Brain architecture of the largest living land arthropod, the Giant Robber Crab Birgus latro (Crustacea, Anomura, Coenobitidae): evidence for a prominent central olfactory pathway?

<p>Abstract</p> <p>Background</p> <p>Several lineages within the Crustacea conquered land independently during evolution, thereby requiring physiological adaptations for a semi-terrestrial or even a fully terrestrial lifestyle. <it>Birgus latro </it>Linnaeus, 1767, the giant robber crab or coconut crab (Anomura, Coenobitidae), is the largest land-living arthropod and inhabits Indo-Pacific islands such as Christmas Island. <it>B. latro </it>has served as a model in numerous studies of physiological aspects related to the conquest of land by crustaceans. From an olfactory point of view, a transition from sea to land means that molecules need to be detected in gas phase instead of in water solution. Previous studies have provided physiological evidence that terrestrial hermit crabs (Coenobitidae) such as <it>B. latro </it>have a sensitive and well differentiated sense of smell. Here we analyze the brain, in particular the olfactory processing areas of <it>B. latro</it>, by morphological analysis followed by 3 D reconstruction and immunocytochemical studies of synaptic proteins and a neuropeptide.</p> <p>Results</p> <p>The primary and secondary olfactory centers dominate the brain of <it>B. latro </it>and together account for ca. 40% of the neuropil volume in its brain. The paired olfactory neuropils are tripartite and composed of more than 1,000 columnar olfactory glomeruli, which are radially arranged around the periphery of the olfactory neuropils. The glomeruli are innervated ca. 90,000 local interneurons and ca. 160,000 projection neurons per side. The secondary olfactory centers, the paired hemiellipsoid neuropils, are targeted by the axons of these olfactory projection neurons. The projection neuron axonal branches make contact to ca. 250.000 interneurons (per side) associated with the hemiellipsoid neuropils. The hemiellipsoid body neuropil is organized into parallel neuropil lamellae, a design that is quite unusual for decapod crustaceans. The architecture of the optic neuropils and areas associated with antenna two suggest <it>that B. latro </it>has visual and mechanosensory skills that are comparable to those of marine Crustacea.</p> <p>Conclusions</p> <p>In parallel to previous behavioral findings that <it>B. latro </it>has aerial olfaction, our results indicate that their central olfactory pathway is indeed most prominent. Similar findings from the closely related terrestrial hermit crab <it>Coenobita clypeatus </it>suggest that in Coenobitidae, olfaction is a major sensory modality processed by the brain, and that for these animals, exploring the olfactory landscape is vital for survival in their terrestrial habitat. Future studies on terrestrial members of other crustacean taxa such as Isopoda, Amphipoda, Astacida, and Brachyura will shed light on how frequently the establishment of an aerial sense of olfaction evolved in Crustacea during the transition from sea to land. Amounting to ca. 1,000,000, the numbers of interneurons that analyse the olfactory input in <it>B. latro </it>brains surpasses that in other terrestrial arthropods, as e.g. the honeybee <it>Apis mellifera </it>or the moth <it>Manduca sexta</it>, by two orders of magnitude suggesting that <it>B. latro </it>in fact is a land-living arthropod that has devoted a substantial amount of nervous tissue to the sense of smell.</p

Bio-inspired Antennal Tactile Sensing

Vision dominates perception research in robotics and biology, but for many animals, it is not the dominant sensory system. Indeed, arthropods often rely on sensory cues sampled via a pair of passive head-mounted antennae to achieve navigation and control. These mechanosensory structures support multimodal receptors—tactile, hygrometric, thermal, olfactory—enabling a wide range of sensorimotor behaviors. One model biological system, Periplaneta americana cockroach, performs a remarkably robust escape behavior by using its long, slender, flexible antennae to facilitate rapid closed-loop course control. The antenna is a passive, hyper-redundant kinematic linkage that acts as a distributed tactile sensory structure to mediate mechanical interactions with the environment at very high rates. This thesis demonstrates that the antennal mechanics are tuned to enable high-speed, high-bandwidth locomotor control even in total darkness. Despite the extraordinary success of antennal sensing in nature, there are few effective bio-inspired antennae. To incorporate similar antennal sensing capability in agile mobile robots, I developed a tunable bio-inspired modular robotic research antenna and experimentation platform. I also synthesized numerical models to approximate antenna mechanics under relevant boundary conditions, which I verified against my physical model. Both numerical simulations and physical experiments were conducted to isolate fundamental parameters that underly the stability and performance I observed in the biological model. Using a combination of numerical and robotic experiments, in concert with biological experiments conducted by my collaborators, I discovered that several behaviorally relevant characteristics of an antennae are predominantly governed by a combination of (1) the stiffness profile of the antenna and (2) the interaction of hairlike mechano-structures along the length of the antenna. I found that the “right” combination of these features improves the postural stability and the steady state spatial acuity of tactile interaction with the environment. Specifically, antennae with an exponentially decreasing stiffness profile accompanied by distally pointing anisotropic mechano-hairs are ideal for navigation tasks, and greatly facilitate stable high-speed wall following

ENVIRONMENTAL EFFECTS ON BEHAVIOR AND PHYSIOLOGY IN CRAYFISH

Despite dramatic morphological differences between animals from different taxa, several important features in organization and sensory system processing are similar across animals. Because of this similarity, a number of different organisms including mammals, insects, and decapod crustaceans serve as valuable model systems for understanding general principles of environmental effects. This research examines intrinsic and extrinsic factors by behaviorally and physiologically means to identify the impact of environmental conditions on two distinct crayfish species- Procambarus clarkii (surface) and Orconectes australis packardi (cave). The research identified behavioral and physiological responses in these two morphological and genetically distinct species. The studies also examined multiple levels of complexity including social behavior, an autonomic response, chemosensory capabilities and neuronal communication, identified comparative similarities/differences, addressed learning and environmental influences on learning and examined behavioral and cellular responses to high levels of carbon dioxide. I found environmental factors directly influence crayfish behavior of social interactions. Interactions were more aggressive, more intense and more likely to end with a physical confrontation when they took place \u27in water\u27 than \u27out of water\u27. The modified social interaction resulted in a altered fighting strategy. A study on motor task learning was undertaken which showed similar learning trends among these crayfish species despite their reliance on different sensory modalities. I also demonstrated learning was dependent on perceived stress by the organism. Previously trained crayfish inhibited from completing a task showed significant increase in an autonomic stress response. Studies on the behavioral and physiological responses to CO2 revealed that high [CO2] is a repellent in a concentration dependent manner. The autonomic responses in heart rate and an escape tailflip reflex shows complete cessation with high [CO2]. A mechanistic effect of CO2 is by blocking glutamate receptors at the neuromuscular junction and through inhibition of the motor nerve within the CNS

Mechanisms of Navigation in Fiddler Crabs: An Analysis of Allocentric and Egocentric Contributions

Navigation in biological systems is a complex task-set that involves learning processes and may include constructing representations of features of their environment. Across the animal kingdom, different learning mechanisms have evolved to similar spatial problems. The extent to which mechanisms are conserved across taxa are an important research area that can guide our understanding of the cognitive dimensions of navigation. Recent studies of mammals, birds, and arthropods has found that these animals often attend to multiple forms of sensory cues, and to either integrate the solutions generated by these cues, or at times prefer one form of cue over another. This dissertation examines the fiddler crab (Uca pugilator), a burrow-homing arthropod whose ecology and behavior engender evolutionary pressures that favor spatial memory to determine which these kinds of multi-modal integrative processes are at they employ. Previous field studies give indications of complexity beyond simple route reversal methods. U. pugilator are a species that share and likely resemble a basal ancestor to the insect taxa that have proved fruitful to the study of navigation. The results of this dissertation suggest that the ability to employ and integrate solutions from multiple navigational mechanisms is evolutionarily old and conserved across a wide range of taxa. Four experiments are presented that employ a place learning paradigm to examine the roles of externally (allocentric) and internally (egocentric) generated sensory cues in the construction of fiddler crab navigational strategies. Three of these experiments provide evidence for a preexisting taxis in these animals that dictates they approach certain visual stimuli, and two of these experiments provide evidence of an allocentrically informed associative process in navigating fiddler crabs, a finding not before seen in a laboratory study of these animals. Taken together the results of this dissertation suggest that fiddler crabs possess some form of cognitive representation of the external world, which is informed by multiple sensory modalities, and extends beyond response learning and path integration

Social cognition in domestic horses (Equus caballus)

The social intelligence hypothesis states that the main selection pressures driving increases in brain-to-body ratio are social rather than ecological. The domestic horse is an ideal animal to study within this framework because horses possess rich social lives but inhabit simple ecological environments. Here I assess the abilities of horses within two broad areas of social cognition; the classification of, and the use of information obtained from, social partners. In Section One I demonstrate that horses are capable of cross-modal individual recognition of conspecifics, an ability not previously demonstrated conclusively outside of humans. This ability extends to identifying familiar human companions suggesting that recognition systems are highly plastic in the individuals they can encode. These results also provide the first insights into the brain mechanisms involved in this process by revealing a clear left hemisphere bias in discriminatory ability. In Section Two I investigate the extent to which horses are capable of reading human attentional and communicative cues. It has been suggested that this skill was selected for through the process of domestication, however there have been no systematic studies of domestic animals other than the domestic dog. I found that horses were indeed highly skilled at determining if people were paying attention to them. In contrast they tended to only use basic stimulus enhancement cues to choose a rewarded bucket. A further study of young horses indicated that the ability to detect human attention requires significant experience to develop fully whereas the ability to use stimulus enhancement cues in an object choice task appears to require far less (if any) experience to develop. Overall my thesis extends our knowledge of comparative social cognition and in particular our knowledge of social cognition in horses. Taken together, these results clearly demonstrate that horses do indeed possess some complex socio-cognitive skills

The maintenance and spread of cooperation in social networks in natural populations

Social interaction is crucial for social animals to thrive, but it comes with both benefits and costs. The social network approach helps to identify how social processes like cooperation are influenced by individual and population-level characteristics. Factors like individual recognition, familiarity, and social stability play a role in promoting tolerance and cooperation; and understanding these factors can provide insights into how social networks, cooperation, and other social behaviors evolve in different species. In this thesis, I examine how social processes interact with spatial and temporal factors to influence fitness and other outcomes in wild birds (Paridae). I begin by establishing a theoretical foundation for my work by reviewing the concepts of cooperation and individual recognition and how they can be better understood through a social network approach (Chapters 2 & 3). Using experimental data, I explore the processes that may impact cooperative behavior by examining leading behavior during mixed-species foraging (Chapter 4) and demonstrate that motivation and dominance, as well as spatio- temporal effects, are determinants of this behavior. Drawing on these insights, I then use long-term breeding data to show that familiarity among neighbors as well as familiarity within breeding pairs influences fitness (Chapter 5). I also demonstrate that different kinds of social relationships have different effects on reproductive outcomes (Chapter 6). All in all, I demonstrate that social connections have implications for fitness, and highlight the importance of accounting for spatial, temporal, and cognitive components when studying the ecology and evolution of sociality

Behavioural responses by marine fishes and macroinvertebrates to underwater noise

The aim of this thesis was to explore and evaluate the key behavioural responses of coastal UK marine fishes and macroinvertebrates to anthropogenic noise. Work focussed upon two key aspects, water-borne acoustics and the relatively unstudied substrate-borne vibration, with a combination of laboratory and field work using grouped and solitary individuals. A literature review on underwater vibroacoustics, detection abilities, anthropogenic noise sources and the effects of such stimuli was provided (Chapter 1).Playbacks were undertaken in the field using a purpose-built underwater transducer array capable of accurately reproducing man-made signatures (Chapter 2 – 3). The behavioural responses of wild, unrestrained schooling pelagic fish to impulsive sound were observed using an acoustic observation system. Precise exposure levels were linked to specific responses, with dose response curves produced for two pelagic species of varied hearing abilities. Baited remote underwater video (BRUV) was used to observe the behavioural responses of free-ranging individual fish and crustaceans exposed to impulsive sound and shipping noise. In both cases responses varied according to the level of sound, the type of school and the species.In the laboratory, animals were exposed to sinusoidal vibratory signals using a fully calibrated electromagnetic shaker system. The sensitivity of unconditioned invertebrates (crustaceans and molluscs) to substrate-borne vibration was quantified with controlled vibratory exposures, allowing the production of a sensory threshold curve for three species (Chapters 4 - 5). Response variation was described in terms of two behavioural indicators, and related to consistency within individuals (personality), morphological parameters and time in the laboratory prior to tests. Further work investigated the response of sessile invertebrates to vibration, with the observations fully described in terms of response occurrence, duration and variation for both grouped and solitary animals.The responses described in each chapter were related to actual measurements of anthropogenic noise sources in terms of water-borne and substrate-borne energy, allowing behavioural responses to be translated to actual conditions. The data here provide evidence for the levels of playback sound to induce a behavioural response, and are fully reproducible to allow further testing of the responsiveness of fish to different sound levels and signatures. Furthermore, the data are a first step towards understanding the sensitivity of benthic invertebrates to substrate-borne vibration and indicate that the effects of substrate transmission should not be overlooked when investigating the effects of noise pollution on the marine environment. The results from the current work, along with the recommendations for future work, will be important to aid the filling of the ‘information gaps’ that exist within the underwater bioacoustics field