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

Chemosensory Basis of Feeding Behavior in Pacific White Shrimp, Litopenaeus vannamei

AbstractThe Pacific white shrimp, Litopenaeus vannamei, is important as the principal species in the worldwide aquaculture of shrimp. It has also become a model in the study of crustacean biology, especially because it is one of the first decapod crustaceans to have its genome sequenced. This study examined an aspect of the sensory biology of this shrimp that is important in its aquaculture, by describing its peripheral chemical sensors and how they are used in acquiring and consuming food pellets. We used scanning electron microscopy to describe the diversity of sensilla on the shrimp's major chemosensory organs: antennules, antennae, mouthparts, and legs. Using behavioral studies on animals with selective sensory ablations, we then explored the roles that these chemosensory organs play in the shrimp's search for, and acquisition and ingestion of, food pellets. We found that the antennules mediate odor-activated searching for pellets, with both the lateral and medial antennular flagella contributing to this behavior and thus demonstrating that both aesthetasc (olfactory) and distributed chemosensors on the antennules can mediate this behavior. Once the shrimp finds and grasps the food pellet, the antennular chemoreceptors no longer play a role, and then the chemoreceptors on the mouthparts and legs control ingestion of the pellets. This sequence of chemosensory control of feeding in L. vannamei, a dendrobranchiate crustacean with small antennules and an ability to live and feed in both benthic and pelagic environments, is generally similar to that of the better-studied, large-antennuled, benthic reptantian crustaceans, including spiny lobsters (Achelata), clawed lobsters and crayfish (Astacidea), and crabs (Meirua).

Interpreting the Spatial-Temporal Structure of Turbulent Chemical Plumes Utilized in Odor Tracking by Lobsters

Olfactory systems in animals play a major role in finding food and mates, avoiding predators, and communication. Chemical tracking in odorant plumes has typically been considered a spatial information problem where individuals navigate towards higher concentration. Recent research involving chemosensory neurons in the spiny lobster, Panulirus argus, show they possess rhythmically active or ‘bursting’ olfactory receptor neurons that respond to the intermittency in the odor signal. This suggests a possible, previously unexplored olfactory search strategy that enables lobsters to utilize the temporal variability within a turbulent plume to track the source. This study utilized computational fluid dynamics to simulate the turbulent dispersal of odorants and assess a number of search strategies thought to aid lobsters. These strategies include quantification of concentration magnitude using chemosensory antennules and leg chemosensors, simultaneous sampling of water velocities using antennule mechanosensors, and utilization of antennules to quantify intermittency of the odorant plume. Results show that lobsters can utilize intermittency in the odorant signal to track an odorant plume faster and with greater success in finding the source than utilizing concentration alone. However, the additional use of lobster leg chemosensors reduced search time compared to both antennule intermittency and concentration strategies alone by providing spatially separated odorant sensors along the body.

Crustacean olfactory systems: A comparative review and a crustacean perspective on olfaction in insects

Malacostracan crustaceans display a large diversity of sizes, morphs and life styles. However, only a few representatives of decapod taxa have served as models for analyzing crustacean olfaction, such as crayfish and spiny lobsters. Crustaceans bear multiple parallel chemosensory pathways represented by different populations of unimodal chemosensory and bimodal chemo- and mechanosensory sensilla on the mouthparts, the walking limbs and primarily on their two pairs of antennae. Here, we focus on the olfactory pathway associated with the unimodal chemosensory sensilla on the first antennal pair, the aesthetascs. We explore the diverse arrangement of these sensilla across malacostracan taxa and point out evolutionary transformations which occurred in the central olfactory pathway. We discuss the evolution of chemoreceptor proteins, comparative aspects of active chemoreception and the temporal resolution of crustacean olfactory system. Viewing the evolution of crustacean brains in light of energetic constraints can help us understand their functional morphology and suggests that in various crustacean lineages, the brains were simplified convergently because of metabolic limitations. Comparing the wiring of afferents, interneurons and output neurons within the olfactory glomeruli suggests a deep homology of insect and crustacean olfactory systems. However, both taxa followed distinct lineages during the evolutionary elaboration of their olfactory systems. A comparison with insects suggests their olfactory systems ö especially that of the vinegar fly ö to be superb examples for "economy of design". Such a comparison also inspires new thoughts about olfactory coding and the functioning of malacostracan olfactory systems in general.

Antennular Morphology and Contribution of Aesthetascs in the Detection of Food-related Compounds in the Shrimp Palaemon adspersus Rathke, 1837 (Decapoda: Palaemonidae)

Shrimp are an essential ecological component of marine ecosystems, and have commercial importance for human consumption and aquaculture. Like other decapod crustaceans, shrimp rely on chemical senses to detect and localize food resources by means of chemosensilla that are located mainly on the cephalothoracic appendages. Using the shrimp Palaemon adspersus, a model organism with omnivorous feeding behavior, we aimed to provide comparative information on the role of aesthetascs, antennular sensilla, and flicking behavior in food detection. To this end, we examined i) the morphology of antennular sensilla by field emission scanning electron microscopy, ii) the shrimp's sensitivity to a number of food-related compounds (amino acids and sugars) by means of whole-animal bioassays, and iii) the contribution of the aesthetasc sensilla to food detection. Our results showed that, aside from the aesthetascs, only three other main morphotypes of setae with chemoreceptive features were present in the antennules, thus accounting for relatively simple sensillar equipment. Nevertheless, we found broad-spectrum sensitivity of the shrimp to a number of amino acids (i.e., isoleucine, leucine, methionine, phenylalanine, glycine, tryptophan, cysteine, and tyrosine) and carbohydrates (trehalose, maltose, cellobiose, and fructose) that was consistent with the omnivorous or scavenging habits of the animal. Although aesthetasc ablation attenuated flicking behavior in a chemical stimulus-independent manner, success in detection and short-range localization of food did not rely on the presence of aesthetasc sensilla. This finding confirms the existence of a non-aesthetasc alternative pathway for feeding, with functional redundancy in simple generalist feeder models such as shrimp.

Finding food: how marine invertebrates use chemical cues to track and select food

This review covers recent research on how marine invertebrates use chemical cues to find and select food.

Effects of antennule morphology and flicking kinematics on flow and odor sampling by the freshwater crayfish, Procambarus clarkii

The flow structure around the lateral antennular flagellum of the freshwater crayfish, Procambarus clarkii, was quantified to determine how antennule morphology and flicking kinematics affect fine-scale flow surrounding their chemosensory sensilla, called aesthetascs. Particle image velocimetry was used to measure velocity and vorticity of flow between aesthetascs of dynamically scaled physical models of P. clarkii antennules. Results revealed that the spacing between aesthetascs and antennule flicking speed induces substantial changes in fluid flow near aesthetascs. The downstroke flicking motion of the antennule occurs at a peak speed of 2.7cm/s. The returnstroke occurs at approximately 70% of this speed, but the fluid velocity between aesthetascs during the returnstroke is approximately 15% compared with the downstroke. The significant decrease in fluid flow near aesthetascs results from the reduced antennule speed and from the coupled interaction of boundary layers of the aesthetascs and antennule during the returnstroke. Odorant-laden fluid captured during the downstroke is retained between the aesthetascs during the slower returnstroke, and sufficient time occurs for odorant molecules to molecularly diffuse to aesthetasc surfaces. In addition, locally generated vorticity was observed near the tip of the aesthetascs, which may induce odorant transport to aesthetasc surfaces and enhance olfactory response times to odors.

On the brain of a crustacean: a morphological analysis of CaMKII expression and its relation to sensory and motor pathways

Calcium/calmodulin kinase II (CaMKII) is a Ca²⁺-activated enzyme that is abundant in vertebrate and invertebrate brains. However, its characterization is poorly addressed in the nervous system of crustaceans, and, to our knowledge, no studies have determined the microanatomical location of CaMKII in a crustacean species. In this study, we found labeling of CaMKII in the eyestalk and brain of the prawn Macrobrachium acanthurus, by means of immunohistochemistry and Western blotting. Antibodies against neuron (ß tubulin III), glutamate receptor (GluA1), and FMRFamide were used in order to further characterize the CaMKII-labeled cells in the brain. In the eyestalk, strong labeling with CaMKII was observed in the photoreceptors. These cells, especially in the rhabdom, were also reactive to anti-ß tubulin III, whereas the pigment cells were labeled with anti-CaMKII. GluA1 co-located with CaMKII in the photoreceptors. Also, CaMKII appeared in the same sites as FMRFamide in the deutocerebrum, including the olfactory lobe, and in the tritocerebrum, specifically in the antennular neuropil, indicating that the synaptic areas in these regions may be related to sensory-motor processing. In the brain, the identification of cells and regions that express CaMKII contributes to the understanding of the processing of neural connections and the modulating role of CaMKII in decapod crustaceans.

Defense through sensory inactivation: sea hare ink reduces sensory and motor responses of spiny lobsters to food odors

Antipredator defenses are ubiquitous and diverse. Ink secretion of sea hares (Aplysia) is an antipredator defense acting through the chemical senses of predators by different mechanisms. The most common mechanism is ink acting as an unpalatable repellent. Less common is ink secretion acting as a decoy (phagomimic) that misdirects predators' attacks. In this study, we tested another possible mechanism – sensory inactivation – in which ink inactivates the predator's reception of food odors associated with would-be prey. We tested this hypothesis using spiny lobsters, Panulirus argus, as model predators. Ink secretion is composed of two glandular products, one being opaline, a viscous substance containing concentrations of hundreds of millimolar of total free amino acids. Opaline sticks to antennules, mouthparts and other chemosensory appendages of lobsters, physically blocking access of food odors to the predator's chemosensors, or over-stimulating (short term) and adapting (long term) the chemosensors. We tested the sensory inactivation hypotheses by treating the antennules with opaline and mimics of its physical and/or chemical properties. We compared the effects of these treatments on responses to a food odor for chemoreceptor neurons in isolated antennules, as a measure of effect on chemosensory input, and for antennular motor responses of intact lobsters, as a measure of effect on chemically driven motor behavior. Our results indicate that opaline reduces the output of chemosensors by physically blocking reception of and response to food odors, and this has an impact on motor responses of lobsters. This is the first experimental demonstration of inactivation of peripheral sensors as an antipredatory defense.

Dine or dash? Turbulence inhibits blue crab navigation in attractive–aversive odor plumes by altering signal structure encoded by the olfactory pathway

Blue crabs can distinguish and navigate to attractive (food) odors even when aversive odors (injured crab metabolites) are released nearby. Blue crabs in these conditions detect the aversive odor and avoid it, but find the attractive source with nearly the same success rate as when the attractive source is presented alone. Spatially and temporally distinct odor filaments appear to signal to foragers that the two odor sources are not co-located, and hence navigating to the attractive odor entails an acceptable risk of predation. However, environmentally produced turbulence suppresses tracking by homogenizing the two odors; blue crabs fail to track to the attractive source when the aversive source is present, even though turbulence does not substantially inhibit tracking to the attractive source alone. Removal of sensory input from aesthetascs on the antennules, but not chemosensors on the legs, rescues navigation to attractive–aversive dual plumes in turbulent conditions. These results suggest that mixing in the natural environment may amplify the effects of predators by suppressing tracking to food odors when aversive cues are present, and that the olfactory pathway mediates the response.

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.

Numerical simulations of odorant detection by biologically inspired sensor arrays

The antennules of many marine crustaceans enable them to rapidly locate sources of odorant in turbulent environmental flows and may provide biological inspiration for engineered plume sampling systems. A substantial gap in knowledge concerns how the physical interaction between a sensing device and the chemical filaments forming a turbulent plume affects odorant detection and filters the information content of the plume. We modeled biological arrays of chemosensory hairs as infinite arrays of odorant flux-detecting cylinders and simulated the fluid flow around and odorant flux into the hair-like sensors as they intercepted a single odorant filament. As array geometry and sampling kinematics were varied, we quantified distortion of the flux time series relative to the spatial shape of the original odorant filament as well as flux metrics that may be important to both organisms and engineered systems attempting to measure plume structure and/or identify chemical composition. The most important predictor of signal distortion is the ratio of sensor diameter to odorant filament width. Achieving high peak properties (e.g. sharpness) of the flux time series and maximizing the total number of odorant molecules detected appear to be mutually exclusive design goals. Sensor arrays inspired specifically by the spiny lobster Panulirus argus and mantis shrimp Gonodactylaceus falcatus introduce little signal distortion but these species' neural systems may not be able to resolve plume structure at the level of individual filaments via temporal properties of the odorant flux. Current chemical sensors are similarly constrained. Our results suggest either that the spatial distribution of flux across the aesthetasc array is utilized by P. argus and G. falcatus, or that such high spatiotemporal resolution is unnecessary for effective plume tracking.

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.

Getting ahead: context-dependent responses to odorant filaments drive along-stream progress during odor tracking in blue crabs

The chemosensory signal structure governing the upstream progress of blue crabs to an odorant source was examined. We used a three-dimensional laser-induced fluorescence system to collect chemical concentration data simultaneously with behavior observations of actively tracking blue crabs (Callinectes sapidus) in a variety of plume types. This allowed us to directly link chemical signal properties at the antennules and legs to subsequent upstream motion while altering the spatial and temporal intermittency characteristics of the sensory field. Our results suggest that odorant stimuli elicit responses in a binary fashion by causing upstream motion, provided the concentration at the antennules exceeds a specific threshold. In particular, we observed a significant association between crab velocity changes and odorant spike encounters defined using a threshold that is scaled to the mean of the instantaneous maximum concentration. Thresholds were different for each crab, indicating a context-sensitive response to signal dynamics. Our data also indicate that high frequency of odorant spike encounters terminate upstream movement. Further, the data provide evidence that the previous state of the crab and prior stimulus history influence the behavioral response (i.e. the response is context dependent). Two examples are: (1) crabs receiving prior odorant spikes attained elevated velocity more quickly in response to subsequent spikes; and (2) prior acceleration or deceleration of the crab influenced the response time period to a particular odorant spike. Finally, information from both leg and antennule chemosensors interact, suggesting parallel processing of odorant spike properties during navigation.

The computational worm: spatial orientation and its neuronal basis in C. elegans

Spatial orientation behaviors in animals are fundamental for survival but poorly understood at the neuronal level. The nematode Caenorhabditis elegans orients to a wide range of stimuli and has a numerically small and well-described nervous system making it advantageous for investigating the mechanisms of spatial orientation. Recent work by the C. elegans research community has identified essential computational elements of the neural circuits underlying two orientation strategies that operate in five different sensory modalities. Analysis of these circuits reveals novel motifs including simple circuits for computing temporal derivatives of sensory input and for integrating sensory input with behavioral state to generate adaptive behavior. These motifs constitute hypotheses concerning the identity and functionality of circuits controlling spatial orientation in higher organisms.

Robot navigation: implications from search strategies in exploring crayfish

Biomimetic applications play an important role in informing the field of robotics. One aspect is navigation – a skill automobile robots require to perform useful tasks. A sub-area of this is search strategies, e.g. for search and rescue, demining, exploring surfaces of other planets or as a default strategy when other navigation mechanisms fail. Despite that, only a few approaches have been made to transfer biological knowledge of search mechanisms on surfaces along the ground into biomimetic applications. To provide insight for robot navigation strategies, this study describes the paths a crayfish used to explore terrain. We tracked movement when different sets of sensory input were available. We then tested this algorithm with a computer model crayfish and concluded that the movement ofC. destructorhas a specialised walking strategy that could provide a suitable baseline algorithm for autonomous mobile robots during navigation.

Identification of chemosensory sensilla mediating antennular flicking behavior in Panulirus argus, the Caribbean spiny lobster

Crustaceans sample odorants by a rapid series of flicks of the two flagella composing the distal segments of each of the paired antennules. The lateral flagella contain aesthetasc sensilla that house unimodal chemosensory neurons. Nine types of nonaesthetasc setae with putative chemosensory and mechanosensory functions are distributed on the lateral and medial flagella. Sensory neurons in aesthetascs and nonaesthetasc sensilla terminate in separate regions of the brain, the olfactory lobe, and the lateral antennular neuropil, resulting in two odorant-processing pathways. Distilled water ablation of flagella and excision of specific setae were used to identify chemosensory sensilla mediating antennular flick behavior in Panulirus argus. The flick rates of sham-ablated and ablated or excised lobsters toward squid extract were compared. Complete attenuation of flick response to squid extract occurred as a result of (1) distilled water ablation of lateral flagella, (2) excision of aesthetascs and asymmetric sensilla, and (3) excision of aesthetascs. Distilled water ablation of medial flagella resulted in a mean flick rate 52% of that observed for sham-ablated lobsters toward squid extract. Flicking was unaffected by excision of asymmetric, guard, or companion sensilla. We propose that odorant mediation of flicking behavior requires both the aesthetasc and nonaesthetasc pathways.

Neural processing, perception, and behavioral responses to natural chemical stimuli by fish and crustaceans

This manuscript reviews the chemical ecology of two of the major aquatic animal models, fish and crustaceans, in the study of chemoreception. By necessity, it is restricted in scope, with most emphasis placed on teleost fish and decapod crustaceans. First, we describe the nature of the chemical world perceived by fish and crustaceans, giving examples of the abilities of these animals to analyze complex natural odors. Fish and crustaceans share the same environments and have evolved some similar chemosensory features: the ability to detect and discern mixtures of small metabolites in highly variable backgrounds and to use this information to identify food, mates, predators, and habitat. Next, we give examples of the molecular nature of some of these natural products, including a description of methodologies used to identify them. Both fish and crustaceans use their olfactory and gustatory systems to detect amino acids, amines, and nucleotides, among many other compounds, while fish olfactory systems also detect mixtures of sex steroids and prostaglandins with high specificity and sensitivity. Third, we discuss the importance of plasticity in chemical sensing by fish and crustaceans. Finally, we conclude with a description of how natural chemical stimuli are processed by chemosensory systems. In both fishes and crustaceans, the olfactory system is especially adept at mixture discrimination, while gustation is well suited to facilitate precise localization and ingestion of food. The behaviors of both fish and crustaceans can be defined by the chemical worlds in which they live and the abilities of their nervous systems to detect and identify specific features in their domains. An understanding of these worlds and the sensory systems that provide the animals with information about them provides insight into the chemical ecology of these species.

The sensory ecology of ocean navigation

How animals guide themselves across vast expanses of open ocean, sometimes to specific geographic areas, has remained an enduring mystery of behavioral biology. In this review we briefly contrast underwater oceanic navigation with terrestrial navigation and summarize the advantages and constraints of different approaches used to analyze animal navigation in the sea. In addition, we highlight studies and techniques that have begun to unravel the sensory cues that underlie navigation in sea turtles, salmon and other ocean migrants. Environmental signals of importance include geomagnetic, chemical and hydrodynamic cues, perhaps supplemented in some cases by celestial cues or other sources of information that remain to be discovered. An interesting similarity between sea turtles and salmon is that both have been hypothesized to complete long-distance reproductive migrations using navigational systems composed of two different suites of mechanisms that function sequentially over different spatial scales. The basic organization of navigation in these two groups of animals may be functionally similar, and perhaps also representative of other long-distance ocean navigators.

Structural and functional changes in regenerating antennules in the crayfish Orconectes sanborni

Crayfish rely on the chemosensory neurons in their antennules to help them find food and habitat and to mediate social interactions. These structures often sustain damage from aggressive interactions or from the environment, but they have the ability to regenerate. In this study, we examine whether the effects of antennule ablation and regeneration on odor-tracking ability correlate with structural changes in the antennule that occur during regeneration. We initiated the regeneration process by removing the right antennules from 55 individuals of Orconectes sanborni. We developed a method to nondestructively sample the regenerating antennules so that we could follow the growth of new antennular tissue in the same animals over time. We used dental epoxy to make molds of the regenerating antennule after each molt. We then made resin positives, which were visualized using scanning electron microscopy. Structural parameters including aesthetasc length, diameter, segment length, and number per row were measured from scanning electron micrographs using Image J software. Crayfish were tested in a tabletop water Y-maze before and after surgery and after each molt to assess their ability to track food odors. The structural and the behavioral data indicate that the antennules possessed many aspects of their original structure by the end of the second molt. Flicking of antennules, investigation of substrate, success rate at finding the odor-containing Y-maze branch, and time to completion of Y-maze regained pre-antennulectomy values by the end of the third molt.

Video tracking in the extreme: video analysis for nocturnal underwater animal movement

Computer analysis of video footage is one option for recording locomotor behavior for a range of neurophysiological and behavioral studies. This technique is reasonably well established and accepted, but its use for some behavioral analyses remains a challenge. For example, filming through water can lead to reflection, and filming nocturnal activity can reduce resolution and clarity of filmed images. The aim of this study was to develop a noninvasive method for recording nocturnal activity in aquatic decapods and test the accuracy of analysis by video tracking software. We selected crayfish, Cherax destructor, because they are often active at night, they live underwater, and data on their locomotion is important for answering biological and physiological questions such as how they explore and navigate. We constructed recording arenas and filmed animals in infrared light. Wethen compared human observer data and software-acquired values. In this article, we outline important apparatus and software issues to obtain reliable computer tracking.

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.

Source and specificity of chemical cues mediating shelter preference of Caribbean spiny lobsters (Panulirus argus)

Caribbean spiny lobsters display a diversity of social behaviors, one of the most prevalent of which is gregarious diurnal sheltering. Previous research has demonstrated that shelter selection is chemically mediated, but the source of release and the identity of the aggregation signal are unknown. In this study, we investigated the source and specificity of the aggregation signal in Caribbean spiny lobsters, Panulirus argus. We developed a relatively rapid test of shelter choice in a 5000-l laboratory flume that simulated flow conditions in the spiny lobster's natural environment, and used it to examine the shelter preference of the animals in response to a variety of odorants. We found that both males and females associated preferentially with shelters emanating conspecific urine of either sex, but not with shelters emanating seawater, food odors, or the scent of a predatory octopus. These results demonstrate specificity in the cues mediating sheltering behavior and show that urine is at least one source of the aggregation signal.

Effects of manganese on chemically induced food search behaviour of the Norway lobster, Nephrops norvegicus (L.)

The decapod Norway lobster, Nephrops norvegicus (L.), lives on muddy sediments rich in manganese (Mn). In hypoxic conditions, manganese is reduced and released from the sediment, so increased concentrations of dissolved Mn(2+) become bioavailable. In excess, manganese acts as a neurotoxin and may inhibit vital functions of benthic organisms, such as muscle contraction. We investigated in a laboratory flume experiment, the effect of environmentally realistic concentrations of manganese (0.1 and 0.2mM for 12 days) on the food search behaviour of N. norvegicus. We found that lobsters exposed to manganese had a more than doubled reaction time to food odour stimuli compared to the controls (p<0.05). In addition, manganese exposure reduced the number of N. norvegicus reaching the food stimuli source. Compared to the controls where 86% reached the stimuli source, only about half of the lobsters exposed to 0.1mM Mn and one-third of the lobsters exposed to 0.2mM Mn reached the stimuli source (p<0.05 and 0.001, respectively). There was no significant difference between treatments in the number of lobsters leaving their shelter or in the time from reaction until leaving the shelter and there was no difference in search time for those animals that eventually did locate the stimuli source. This study shows that environmentally realistic manganese concentrations affect parts of the food search behaviour of N. norvegicus, likely due to impaired chemosensory ability or reduced motivation for feeding. Thus, the ability of N. norvegicus to detect and find food can be reduced in areas with high manganese concentrations, with possible consequences on individual and population levels.

Rosette-type tegumental glands associated with aesthetasc sensilla in the olfactory organ of the Caribbean spiny lobster, Panulirus argus

The lateral antennular flagellum of decapod crustaceans bears unique olfactory sensilla, namely the aesthetascs, and other sensilla types. In this study, we identify a new major tissue in the lateral flagellum of the Caribbean spiny lobster, Panulirus argus, namely "aesthetasc tegumental glands" (ATGs), based on immunostaining with antibodies against CUB serine protease (Csp), in situ hybridization with csp-specific probes, labeling with the F-actin marker phalloidin, labeling with the nuclear marker Hoechst 33258, and staining with methylene blue. Each ATG has 12-20 secretory cells arranged in a rosette. Each secretory cell has a Csp-immunoreactive basal portion and an apical portion containing granular material (metachromatic staining indicative of acid mucopolysaccharides). At the center of each secretory rosette is a phalloidin-positive common locus that gives rise to a main drainage duct projecting toward the cuticle. Scanning electron and light microscopy show that thin ducts traverse the cuticle and connect to "peg pores" proximal to the bases of the aesthetascs, with 3.4 peg pores per aesthetasc. Since the number of common loci is correlated with the number of peg pores, we conclude that each pore represents the outlet of one ATG, and that the secretions are released from them. We conclude further that ATGs and aesthetascs are functionally linked. We hypothesize that ATG secretions have antifouling and/or friction-reducing properties, and that they are spread over the surface of the aesthetascs by antennular grooming. A review of the literature suggests that ATGs are common in decapod crustacean antennules, and that rosette glands and grooming might be functionally coupled in other body areas.

The olfactory pathway for individual recognition in the American lobster Homarus americanus

Individual recognition in the lobster Homarus americanus (Milne-Edwards), is based on detection of urine pheromones via chemoreceptors of the lateral antennular flagellum. The specific sensory pathway mediating this recognition is not known. Most of the chemoreceptor cells of this flagellum are found in the unimodal aesthetasc sensilla and project specifically to the glomeruli of the olfactory lobe in the brain. Additional chemoreceptor cells are located among mechanoreceptor cells in bimodal sensilla, including the guard hairs; they do not project to the olfactory lobe. This neuroanatomy suggested that aesthetascs were essential to all complex chemosensory tasks until it was shown that spiny lobsters Panulirus argus can still perform complex food odor discrimination and localization tasks without aesthetascs. Here, we demonstrate that the aesthetascs of H. americanus contain the chemoreceptors necessary for individual recognition of familiar opponents. In contrast to intact and guard hair-shaved animals, lobsters with aesthetascs removed did not recognize previous opponents as shown by second encounters statistically similar in length and aggression to first-encounter fights. Non-aesthetasc chemosensory pathways were incapable of rescuing opponent recognition. Subsequent lesion of all remaining chemoreceptor cells (by immersion in distilled water) abolished recognition and renewed fighting.

The fluid mechanics of arthropod sniffing in turbulent odor plumes

Many arthropods capture odorant molecules from the environment using antennae or antennules bearing arrays of chemosensory hairs. The penetration of odorant-carrying water or air into the spaces between these chemosensory hairs depends on the speed at which they are moved through the surrounding fluid. Therefore, antennule flicking by crustaceans and wing fanning by insects can have a profound impact on the odorant encounter rates of the chemosensory sensilla they bear; flicking and fanning are examples of sniffing. Odors are dispersed in the environment by turbulent wind or water currents. On the scale of an antenna or antennule, an odor plume is not a diffuse cloud but rather is a series of fine filaments of scent swirling in odor-free water. The spatiotemporal pattern of these filaments depends on distance from the odor source. The physical interaction of a hair-bearing arthropod antennule with the surrounding fluid affects the temporal patterns of odor concentration an animal intercepts when it sniffs in a turbulent odor plume.

Non-olfactory chemoreceptors in asymmetric setae activate antennular grooming behavior in the Caribbean spiny lobster Panulirus argus

In the spiny lobster Panulirus argus the antennules carrying olfactory sensilla called aesthetascs and several types of other non-olfactory sensilla accompanying them are frequently groomed by the third maxillipeds in a stereotyped behavioral pattern. This behavior can be elicited by chemical stimulation with l-glutamate. Using selective sensillar ablations, we tested whether this behavior is driven by the numerous aesthetascs, which have been implicated as mediating this chemically elicited antennular grooming behavior in a previous investigation, or other, less numerous sensilla called asymmetric setae, which are tightly associated with aesthetascs. The selective sensilla ablations showed that the asymmetric setae are necessary and sufficient for driving chemically elicited antennular grooming. Bilateral elimination of the ca. 160 asymmetric setae almost completely abolished the behavior, whereas bilateral elimination of the ca. 2600 aesthetascs or of another type of sensilla associated with them (guard setae) did not cause a reduction in chemically elicited antennular grooming. Microscopical analysis of the morphological properties of the asymmetric setae revealed the presence of a terminal pore at the tip of the seta and a phalloidin-positive scolopale below its base. Since these structures have been identified in decapod crustaceans as modality-specific structures of bimodal chemo- and mechanosensory sensilla, we conclude that the asymmetric setae belong to this type of sensilla and thus have the appropriate features to function as chemoreceptors in the elicitation of antennular grooming. The identification of asymmetric setae and not aesthetascs as the drivers of chemically elicited antennular grooming suggests that it is not the olfactory pathway in the brain but a parallel pathway, constituted mainly by the lateral antennular neuropils, that is the neuronal substrate of this behavior. The lateral antennular neuropils receive non-olfactory sensory input from the antennule and contain the major arborizations of antennular motoneurons, allowing that direct sensory-motor coupling is involved in mediating the chemical elicitation of antennular grooming behavior.