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.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659National Science Foundation http://dx.doi.org/10.13039/100000001Peer Reviewe

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

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

Abstract 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

First description of epimorphic development in Antarctic Pallenopsidae (Arthropoda, Pycnogonida) with insights into the evolution of the four-articled sea spider cheliphore

Abstract Background Sea spiders (Pycnogonida) are an abundant faunal element of the Southern Ocean (SO). Several recent phylogeographical studies focused on the remarkably diverse SO pycnogonid fauna, resulting in the identification of new species in previously ill-defined species complexes, insights into their genetic population substructures, and hypotheses on glacial refugia and recolonization events after the last ice age. However, knowledge on the life history of many SO pycnogonids is fragmentary, and early ontogenetic stages often remain poorly documented. This impedes assessing the impact of different developmental pathways on pycnogonid dispersal and distributions and also hinders pycnogonid-wide comparison of developmental features from a phylogenetic-evolutionary angle. Results Using scanning electron microscopy (SEM) and fluorescent nuclear staining, we studied embryonic stages and postembryonic instars of three SO representatives of the taxon Pallenopsidae (Pallenopsis villosa, P. hodgsoni, P. vanhoeffeni), the development of which being largely unknown. The eggs are large and yolk-rich, and the hatching stage is an advanced lecithotrophic instar that stays attached to the father for additional molts. The first free-living instar is deduced to possess at least three functional walking leg pairs. Despite gross morphological similarities between the congeners, each instar can be reliably assigned to a species based on body size, shape of ocular tubercle and proboscis, structure of the attachment gland processes, and seta patterns on cheliphore and walking legs. Conclusions We encourage combination of SEM with fluorescent markers in developmental studies on ethanol-preserved and/or long term-stored pycnogonid material, as this reveals internal differentiation processes in addition to external morphology. Using this approach, we describe the first known cases of pallenopsid development with epimorphic tendencies, which stand in contrast to the small hatching larvae in other Pallenopsidae. Evaluation against current phylogenetic hypotheses indicates multiple gains of epimorphic development within Pycnogonida. Further, we suggest that the type of development may impact pycnogonid distribution ranges, since free-living larvae potentially have a better dispersal capability than lecithotrophic attaching instars. Finally, we discuss the bearing of pycnogonid cheliphore development on the evolution of the raptorial first limb pair in Chelicerata and support a multi-articled adult limb as the plesiomorphic state of the chelicerate crown group, arising ontogenetically via postembryonic segmentation of a three-articled embryonic limb

New species of Australian Pseudopallene (Pycnogonida: Callipallenidae) based on live colouration, morphology and DNA

Arango, Claudia P., Brenneis, Georg (2013): New species of Australian Pseudopallene (Pycnogonida: Callipallenidae) based on live colouration, morphology and DNA. Zootaxa 3616 (5): 401-436, DOI: 10.11646/zootaxa.3616.5.

The ‘Ventral Organs’ of Pycnogonida (Arthropoda) Are Neurogenic Niches of Late Embryonic and Post-Embryonic Nervous System Development

<div><p>Early neurogenesis in arthropods has been in the focus of numerous studies, its cellular basis, spatio-temporal dynamics and underlying genetic network being by now comparably well characterized for representatives of chelicerates, myriapods, hexapods and crustaceans. By contrast, neurogenesis during late embryonic and/or post-embryonic development has received less attention, especially in myriapods and chelicerates. Here, we apply <i>(i)</i> immunolabeling, <i>(ii)</i> histology and <i>(iii)</i> scanning electron microscopy to study post-embryonic ventral nerve cord development in <i>Pseudopallene</i> sp., a representative of the sea spiders (Pycnogonida), the presumable sister group of the remaining chelicerates. During early post-embryonic development, large neural stem cells give rise to additional ganglion cell material in segmentally paired invaginations in the ventral ectoderm. These ectodermal cell regions – traditionally designated as ‘ventral organs’ – detach from the surface into the interior and persist as apical cell clusters on the ventral ganglion side. Each cluster is a post-embryonic neurogenic niche that features a tiny central cavity and initially still houses larger neural stem cells. The cluster stays connected to the underlying ganglionic somata cortex via an anterior and a posterior cell stream. Cell proliferation remains restricted to the cluster and streams, and migration of newly produced cells along the streams seems to account for increasing ganglion cell numbers in the cortex. The pycnogonid cluster-stream-systems show striking similarities to the life-long neurogenic system of decapod crustaceans, and due to their close vicinity to glomerulus-like neuropils, we consider their possible involvement in post-embryonic (perhaps even adult) replenishment of olfactory neurons – as in decapods. An instance of a potentially similar post-embryonic/adult neurogenic system in the arthropod outgroup Onychophora is discussed. Additionally, we document two transient posterior ganglia in the ventral nerve cord of <i>Pseudopallene</i> sp. and evaluate this finding in light of the often discussed reduction of a segmented ‘opisthosoma’ during pycnogonid evolution.</p></div

Mitotic activity in detached and nascent apical CSSs in the VNC of <i>Pseudopallene</i> sp. (PS 2).

<p>Acetylated tubulin (red) and PH3 (yellow) labeling of VNCs with Hoechst (blue) counterstain. <b>A</b>: Composite image of VNC, ventral view. Imaris volume (MIP), Hoechst counterstain not shown, stippled lines mark borders of separate images. Note that mitoses in the anterior ‘embryonic’ ganglia (seg-2 wlg) are restricted to the detached apical clusters. <b>B–F</b>: Optical sections through detached and nascent apical clusters and underlying segmental ganglia (anlagen). Sagittal (<b>B</b>,<b>D</b>,<b>F</b>) or transverse (<b>C</b>,<b>E</b>) sections above dashed lines, horizontal sections below dashed lines. Stars indicate ganglionic neuropil. White arrows mark anterior and posterior cell streams. Open arrowheads indicate sub-apical mitoses close to or within the cell streams. Asterisks (<b>E</b>,<b>F</b>) highlight selected apical NSCs that are not in mitosis. Note tiny central cavities and still detectable differences in cell sizes and nuclear staining intensities in the detached anterior clusters (pa-2 wlg). The paired nascent apical clusters of the anlage of walking leg ganglion 3 are characterized by a nuclei-free apical invagination. <b>H</b>: Optical sections through primordium of walking leg ganglion 4. Transverse section above dashed line, horizontal section below dashed line. Arrows indicate basal anlagen of the longitudinal connectives. Note tangential apical mitosis and lack of large NSCs in this early neurogenic phase. Abbreviations: cec  =  circum-esophageal connective, ov  =  ovigeral neuromere, pa  =  palpal neuromere, seg  =  sub-esophageal ganglion, wlg  =  walking leg ganglion (anlage).</p

Mitotic activity of apical NSCs and sub-apical INPs in walking leg ganglia 1 and 2 during PS 1 of <i>Pseudopallene</i> sp.

<p>Histological sections (<b>A–D</b>) and optical sections of tubulin (red)- and PH3 (yellow)-labeled specimens with Hoechst (blue) counterstain (<b>E–H</b>). <b>A–D</b>: Sagittal sections at different levels through walking leg ganglion 1 in early PS 1. Stars mark ganglionic neuropil. Arrows indicate lumen of apical pits. White arrowheads highlight divisions of selected NSCs, the morphological asymmetry of the divisions being visible in late mitotic stages (<b>A</b>,<b>C</b>). Black arrowheads mark symmetrical divisions of sub-apical INPs (<b>A</b>,<b>D</b>). Big gray arrow marks pycnotic body (<b>C</b>). <b>E–H</b>: Horizontal sections through apical pits in early PS 1 and detached apical cell clusters and streams in late PS 1. Acetylated tubulin (<b>E,F</b>) and tyrosine tubulin (<b>G,H</b>) labeling. Stippled outlines mark extensions of nascent apical clusters and ganglion anlagen proper (<b>E</b>,<b>F</b>) or anterior and posterior cell streams (<b>H</b>). Asterisks label NSCs. <b>E</b>: Walking leg ganglion 1, section slightly basal to the epidermal cells covering the rim of the pit. <b>F</b>: Walking leg ganglion 1, section through the bottom of the pit being characterized by massive proliferation of the lining NSCs (bottom half of image). Anteriorly, open arrowheads indicate divisions of INPs. At this stage, each nascent apical cluster extends across the postero-ventral side of the corresponding hemi-ganglion proper. <b>G</b>: Walking leg ganglion 2, section through detached apical cell clusters at the level of the central cavities. Note size differences of NSCs and the smaller intermingled cells with more brightly stained nuclei. White spots label epidermis cells. <b>H</b>: Walking leg ganglion 2, section through the more defined anterior and posterior cell streams that extend into the ganglion proper. Note INP divisions within the streams. Abbreviations: PS  =  post-embryonic stage, wlg  =  walking leg ganglion (anlage).</p