Serotonin-immunoreactive neurons in scorpion pectine neuropils: similarities to insect and crustacean primary olfactory centres?

Comparative biology of spatial navigation in three arachnid orders (Amblypygi, Araneae, and Scorpiones)

From both comparative biology and translational research perspectives, there is escalating interest in understanding how animals navigate their environments. Considerable work is being directed towards understanding the sensory transduction and neural processing of environmental stimuli that guide animals to, for example, food and shelter. While much has been learned about the spatial orientation behavior, sensory cues, and neurophysiology of champion navigators such as bees and ants, many other, often overlooked animal species possess extraordinary sensory and spatial capabilities that can broaden our understanding of the behavioral and neural mechanisms of animal navigation. For example, arachnids are predators that often return to retreats after hunting excursions. Many of these arachnid central-place foragers are large and highly conducive to scientific investigation. In this review we highlight research on three orders within the Class Arachnida: Amblypygi (whip spiders), Araneae (spiders), and Scorpiones (scorpions). For each, we describe (I) their natural history and spatial navigation, (II) how they sense the world, (III) what information they use to navigate, and (IV) how they process information for navigation. We discuss similarities and differences among the groups and highlight potential avenues for future research.

Mechanosensory pathways of scorpion pecten hair sensillae-Adjustment of body height and pecten position

Scorpions' sensory abilities are intriguing, especially the rather enigmatic ventral comb-like chemo- and mechanosensory organs, the so-called pectines. Attached ventrally to the second mesosomal segment just posterior to the coxae of the fourth walking leg pair, the pectines consist of the lamellae, the fulcra, and a variable number of pecten teeth. The latter contain the bimodal peg sensillae, used for probing the substrate with regard to chemo- and mechanosensory cues simultaneously. In addition, the lamellae, the fulcra and the pecten teeth are equipped with pecten hair sensillae (PHS) to gather mechanosensory information. Previously, we have analyzed the neuronal pathway associated with the peg sensillae unraveling their somatotopic projection pattern in dedicated pecten neuropils. Little is known, however, regarding the projections of PHS within the scorpion nervous system. Behavioral and electrophysiological assays showed involvement of PHS in reflexive responses but how the information is integrated remains unresolved. Here, we unravel the innervation pattern of the mechanosensory pecten hair afferents in Mesobuthus eupeus and Euscorpius italicus. By using immunofluorescent labeling and injection of Neurobiotin tracer, we identify extensive arborizations of afferents, including (i) ventral neuropils, (ii) somatotopically organized multisegmental sensory tracts, (iii) contralateral branches via commissures, and (iv) direct ipsilateral innervation of walking leg neuromeres 3 and 4. Our results suggest that PHS function as sensors to elicit reflexive adjustment of body height and obstacle avoidance, mediating accurate pecten teeth alignment to guarantee functionality of pectines, which are involved in fundamental capacities like mating or navigation.

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.

Structure of the pecten neuropil pathway and its innervation by bimodal peg afferents in two scorpion species

The pectines of scorpions are comb-like structures, located ventrally behind the fourth walking legs and consisting of variable numbers of teeth, or pegs, which contain thousands of bimodal peg sensillae. The associated neuropils are situated ventrally in the synganglion, extending between the second and fourth walking leg neuromeres. While the general morphology is consistent among scorpions, taxon-specific differences in pecten and neuropil structure remain elusive but are crucial for a better understanding of chemosensory processing. We analysed two scorpion species (Mesobuthus eupeus and Heterometrus petersii) regarding their pecten neuropil anatomy and the respective peg afferent innervation with anterograde and lipophilic tracing experiments, combined with immunohistochemistry and confocal laser-scanning microscopy. The pecten neuropils consisted of three subcompartments: a posterior pecten neuropil, an anterior pecten neuropil and a hitherto unknown accessory pecten neuropil. These subregions exhibited taxon-specific variations with regard to compartmentalisation and structure. Most notable were structural differences in the anterior pecten neuropils that ranged from ovoid shape and strong fragmentation in Heterometrus petersii to elongated shape with little compartmentalisation in Mesobuthus eupeus. Labelling the afferents of distinct pegs revealed a topographic organisation of the bimodal projections along a medio-lateral axis. At the same time, all subregions along the posterior-anterior axis were innervated by a single peg's afferents. The somatotopic projection pattern of bimodal sensillae appears to be common among arachnids, including scorpions. This includes the structure and organisation of the respective neuropils and the somatotopic projection patterns of chemosensory afferents. Nonetheless, the scorpion pecten pathway exhibits unique features, e.g. glomerular compartmentalisation superimposed on somatotopy, that are assumed to allow high resolution of substrate-borne chemical gradients.

The reniform body: An integrative lateral protocerebral neuropil complex of Eumalacostraca identified in Stomatopoda and Brachyura

Mantis shrimps (Stomatopoda) possess in common with other crustaceans, and with Hexapoda, specific neuroanatomical attributes of the protocerebrum, the most anterior part of the arthropod brain. These attributes include assemblages of interconnected centers called the central body complex and in the lateral protocerebra, situated in the eyestalks, paired mushroom bodies. The phenotypic homologues of these centers across Panarthropoda support the view that ancestral integrative circuits crucial to action selection and memory have persisted since the early Cambrian or late Ediacaran. However, the discovery of another prominent integrative neuropil in the stomatopod lateral protocerebrum raises the question whether it is unique to Stomatopoda or at least most developed in this lineage, which may have originated in the upper Ordovician or early Devonian. Here, we describe the neuroanatomical structure of this center, called the reniform body. Using confocal microscopy and classical silver staining, we demonstrate that the reniform body receives inputs from multiple sources, including the optic lobe's lobula. Although the mushroom body also receives projections from the lobula, it is entirely distinct from the reniform body, albeit connected to it by discrete tracts. We discuss the implications of their coexistence in Stomatopoda, the occurrence of the reniform body in another eumalacostracan lineage and what this may mean for our understanding of brain functionality in Pancrustacea.

Scorpions pectines - Idiosyncratic chemo- and mechanosensory organs

Scorpions possess specialised chemosensory appendages, the pectines. These comb-shaped limbs are located ventrally behind the walking legs. Like the antennae of mandibulate arthropods, they also serve a mechanosensory function. However, more than 90% of the sometimes well above 100,000 sensory neurons projecting from a pectine to the central nervous system are chemosensory. There are two primary projection neuropils. The posterior one, immediately adjacent to the pectine nerve entrance, has an intriguing substructure reminiscent of the olfactory glomeruli observed in the primary chemosensory neuropils of many arthropods and indeed of most bilaterian animals. There are further similarities, particularly to the antennal lobes of mandibulate arthropods, including dense innervation by a relatively small number of putative serotonergic interneurons and the presence of GABA immunoreactivity, indicative of inhibitory interactions. Scorpion idiosyncrasies include the flattened shape and broad size range of the glomerulus-like neuropil compartments. Further, these compartments are often not clearly delimited and form layers in the neuropil that are arranged like onion peels. In summary, the pectine appendages of scorpions and their central nervous projections appear as promising study subjects, particularly regarding comparative examination of chemosensory representation and processing strategies. The possibility of combined, rather than discrete, representations of chemo- and mechanosensory inputs should merit further study.

Serotonergic neurons in the ventral nerve cord of Chilopoda - a mandibulate pattern of individually identifiable neurons

BACKGROUND

Given the numerous hypotheses concerning arthropod phylogeny, independent data are needed to supplement knowledge based on traditional external morphology and modern molecular sequence information. One promising approach involves comparisons of the structure and development of the nervous system. Along these lines, the morphology of serotonin-immunoreactive neurons in the ventral nerve cord has been investigated in numerous tetraconate taxa (Crustacea and Hexapoda). It has been shown that these neurons can be identified individually due to their comparably low number, characteristic soma position, and neurite morphology, thus making it possible to establish homologies at the single cell level. Within Chilopoda (centipedes), detailed analyses of major branching patterns of serotonin-immunoreactive neurons are missing, but are crucial for developing meaningful conclusions on the homology of single cells.

RESULTS

In the present study, we re-investigated the distribution and projection patterns of serotonin-immunoreactive neurons in the ventral nerve cord of three centipede species: Scutigera coleoptrata, Lithobius forficatus, and Scolopendra oraniensis. The centipede serotonergic system in the ventral nerve cord contains defined groups of individually identifiable neurons. An anterior and two posterior immunoreactive neurons per hemiganglion with contralateral projections, a pair of ipsilateral projecting lateral neurons (an autapomorphic character for Chilopoda), as well as a postero-lateral group of an unclear number of cells are present in the ground pattern of Chilopoda.

CONCLUSIONS

Comparisons to the patterns of serotonin-immunoreactive neurons of tetraconate taxa support the homology of anterior and posterior neurons. Our results thus support a sister group relationship of Myriapoda and Tetraconata and, further, a mandibulate ground pattern of individually identifiable serotonin-immunoreactive neurons in the ventral nerve cord. Medial neurons are not considered to be part of the tetraconate ground pattern, but could favor the 'Miracrustacea hypothesis', uniting Remipedia, Cephalocarida, and Hexapoda.

The synganglion of the jumping spider Marpissa muscosa (Arachnida: Salticidae): Insights from histology, immunohistochemistry and microCT analysis

Jumping spiders are known for their extraordinary cognitive abilities. The underlying nervous system structures, however, are largely unknown. Here, we explore and describe the anatomy of the brain in the jumping spider Marpissa muscosa (Clerck, 1757) by means of paraffin histology, X-ray microCT analysis and immunohistochemistry as well as three-dimensional reconstruction. In the prosoma, the CNS is a clearly demarcated mass that surrounds the esophagus. The anteriormost neuromere, the protocerebrum, comprises nine bilaterally paired neuropils, including the mushroom bodies and one unpaired midline neuropil, the arcuate body. Further ventrally, the synganglion comprises the cheliceral (deutocerebrum) and pedipalpal neuropils (tritocerebrum). Synapsin-immunoreactivity in all neuropils is generally strong, while allatostatin-immunoreactivity is mostly present in association with the arcuate body and the stomodeal bridge. The most prominent neuropils in the spider brain, the mushroom bodies and the arcuate body, were suggested to be higher integrating centers of the arthropod brain. The mushroom body in M. muscosa is connected to first and second order visual neuropils of the lateral eyes, and the arcuate body to the second order neuropils of the anterior median eyes (primary eyes) through a visual tract. The connection of both, visual neuropils and eyes and arcuate body, as well as their large size corroborates the hypothesis that these neuropils play an important role in cognition and locomotion control of jumping spiders. In addition, we show that the architecture of the brain of M. muscosa and some previously investigated salticids differs significantly from that of the wandering spider Cupiennius salei, especially with regard to structure and arrangement of visual neuropils and mushroom body. Thus, we need to explore the anatomical conformities and specificities of the brains of different spider taxa in order to understand evolutionary transformations of the arthropod brain.

Serotonin-immunoreactivity in the ventral nerve cord of Pycnogonida--support for individually identifiable neurons as ancestral feature of the arthropod nervous system

BACKGROUND

The arthropod ventral nerve cord features a comparably low number of serotonin-immunoreactive neurons, occurring in segmentally repeated arrays. In different crustaceans and hexapods, these neurons have been individually identified and even inter-specifically homologized, based on their soma positions and neurite morphologies. Stereotypic sets of serotonin-immunoreactive neurons are also present in myriapods, whereas in the investigated chelicerates segmental neuron clusters with higher and variable cell numbers have been reported. This led to the suggestion that individually identifiable serotonin-immunoreactive neurons are an apomorphic feature of the Mandibulata. To test the validity of this neurophylogenetic hypothesis, we studied serotonin-immunoreactivity in three species of Pycnogonida (sea spiders). This group of marine arthropods is nowadays most plausibly resolved as sister group to all other extant chelicerates, rendering its investigation crucial for a reliable reconstruction of arthropod nervous system evolution.

RESULTS

In all three investigated pycnogonids, the ventral walking leg ganglia contain different types of serotonin-immunoreactive neurons, the somata of which occurring mostly singly or in pairs within the ganglionic cortex. Several of these neurons are readily and consistently identifiable due to their stereotypic soma position and characteristic neurite morphology. They can be clearly homologized across different ganglia and different specimens as well as across the three species. Based on these homologous neurons, we reconstruct for their last common ancestor (presumably the pycnogonid stem species) a minimal repertoire of at least seven identified serotonin-immunoreactive neurons per hemiganglion. Beyond that, each studied species features specific pattern variations, which include also some neurons that were not reliably labeled in all specimens.

CONCLUSIONS

Our results unequivocally demonstrate the presence of individually identifiable serotonin-immunoreactive neurons in the pycnogonid ventral nerve cord. Accordingly, the validity of this neuroanatomical feature as apomorphy of Mandibulata is questioned and we suggest it to be ancestral for arthropods instead. The pronounced disparities between the segmental pattern in pycnogonids and the one of studied euchelicerates call for denser sampling within the latter taxon. By contrast, overall similarities between the pycnogonid and myriapod patterns may be indicative of single cell homologies in these two taxa. This notion awaits further substantiation from future studies.

A wonderful network unraveled - Detailed description of capillaries in the prosomal ganglion of scorpions

INTRODUCTION

Though it has long been known that the prosomal ganglion of scorpions is supplied by a dense system of arteries, the pattern of this network has never been described and analyzed in detail. Using MicroCT in combination with computer aided 3D-reconstruction we provide the first detailed description of the pattern of arteries in the prosomal ganglion of Brotheas granulatus (Scorpiones, Chactidae) and other scorpion species.

RESULTS

The entire prosomal ganglion in scorpions is supplied by a network of arteries that branch off the major arteries of the anterior aorta system. The most prominent of these are the nine transganglionic arteries which run through the nerve mass along the midline of the body and branch terminally, i.e. below the neuropils, into smaller arteries. These arteries reticulate into a dense network between the surrounding somata and the centrally located neuropil structures of the ganglion.

CONCLUSIONS

We demonstrate the presence in the prosomal ganglion of scorpions of a capillary system made up of afferent arteries which deliver hemolymph into the ganglion and efferent arteries which transport the hemolymph out of the ganglion. Adopting the structural definition used for vertebrate circulatory systems, this capillary network can also be termed a bipolar rete mirabile (located as it is between afferent and efferent arteries) analogous to those found in vertebrates and some echinoderms. Within the rete mirabile of the scorpion prosomal ganglion, some regions (i.e. neuropils) are better supplied than others. The structural information provided here can now be used in functional neuronal studies to determine the physiological and computational significance of the various neuropils in the complex scorpion nervous system.

Serotonin-immunoreactive neurons in the ventral nerve cord of Remipedia (Crustacea): support for a sister group relationship of Remipedia and Hexapoda?

Background

Remipedia were initially seen as a primitive taxon within Pancrustacea based on characters considered ancestral, such as the homonomously segmented trunk. Meanwhile, several morphological and molecular studies proposed a more derived position of Remipedia within Pancrustacea, including a sister group relationship to Hexapoda. Because of these conflicting hypotheses, fresh data are crucial to contribute new insights into euarthropod phylogeny. The architecture of individually identifiable serotonin-immunoreactive neurons has successfully been used for phylogenetic considerations in Euarthropoda. Here, we identified neurons in three species of Remipedia with an antiserum against serotonin and compared our findings to reconstructed ground patterns in other euarthropod taxa. Additionally, we traced neurite connectivity and neuropil outlines using antisera against acetylated α-tubulin and synapsin.

Results

The ventral nerve cord of Remipedia displays a typical rope-ladder-like arrangement of separate metameric ganglia linked by paired longitudinally projecting connectives. The peripheral projections comprise an intersegmental nerve, consisting of two branches that fuse shortly after exiting the connectives, and the segmental anterior and posterior nerve. The distribution and morphology of serotonin-immunoreactive interneurons in the trunk segments is highly conserved within the remipede species we analyzed, which allows for the reconstruction of a ground pattern: two posterior and one anterior pair of serotonin-immunoreactive neurons that possess a single contralateral projection. Additionally, three pairs of immunoreactive neurons are found in the medial part of each hemiganglion. In one species (Cryptocorynetes haptodiscus), the anterior pair of immunoreactive neurons is missing.

Conclusions

The anatomy of the remipede ventral nerve cord with its separate metameric ganglia mirrors the external morphology of the animal’s trunk. The rope-ladder-like structure and principal architecture of the segmental ganglia in Remipedia corresponds closely to that of other Euarthropoda. A comparison of the serotonin-immunoreactive cell arrangement of Remipedia to reconstructed ground patterns of major euarthropod taxa supports a homology of the anterior and posterior neurons in Pancrustacea. These neurons in Remipedia possess unbranched projections across the midline, pointing towards similarities to the hexapod pattern. Our findings are in line with a growing number of phylogenetic investigations proposing Remipedia to be a rather derived crustacean lineage that perhaps has close affinities to Hexapoda.

Serotonin immunoreactive interneurons in the brain of the Remipedia: new insights into the phylogenetic affinities of an enigmatic crustacean taxon

BACKGROUND

Remipedia, a group of homonomously segmented, cave-dwelling, eyeless arthropods have been regarded as basal crustaceans in most early morphological and taxonomic studies. However, molecular sequence information together with the discovery of a highly differentiated brain led to a reconsideration of their phylogenetic position. Various conflicting hypotheses have been proposed including the claim for a basal position of Remipedia up to a close relationship with Malacostraca or Hexapoda. To provide new morphological characters that may allow phylogenetic insights, we have analyzed the architecture of the remipede brain in more detail using immunocytochemistry (serotonin, acetylated α-tubulin, synapsin) combined with confocal laser-scanning microscopy and image reconstruction techniques. This approach allows for a comprehensive neuroanatomical comparison with other crustacean and hexapod taxa.

RESULTS

The dominant structures of the brain are the deutocerebral olfactory neuropils, which are linked by the olfactory globular tracts to the protocerebral hemiellipsoid bodies. The olfactory globular tracts form a characteristic chiasm in the center of the brain. In Speleonectes tulumensis, each brain hemisphere contains about 120 serotonin immunoreactive neurons, which are distributed in distinct cell groups supplying fine, profusely branching neurites to 16 neuropilar domains. The olfactory neuropil comprises more than 300 spherical olfactory glomeruli arranged in sublobes. Eight serotonin immunoreactive neurons homogeneously innervate the olfactory glomeruli. In the protocerebrum, serotonin immunoreactivity revealed several structures, which, based on their position and connectivity resemble a central complex comprising a central body, a protocerebral bridge, W-, X-, Y-, Z-tracts, and lateral accessory lobes.

CONCLUSIONS

The brain of Remipedia shows several plesiomorphic features shared with other Mandibulata, such as deutocerebral olfactory neuropils with a glomerular organization, innervations by serotonin immunoreactive interneurons, and connections to protocerebral neuropils. Also, we provided tentative evidence for W-, X-, Y-, Z-tracts in the remipedian central complex like in the brain of Malacostraca, and Hexapoda. Furthermore, Remipedia display several synapomorphies with Malacostraca supporting a sister group relationship between both taxa. These homologies include a chiasm of the olfactory globular tract, which connects the olfactory neuropils with the lateral protocerebrum and the presence of hemiellipsoid bodies. Even though a growing number of molecular investigations unites Remipedia and Cephalocarida, our neuroanatomical comparison does not provide support for such a sister group relationship.