Why do animals have so many receptors? The role of multiple chemosensors in animal perception

Are you scared yet? Variations to cue indices elicit differential prey behavioral responses even when gape-limited predators are relatively small

Anti-predator behavior is often evoked based on measurements of risk calculated from sensory cues emanating from predators independent of physical attack. Yet, the exact sensory indices of cues utilized in risk assessment remain largely unknown. To examine how different predatory cue indices of information are used in risk assessment, we presented prey with various cues from sub-lethal gape-limited predators. Rusty crayfish (<i>Faxonius rusticus</i> (Girard, 1852)) were exposed to predatory odors from sub-lethal sized largemouth bass (<i>Micropterus salmoides</i> (Lacepѐde, 1802)) to test effects of changing predator abundance, relative size relationships, and total predator length in flow through mesocosms. Foraging, shelter use, and movement behavior were used to measure cue effects. Foraging time depended jointly upon predator abundance and total predator size (p = 0.030). Specifically, high predator abundance resulted in decreased foraging efforts as gape ratio increased. Similarly, sheltering time depended on the interaction between predator abundance and gape ratio when predator abundance was highest (p = 0.020). Crayfish significantly increased exploration time when gape ratio increased (p = 0.010). Thus, this study shows crayfish can utilize different indices of predatory cues, namely total predator abundance and relative size ratios, in risk assessment but do so in context specific ways.

Do Hydrothermal Shrimp Smell Vents?

Deep-sea species endemic to hydrothermal vents face the critical challenge of detecting active sites in a vast environment devoid of sunlight. This certainly requires specific sensory abilities, among which olfaction could be a relevant sensory modality, since chemical compounds in hydrothermal fluids or food odors could potentially serve as orientation cues. The temperature of the vent fluid might also be used for locating vent sites. The objective of this study is to observe the following key behaviors of olfaction in hydrothermal shrimp, which could provide an insight into their olfactory capacities: (1) grooming behavior; (2) attraction to environmental cues (food odors and fluid markers). We designed experiments at both deep-sea and atmospheric pressure to assess the behavior of the vent shrimp Rimicaris exoculata and Mirocaris fortunata, as well as of the coastal species Palaemon elegans and Palaemon serratus for comparison. Here, we show that hydrothermal shrimp groom their sensory appendages similarly to other crustaceans, but this does not clean the dense bacterial biofilm that covers the olfactory structures. These shrimp have previously been shown to possess functional sensory structures, and to detect the environmental olfactory signals tested, but we do not observe significant attraction behavior here. Only temperature, as a signature of vent fluids, clearly attracts vent shrimp and thus is confirmed to be a relevant signal for orientation in their environment.

Humane Slaughter of Edible Decapod Crustaceans

Simple Summary

Decapods respond to noxious stimuli in ways that are consistent with the experience of pain; thus, we accept the need to provide a legal framework for their protection when they are used for human food. We review the main methods used to slaughter the major decapod crustaceans, highlighting problems posed by each method for animal welfare. The aim is to identify methods that are the least likely to cause suffering. These methods can then be recommended, whereas other methods that are more likely to cause suffering may be banned. We thus request changes in the legal status of this group of animals, to protect them from slaughter techniques that are not viewed as being acceptable.

Abstract

Vast numbers of crustaceans are produced by aquaculture and caught in fisheries to meet the increasing demand for seafood and freshwater crustaceans. Simultaneously, the public is increasingly concerned about current methods employed in their handling and killing. Recent evidence has shown that decapod crustaceans probably have the capacity to suffer because they show responses consistent with pain and have a relatively complex cognitive capacity. For these reasons, they should receive protection. Despite the large numbers of crustaceans transported and slaughtered, legislation protecting their welfare, by using agreed, standardized methods, is lacking. We review various stunning and killing systems proposed for crustaceans, and assess welfare concerns. We suggest the use of methods least likely to cause suffering and call for the implementation of welfare guidelines covering the slaughter of these economically important animals.

The Crustacean Antennule: A Complex Organ Adapted for Lifelong Function in Diverse Environments and Lifestyles

AbstractThe crustacean first antenna, or antennule, has been an experimental model for studying sensory biology for over 150 years. Investigations have led to a clearer understanding of the functional organization of the antennule as an olfactory organ but also to a realization that the antennule is much more than that. Across the Crustacea, the antennules take on many forms and functions. As an example, the antennule of reptantian decapods has many types of sensilla, each with distinct structure and function and with hundreds of thousands of chemosensory neurons expressing hundreds of genes that code for diverse classes of receptor proteins. Together, these antennular sensilla represent multiple chemosensory pathways, each with its own central connections and functions. The antennule also has a diversity of sensors of mechanical stimuli, including vibrations, touch, water flow, and the animal's own movements. The antennule likely also detects other environmental cues, such as temperature, oxygen, pH, salinity, and noxious stimuli. Furthermore, the antennule is a motor organ-it is flicked to temporally and spatially sample the animal's chemo-mechanical surroundings-and this information is used in resolving the structure of chemical plumes and locating the odor source. The antennule is also adapted to maintain lifelong function in a changing environment. For example, it has specific secretory glands, grooming structures, and behaviors to stay clean and functional. Antennular sensilla and the annuli on which they reside are also added and replaced, leading to a complete turnover of the antennule over several molts. Thus, the antennule is a complex and dynamic sensory-motor integrator that is intricately engaged in most aspects of the lives of crustaceans.

Ultrastructure of Antennal Morphology and Sensilla of Teak Skeletonizer, Eutectona machaeralis Walker (Lepidoptera: Crambidae)

The leaf skeletonizer, Eutectona machaeralis (Walker) (Lepidoptera: Crambidae), is a severe insect pest of teak trees (Tectona grandis L.f.) in China. To provide some basic evidence for future semiochemical-based management strategies of E. machaeralis, the morphology, ultrastructure, and distribution of antennal sensilla of adults were observed under scanning and transmission electron microscopy. The shape and structure of antenna were similar between males and females, both being filiform. However, the antennal length of males was significantly longer than that of females. Eight morphological sensilla types were observed in both sexes: Böhm's bristles, sensilla trichodea, sensilla basiconica, sensilla chaetica, sensilla styloconica, sensilla coeloconica, sensilla auricillica, and sensilla squamiformia. Significant sexual dimorphism of the sensilla dimensions was found, especially in sensillar length. The putative and potential functions of the different sensilla types are discussed based on the fine structures of the cuticular walls and dendrites of the different sensilla types. We expect these results to help lay a solid foundation for future functional research and develop further investigations of E. machaeralis.

Sensorial Hierarchy in Octopus vulgaris's Food Choice: Chemical vs. Visual

Simple Summary

Coleoids are cephalopods endowed with a highly sophisticated nervous system with keen sense organs and an exceptionally large brain that includes more than 30 differentiated lobes. Within this group, Octopus vulgaris, well known as an intelligent soft-bodied animal, has a significant number of lobes in the nervous system dedicated to decoding and integrating visual, tactile, and chemosensory perceptions. In this study, we aimed to understand the key role of chemical and visual cues during food selection in O. vulgaris. We first defined the preferred food, and subsequently, we set up five different problem-solving tasks, in which the animal’s choice is guided by visual and chemosensory signals, either alone or together, to evaluate whether individual O. vulgaris uses a sensorial hierarchy. Our behavioural experiments show that this species does integrate different sensory information from chemical and visual cues during food selection; however, our results indicate that chemical perception provides accurate and faster information leading to food choice. This research opens new perspectives on O. vulgaris’ predation strategies.

Abstract

Octopus vulgaris possesses highly sophisticated sense organs, processed by the nervous system to generate appropriate behaviours such as finding food, avoiding predators, identifying conspecifics, and locating suitable habitat. Octopus uses multiple sensory modalities during the searching and selection of food, in particular, the chemosensory and visual cues. Here, we examined food choice in O. vulgaris in two ways: (1) We tested octopus’s food preference among three different kinds of food, and established anchovy as the preferred choice (66.67%, Friedman test p < 0.05); (2) We exposed octopus to a set of five behavioural experiments in order to establish the sensorial hierarchy in food choice, and to evaluate the performance based on the visual and chemical cues, alone or together. Our data show that O. vulgaris integrates sensory information from chemical and visual cues during food choice. Nevertheless, food choice resulted in being more dependent on chemical cues than visual ones (88.9%, Friedman test p < 0.05), with a consistent decrease of the time spent identifying the preferred food. These results define the role played by the senses with a sensorial hierarchy in food choice, opening new perspectives on the O. vulgaris’ predation strategies in the wild, which until today were considered to rely mainly on visual cues.

Comparative Study of Chemosensory Organs of Shrimp From Hydrothermal Vent and Coastal Environments

The detection of chemical signals is involved in a variety of crustacean behaviors, such as social interactions, search and evaluation of food and navigation in the environment. At hydrothermal vents, endemic shrimp may use the chemical signature of vent fluids to locate active edifices, however little is known on their sensory perception in these remote deep-sea habitats. Here, we present the first comparative description of the sensilla on the antennules and antennae of 4 hydrothermal vent shrimp (Rimicaris exoculata, Mirocaris fortunata, Chorocaris chacei, and Alvinocaris markensis) and of a closely related coastal shrimp (Palaemon elegans). These observations revealed no specific adaptation regarding the size or number of aesthetascs (specialized unimodal olfactory sensilla) between hydrothermal and coastal species. We also identified partial sequences of the ionotropic receptor IR25a, a co-receptor putatively involved in olfaction, in 3 coastal and 4 hydrothermal shrimp species, and showed that it is mainly expressed in the lateral flagella of the antennules that bear the unimodal chemosensilla aesthetascs.

Modelling a historic oil-tank fire allows an estimation of the sensitivity of the infrared receptors in pyrophilous Melanophila beetles

Pyrophilous jewel beetles of the genus Melanophila approach forest fires and there is considerable evidence that these beetles can detect fires from great distances of more than 60 km. Because Melanophila beetles are equipped with infrared receptors and are also attracted by hot surfaces it can be concluded that these infrared receptors are used for fire detection.

The sensitivity of the IR receptors is still unknown. The lowest threshold published so far is 0.6 W/m² which, however, cannot explain the detection of forest fires by IR radiation from distances larger than approximately 10 km. To investigate the possible sensitivity of the IR receptors we assumed that beetles use IR radiation for remote fire detection and we made use of a historic report about a big oil-tank fire in Coalinga, California, in 1924. IR emission of an oil-tank fire can be calculated by “pool fire” simulations which now are used for fire safety and risk analysis. Assuming that beetles were lured to the fire from the nearest forests 25 and 130 km away, our results show that detection from a distance of 25 km requires a threshold of the IR receptors of at least 3×10⁻² W/m². According to our investigations most beetles became aware of the fire from a distance of 130 km. In this case the threshold has to be 1.3×10⁻⁴ W/m². Because such low IR intensities are buried in thermal noise we suggest that the infrared sensory system of Melanophila beetles utilizes stochastic resonance for the detection of weak IR radiation. Our simulations also suggest that the biological IR receptors might be even more sensitive than uncooled technical IR sensors. Thus a closer look into the mode of operation of the Melanophila IR receptors seems promising for the development of novel IR sensors.

Oesophageal chemoreceptors of blue crabs, Callinectes sapidus, sense chemical deterrents and can block ingestion of food

Decapod crustaceans such as blue crabs possess a variety of chemoreceptors that control different stages of the feeding process. All these chemoreceptors are putative targets for feeding deterrents that cause animals to avoid or reject otherwise palatable food. As a first step towards characterizing the chemoreceptors that mediate the effect of deterrents, we used a behavioral approach to investigate their precise location. Data presented here demonstrate that chemoreceptors located on the antennules, pereiopods and mouthparts do not mediate the food-rejection effects of a variety of deterrents, both natural and artificial to crabs. Crabs always searched for deterrent-laced food and took it to their oral region. The deterrent effect was manifested as either rejection or extensive manipulation, but in both cases crabs bit the food. The biting behavior is relevant because the introduction of food into the oral cavity ensured that the deterrents gained access to the oesophageal taste receptors, and so we conclude that they are the ones mediating rejection. Additional support comes from the fact that a variety of deterrent compounds evoked oesophageal dilatation, which is mediated by oesophageal receptors and has been linked to food rejection. Further, there is a positive correlation between a compound’s ability to elicit rejection and its ability to evoke oesophageal dilatation. The fact that deterrents do not act at a distance is in accordance with the limited solubility of most known feeding deterrents, and likely influences predator–prey interactions and their outcome: prey organisms will be attacked and bitten before deterrents become relevant.

The spatial and temporal patterns of odors sampled by lobsters and crabs in a turbulent plume

Odors are dispersed across aquatic habitats by turbulent water flow as filamentous, intermittent plumes. Many crustaceans sniff (take discrete samples of ambient water and the odors it carries) by flicking their olfactory antennules. We used planar laser-induced fluorescence to investigate how flicking antennules of different morphologies (long antennules of spiny lobsters, Panulirus argus; short antennules of blue crabs, Callinectes sapidus) sample fluctuating odor signals at different positions in a turbulent odor plume in a flume to determine whether the patterns of concentrations captured can provide information about an animal's position relative to the odor source. Lobster antennules intercept odors during a greater percentage of flicks and encounter higher peak concentrations than do crab antennules, but because crabs flick at higher frequency, the duration of odor-free gaps between encountered odor pulses is similar. For flicking antennules there were longer time gaps between odor encounters as the downstream distance to the odor source decreases, but shorter gaps along the plume centerline than near the edge. In contrast to the case for antennule flicking, almost all odor-free gaps were <500 ms at all positions in the plume if concentration was measured continuously at the same height as the antennules. Variance in concentration is lower and mean concentration is greater near the substratum, where leg chemosensors continuously sample the plume, than in the water where antennules sniff. Concentrations sampled by legs increase as an animal nears an odor source, but decrease for antennules. Both legs and antennules encounter higher concentrations near the centerline than at the edge of the plume.

Functionally redundant peg sensilla on the scorpion pecten

All scorpions have two mid-ventral organs called pectines. Each pecten has thousands of pore-tipped sensilla sensitive to a variety of volatile organic and water-based stimulants. However, it was previously unknown whether individual sensilla were functionally identical or different. The information enhancement hypothesis predicts that all sensilla have similar chemosensitivities such that each is a unit of a parallel processing system. The information segmentation hypothesis states that sensilla differ in their chemosensitivities, a functional arrangement akin to the glomeruli-specific chemical detection system in the moth or human olfactory sense. In this study, we tested these hypotheses by extracellularly tip-recording sensillar responses to three aqueous tastants: 0.01 M KCl, 0.1 M citric acid, and 40% ethanol by volume. We isolated stimulation to one sensillum at a time and compared the chemoresponses. Sensilla appeared to respond similarly to the same stimulant (i.e., sensillar tip-recordings revealed activity of the same cell types), although sometimes a few sensilla responded with higher spike rates than the others. We conclude that our data primarily support the information enhancement hypothesis but for future tests of sensillar function we suggest a new hybrid model, which proposes that a few specialized sensilla exist among a mostly uniform field of identical sensilla.

Eukaryotic vs. prokaryotic chemosensory systems

In the last decades, microbiologists demonstrated that microorganisms possess chemosensory capabilities and communicate with each other via chemical signals. In parallel, it was demonstrated that solitary eukaryotic chemosensory cells are diffusely located on the mucosae of digestive and respiratory apparatuses. It is now evident that on the mucosal surfaces of vertebrates, two chemoreceptorial systems (i.e. eukaryotic and prokaryotic) coexist in a common microenvironment. To date, it is not known if the two chemosensory systems reciprocally interact and compete for detection of chemical cues. This appears to be a fruitful field of study and future researches must consider that the mucosal epithelia possess more chemosensory capabilities than previously supposed.

Recent advances in biomimetic sensing technologies

The importance of biological materials has long been recognized from the molecular level to higher levels of organization. Whereas, in traditional engineering, hardness and stiffness are considered desirable properties in a material, biology makes considerable and advantageous use of softer, more pliable resources. The development, structure and mechanics of these materials are well documented and will not be covered here. The purpose of this paper is, however, to demonstrate the importance of such materials and, in particular, the functional structures they form. Using only a few simple building blocks, nature is able to develop a plethora of diverse materials, each with a very different set of mechanical properties and from which a seemingly impossibly large number of assorted structures are formed. There is little doubt that this is made possible by the fact that the majority of biological 'materials' or 'structures' are based on fibres and that these fibres provide opportunities for functional hierarchies. We show how these structures have inspired a new generation of innovative technologies in the science and engineering community. Particular attention is given to the use of insects as models for biomimetically inspired innovations.

Social domination increases neuronal survival in the brain of juvenile crayfish Procambarus clarkii

Olfactory cues are among the sensory inputs that crayfish use in establishing dominance hierarchies. Throughout their lives, new neurons are continuously added into brain cell clusters 9 and 10, which contain somata of olfactory local and projection interneurons, respectively. Using markers for DNA synthesis (bromodeoxyuridine) and mitosis (phospho-histone-3), we tested juvenile crayfish (Procambarus clarkii) to examine effects of pairwise social experience on proliferation and survival of cells in these brain regions. Proliferating and mitotic cells appeared within restricted neurogenic areas in both clusters and in ;tails' extending from them. These tails, embedded in tubulin-positive strands, are linked by a patch of cells. Neither cell proliferation nor mitotic activity was affected by social dominance. Cell survival of neuronal precursors was affected by dominance: compared to dominants, subordinates had fewer newborn cells surviving in cluster 9 after 14 days of social experience. Social experience also affected body growth rate, but the effect of social experience on neurogenesis remained when differences in body growth rate were statistically controlled. We conclude that social domination enhances survival of new olfactory interneuronal precursors compared to social subordination but not compared to social isolation.

The olfactory pathway mediates sheltering behavior of Caribbean spiny lobsters, Panulirus argus, to conspecific urine signals

The "noses" of diverse taxa are organized into different subsystems whose functions are often not well understood. The "nose" of decapod crustaceans is organized into two parallel pathways that originate in different populations of antennular sensilla and project to specific neuropils in the brain-the aesthetasc/olfactory lobe pathway and the non-aesthetasc/lateral antennular neuropil pathway. In this study, we investigated the role of these pathways in mediating shelter selection of Caribbean spiny lobsters, Panulirus argus, in response to conspecific urine signals. We compared the behavior of ablated animals and intact controls. Our results show that control and non-aesthetasc ablated lobsters have a significant overall preference for shelters emanating urine over control shelters. Thus the non-aesthetasc pathway does not play a critical role in shelter selection. In contrast, spiny lobsters with aesthetascs ablated did not show a preference for either shelter, suggesting that the aesthetasc/olfactory pathway is important for processing social odors. Our results show a difference in the function of these dual chemosensory pathways in responding to social cues, with the aesthetasc/olfactory lobe pathway playing a major role. We discuss our results in the context of why the noses of many animals contain multiple parallel chemosensory systems.

Exploring with damaged antennae: do crayfish compensate for injuries?

Appendages are important sources of sensory information for all animals that possess them but they are commonly damaged in nature. We describe how the tactile system of the crayfish Cherax destructor functioned when subjected to the kind of damage found in wild-caught or cultured animals. Touch information was methodically varied by the removal of antennae and chelae. The resulting behaviour was analysed in a T-maze. Crayfish with a single antenna ablated turned toward the intact appendage, however, those with only a partial ablation did not, suggesting that a tactile information threshold exists for normal behaviour. When exposed to the same environment after an antennal ablation but with no prior experience in that terrain, crayfish also turned toward the side of the intact antenna. By contrast, when animals with experience obtained in a previous trial with intact antennae were tested after ablation of one antenna, they did not turn into one arm of the maze more than the other. These two outcomes indicate that behaviour is affected by an interaction between the time at which an injury occurs and an animal's knowledge of the topography, and that an injury may affect learning. We also tested to see if other appendages could provide tactile information to compensate for antennal loss. Input from the chelae did not affect the turning behaviour of crayfish in the maze.

The taste cell-related diffuse chemosensory system

Elements expressing the molecular mechanisms of gustatory transduction have been described in several organs in the digestive and respiratory apparatuses. These taste cell-related elements are isolated cells, which are not grouped in buds, and they have been interpreted as chemoreceptors. Their presence in epithelia of endodermal origin suggests the existence of a diffuse chemosensory system (DCS) sharing common signaling mechanisms with the "classic" taste organs. The elements of this taste cell-related DCS display a site-related morphologic polymorphism, and in the past they have been indicated with various names (e.g., brush, tuft, caveolated, fibrillo-vesicular or solitary chemosensory cells). It may be that the taste cell-related DCS is like an iceberg: the taste buds are probably only the most visible portion, with most of the iceberg more caudally located in the form of solitary chemosensory cells or chemosensory clusters. Comparative anatomical studies in lower vertebrates suggest that this 'submerged' portion may represent the most phylogenetically ancient component of the system, which is probably involved in defensive or digestive mechanisms. In the taste buds, the presence of several cell subtypes and of a wide range of molecular mechanisms permits precise food analysis. The larger, 'submerged' portion of the iceberg is composed of a polymorphic population of isolated elements or cell clusters in which the molecular cascade of cell signaling needs to be explored in detail. The little data we have strongly suggests a close relationship with taste cells. Morphological and biochemical considerations suggest that the DCS is a potential new drug target. Modulation of the respiratory and digestive apparatuses through substances, which act on the molecular receptors of this chemoreceptive system, could be a new frontier in drug discovery.

Dual antennular chemosensory pathways can mediate orientation by Caribbean spiny lobsters in naturalistic flow conditions

Benthic crustaceans rely on chemical stimuli to mediate a diversity of behaviors ranging from food localization and predator avoidance to den selection, conspecific interactions and grooming. To accomplish these tasks, Caribbean spiny lobsters (Panulirus argus) rely on a complex chemosensory system that is organized into two parallel chemosensory pathways originating in diverse populations of antennular sensilla and projecting to distinct neuropils within the brain. Chemosensory neurons associated with aesthetasc sensilla project to the glomerular olfactory lobes (the aesthetasc pathway), whereas those associated with non-aesthetasc sensilla project to the stratified lateral antennular neuropils and the unstructured median antennular neuropil (the non-aesthetasc pathway). Although the pathways differ anatomically, unique roles for each in odor-mediated behaviors have not been established. This study investigates the importance of each pathway for orientation by determining whether aesthetasc or non-aesthetasc sensilla are necessary and sufficient for a lobster to locate the source of a 2 m-distant food odor stimulus in a 5000-liter seawater flume under controlled flow conditions. To assess the importance of each pathway for this task, we selectively ablated specific populations of sensilla on the antennular flagella and compared the searching behavior of ablated animals to that of intact controls. Our results show that either the aesthetasc or the non-aesthetasc pathway alone is sufficient to mediate the behavior and that neither pathway alone is necessary. Under the current experimental conditions, there appears to be a high degree of functional overlap between the pathways for food localization behavior.

Chemical and visual communication during mate searching in rock shrimp

Mate searching in crustaceans depends on different communicational cues, of which chemical and visual cues are most important. Herein we examined the role of chemical and visual communication during mate searching and assessment in the rock shrimp Rhynchocinetes typus. Adult male rock shrimp experience major ontogenetic changes. The terminal molt stages (named "robustus") are dominant and capable of monopolizing females during the mating process. Previous studies had shown that most females preferably mate with robustus males, but how these dominant males and receptive females find each other is uncertain, and is the question we examined herein. In a Y-maze designed to test for the importance of waterborne chemical cues, we observed that females approached the robustus male significantly more often than the typus male. Robustus males, however, were unable to locate receptive females via chemical signals. Using an experimental set-up that allowed testing for the importance of visual cues, we demonstrated that receptive females do not use visual cues to select robustus males, but robustus males use visual cues to find receptive females. Visual cues used by the robustus males were the tumults created by agitated aggregations of subordinate typus males around the receptive females. These results indicate a strong link between sexual communication and the mating system of rock shrimp in which dominant males monopolize receptive females. We found that females and males use different (sex-specific) communicational cues during mate searching and assessment, and that the sexual communication of rock shrimp is similar to that of the American lobster, where females are first attracted to the dominant males by chemical cues emitted by these males. A brief comparison between these two species shows that female behaviors during sexual communication contribute strongly to the outcome of mate searching and assessment.

Novel, secondary sensory cell organ in ascidians: in search of the ancestor of the vertebrate lateral line

A new mechanoreceptor organ, the "coronal organ," located in the oral siphon, is described by light and electron microscopy in the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. It is composed of a line of sensory cells (hair cells), accompanied by supporting cells, that runs continuously along the margin of the velum and tentacles of the siphon. These hair cells resemble those of the vertebrate lateral line or, in general, the acoustico-lateralis system, because they bear a single cilium, located centrally or eccentrically to a hair bundle of numerous stereovilli. In contrast to other sensory cells of ascidians, the coronal hair cells are secondary sensory cells, since they lack axonal processes directed towards the cerebral ganglion. Moreover, at their base they form synapses with nerve fibers, most of which exhibit acetylcholinesterase activity. The absence of axonal extensions was confirmed by experiments with lipophilic dyes. Different kinds of synapses were recognized: usually, each hair cell forms a few afferent synapses with dendrites of neurons located in the ganglion; efferent synapses, both axo-somatic (between an axon coming from the ganglion and the hair cell) and axo-dendritic (between an axon coming from the ganglion and an afferent fiber) were occasionally found. The presence of secondary sensory cells in ascidians is discussed in relation to the evolution of sensory cells and placodes in vertebrates. It is proposed that the coronal organ in urochordates is homologous to the vertebrate acoustico-lateralis system.

Amputation-induced activity of progenitor cells leads to rapid regeneration of olfactory tissue in lobsters

Lobsters have a self-renewing olfactory system and, like many animals, continuously replace old or dying olfactory receptor neurons. In addition, lobsters are able to regenerate the peripheral olfactory system even after complete loss. The olfactory sensors in lobsters are located distally on a pair of antennules. These antennules are often damaged, but this has little impact on the lobster's sense of smell because damaged olfactory tissue is rapidly replaced. In this study, we investigated damage-induced regeneration of the olfactory system by measuring cell proliferation following controlled amputation. We show that amputation-induced regeneration occurs as a result of up-regulating the normal development of olfactory sensors. A unique feature of up-regulated development is the formation of patches of proliferating cells within the antennular epithelium. Epithelial patches were typically formed between 3 and 10 days postamputation on the amputated side. They were characterized by their: proximal position with respect to developing clusters of olfactory receptor neurons (ORNs); tendency to form two discrete patches within the borders of each existing annulus; cell size, which was approximately twice that of mature ORNs; and location within the ventral epithelium. The development of epithelial patches was immediately followed by proliferation of clusters of ORNs and associated glial cells, and the level of this proliferation increased significantly during the premolt stage of the lobster's molt cycle. These epithelial patches may represent populations of precursor cells, because they develop in response to amputation and immediately precede development of cell clusters composed of ORNs and glia. Possible regulatory signals controlling epithelial patch development are discussed.

Role of chemical and visual cues in food recognition by leatherback posthatchlings (Dermochelys coriacea L)

We raised leatherback posthatchlings in the laboratory for up to 7 weeks to study the role of visual and chemical cues in food recognition and food-seeking behavior. Turtles were reared on a formulated (artificial gelatinous) diet and had no contact with test materials until experiments began. Subjects were presented with visual cues (a plastic jellyfish; white plastic shapes [circle, square, diamond] similar in surface area to the plastic model), chemical cues (homogenates of lion's mane jellyfish, Cyanea capillata; moon jellyfish, Aurelia aurita; and a ctenophore, Ocyropsis sp., introduced through a water filter outflow), and visual and chemical cues presented simultaneously. Visual stimuli evoked an increase in swimming activity, biting, diving, and orientation toward the object. Chemical cues elicited an increase in biting, and orientation into water currents (rheotaxis). When chemical and visual stimuli were combined, turtles ignored currents and oriented toward the visual stimuli. We conclude that both cues are used to search for, and locate, food but that visual cues may be of primary importance. We hypothesize that under natural conditions turtles locate food visually, then, as a consequence of feeding, associate chemical with visual cues. Chemical cues then may function alone as a feeding attractant.

How lobsters, crayfishes, and crabs locate sources of odor: current perspectives and future directions

Olfactory orientation poses many challenges for crustaceans in marine environments. Recent behavioral experiments lead to a new understanding of the role of multiple sensory appendages, whereas application of non-invasive chemical visualization techniques and biomimetic robotics have allowed researchers to correlate the stimulus environment with behavior and to directly test proposed orientation mechanisms in decapod crustaceans.

Contact chemoreception in feeding by phytophagous insects

Gustatory receptors associated with feeding in phytophagous insects are broadly categorized as phagostimulatory or deterrent. No phytophagous insect is known that tastes all its essential nutrients, and the ability to discriminate between nutrients is limited. The insects acquire a nutritional balance largely "adventitiously" because leaves have an appropriate chemical composition. Sugars are the most important phagostimulants. Plant secondary compounds are most often deterrent but stimulate phagostimulatory cells if they serve as host-indicating sign stimuli, or if they are sequestered for defense or used as pheromone precursors. The stimulating effects of chemicals are greatly affected by other chemicals in mixtures like those to which the sensilla are normally exposed. Host plant selection depends on the balance of phagostimulatory and deterrent inputs with, in some oligophagous and monophagous species, a dominating role of a host-related chemical. Evolution of phytophagy has probably involved a change in emphasis in the gustatory system, not fundamentally new developments. The precise role of the gustatory systems remains unclear. In grasshoppers, it probably governs food selection and the amounts eaten, but in caterpillars there is some evidence that central feedbacks are also involved in regulating the amount eaten.

Selective ablation of antennular sensilla on the Caribbean spiny lobsterPanulirus argussuggests that dual antennular chemosensory pathways mediate odorant activation of searching and localization of food

In spiny lobsters and other decapod crustaceans, odorant-mediated searching behavior patterns are driven primarily by chemosensory neurons in the antennules. Two groups of antennular chemosensory neurons can be distinguished on the basis of the sensilla that they innervate and their central projections: those that innervate the aesthetasc sensilla on the lateral flagella and project into the glomerularly organized olfactory lobes, and those that innervate other (i.e. non-aesthetasc) sensilla on both lateral and medial flagella and project into the stratified and non-glomerularly organized lateral antennular neuropils. By ablating different groups of antennular sensory neurons or sensilla, we examined the role of aesthetasc and non-aesthetasc chemosensory neurons in regulating local searching behavior of Caribbean spiny lobsters, Panulirus argus, for food (squid) in a low-flow environment. The results show that odorant-mediated activation of searching and localization of food under these conditions requires only a subset of functional antennular chemosensory neurons, since neither aesthetasc chemosensory neurons nor non-aesthetasc chemosensory neurons are by themselves necessary for these types of behavior. However, ablation of aesthetasc chemosensory neurons together with subsets of non-aesthetasc chemosensory neurons from either the medial or lateral flagella impairs the ability of lobsters to locate the food. This reveals a large degree of functional redundancy but also some complementary functions between aesthetasc and non-aesthetasc chemosensory neurons, and hence between these dual antennular chemosensory pathways, in odorant-mediated searching behavior of lobsters under these conditions.

Invertebrate-Inspired sensory-motor systems and autonomous, olfactory-guided exploration

The localization of resources in a natural environment is a multifaceted problem faced by both invertebrate animals and autonomous robots. At a first approximation, locomotion through natural environments must be guided by reliable sensory information. But natural environments can be unpredictable, so from time to time, information from any one sensory modality is likely to become temporarily unreliable. Fortunately, compensating mechanisms ensure that such signals are replaced or disambiguated by information from more reliable modalities. For invertebrates and robots to rely primarily on chemical senses has advantages and pitfalls, and these are discussed. The role of turbulence, which makes tracking a single odor to its source a complex problem, is contrasted with the high-fidelity identification of stimulus quality by the invertebrate chemoreceptor and by artificial sensors.