovician or even earlier. Its conserved layout exhibits similarities to the median eye pathway in euchelicerates, especially in xiphosurans, with which pycnogonids share two median eye pairs that differentiate consecutively during development and target one visual neuropil upstream of the arcuate body. Given multiple losses of median and/or lateral eyes in chelicerates, and the tightly linked reduction of visual processing centers, interconnections between median and lateral visual neuropils in xiphosurans and arachnopulmonates are critically discussed, representing a plausible ancestral condition of taxa that have retained both eye types.

What Is an “Arachnid”? Consensus, Consilience, and Confirmation Bias in the Phylogenetics of Chelicerata

The basal phylogeny of Chelicerata is one of the opaquest parts of the animal Tree of Life, defying resolution despite application of thousands of loci and millions of sites. At the forefront of the debate over chelicerate relationships is the monophyly of Arachnida, which has been refuted by most analyses of molecular sequence data. A number of phylogenomic datasets have suggested that Xiphosura (horseshoe crabs) are derived arachnids, refuting the traditional understanding of arachnid monophyly. This result is regarded as controversial, not least by paleontologists and morphologists, due to the widespread perception that arachnid monophyly is unambiguously supported by morphological data. Moreover, some molecular datasets have been able to recover arachnid monophyly, galvanizing the belief that any result that challenges arachnid monophyly is artefactual. Here, we explore the problems of distinguishing phylogenetic signal from noise through a series of in silico experiments, focusing on datasets that have recently supported arachnid monophyly. We assess the claim that filtering by saturation rate is a valid criterion for recovering Arachnida. We demonstrate that neither saturation rate, nor the ability to assemble a molecular phylogenetic dataset supporting a given outcome with maximal nodal support, is a guarantor of phylogenetic accuracy. Separately, we review empirical morphological phylogenetic datasets to examine characters supporting Arachnida and the downstream implication of a single colonization of terrestrial habitats. We show that morphological support of arachnid monophyly is contingent upon a small number of ambiguous or incorrectly coded characters, most of these tautologically linked to adaptation to terrestrial habitats.

Phylogenomic Resolution of Sea Spider Diversification through Integration of Multiple Data Classes

Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. Previous efforts based on a handful of genes have yielded unstable tree topologies. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and 3 nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the robust-bodied family Pycnogonidae. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.

A postlarval instar of Phoxichilidium femoratum (Pycnogonida, Phoxichilidiidae) with an exceptional malformation

Individuals of the marine chelicerate lineage Pycnogonida (sea spiders) show considerable regenerative capabilities after appendage injury or loss. In their natural habitats, especially the long legs of sea spiders are commonly lost and regenerated, as is evidenced by the frequent encounter of specimens with missing or miniature legs. In contrast to this, the collection of individuals with abnormally developed appendages or trunk regions is comparably rare. Here, we studied a remarkable malformation in a postlarval instar of the species Phoxichilidium femoratum (Rathke, 1799) and describe the external morphology and internal organization of the specimen using a combination of fluorescent histochemistry and scanning electron microscopy. The individual completely lacks the last trunk segment with leg pair 4 and the normally penultimate trunk segment bears only a single aberrant appendage resembling an extension of the anteroposterior body axis. Externally, the proximal units of the articulated appendage are unpaired, but further distally a bifurcation into two equally developed leg-like branches is found. Three-dimensional reconstruction of the musculature reveals components of two regular leg muscle sets in several of the proximal articles. This confirms interpretation of the entire appendage as a malformed leg and reveals an externally hidden paired organization along its entire proximodistal axis. To explain the origin of this unique malformation, early pioneering studies on the regenerative potential of pycnogonids are evaluated and (a) an injury-induced partial fusion of the developing limb buds of leg pair 3, as well as (b) irregular leg regeneration following near complete loss of trunk segments 3 and 4 are discussed. Which of the two hypotheses is more realistic remains to be tested by dedicated experimental approaches. These will have to rely on pycnogonid species with established laboratory husbandry in order to overcome the limitations of the few short-term regeneration studies performed to date.

The more the merrier: unparalleled sympatric species richness in a sea spider genus (Pycnogonida : Callipallenidae : Pallenella) from Tasmanian waters

Southern Australian waters feature remarkably diverse assemblages of the sea spider family Callipallenidae Hilton, 1942. The most speciose of the three Australian-endemic genera currently recognised has been known as Meridionale Staples, 2014, but is here reinstated under the name Pallenella Schimkewitsch, 1909 based on its type species Pallenella laevis (Hoek, 1881). This genus includes several brightly coloured forms that occur in high abundance on arborescent bryozoans. However, considerable similarity of congeners and scarcity of diagnostic characters continue to render species delineation in this genus challenging. Using an integrative taxonomic approach, we combine detailed morphological investigation with analysis of two genetic markers (mitochondrial cytochrome c oxidase subunit I, and nuclear rDNA including internal transcribed spacers 1 and 2) to explore the extraordinary species richness of the genus Pallenella in south-east Tasmania. In agreement with our morphology-based segregation of different species and morphotypes, we recovered well-supported corresponding clades in the genetic analyses. Strong mito-nuclear concordance in the two markers supports the inference of sustained reproductive isolation between the sympatrically occurring forms. Based on these findings, we distinguish a total of 13 Tasmanian congeners, representing the most diverse assemblage of sympatric species in the same microhabitat reported for a single pycnogonid genus. Within this assemblage, we (1) record the type species P. laevis for the first time after almost 150 years, (2) delineate the two Tasmanian morphotypes of the provisional ‘variabilis’ complex, and (3) describe two species new to science (P. karenae, sp. nov., P. baroni, sp. nov.). Despite considerable genetic divergences between most congeners, only few and often subtle characters are found to be suitable for morphology-based delineation. Notably, colouration of living specimens is suggested to be informative in some cases. For morphology-based species identification of preserved specimens, a key relying on combinations of characters rather than single diagnostic features is proposed.

Adult neurogenesis in crayfish: Origin, expansion, and migration of neural progenitor lineages in a pseudostratified neuroepithelium

Two decades after the discovery of adult-born neurons in the brains of decapod crustaceans, the deutocerebral proliferative system (DPS) producing these neural lineages has become a model of adult neurogenesis in invertebrates. Studies on crayfish have provided substantial insights into the anatomy, cellular dynamics, and regulation of the DPS. Contrary to traditional thinking, recent evidence suggests that the neurogenic niche in the crayfish DPS lacks self-renewing stem cells, its cell pool being instead sustained via integration of hemocytes generated by the innate immune system. Here, we investigated the origin, division and migration patterns of the adult-born neural progenitor (NP) lineages in detail. We show that the niche cell pool is not only replenished by hemocyte integration but also by limited numbers of symmetric cell divisions with some characteristics reminiscent of interkinetic nuclear migration. Once specified in the niche, first generation NPs act as transit-amplifying intermediate NPs that eventually exit and produce multicellular clones as they move along migratory streams toward target brain areas. Different clones may migrate simultaneously in the streams but occupy separate tracks and show spatio-temporally flexible division patterns. Based on this, we propose an extended DPS model that emphasizes structural similarities to pseudostratified neuroepithelia in other arthropods and vertebrates. This model includes hemocyte integration and intrinsic cell proliferation to synergistically counteract niche cell pool depletion during the animal's lifespan. Further, we discuss parallels to recent findings on mammalian adult neurogenesis, as both systems seem to exhibit a similar decoupling of proliferative replenishment divisions and consuming neurogenic divisions.

X-ray imaging of a water bear offers a new look at tardigrade internal anatomy

BACKGROUND

Tardigrades (water bears) are microscopic invertebrates of which the anatomy has been well studied using traditional techniques, but a comprehensive three-dimensional reconstruction has never been performed. In order to close this gap, we employed X-ray computed tomography (CT), a technique that is becoming increasingly popular in zoology for producing high-resolution, three-dimensional (3D) scans of whole specimens. While CT has long been used to scan larger samples, its use in some microscopic animals can be problematic, as they are often too small for conventional CT yet too large for high-resolution, optics-based soft X-ray microscopy. This size gap continues to be narrowed with advancements in technology, with high-resolution imaging now being possible using both large synchrotron devices and, more recently, laboratory-based instruments.

RESULTS

Here we use a recently developed prototype lab-based nano-computed tomography device to image a 152 μm-long tardigrade at high resolution (200-270 nm pixel size). The resulting dataset allowed us to visualize the anatomy of the tardigrade in 3D and analyze the spatial relationships of the internal structures. Segmentation of the major structures of the body enabled the direct measurement of their respective volumes. Furthermore, we segmented every storage cell individually and quantified their volume distribution. We compare our measurements to those from published studies in which other techniques were used.

CONCLUSIONS

The data presented herein demonstrate the utility of CT imaging as a powerful supplementary tool for studies of tardigrade anatomy, especially for quantitative volume measurements. This nanoCT study represents the smallest complete animal ever imaged using CT, and offers new 3D insights into the spatial relationships of the internal organs of water bears.