Journal of Comparative Neurology

All insects studied to date possess a centrally located group of neuropils, known collectively as the central complex, that has been implicated in sensory integration and motor action selection. Among the functions prescribed to the central complex, none is perhaps as intriguing as its role in orientation and navigation. Neurobiological correlates of both current and desired headings have been described in insect CXs. Despite the diversity of arthropods, understanding of the CX as a navigational center originates entirely from terrestrial insects. Stomatopod crustaceans, commonly referred to as mantis shrimps, form an order of predatory marine crustaceans with intricate and diverse visual systems that maintain the distinction of being the only fully aquatic animal known to utilize the navigational strategy of path integration. They utilize idiothetic, celestial, and landmark cues to orient in the benthos. Here, we investigate the neuroanatomy of adult and developing mantis shrimp central complexes and associated neuropils to begin understanding this brain region in a sensorially and behaviorally complex crustacean.

FMRFamide is a neuropeptide that is widely distributed in invertebrates and known to be involved in many physiological functions. Previously we noted marked differences in expression of the fmrf gene in the stellate ganglion of Doryteuthis pealei* compared to the central nervous system. In this study we aimed to examen the brain systems of Doryteuthis pealei* for the presence and distribution of fmrf-expressing cells and fiber networks. Late squid embryos and hatchlings were examined by in situ hybridization and immunohistochemistry in whole mounts and tissue sections. All central lobes contained limited numbers of scattered neurons expressing fmrf, but the FMRFamide-containing fiber systems were abundant and extensive, mostly present in the neuropil of lobes. Main clusters of neurons were located in the magnocellular and chromatophore lobes of the posterior subesophageal mass (PSM), and in dorsal aspects of the basal lobe (BL). Dense FMRFamide-immunoreactive fibers were particularly seen in the optic lobe (OL), medial and posterior supraesophageal masses (MSM and SPM) often with a commissural organization. The data show that the central lobes of Doryteuthis pealei hatchlings have a matured FMRFamide system organized in a limited number of centers, but with widely distributed efferents. This suggests that FMRFamide neurons are already functionally engaged in the late embryo. The localization indicates that control of chromatophores and fin movement are amongst these functions.

Elephants have elaborate trunk skills and large, but poorly understood brains. Here we study trunk representations in elephant trigeminal nuclei, which form large protrusions on the ventral brainstem. These ventral brainstem protrusions have previously been referred to as inferior olive, but a delineation of the olivo-cerebellar tract reveals these (trigeminal) nuclei are not connected to the cerebellum via climbing fibers. In contrast, the olivo-cerebellar tract connects to a large dorsolateral nucleus with a serrated cellular architecture, the putative elephant inferior olive. Dense vascularization and intense cytochrome-oxidase reactivity distinguish several elongated trigeminal putative trunk modules, which repeat in the anterior-posterior direction. We focus on the most anterior and largest of these units, the putative nucleus principalis trunk module. Module neuron density is low and non-neural cells outnumber neurons by [~]108:1. Dendritic trees are elongated along the axis of axon bundles (myelin stripes) transversing the trunk module. Synchrotron X-ray-phase-contrast tomography suggests myelin-stripe-axons transverse the trunk module. We show a remarkable correspondence of trunk module myelin stripes and trunk folds. Myelin stripes show little relation to trigeminal neurons and stripe-axons appear to often go nowhere; we suggest that myelin stripes might serve to separate trunk-fold domains rather than to connect neurons. Myelin-stripes-to-folds mapping allowed to determine neural magnification factors, which changed from 1000:1 proximally to 5:1 in the trunk finger. Asian elephants have fewer ([~]640,000) trunk-module neurons than Africans ([~]740,000) and show enlarged representations of trunk parts involved in object wrapping. The elephant trigeminal trunk module is exquisitely organized into trunk-fold-related units.

Organisms are exposed to predictable daily and seasonal environmental oscillations. Biological clocks enable organisms to anticipate these changes and coordinate physiology and behaviour accordingly. While circadian mechanisms are well studied in terrestrial model organisms, little is known about the neuronal organisation of biological clocks in ecologically important species, especially in the marine environment. Antarctic krill (Euphausia superba) is central to the functioning of the Southern Ocean ecosystem and relies on precise timing to cope with the extreme, high-latitude fluctuations in photoperiod, food availability, and sea-ice cover in its habitat. Despite evidence for circadian and seasonal rhythms in krill behaviour and physiology, the neuronal architecture underlying these timing processes has remained unresolved. In this study, we use in situ hybridisation and antibody staining to characterise the circadian clock in the krill brain. Immunostaining with an antibody against crustacean {beta}-Pigment-dispersing hormone ({beta}-PDH) reveals distinct clusters of PDH-positive neurons in the optic lobes and dorsal central brain, along with an extensive PDH-positive fibre network. We further localise transcripts of the core clock genes cryptochrome-2 (cry2) and period (per) in cell clusters in the optic lobes, which also include the PDH-positive neurons. More specifically, PDH-positive neurons are a subgroup of the cry2 and per-positive cells. Together, these findings provide the first description of the neuronal architecture of the circadian clock in Antarctic krill and establish essential groundwork for future studies on biological timing, environmental adaptation, and the resilience of this key species in a rapidly changing Southern Ocean.

The vomeronasal system (VNS) is critical for detecting pheromonal cues that modulate sociosexual behaviors. Despite its central role in chemical communication, our understanding of its anatomical and functional variability across mammals remains incomplete. This study provides the first detailed characterization of the VNS in the Iberian mole (Talpa occidentalis), a fossorial species endemic to the Iberian Peninsula. We performed a morphofunctional and neurochemical analysis of the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB) using histology, immunohistochemistry, and lectin histochemistry. The VNO in T. occidentalis exhibited an unusual circular lumen lined by a uniform sensory epithelium, lacking the dual epithelial organization seen in most species. The vomeronasal cartilage was limited in extent and did not form the typical J-shaped structure. Importantly, no evidence of a vomeronasal pump was found, suggesting alternative mechanisms for semiochemical entry, likely facilitated by the organs anatomical position and continuous receptor distribution. Immunohistochemical analysis revealed strong expression of Gi2 and G{Upsilon}8 in sensory neurons, with weaker G0 expression, suggesting predominance of V1R-type signal transduction. The AOB, though small, exhibited clear lamination and specific marker localization (Gi2, OMP, CR, MAP2), indicating robust functional organization. Lectin binding revealed specific glycosylation patterns in the glomerular layer, with STL and LEA marking synaptic regions. These findings uncover unprecedented anatomical and molecular features in the VNS of T. occidentalis, positioning this species as a valuable model for studying vomeronasal diversity and evolution among Laurasiatherian mammals.

The octopus peduncle complex is an agglomeration of neural structures with remarkably diverse functional roles. The complex rests on the optic tract, between the optic lobe and the central brain, and comprises the peduncle lobe proper, the olfactory lobe, and the optic gland. The peduncle lobe regulates visuomotor behaviors, the optic glands control sexual maturation and maternal death, and the olfactory lobe is thought to receive input from the olfactory organ. Recent transcriptomic and metabolomic studies have identified candidate peptide and steroid ligands in the Octopus bimaculoides optic gland. With gene expression for these ligands and their biosynthetic enzymes, we show that optic gland neurochemistry extends beyond the traditional optic gland secretory tissue and into lobular territories. A key finding is that the classically defined olfactory lobe is itself a heterogenous territory and includes steroidogenic territories that overlap with cells expressing molluscan neuropeptides and the synthetic enzyme dopamine beta-hydroxylase.

Chemical communication is essential for mammalian survival from the earliest stages of life, yet most of what is known about the prenatal development of the olfactory and vomeronasal systems comes from laboratory rodents. These models, while invaluable, may not fully represent the developmental trajectories of wild species living under natural ecological pressures. Here we investigated the fetal development of the nasal chemosensory systems in the fossorial water vole (Arvicola scherman), a free-living arvicoline rodent with a highly subterranean lifestyle. We analyzed fetuses at embryonic days E17 and E21 (term) using classical histology, immunohistochemistry (markers: Gi2, Go, G{gamma}8, CB, CR, PGP 9.5, GAP-43, {beta}-tubulin, MAP2), and lectin histochemistry (UEA, LEA, SBA, STA, DBA). This combined approach enabled us to assess structural maturation, neuronal differentiation, and the temporal dynamics of glycoconjugate expression in the vomeronasal organ (VNO), olfactory epithelium (OE), and the main (MOB) and accessory olfactory bulbs (AOB). By E21, the MOB displayed a six-layered adult-like organization with well-defined glomeruli and interneuronal populations, whereas the AOB showed delayed morphological maturation but already exhibited selective molecular signatures in its nerve and superficial layers. Prenatally, the VNO underwent conspicuous structural differentiation, including stratification of the sensory epithelium, robust axonal fasciculation, and early development of vomeronasal glands. Immunohistochemical analysis revealed early expression of G-protein subunits and calcium-binding proteins, indicating premature pathway specification and interneuronal circuit formation. Lectin labeling provided additional insights: SBA emerged as a highly selective marker of the vomeronasal pathway; UEA highlighted early compartmentalization of vomeronasal projections; LEA showed a conserved, pan-chemosensory binding pattern across systems; and DBA, despite its lower specificity, revealed late-onset reactivity in postmitotic neurons. Together, these findings demonstrate that A. scherman exhibits a remarkably accelerated prenatal maturation of its chemosensory systems compared with laboratory rodents. This early functional readiness likely reflects adaptive pressures of a fossorial lifestyle, emphasizing the importance of incorporating wild species into developmental neurobiology to refine our understanding of mammalian chemosensory evolution.

To elucidate causality underlying the evolution of large brains in cetaceans, we examined the brains of 16 cetartiodactyl species for evidence of non-shivering thermogenesis. In comparison to the artiodactyl brain, the cetacean brain exhibits an expanded expression of uncoupling protein 1 (UCP1, UCPs being mitochondrial inner membrane proteins that dissipate the proton gradient to generate heat) in cortical neurons, localization of UCP4 within a substantial proportion of glia throughout the brain, and an increased density of noradrenergic axonal boutons (noradrenaline functioning to control concentrations of and activate UCPs). Thus, cetacean brains possess multiple characteristics indicative of intensified thermogenetic functionality that can be related to their current and historical obligatory aquatic niche. These findings necessitate reassessment of our concepts regarding the reasons for large brain evolution and associated functional capacities in cetaceans.

Octopus arms, notable for their complex anatomy and remarkable flexibility, have sparked significant interest within the neuroscience community. However, there remains a dearth of knowledge about the molecular and functional identities of various cell types in the arms nervous system. To address this gap, we used hybridization chain reaction (HCR) to identify distinct neuronal types in the arms of the pygmy octopus, Octopus bocki, including putative dopaminergic, octopaminergic, serotonergic, GABAergic, glutamatergic, cholinergic, and peptidergic neurons. We obtained high-resolution multiplexed fluorescent images at 0.28x0.28x1.0 M voxel size from 10 arm base and arm tip cross sections (each 50 M thick) and created three-dimensional reconstructions of the axial ganglia, illustrating the spatial distribution of multiple neuronal populations. Our analysis unveiled anatomically distinct and molecularly diverse scattered neurons, while also highlighting multiple populations of dense small excitatory neurons that appear uniformly distributed throughout the cortical layer. Our data provide new insights into how different types of neurons may contribute to the ability of an octopus to interact with its environment and execute complex tasks. In addition, our findings establish a benchmark for future studies, allowing pioneering exploration of octopus arm molecular neuroanatomy, and offering exciting new avenues in invertebrate neuroscience research.

The habenula (Hb), which contains medial and lateral subdivisions (MHb and LHb, respectively), has high intensity mu opioid binding and receptor (MOR) expression, yet the details of MOR localization across these regions remains debated. MHb and LHb participate in largely non-overlapping neural circuits, therefore accurately resolving MOR expression across MHb and LHb is critical for understanding how MOR ligands impact behaviors. Here we utilized in situ hybridization (ISH) and immunocytochemistry (ICC) to systematically map Oprm1 mRNA and MOR protein throughout the habenular complex. We studied both rat and mouse tissue to evaluate conserved expression across two common research species. Importantly, we found mRNA and protein in both the MHb and LHb in both. In rat, 39 {+/-} 3% (MHb) and 21 {+/-} 4% (LHb) of cells expressed Oprm1. These proportions were greater in mouse: 57 {+/-} 1% (MHb) and 32 {+/-} 4% (LHb). Within each species, Oprm1 labeling density per positive cell was greater in MHb compared to LHb (p < 0.0001 for rat and mouse). The highest intensity labeling was localized along the lateral edge of the MHb for both methods. ICC showed MOR localized to fibers and somata in MHb and LHb. In LHb, MOR labeling was most dense in intermediate sections along the anterior-posterior (AP) axis. In rats we also observed higher density labeling in dorsal LHb at intermediate AP levels and medial LHb more posteriorly. These results indicate that both MHb and LHb can contribute to MOR mediated actions through their respective circuits. Key PointsO_LIMu opioid receptor mRNA and protein is expressed in both the medial and lateral habenulae in rat and mouse. C_LIO_LIIn the medial habenula, most mu opioid receptor mRNA and protein was detected along its lateral border. C_LIO_LIAcross samples, Oprm1+ cells in the MHb contained more mRNA puncta per cell compared to lateral habenula cells. C_LI

The amygdala is a complex brain structure in the vertebrate telencephalon, essential for regulating social behaviors, emotions and (social) cognition. In contrast to the vast majority of neuron types described in the many nuclei of the mammalian amygdala, little is known about the neuronal diversity in non-mammals, making reconstruction of its evolution particularly difficult. Here, we characterize glutamatergic neuron types in the amygdala of the salamander Pleurodeles waltl. Our single-cell RNA sequencing data indicate the existence of at least ten distinct types and subtypes of glutamatergic neurons in the salamander amygdala. In situ hybridization for marker genes indicates that these neuron types are located in three major subdivisions: the lateral amygdala, the medial amygdala, and a newly-defined area demarcated by high expression of the transcription factor Sim1. The gene expression profiles of these neuron types suggest similarities with specific neuron types in the sauropsid and mammalian amygdala, and in particular the evolutionary conservation of Sim1-expressing amygdalar neurons in tetrapods. Taken together, our results reveal a surprising diversity of glutamatergic neuron types in the amygdala of salamanders, despite the anatomical simplicity of their brain.

Nitric oxide (NO) is a diffusible neuromodulator with roles in synaptic plasticity and memory flexibility, exerting its primary effects via the enzyme soluble guanylate cyclase (sGC). Despite its well-documented functions in mammals and insects, little is known about the neuroanatomical distribution and functional relevance of NO in birds, particularly in relation to dopaminergic systems. This study used histochemical and immunohistochemical techniques to map the distribution of NO-synthesizing neurons--identified by NADPH-diaphorase (NADPH-d) and nNOS activity--and their relation to sGC and tyrosine hydroxylase (TH)-positive dopaminergic pathways in the pigeon brain. We found extensive NADPH-d labeling throughout forebrain, midbrain, and hindbrain regions. Among TH-positive midbrain structures, the locus coeruleus exhibited high colocalization with nNOS, while moderate colocalization was seen in the ventral tegmental area substantia grisea centralis and substantia nigra. Notably, a significant proportion of sGC-positive neurons was targeted by TH and NADPH-d positive fibres in the pigeon NCL. Our findings support the potential for NO-dopamine interactions in avian species, reminiscent of memory-related mechanisms in Drosophila melanogaster, and contribute to an understanding of conserved pathways that may underlie flexible learning and memory processing during navigation or related tasks across vertebrates. This work also offers insight into comparative NADPH-d distribution among avian species, with implications for aging, spatial learning, and memory formation.

Striatal neurons play a central role in vertebrate action selection; however, their location in larval zebrafish is not well defined. We assayed for conserved striatal markers in the zebrafish subpallium using fluorescent in situ hybridization (FISH) and immunohistochemistry. Whole mount FISH revealed an inhibitory neuronal cluster rostral to the anterior commissure that expresses tac1, a gene encoding substance P. This molecular profile is shared by mammalian striatal direct pathway neurons. A second partially overlapping population of inhibitory neurons was identified that expresses penka, a gene encoding enkephalin. This molecular profile is shared by striatal indirect pathway neurons. Immunostaining for substance P and enkephalin confirmed the presence of these peptides in the subpallium. The tac1 and penka populations were both found to increase linearly across larval stages. Together, these findings support the existence of a striatal homologue in larval zebrafish that grows to match the development and increasing behavioural complexity of the organism.

The mammalian olfactory systems can be divided into several subsystems based on the anatomical location of their neuroreceptor cells and the family of receptors they express. The more in depth studied systems are the main olfactory system and the vomeronasal system, whose first integrative enters are the main and the accessory olfactory bulb, respectively. In addition, there is a range of olfactory subsystems which converge to the transition zone located between the main olfactory bulb and the accessory olfactory bulb., which has been termed as olfactory limbus (OL) and includes specialized glomeruli which receive uncanonical sensory afferences and interact with the MOB and AOB. Beyond the laboratory rodents, there is a lack of information regarding the olfactory subsystems of carnivores. We have focused on the specific study of the olfactory limbus of the fox, performing serial histological sections, general and specific histological stainings, including both double and simple immunohistochemical and lectin-histochemical labeling techniques. As a result, we have been able to determine that the OL of the fox shows an uncommon development with a high degree of development and complexity. This makes this species a novel mammalian model that could provide a wider understanding of non-canonical pathways involved in the processing of chemosensory cues.

In spinal cord and medulla, ependymal cells re organized in a monolayer forming the central canal (cc). In rodents, this region, also known as a stem cell niche, was shown to contain cerebrospinal fluid-contacting neurons (CSF-cNs). These neurons are GABAergic and because of their chemo- and mechanosensory properties they would represent a novel sensory system intrinsic to the central nervous system. In primates, little is known about these neurons and more generally about the region around the cc. Here, using immunohistochemical approaches, we investigated the organization of the cc region and CSF-cN properties in Macaca mulatta Rhesus monkey. In contrast to rodent, we observe along the whole medullo-spinal axis a large zone around the cc delimited by long radial ependymal fibers that is enriched with astrocytes and microglia but largely devoid of neuronal elements except for CSF-cNs. These primate CSF-cNs share with rodent CSF-cNs similar morphological and phenotypical features with a largely immature profile. Our data suggest that they extend their axons in the longitudinal axis to form fiber bundles close to the cc and we further show that CSF-cNs receive GABAergic and serotoninergic synaptic contacts on their soma and dendrite. Taken together our results reveal in Rh. monkey a specific organization of the region around the cc potentially forming a buffer zone between CSF and parenchyma where CSF-cNs would play a crucial role in the detection of CSF signals and their transmission to the central nervous system, a role that would need to be further investigated.

For decades we have known that the brain "drains" through the subarachnoid space following a route that crosses the cribriform plate to the nasal mucosa and cervical lymph nodes. Yet little is known about the potential role of the olfactory epithelia and associated lymphatic vasculature in the immune response. To better understand the immune response in the olfactory organs we used cell-specific fluorescent reporter lines in dissected, intact adult brains to visualize blood-lymphatic vasculature and neutrophils in the olfactory sensory system. Here we show that the extensive blood vasculature of the olfactory organs is associated with a lymphatic cell type resembling high endothelial venules (HEVs) of the lymph nodes in mammals and a second resembling Mural Lymphatic Endothelial Cells (muLECs) that extended from the brain to the peripheral olfactory epithelia. Surprisingly, the olfactory organs contained the only neutrophil populations observed in the brain. Damage to the olfactory epithelia resulted in a rapid increase of neutrophils within the olfactory organs as well as the appearance of neutrophils in the brain suggesting that neutrophils enter the brain in response to damage. Analysis of cell division during and after damage showed an increase in BrdU labeling in the olfactory epithelia and a subset of the neutrophils. Our results reveal a unique population of neutrophils in the olfactory organs that are associated with an extensive lymphatic vasculature suggesting a dual olfactory-immune function for this unique sensory system. HighlightsO_LIThe olfactory organ is the only region of the brain that contains resident neutrophils in the adult animal. C_LIO_LIDamage to olfactory sensory neurons triggers a rapid mobilization of neutrophils within the olfactory organ and in the central nervous system. C_LIO_LITwo types of lymphatic vasculature resembling Mural Lymphatic Endothelial Cells (muLEC) and High Endothelial Venules (HEV) are present in the olfactory sensory system. C_LIO_LILymphatic vasculature resembling Mural Lymphatic Endothelial Cells (muLEC) wrap the olfactory bulbs and extend across the cribriform plate to olfactory epithelia. C_LI

Cephalopod arms are controlled by a distributed peripheral nervous system comprising the axial nerve cord (ANC), subacetabular ganglia associated with each sucker, four longitudinal intramuscular nerve cords (INCs) embedded within the arm musculature and oblique connectives (OCs) running between INCs. Despite their prominent anatomical position and proposed roles in local sensorimotor integration and inter-arm coordination, the INCs remain poorly characterized with respect to cell-type composition and molecular identity. Here, we report the first integrative characterization of INC structure and composition in Octopus bocki by combining serial block-face scanning electron microscopy (SBEM) with multiplexed hybridization chain reaction (HCR) in situ labeling. We show that oral and aboral INCs share a consistent internal organization comprising distinct cell body regions, a peripheral tract layer, and a central synaptic zone. Both oral and aboral cords contain morphologically diverse cell populations, including abundant bipolar neurons with long unbranched processes and a second class of neurons with highly branched processes bearing bouton-like enlargements. On the molecular level, the sampled INCs are enriched for glutamatergic and buccalin-positive cells, and express abundant glia-associated transcripts. In contrast to the ANC, cholinergic, dopaminergic, serotonergic, and octopaminergic markers were not detected above background. We also characterize the relationship between the INCs and adjacent oblique connectives (OCs), showing that these structures run in close proximity but remain physically separate within the sampled high-resolution volume, with no shared fibers or crossing processes detected across the observed boundary. Together, these data establish a cellular and molecular framework for the INCs and clarify their relationship to neighboring peripheral pathways.

To elicit voluntary movements and integrative reflexes underlying behavior, the brain sends command signals to the spinal cord via specialized long-range descending neurons, known as spinal-projecting neurons (SPNs). The vast and widespread distribution of SPNs, combined with their complex long-distance connectivity, poses a significant challenge for mapping their anatomical organization and associating specific populations with distinct functions. Here we took advantage of the transparency and genetic accessibility of larval zebrafish to uncover the fundamental principles of SPN anatomical organization in a Teleost. Using an optical backfilling method relying on photoactivable GFP, we generate a whole-brain map of all neurons sending axons towards the spinal cord. This approach reveals far more SPNs than previously described through conventional strategies, offering an unparalleled opportunity to revisit distinct spinal-projecting nuclei distributed across hindbrain, midbrain, and diencephalic structures. Combining information on cell location, morphology, and projection patterns, we propose tentative homological designations for zebrafish of SPN nuclei based on established descriptions in mammals and other vertebrates.

Evolutionary changes in catecholamine neurotransmitters such as dopamine and noradrenaline can lead to habitat specific behaviors. We used tyrosine hydroxylase, a conserved precursor to the biosynthesis of dopamine and noradrenaline, to compare catecholaminergic neurons in the brain of a species undergoing allopatric speciation. The teleost fish Astyanax mexicanus is extant in two readily available forms, an ancestral river dwelling form (surface) and various derived blind cave forms (cavefish). Adaptation to nutrient poor cave life without predation has led to marked differences in the behavior of this species. The cavefish has lost defensive responses, such as stimulus aversion, found in the ancestral surface fish and instead displays enhanced food seeking behaviors. This is reflected by an increase in catecholamine immunoreactivity in the cavefish brain in regions associated with non-visual sensory perception, motor control pathways, attention, and endocrine release. These neuroanatomical regions include the olfactory system, the basal telencephalon, the preoptic nuclei, the posterior tuberculum, caudal hypothalamus, and isthmus. These results indicate that the evolutionary shift from aversive defensive responses to attractive exploratory behaviors was driven by increases in the size and/or quantity of catecholaminergic neurons in the cavefish brain.

Recent studies have shown that symbiotic bacteria can have drastic effects on host neurobiology, but few simple, accessible models currently exist in which to study these interactions. Hawaiian bobtail squid (Euprymna scolopes) participate in a binary symbiosis with the bacterium Vibrio fischeri, a population of which resides in a specialized hindgut-derived organ called the light organ. Upon colonization by V. fischeri, the light organ undergoes transcriptional changes that suggest neurons are impacted by the initiation of symbiosis, but the nascent light organs innervation has remained uncharacterized. Here, we show that the light organ-associated nervous system (LONS) in hatchling E. scolopes is a remarkably complex segment of the peripheral nervous system. The LONS is largely plexiform and originates from two primary nerves connected by a local commissure. The abundance of synapsin-like immunoreactivity (-lir) indicates that the lobe plexus is highly interconnected. We also highlight a small number of serotonin-lir neurites that innervate the anterior appendages whose developmental fate may be directly affected by symbiont-driven light organ morphogenesis. Finally, we present evidence that a limited but diverse population of neurons reside within the light organ and are often located near internal symbiont-interacting structures. This description of the E. scolopes LONS serves to provide a foundation from which to investigate how beneficial bacterial symbionts affect host peripheral neurobiology in a tractable model system.