A network of visual motion-sensitive neurons for computing object position in an arthropod

Threatening stimuli elicit a sequential cardiac pattern in arthropods

In order to cope with the challenges of living in dynamic environments, animals rapidly adjust their behaviors in coordination with different physiological responses. Here, we studied whether threatening visual stimuli evoke different heart rate patterns in arthropods and whether these patterns are related with defensive behaviors. We identified two sequential phases of crab's cardiac response that occur with a similar timescale to that of the motor arrest and later escape response. The first phase was modulated by low salience stimuli and persisted throughout spaced stimulus presentation. The second phase was modulated by high-contrast stimuli and reduced by repetitive stimulus presentation. The overall correspondence between cardiac and motor responses suggests that the first cardiac response phase might be related to motor arrest while the second to the escape response. We show that in the face of threat arthropods coordinate their behavior and cardiac activity in a rapid and flexible manner.

Neuroethology in South America: past, present and future

South America is a vast continent endowed with extraordinary biodiversity that offers abundant opportunities for neuroethological research. Although neuroethology is still emerging in the region, the number of research groups studying South American species to unveil the neural organization of natural behaviors has grown considerably during the last decade. In this Perspective, we provide an account of the roots and strategies that led to the present state of neuroethology in the Southern Cone of America, with a forward-looking vision of its role in education and its international recognition. Hopefully, our Perspective will serve to further promote the study of natural behaviors across South America, as well as in other scarcely explored regions of the world.

Dual Receptive Fields Underlying Target and Wide-Field Motion Sensitivity in Looming-Sensitive Descending Neurons

Responding rapidly to visual stimuli is fundamental for many animals. For example, predatory birds and insects alike have amazing target detection abilities, with incredibly short neural and behavioral delays, enabling efficient prey capture. Similarly, looming objects need to be rapidly avoided to ensure immediate survival, as these could represent approaching predators. Male Eristalis tenax hoverflies are nonpredatory, highly territorial insects that perform high-speed pursuits of conspecifics and other territorial intruders. During the initial stages of the pursuit, the retinal projection of the target is very small, but this grows to a larger object before physical interaction. Supporting such behaviors, E. tenax and other insects have both target-tuned and loom-sensitive neurons in the optic lobes and the descending pathways. We here show that these visual stimuli are not necessarily encoded in parallel. Indeed, we describe a class of descending neurons that respond to small targets, to looming and to wide-field stimuli. We show that these descending neurons have two distinct receptive fields where the dorsal receptive field is sensitive to the motion of small targets and the ventral receptive field responds to larger objects or wide-field stimuli. Our data suggest that the two receptive fields have different presynaptic input, where the inputs are not linearly summed. This novel and unique arrangement could support different behaviors, including obstacle avoidance, flower landing, and target pursuit or capture.

Predatory behavior under monocular and binocular conditions in the semiterrestrial crab Neohelice granulata

Introduction

Neohelice granulata crabs live in mudflats where they prey upon smaller crabs. Predatory behavior can be elicited in the laboratory by a dummy moving at ground level in an artificial arena. Previous research found that crabs do not use apparent dummy size nor its retinal speed as a criterion to initiate attacks, relying instead on actual size and distance to the target. To estimate the distance to an object on the ground, Neohelice could rely on angular declination below the horizon or, since they are broad-fronted with eye stalks far apart, on stereopsis. Unlike other animals, binocular vision does not widen the visual field of crabs since they already cover 360° monocularly. There exist nonetheless areas of the eye with increased resolution.

Methods

We tested how predatory responses towards the dummy changed when animals' vision was monocular (one eye occluded by opaque black paint) compared to binocular.

Results

Even though monocular crabs could still perform predatory behaviors, we found a steep reduction in the number of attacks. Predatory performance defined by the probability of completing the attacks and the success rate (the probability of making contact with the dummy once the attack was initiated) was impaired too. Monocular crabs tended to use frontal, ballistic jumps (lunge behavior) less, and the accuracy of those attacks was reduced. Monocular crabs used prey interception (moving toward the dummy while it approached the crab) more frequently, favoring attacks when the dummy was ipsilateral to the viewing eye. Instead, binocular crabs' responses were balanced in the right and left hemifields. Both groups mainly approached the dummy using the lateral field of view, securing speed of response.

Conclusion

Although two eyes are not strictly necessary for eliciting predatory responses, binocularity is associated with more frequent and precise attacks.

Matched function of the neuropil processing optic flow in flies and crabs: the lobula plate mediates optomotor responses in Neohelice granulata

When an animal rotates (whether it is an arthropod, a fish, a bird or a human) a drift of the visual panorama occurs over its retina, termed optic flow. The image is stabilized by compensatory behaviours (driven by the movement of the eyes, head or the whole body depending on the animal) collectively termed optomotor responses. The dipteran lobula plate has been consistently linked with optic flow processing and the control of optomotor responses. Crabs have a neuropil similarly located and interconnected in the optic lobes, therefore referred to as a lobula plate too. Here we show that the crabs' lobula plate is required for normal optomotor responses since the response was lost or severely impaired in animals whose lobula plate had been lesioned. The effect was behaviour-specific, since avoidance responses to approaching visual stimuli were not affected. Crabs require simpler optic flow processing than flies (because they move slower and in two-dimensional instead of three-dimensional space), consequently their lobula plates are relatively smaller. Nonetheless, they perform the same essential role in the visual control of behaviour. Our findings add a fundamental piece to the current debate on the evolutionary relationship between the lobula plates of insects and crustaceans.

Behavioural and neural responses of crabs show evidence for selective attention in predator avoidance

Selective attention, the ability to focus on a specific stimulus and suppress distractions, plays a fundamental role for animals in many contexts, such as mating, feeding, and predation. Within natural environments, animals are often confronted with multiple stimuli of potential importance. Such a situation significantly complicates the decision-making process and imposes conflicting information on neural systems. In the context of predation, selectively attending to one of multiple threats is one possible solution. However, how animals make such escape decisions is rarely studied. A previous field study on the fiddler crab, Gelasimus dampieri, provided evidence of selective attention in the context of escape decisions. To identify the underlying mechanisms that guide their escape decisions, we measured the crabs' behavioural and neural responses to either a single, or two simultaneously approaching looming stimuli. The two stimuli were either identical or differed in contrast to represent different levels of threat certainty. Although our behavioural data provides some evidence that crabs perceive signals from both stimuli, we show that both the crabs and their looming-sensitive neurons almost exclusively respond to only one of two simultaneous threats. The crabs' body orientation played an important role in their decision about which stimulus to run away from. When faced with two stimuli of differing contrasts, both neurons and crabs were much more likely to respond to the stimulus with the higher contrast. Our data provides evidence that the crabs' looming-sensitive neurons play an important part in the mechanism that drives their selective attention in the context of predation. Our results support previous suggestions that the crabs' escape direction is calculated downstream of their looming-sensitive neurons by means of a population vector of the looming sensitive neuronal ensemble.

Immunocytochemical Localization of Enzymes Involved in Dopamine, Serotonin, and Acetylcholine Synthesis in the Optic Neuropils and Neuroendocrine System of Eyestalks of Paralithodes camtschaticus

Identifying the neurotransmitters secreted by specific neurons in crustacean eyestalks is crucial to understanding their physiological roles. Here, we combined immunocytochemistry with confocal microscopy and identified the neurotransmitters dopamine (DA), serotonin (5-HT), and acetylcholine (ACh) in the optic neuropils and X-organ sinus gland (XO-SG) complex of the eyestalks of Paralithodes camtschaticus (red king crab). The distribution of Ach neurons was studied by choline acetyltransferase (ChAT) immunohistochemistry and compared with that of DA neurons examined in the same or adjacent sections by tyrosine hydroxylase (TH) immunohistochemistry. We detected 5-HT, TH, and ChAT in columnar, amacrine, and tangential neurons in the optic neuropils and established the presence of immunoreactive fibers and neurons in the terminal medulla in the XO region of the lateral protocerebrum. Additionally, we detected ChAT and 5-HT in the endogenous cells of the SG of P. camtschaticus for the first time. Furthermore, localization of 5-HT- and ChAT-positive cells in the SG indicated that these neurotransmitters locally modulate the secretion of neurohormones that are synthesized in the XO. These findings establish the presence of several neurotransmitters in the XO-SG complex of P. camtschaticus.

A Looming Spatial Localization Neural Network Inspired by MLG1 Neurons in the Crab Neohelice

Similar to most visual animals, the crab Neohelice granulata relies predominantly on visual information to escape from predators, to track prey and for selecting mates. It, therefore, needs specialized neurons to process visual information and determine the spatial location of looming objects. In the crab Neohelice granulata, the Monostratified Lobula Giant type1 (MLG1) neurons have been found to manifest looming sensitivity with finely tuned capabilities of encoding spatial location information. MLG1s neuronal ensemble can not only perceive the location of a looming stimulus, but are also thought to be able to influence the direction of movement continuously, for example, escaping from a threatening, looming target in relation to its position. Such specific characteristics make the MLG1s unique compared to normal looming detection neurons in invertebrates which can not localize spatial looming. Modeling the MLG1s ensemble is not only critical for elucidating the mechanisms underlying the functionality of such neural circuits, but also important for developing new autonomous, efficient, directionally reactive collision avoidance systems for robots and vehicles. However, little computational modeling has been done for implementing looming spatial localization analogous to the specific functionality of MLG1s ensemble. To bridge this gap, we propose a model of MLG1s and their pre-synaptic visual neural network to detect the spatial location of looming objects. The model consists of 16 homogeneous sectors arranged in a circular field inspired by the natural arrangement of 16 MLG1s' receptive fields to encode and convey spatial information concerning looming objects with dynamic expanding edges in different locations of the visual field. Responses of the proposed model to systematic real-world visual stimuli match many of the biological characteristics of MLG1 neurons. The systematic experiments demonstrate that our proposed MLG1s model works effectively and robustly to perceive and localize looming information, which could be a promising candidate for intelligent machines interacting within dynamic environments free of collision. This study also sheds light upon a new type of neuromorphic visual sensor strategy that can extract looming objects with locational information in a quick and reliable manner.

Neural organization of the third optic neuropil, the lobula, in the highly visual semiterrestrial crab Neohelice granulata

The visual neuropils (lamina, medulla and lobula complex), of malacostracan crustaceans and hexapods have many organizational principles, cell types and functional properties in common. Information about the cellular elements that compose the crustacean lobula is scarce especially when focusing on small columnar cells. Semiterrestrial crabs possess a highly developed visual system and display conspicuous visually guided behaviors. In particular, Neohelice granulata has been previously used to describe the cellular components of the first two optic neuropils using Golgi impregnation technique. Here, we present a comprehensive description of individual elements composing the third optic neuropil, the lobula, of that same species. We characterized a wide variety of elements (140 types) including input terminals and lobula columnar, centrifugal and input columnar elements. Results reveal a very dense and complex neuropil. We found a frequently impregnated input element (suggesting a supernumerary cartridge representation) that arborizes in the third layer of the lobula and that presents four variants each with ramifications organized following one of the four cardinal axes suggesting a role in directional processing. We also describe input elements with two neurites branching in the third layer, probably connecting with the medulla and lobula plate. These facts suggest that this layer is involved in the directional motion detection pathway in crabs. We analyze and discuss our findings considering the similarities and differences found between the layered organization and components of this crustacean lobula and the lobula of insects. This article is protected by copyright. All rights reserved.

Strange eyes, stranger brains: exceptional diversity of optic lobe organization in midwater crustaceans

Nervous systems across Animalia not only share a common blueprint at the biophysical and molecular level, but even between diverse groups of animals the structure and neuronal organization of several brain regions are strikingly conserved. Despite variation in the morphology and complexity of eyes across malacostracan crustaceans, many studies have shown that the organization of malacostracan optic lobes is highly conserved. Here, we report results of divergent evolution to this 'neural ground pattern' discovered in hyperiid amphipods, a relatively small group of holopelagic malacostracan crustaceans that possess an unusually wide diversity of compound eyes. We show that the structure and organization of hyperiid optic lobes has not only diverged from the malacostracan ground pattern, but is also highly variable between closely related genera. Our findings demonstrate a variety of trade-offs between sensory systems of hyperiids and even within the visual system alone, thus providing evidence that selection has modified individual components of the central nervous system to generate distinct combinations of visual centres in the hyperiid optic lobes. Our results provide new insights into the patterns of brain evolution among animals that live under extreme conditions.

Convergent evolution of optic lobe neuropil in Pancrustacea

A prevailing opinion since 1926 has been that optic lobe organization in malacostracan crustaceans and insects reflects a corresponding organization in their common ancestor. Support for this refers to malacostracans and insects both possessing three, in some instances four, nested retinotopic neuropils beneath their compound eyes. Historically, the rationale for claiming homology of malacostracan and insect optic lobes referred to those commonalities, and to comparable arrangements of neurons. However, recent molecular phylogenetics has firmly established that Malacostraca belong to Multicrustacea, whereas Hexapoda and its related taxa Cephalocarida, Branchiopoda, and Remipedia belong to the phyletically distinct clade Allotriocarida. Insects are more closely related to remipedes than are either to malacostracans. Reconciling neuroanatomy with molecular phylogenies has been complicated by studies showing that the midbrains of remipedes share many attributes with the midbrains of malacostracans. Here we review the organization of the optic lobes in Malacostraca and Insecta to inquire which of their characters correspond genealogically across Pancrustacea and which characters do not. We demonstrate that neuroanatomical characters pertaining to the third optic lobe neuropil, called the lobula complex, may indicate convergent evolution. Distinctions of the malacostracan and insect lobula complexes are sufficient to align neuroanatomical descriptions of the pancrustacean optic lobes within the constraints of molecular-based phylogenies.

The lobula plate is exclusive to insects

Just one superorder of insects is known to possess a neuronal network that mediates extremely rapid reactions in flight in response to changes in optic flow. Research on the identity and functional organization of this network has over the course of almost half a century focused exclusively on the order Diptera, a member of the approximately 300-million-year-old clade Holometabola defined by its mode of development. However, it has been broadly claimed that the pivotal neuropil containing the network, the lobula plate, originated in the Cambrian before the divergence of Hexapoda and Crustacea from a mandibulate ancestor. This essay defines the traits that designate the lobula plate and argues against a homologue in Crustacea. It proposes that the origin of the lobula plate is relatively recent and may relate to the origin of flight.

Multielectrode Recordings From Identified Neurons Involved in Visually Elicited Escape Behavior

A major challenge in current neuroscience is to understand the concerted functioning of distinct neurons involved in a particular behavior. This goal first requires achieving an adequate characterization of the behavior as well as an identification of the key neuronal elements associated with that action. Such conditions have been considerably attained for the escape response to visual stimuli in the crab Neohelice. During the last two decades a combination of in vivo intracellular recordings and staining with behavioral experiments and modeling, led us to postulate that a microcircuit formed by four classes of identified lobula giant (LG) neurons operates as a decision-making node for several important visually-guided components of the crab's escape behavior. However, these studies were done by recording LG neurons individually. To investigate the combined operations performed by the group of LG neurons, we began to use multielectrode recordings. Here we describe the methodology and show results of simultaneously recorded activity from different lobula elements. The different LG classes can be distinguished by their differential responses to particular visual stimuli. By comparing the response profiles of extracellular recorded units with intracellular recorded responses to the same stimuli, two of the four LG classes could be faithfully recognized. Additionally, we recorded units with stimulus preferences different from those exhibited by the LG neurons. Among these, we found units sensitive to optic flow with marked directional preference. Units classified within a single group according to their response profiles exhibited similar spike waveforms and similar auto-correlograms, but which, on the other hand, differed from those of groups with different response profiles. Additionally, cross-correlograms revealed excitatory as well as inhibitory relationships between recognizable units. Thus, the extracellular multielectrode methodology allowed us to stably record from previously identified neurons as well as from undescribed elements of the brain of the crab. Moreover, simultaneous multiunit recording allowed beginning to disclose the connections between central elements of the visual circuits. This work provides an entry point into studying the neural networks underlying the control of visually guided behaviors in the crab brain.

Direction Selective Neurons Responsive to Horizontal Motion in a Crab Reflect an Adaptation to Prevailing Movements in Flat Environments

All animals need information about the direction of motion to be able to track the trajectory of a target (prey, predator, cospecific) or to control the course of navigation. This information is provided by direction selective (DS) neurons, which respond to images moving in a unique direction. DS neurons have been described in numerous species including many arthropods. In these animals, the majority of the studies have focused on DS neurons dedicated to processing the optic flow generated during navigation. In contrast, only a few studies were performed on DS neurons related to object motion processing. The crab Neohelice is an established experimental model for the study of neurons involved in visually-guided behaviors. Here, we describe in male crabs of this species a new group of DS neurons that are highly directionally selective to moving objects. The neurons were physiologically and morphologically characterized by intracellular recording and staining in the optic lobe of intact animals. Because of their arborization in the lobula complex, we called these cells lobula complex directional cells (LCDCs). LCDCs also arborize in a previously undescribed small neuropil of the lateral protocerebrum. LCDCs are responsive only to horizontal motion. This nicely fits in the behavioral adaptations of a crab inhabiting a flat, densely crowded environment, where most object motions are generated by neighboring crabs moving along the horizontal plane.SIGNIFICANCE STATEMENT Direction selective (DS) neurons are key to a variety of visual behaviors including, target tracking (preys, predators, cospecifics) and course control. Here, we describe the physiology and morphology of a new group of remarkably directional neurons exclusively responsive to horizontal motion in crabs. These neurons arborize in the lobula complex and in a previously undescribed small neuropil of the lateral protocerebrum. The strong sensitivity of these cells for horizontal motion represents a clear example of functional neuronal adaptation to the lifestyle of an animal inhabiting a flat environment.

Visual determinants of prey chasing behavior in a mudflat crab

The crab Neohelice granulata inhabits mudflats where it is preyed upon by gulls and, conversely, preys on smaller crabs. Therefore, on seeing moving stimuli, this crab can behave as prey or predator. The crab escape response to visual stimuli has been extensively investigated from the behavioral to the neuronal level. The predatory response (PR), however, has not yet been explored. Here, we show that this response can be reliably elicited and investigated in a laboratory arena. By using dummies of three different sizes moved on the ground at three different velocities over multiple trials, we identified important stimulation conditions that boost the occurrence of PR and its chances of ending in successful prey capture. PR probability was sustained during the first 10 trials of our experiments but then declined. PR was elicited with high probability by the medium size dummy, less effectively by the small dummy, and hardly brought about by the large dummy, which mostly elicited avoidance responses. A GLMM analysis indicated that the dummy size and the tracking line distance were two strong determinants for eliciting PR. The rate of successful captures, however, mainly depended on the dummy velocity. Our results suggest that crabs are capable of assessing the distance to the dummy and its absolute size. The PR characterized here, in connection with the substantial knowledge of the visual processing associated with the escape response, provides excellent opportunities for comparative analyses of the organization of two distinct visually guided behaviors in a single animal.

Masters of communication: The brain of the banded cleaner shrimp Stenopus hispidus (Olivier, 1811) with an emphasis on sensory processing areas

The pan-tropic cleaner shrimp Stenopus hispidus (Crustacea, Stenopodidea) is famous for its specific cleaning behavior in association with client fish and an exclusively monogamous life-style. Cleaner shrimps feature a broad communicative repertoire, which is considered to depend on superb motor skills and the underlying mechanosensory circuits in combination with sensory organs. Their most prominent head appendages are the two pairs of very long biramous antennules and antennae, which are used both for attracting client fish and for intraspecific communication. Here, we studied the brain anatomy of several specimens of S. hispidus using histological sections, immunohistochemical labeling as well as X-ray microtomography in combination with 3D reconstructions. Furthermore, we investigated the morphology of antennules and antennae using fluorescence and scanning electron microscopy. Our analyses show that in addition to the complex organization of the multimodal processing centers, especially chemomechanosensory neuropils associated with the antennule and antenna are markedly pronounced when compared to the other neuropils of the central brain. We suggest that in their brains, three topographic maps are present corresponding to the sensory appendages. The brain areas which provide the neuronal substrate for these maps share distinct structural similarities to a unique extent in decapods, such as size and characteristic striated and perpendicular layering. We discuss our findings with respect to the sensory landscape within animal's habitat. In an evolutionary perspective, the cleaner shrimp's brain is an excellent example of how sensory potential and functional demands shape the architecture of primary chemomechanosensory processing areas.

A multimodal pathway including the basal ganglia in the feline brain

The purpose of this paper is to give an overview of our present knowledge about the feline tecto-thalamo-basal ganglia cortical sensory pathway. We reviewed morphological and electrophysiological studies of the cortical areas, located in ventral bank of the anterior ectosylvian sulcus as well as the region of the insular cortex, the suprageniculate nucleus of the thalamus, caudate nucleus, and the substantia nigra. Microelectrode studies revealed common receptive field properties in all these structures. The receptive fields were extremely large and multisensory, with pronounced sensitivity to motion of visual stimuli. They often demonstrated directional and velocity selectivity. Preference for small visual stimuli was also a frequent finding. However, orientation sensitivity was absent. It became obvious that the structures of the investigated sensory loop exhibit a unique kind of information processing, not found anywhere else in the feline visual system.

Unidirectional Optomotor Responses and Eye Dominance in Two Species of Crabs

Animals, from invertebrates to humans, stabilize the panoramic optic flow through compensatory movements of the eyes, the head or the whole body, a behavior known as optomotor response (OR). The same optic flow moved clockwise or anticlockwise elicits equivalent compensatory right or left turning movements, respectively. However, if stimulated monocularly, many animals show a unique effective direction of motion, i.e., a unidirectional OR. This phenomenon has been reported in various species from mammals to birds, reptiles, and amphibious, but among invertebrates, it has only been tested in flies, where the directional sensitivity is opposite to that found in vertebrates. Although OR has been extensively investigated in crabs, directional sensitivity has never been analyzed. Here, we present results of behavioral experiments aimed at exploring the directional sensitivity of the OR in two crab species belonging to different families: the varunid mud crab Neohelice granulata and the ocypode fiddler crab Uca uruguayensis. By using different conditions of visual perception (binocular, left or right monocular) and direction of flow field motion (clockwise, anticlockwise), we found in both species that in monocular conditions, OR is effectively displayed only with progressive (front-to-back) motion stimulation. Binocularly elicited responses were directional insensitive and significantly weaker than monocular responses. These results are coincident with those described in flies and suggest a commonality in the circuit underlying this behavior among arthropods. Additionally, we found the existence of a remarkable eye dominance for the OR, which is associated to the size of the larger claw. This is more evident in the fiddler crab where the difference between the two claws is huge.

Binocular Neuronal Processing of Object Motion in an Arthropod

Animals use binocular information to guide many behaviors. In highly visual arthropods, complex binocular computations involved in processing panoramic optic flow generated during self-motion occur in the optic neuropils. However, the extent to which binocular processing of object motion occurs in these neuropils remains unknown. We investigated this in a crab, where the distance between the eyes and the extensive overlapping of their visual fields advocate for the use of binocular processing. By performing in vivo intracellular recordings from the lobula (third optic neuropil) of male crabs, we assessed responses of object-motion-sensitive neurons to ipsilateral or contralateral moving objects under binocular and monocular conditions. Most recorded neurons responded to stimuli seen independently with either eye, proving that each lobula receives profuse visual information from both eyes. The contribution of each eye to the binocular response varies among neurons, from those receiving comparable inputs from both eyes to those with mainly ipsilateral or contralateral components, some including contralateral inhibition. Electrophysiological profiles indicated that a similar number of neurons were recorded from their input or their output side. In monocular conditions, the first group showed shorter response delays to ipsilateral than to contralateral stimulation, whereas the second group showed the opposite. These results fit well with neurons conveying centripetal and centrifugal information from and toward the lobula, respectively. Intracellular and massive stainings provided anatomical support for this and for direct connections between the two lobulae, but simultaneous recordings failed to reveal such connections. Simplified model circuits of interocular connections are discussed.SIGNIFICANCE STATEMENT Most active animals became equipped with two eyes, which contributes to functions like depth perception, objects spatial location, and motion processing, all used for guiding behaviors. In visually active arthropods, binocular neural processing of the panoramic optic flow generated during self-motion happens already in the optic neuropils. However, whether binocular processing of single-object motion occurs in these neuropils remained unknown. We investigated this in a crab, where motion-sensitive neurons from the lobula can be recorded in the intact animal. Here we demonstrate that different classes of neurons from the lobula compute binocular information. Our results provide new insight into where and how the visual information acquired by the two eyes is first combined in the brain of an arthropod.

The predator and prey behaviors of crabs: from ecology to neural adaptations

Predator avoidance and prey capture are among the most vital of animal behaviors. They require fast reactions controlled by comparatively straightforward neural circuits often containing giant neurons, which facilitates their study with electrophysiological techniques. Naturally occurring avoidance behaviors, in particular, can be easily and reliably evoked in the laboratory, enabling their neurophysiological investigation. Studies in the laboratory alone, however, can lead to a biased interpretation of an animal's behavior in its natural environment. In this Review, we describe current knowledge – acquired through both laboratory and field studies – on the visually guided escape behavior of the crab Neohelice granulata. Analyses of the behavioral responses to visual stimuli in the laboratory have revealed the main characteristics of the crab's performance, such as the continuous regulation of the speed and direction of the escape run, or the enduring changes in the strength of escape induced by learning and memory. This work, in combination with neuroanatomical and electrophysiological studies, has allowed the identification of various giant neurons, the activity of which reflects most essential aspects of the crabs' avoidance performance. In addition, behavioral analyses performed in the natural environment reveal a more complex picture: crabs make use of much more information than is usually available in laboratory studies. Moreover, field studies have led to the discovery of a robust visually guided chasing behavior in Neohelice. Here, we describe similarities and differences in the results obtained between the field and the laboratory, discuss the sources of any differences and highlight the importance of combining the two approaches.

Visual motion processing subserving behavior in crabs

Motion vision originated during the Cambrian explosion more than 500 million years ago, likely triggered by the race for earliest detection between preys and predators. To successfully evade a predator's attack a prey must react quickly and reliably, which imposes a common constrain to the implementation of escape responses among different species. Thus, neural circuits subserving fast escape responses are usually straightforward and contain giant neurons. This review summarizes knowledge about a small group of motion-sensitive giant neurons thought to be central in guiding the escape performance of crabs to visual stimuli. The flexibility of the escape behavior contrasts with the stiffness of the optomotor response, indicating a task-dependent early segregation of visual pathways.

Predation risk modifies behaviour by shaping the response of identified brain neurons

Interpopulation comparisons in species that show behavioural variations associated with particular ecological disparities offer good opportunities for assessing how environmental factors may foster specific functional adaptations in the brain. Yet, studies on the neural substrate that can account for interpopulation behavioural adaptations are scarce. Predation is one of the strongest driving forces for behavioural evolvability and, consequently, for shaping structural and functional brain adaptations. We analysed the escape response of crabs Neohelice granulata from two isolated populations exposed to different risks of avian predation. Individuals from the high-risk area proved to be more reactive to visual danger stimuli (VDS) than those from an area where predators are rare. Control experiments indicate that the response difference was specific for impending visual threats. Subsequently, we analysed the response to VDS of a group of giant brain neurons that are thought to play a main role in the visually guided escape response of the crab. Neurons from animals of the population with the stronger escape response were more responsive to VDS than neurons from animals of the less reactive population. Our results suggest a robust linkage between the pressure imposed by the predation risk, the response of identified neurons and the behavioural outcome.

Arthropod eyes: The early Cambrian fossil record and divergent evolution of visual systems

Four types of eyes serve the visual neuropils of extant arthropods: compound retinas composed of adjacent facets; a visual surface populated by spaced eyelets; a smooth transparent cuticle providing inwardly directed lens cylinders; and single-lens eyes. The first type is a characteristic of pancrustaceans, the eyes of which comprise lenses arranged as hexagonal or rectilinear arrays, each lens crowning 8-9 photoreceptor neurons. Except for Scutigeromorpha, the second type typifies Myriapoda whose relatively large eyelets surmount numerous photoreceptive rhabdoms stacked together as tiers. Scutigeromorph eyes are facetted, each lens crowning some dozen photoreceptor neurons of a modified apposition-type eye. Extant chelicerate eyes are single-lensed except in xiphosurans, whose lateral eyes comprise a cuticle with a smooth outer surface and an inner one providing regular arrays of lens cylinders. This account discusses whether these disparate eye types speak for or against divergence from one ancestral eye type. Previous considerations of eye evolution, focusing on the eyes of trilobites and on facet proliferation in xiphosurans and myriapods, have proposed that the mode of development of eyes in those taxa is distinct from that of pancrustaceans and is the plesiomorphic condition from which facetted eyes have evolved. But the recent discovery of enormous regularly facetted compound eyes belonging to early Cambrian radiodontans suggests that high-resolution facetted eyes with superior optics may be the ground pattern organization for arthropods, predating the evolution of arthrodization and jointed post-protocerebral appendages. Here we provide evidence that compound eye organization in stem-group euarthropods of the Cambrian can be understood in terms of eye morphologies diverging from this ancestral radiodontan-type ground pattern. We show that in certain Cambrian groups apposition eyes relate to fixed or mobile eyestalks, whereas other groups reveal concomitant evolution of sessile eyes equipped with optics typical of extant xiphosurans. Observations of fossil material, including that of trilobites and eurypterids, support the proposition that the ancestral compound eye was the apposition type. Cambrian arthropods include possible precursors of mandibulate eyes. The latter are the modified compound eyes, now sessile, and their underlying optic lobes exemplified by scutigeromorph chilopods, and the mobile stalked compound eyes and more elaborate optic lobes typifying Pancrustacea. Radical divergence from an ancestral apposition type is demonstrated by the evolution of chelicerate eyes, from doublet sessile-eyed stem-group taxa to special apposition eyes of xiphosurans, the compound eyes of eurypterids, and single-lens eyes of arachnids. Different eye types are discussed with respect to possible modes of life of the extinct species that possessed them, comparing these to extant counterparts and the types of visual centers the eyes might have served.

Object approach computation by a giant neuron and its relation with the speed of escape in the crab Neohelice

Upon detection of an approaching object the crab Neohelice granulata continuously regulates the direction and speed of escape according to ongoing visual information. These visuomotor transformations are thought to be largely accounted for by a small number of motion-sensitive giant neurons projecting from the lobula (third optic neuropil) towards the supraesophageal ganglion. One of these elements, the monostratified lobula giant neurons of type 2 (MLG2), proved to be highly sensitive to looming stimuli (a 2D representation of an object approach). By performing in vivo intracellular recordings we assessed the response of the MLG2 neuron to a variety of looming stimuli representing objects of different sizes and velocities of approach. This allowed us: a) to identify some of the physiological mechanisms involved in the regulation of the MLG2 activity and to test a simplified biophysical model of its response to looming stimuli; b) to identify the stimulus optical parameters encoded by the MLG2, and to formulate a phenomenological model able to predict the temporal course of the neural firing responses to all looming stimuli; c) to incorporate the MLG2 encoded information of the stimulus (in terms of firing rate) into a mathematical model able to fit the speed of the escape run of the animal. The agreement between the model predictions and the actual escape speed measured on a treadmill for all tested stimuli strengthens our interpretation of the computations performed by the MLG2 and of the involvement of this neuron in the regulation of the animal's speed of run while escaping from objects approaching with constant speed.

Comparative analyses of olfactory systems in terrestrial crabs (Brachyura): evidence for aerial olfaction?

Adaptations to a terrestrial lifestyle occurred convergently multiple times during the evolution of the arthropods. This holds also true for the "true crabs" (Brachyura), a taxon that includes several lineages that invaded land independently. During an evolutionary transition from sea to land, animals have to develop a variety of physiological and anatomical adaptations to a terrestrial life style related to respiration, reproduction, development, circulation, ion and water balance. In addition, sensory systems that function in air instead of in water are essential for an animal's life on land. Besides vision and mechanosensory systems, on land, the chemical senses have to be modified substantially in comparison to their function in water. Among arthropods, insects are the most successful ones to evolve aerial olfaction. Various aspects of terrestrial adaptation have also been analyzed in those crustacean lineages that evolved terrestrial representatives including the taxa Anomala, Brachyura, Amphipoda, and Isopoda. We are interested in how the chemical senses of terrestrial crustaceans are modified to function in air. Therefore, in this study, we analyzed the brains and more specifically the structure of the olfactory system of representatives of brachyuran crabs that display different degrees of terrestriality, from exclusively marine to mainly terrestrial. The methods we used included immunohistochemistry, detection of autofluorescence- and confocal microscopy, as well as three-dimensional reconstruction and morphometry. Our comparative approach shows that both the peripheral and central olfactory pathways are reduced in terrestrial members in comparison to their marine relatives, suggesting a limited function of their olfactory system on land. We conclude that for arthropod lineages that invaded land, evolving aerial olfaction is no trivial task.

Differences in the escape response of a grapsid crab in the field and in the laboratory

Escape behaviours of prey animals are frequently used to study the neural control of behaviour. Escape responses are robust, fast, and can be reliably evoked under both field and laboratory conditions. Many escape responses are not as simple as previously suggested, however, and are often modulated by a range of contextual factors. To date it has been unclear to what extent behaviours studied in controlled laboratory experiments are actually representative of the behaviours that occur under more natural conditions. Here we have used the model species, Neohelice granulata, a grapsid crab, to show that there are significant differences between the crabs' escape responses in the field compared to those previously documented in laboratory experiments. These differences are consistent with contextual adjustments such as the availability of a refuge and have clear consequences for understanding the crabs' neural control of behaviour. Furthermore, the methodology used in this study mirrors the methodology previously used in fiddler crab research, allowing us to show that the previously documented differences in escape responses between these grapsid species are real and substantial. Neohelice's responses are delayed and more controlled. Overall, the results highlight the adaptability and flexibility of escape behaviours and provide further evidence that the neural control of behaviour needs to be address in both the laboratory and field context.