Dual antennular chemosensory pathways mediate odor-associative learning and odor discrimination in the Caribbean spiny lobsterPanulirus argus

What are olfaction and gustation, and do all animals have them?

Different animals have distinctive anatomical and physiological properties to their chemical senses that enhance detection and discrimination of relevant chemical cues. Humans and other vertebrates are recognized as having 2 main chemical senses, olfaction and gustation, distinguished from each other by their evolutionarily conserved neuroanatomical organization. This distinction between olfaction and gustation in vertebrates is not based on the medium in which they live because the most ancestral and numerous vertebrates, the fishes, live in an aquatic habitat and thus both olfaction and gustation occur in water and both can be of high sensitivity. The terms olfaction and gustation have also often been applied to the invertebrates, though not based on homology. Consequently, any similarities between olfaction and gustation in the vertebrates and invertebrates have resulted from convergent adaptations or shared constraints during evolution. The untidiness of assigning olfaction and gustation to invertebrates has led some to recommend abandoning the use of these terms and instead unifying them and others into a single category-chemical sense. In our essay, we compare the nature of the chemical senses of diverse animal types and consider their designation as olfaction, oral gustation, extra-oral gustation, or simply chemoreception. Properties that we have found useful in categorizing chemical senses of vertebrates and invertebrates include the nature of peripheral sensory cells, organization of the neuropil in the processing centers, molecular receptor specificity, and function.

Cross-sensory interference assessment after exposure to noise shows different effects in the blue crab olfactory and sound sensing capabilities

Underwater noise pollution is an increasing threat to marine ecosystems. Marine animals use sound in communication and orientation processes. The introduction of anthropogenic noise in their habitat can interfere with sound production and reception as well as with the acquisition of vital information through other sensory systems. In the blue crab (Callinectes sapidus), the statocyst is responsible for acoustic perception, and it is housed at the base of its first pair of antennae (antennule). The sensilla of the distal part of these antennule hosts the olfactory system, which is key for foraging. Given the anatomical proximity of the two sensory regions, we evaluated the possible interference of sound exposure with the crab ability to find food, by using an aquatic maze, and looked at the potential impairment of the righting reflex as well as at ultrastructural damages in statocysts. Although a significant effect was observed when looking at the time used by the animal to recover its habitual position ("righting reflex"), which was associated to lesions in the statocyst sensory epithelia, the time required to find food did not increase after the exposure to sound. When the crabs were exposed to natural sounds (marine background noise and sounds of their predators: Micropogonias undulates and Sciaenops ocellatus) they did not show significant differences in foraging behaviour. Although we found no unequivocal evidence of a negative impact of sound on olfactory capabilities, the study showed a clear righting reflex impairment correlated with ultrastructural damages of the statocysts. We argue that crab populations that cannot easily avoid noise sources due to their specific coastal distributions may incur in significant direct fitness costs (e.g. impairment of complex reflexes). This integrated approach to sound effect assessment could be used as a model for other invertebrate species to effectively monitor noise impact in marine environments.

A method for selective stimulation of leg chemoreceptors in whole crustaceans

The integration of sensory information with adequate motor outputs is critical for animal survival. Here, we present an innovative technique based on a non-invasive closed-circuit device consisting of a perfusion/stimulation chamber chronically applied on a single leg of the crayfish Procambarus clarkii. Using this technique, we focally stimulated the leg inside the chamber and studied the leg-dependent sensory-motor integration involving other sensory appendages, such as antennules and maxillipeds, which remain unstimulated outside the chamber. Results show that the stimulation of a single leg with chemicals, such as disaccharides, is sufficient to trigger a complex search behaviour involving locomotion coupled with the reflex activation of antennules and maxillipeds. This technique can be easily adapted to other decapods and/or other sensory appendages. Thus, it has opened possibilities for studying sensory-motor integration evoked by leg stimulation in whole aquatic animals under natural conditions to complement, with a direct approach, current ablation or silencing techniques.

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).

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.

New electroantennography method on a marine shrimp in water

Antennular chemoreception in aquatic decapods is well studied via the recording of single chemoreceptor neuron activity in the antennule, but global responses of the antennule (or antennae in insects) by electroantennography (EAG) has so far been mainly restricted to aerial conditions. We present here a well-established underwater EAG method to record the global antennule activity in the marine shrimp Palaemon elegans in natural (aqueous) conditions. EAG responses to food extracts, recorded as net positive deviations of the baseline, are reproducible, dose-dependent and exhibit sensory adaptation. This new EAG method opens a large field of possibilities for studying in vivo antennular chemoreception in aquatic decapods, in a global approach to supplement current, more specific techniques.

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.

Structure of the olfactory receptor organs, their GABAergic neural pathways, and modulation of mating behavior, in the giant freshwater prawn, Macrobrachium rosenbergii

In the giant male prawn, Macrobrachium rosenbergii, the olfactory system is thought to be the main pathway for modulating sexual behavior through pheromone perception. In this report, we first used gross anatomical, histological, and SEM methods to describe the structures of the olfactory receptors (sensilla setae), their neural pathways, and possible role in modulating mating behavior. On the surfaces of antennule and antenna filaments there are four types of sensory receptors, viz single spike-like setae, single flagellum-like setae, multiple flagella-like setae, and aesthetascs (ASs). The ASs, which had previously been proposed to be odor receptor setae, are found only on the short filament of lateral antennule (slAn). Each AS on the slAn connects with olfactory receptor neurons (ORNs), whose axons form an outer central antennule nerve (ocAnNv), which then connects with the olfactory neutrophil (ON) of the brain. Thus, the slAn is the major olfactory organ that conveys sensory inputs from each AS to the ON within the deutocerebrum. GABA immunoreactivity was present in ASs, neurons of ORNs, inner central antennular, lateral tegumentary nerve, ocAnNv and the ON, inferring that GABA is the likely neurotransmitter in modulating olfaction. Disruption of the slAn by ablation or covering with Vaseline, resulted in significant reduction of mating behavior, indicating that this organ is crucial for sex pheromone perception. Identification of the active pheromones and further bioassays are now being performed.

Smelling, feeling, tasting and touching: behavioral and neural integration of antennular chemosensory and mechanosensory inputs in the crayfish

Crustaceans possess two pairs of prominent, movable sense organs on the rostral aspect of their bodies termed antennae: (1) a relatively short, usually bifurcate pair, the 1st antennae, also referred to as antennules, and (2) a much longer, uniramous pair, the 2nd antennae, or just 'antennae'. The antennules are equipped with diverse arrays of six or more types of cuticular setae, most of which are believed to have a sensory function. Axons from these structures course within the antennular nerve to the deutocerebrum, a large middle brain region that is known to receive chemoreceptor and mechanoreceptor inputs. In crayfish, axons from two kinds of single sensory-function setae, the olfactory receptor aesthetasc sensilla and as yet unidentified hydrodynamic sensilla, on the lateral antennular flagellum terminate, respectively, within the ipsilateral olfactory lobe and the lateral antennular neuropil of the deutocerebrum, where their activity generates synaptic potentials in local interneurons having dendritic fields that span both of those regions. It has been suggested that the short-latency hydrodynamic input gates or otherwise supplements the olfactory input signals. Much less is known about the functional capabilities of the other sensillar types on the antennular flagella, including the bimodal sensilla: how their inputs are distributed to the various neuropils of the deutocerebrum, whether they target common or separate brain neurons, and the nature, if any, of their functional relationships to the aesthetasc and hydrodynamic sensilla. Integrated processing of chemical and hydrodynamic signals undoubtedly plays an important role in locating odorant sources, perhaps by detecting boundaries of odorant plumes (tropotactic discrimination); other less-plausible strategies include time averaging of turbulent odorant signals and determination of concentration slopes within turbulence-generated odorant patches. These gaps in our understanding present important, but surmountable, experimental challenges for the future.

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.

Behavioral and neurophysiological responses of an insect to changing ratios of constituents in host plant-derived volatile mixtures

Ratios of compounds in host plant odors fluctuate with the phenological stage of the plant. In the present study, we investigated the effect of changing ratios of host plant volatile constituents on herbivore insect attraction and olfactory information processing. We tested a synthetic mixture of bioactive peach shoot volatiles with different concentrations of one of the mixture constituents, benzonitrile, on oriental fruit moth Cydia (=Grapholita) molesta females. Y-tube olfactometer bioassays showed that female attraction to the mixture was maintained while increasing the benzonitrile level up to 100 times. Further increases led to behaviorally ineffective mixtures. Then, we recorded odor-evoked neural activity patterns in the antennal lobes, the main olfactory center of the brain, using calcium imaging. Benzonitrile-containing mixtures elicited strong activation in two glomeruli, which were found to process mixture-related information in specific ways. Activation in one glomerulus directly paralleled behavioral effects of the different ratios tested whereas a deviating pattern was noted in the other glomerulus. Our results indicate that the ratio of constituents in a volatile mixture can be varied to a certain degree without reducing female attraction. Thus, volatile blends in nature might vary quantitatively within a certain range without affecting odor-guided host location. Neurophysiological results showed that the processing of mixture-related information inside the antennal lobes is not uniform across glomeruli. Thus, final processing of this information probably takes place in higher-order brain centers.

Response properties of crayfish antennules to hydrodynamic stimuli: functional differences in the lateral and medial flagella

Antennules have been reported to influence localization of distant food odors, sex discrimination, and agonistic and social behaviors of decapod crustaceans. Although olfaction by the antennules is largely recognized, information on the sensitivity of antennules to hydrodynamic stimuli has been scant. In red swamp crayfish Procambarus clarkii antennules, mechanosensory setae outnumber the chemosensory setae. We studied the mechanosensitivity of crayfish antennules by recording neural activities from isolated antennules in response to sinusoidal dipole stimuli. Both the lateral and the medial flagellum of the antennules responded to hydrodynamic stimuli, although the medial flagellum showed more sensitivity at frequencies higher than 60 Hz. The most dominant setae present on the stimulated site were the simple setal type. Although both lateral and medial flagella are capable of detecting chemical and hydrodynamic cues, results from neural responses, morphological observations and antennular behavior observations indicate that the lateral flagellum of P. clarkii functions as an olfactory organ whereas the medial flagellum complements as a hydrodynamic receptor. It appears that in crayfish antennular sensory processing, crayfish simultaneously use chemical and hydrodynamic information. We have compared our data with the threshold of fish lateral line to the same stimuli and we discuss probable similarities in response properties.

Spiny lobsters use urine-borne olfactory signaling and physical aggressive behaviors to influence social status of conspecifics

Decapod crustaceans, like many other animals, engage in agonistic behaviors that enhance their ability to compete for resources with conspecifics. These agonistic behaviors include the release of chemical signals as well as physical aggressive and submissive behaviors. In this study, we report that Caribbean spiny lobsters, Panulirus argus, use both urine-borne chemical signaling and physical aggressive behaviors during interactions with conspecifics, and that these agonistic behaviors can influence the behavior and eventual social status of the interactants. Spiny lobsters that engaged primarily in physical aggressive behaviors became dominant, whereas spiny lobsters that received these physical aggressive behaviors responded with avoidance behaviors and became subordinates. Dominant animals frequently released urine during social interactions, more than when they were not in contact with subordinates and more than when they were not paired with another animal. Subordinates released urine significantly less often than dominants, and no more than when not paired. Preventing release of urine by catheterizing the animals resulted in an increase in the number and duration of physical interactions, and this increase was primarily driven by dominants initiating interactions through physical aggressive behaviors. Introducing urine from one of the catheterized animals into an aquarium reduced physical aggressive behavior by dominant animals to normal levels. Urine-borne signals alone were capable of inducing avoidance behaviors from solitary spiny lobsters in both laboratory and field conditions. We conclude that urine serves as a chemical signal that communicates social status to the interactants. Ablation experiments showed that that these urine signals are detected primarily by aesthetasc sensilla of the olfactory pathway.

The importance of dietary calcium consumption in two species of semi-terrestrial grapsoid crabs

Calcium (Ca) is essential for crustaceans, due to calcium carbonate (CaCO3) deposition in the new exoskeleton to harden it. The purpose of this work was to study short term Ca balance in terms of dietary Ca ingestion in two phylogenetically related crabs (Superfamily Grapsoidea) showing different degrees of terrestrial adaptations: Sesarma rectum Randall, 1840 and Neohelice granulata (Dana, 1851). Dietary Ca ingestion was studied using purified diets with different Ca concentrations (0, 2.2 and 6.66 % Ca), together with measurements of Ca excretion and Ca hemolymph levels. The results showed that both crabs had the same response to foods containing different levels of Ca, with both species eating more of the high Ca diet. However, S. rectum consumed more per mg body mass at all Ca concentrations (6 mg.g-1 for S. rectum against 3 mg.g-1 for N. granulata). Both species excreted/egested Ca differently: S. rectum excreted Ca proportionally to ingestion, whereas N. granulata maintained constant faecal Ca output at all dietary Ca levels. Moreover, Ca hemolymph levels for crabs fed the different diets were independent of dietary Ca. In conclusion, both S. rectum and N. granulata seem to regulate the consumption of diets containing more Ca, which suggests a fine balance for Ca intake.

Antennule morphology and flicking kinematics facilitate odor sampling by the spiny lobster, Panulirus argus

Many arthropod olfactory appendages bear arrays of hair-like chemosensory sensillae. Odor molecules in the fluid around the animal must reach the surfaces of those hairs to be sensed. We used the lateral flagellum of the olfactory antennule of the spiny lobster, Panulirus argus, as a system to study how the morphology, orientation, and motion of sensilla-bearing appendages affects the small-scale water flow within the hair array. We tested whether antennule flicking enables lobsters to take discrete odor samples by measuring flow fields through an aesthetasc array on a dynamically scaled physical model of a P. argus antennule. Particle image velocimetry revealed that the magnitude and duration of velocity through the aesthetasc array during the rapid flick downstroke is just enough to allow complete replacement of the fluid entrained within the hair array. The complex zig-zag arrangement of aesthetascs hairs, combined with their offset orientation along the antennule, generates flow velocities that are uniform along the length of the hairs. This increases fluid exchange during the flick and reduces the boundary layer thickness surrounding the hairs. The return stroke occurs at about a quarter the speed of the flick, but the velocity of the fluid between the aesthetascs is approximately 25 times slower. The retained fluid during the return stroke remains virtually unstirred and sufficient time occurs for odor molecules to diffuse to aesthetasc surfaces.

Spiny lobsters detect conspecific blood-borne alarm cues exclusively through olfactory sensilla

When attacked by predators, diverse animals actively or passively release molecules that evoke alarm and related anti-predatory behavior by nearby conspecifics. The actively released molecules are alarm pheromones, whereas the passively released molecules are alarm cues. For example, many insects have alarm-signaling systems that involve active release of alarm pheromones from specialized glands and detection of these signals using specific sensors. Many crustaceans passively release alarm cues, but the nature of the cues,sensors and responses is poorly characterized. Here we show in laboratory and field experiments that injured Caribbean spiny lobsters, Panulirus argus, passively release alarm cues via blood (hemolymph) that induce alarm responses in the form of avoidance and suppression of feeding. These cues are detected exclusively through specific olfactory chemosensors,the aesthetasc sensilla. The alarm cues for Caribbean spiny lobsters are not unique to the species but do show some phylogenetic specificity: P. argus responds primarily with alarm behavior to conspecific blood, but with mixed alarm and appetitive behaviors to blood from the congener Panulirus interruptus, or with appetitive behaviors to blood from the blue crab Callinectes sapidus. This study lays the foundation for future neuroethological studies of alarm cue systems in this and other decapod crustaceans.

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.

Ultrastructure and functional morphology of glandular setae and distal claws of cephalic appendages of Speleonectes tanumekes (Crustacea: Remipedia)

Terminal pores on crustacean setae are commonly associated with chemoreception. In this study we present an exception to that association with the description of glandular setae on maxillulary and maxillary endites of the remipede Speleonectes tanumekes. This introduces a function associated with crustacean setae beyond the general functions currently assigned to crustacean setae: sensory functions, mechanical functions, or a combination of these two. Even though the functions of the secretory products are unclear, we suggest means by which these may contribute to feeding behaviors. In addition, we describe glandular features of maxillary and maxillipedal distal claws of the same remipede species. Glandular setae and distal claws appear to share several morphological homologies.

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.

Cross-species comparison of metabolite profiles in chemosensory epithelia: an indication of metabolite roles in chemosensory cells

Comparative studies of chemosensory systems in vertebrates and invertebrates have greatly enhanced our understanding of anatomical and physiological constraints of chemical detection. Immunohistochemical comparisons of chemosensory systems are difficult to make across species due to limited cross-reactivity of mammalian-based antibodies. Immunostaining chemosensory tissues with glutaraldehyde-based antibodies generated against small metabolites in combination with hierarchical cluster analyses provide a novel approach for identifying and classifying cell types regardless of species. We used this "metabolite profiling" technique to determine whether metabolite profiles can be used to identify cell classes within and across different species including mouse, zebrafish, lobster and squid. Within a species, metabolite profiles for distinct cell classes were generally consistent. We found several metabolite-based cell classifications that mirrored function or receptor protein-based classifications. Although profiles of all six metabolites differed across species, we found that specific metabolites were associated with certain cell types. For example, elevated levels of glutathione were characteristic of nonsensory cells from vertebrates, suggesting an antioxidative role in non-neuronal cells in sensory tissues. Collectively, we found significantly different metabolite profiles for distinct cell populations in chemosensory tissue within all of the species studied. Based on their roles in other systems or cells, we discuss the roles of L-arginine, L-aspartate, L-glutamate, glycine, glutathione, and taurine within chemosensory epithelia.

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.

Acidity enhances the effectiveness of active chemical defensive secretions of sea hares, Aplysia californica, against spiny lobsters, Panulirus interruptus

Sea hares such as Aplysia californica, gastropod molluscs lacking a protective shell, can release a purple cloud of chemicals when vigorously attacked by predators. This active chemical defense is composed of two glandular secretions, ink and opaline, both of which contain an array of compounds. This secretion defends sea hares against predators such as California spiny lobsters Panulirus interruptus via multiple mechanisms, one of which is phagomimicry, in which secretions containing feeding chemicals attract and distract predators toward the secretion and away from the sea hare. We show here that ink and opaline are highly acidic, both having a pH of approximately 5. We examined if the acidity of ink and opaline affects their phagomimetic properties. We tested behavioral and electrophysiological responses of chemoreceptor neurons in the olfactory and gustatory organs of P. interruptus, to ink and opaline of A. californica within their natural range of pH values, from approximately 5 to 8. Both behavioral and electrophysiological responses to ink and opaline were enhanced at low pH, and low pH alone accounted for most of this effect. Our data suggest that acidity enhances the phagomimetic chemical defense of sea hares.

Physiological and behavioral effects of chemoreceptors located in different body parts of the swimming crab Callinectes danae

By perfusing their branchial chambers with filtered seawater, we have developed a preparation that allows us to maintain the swimming crab Callinectes danae outside water without any major effects on its cardiac activity. This in turn allowed us to selectively stimulate chemoreceptors located in different body parts, and specifically to discriminate between the receptors located in the branchial chambers and those located in the oral region (mainly in the mouthparts, antennules and antennae). We show that a taurine solution can evoke bradycardia when applied to the oral region or to a combination of the oral region and the branchial chambers. Although the precise localization of the oral region receptors involved remains to be determined, ablation experiments show that the olfactory organs (i.e., the antennules) are not involved. Finally, we show that although stimulating the pereiopods has no effect on the animals' cardiac activity it causes the animals to move, putatively to try to grasp a piece of food, a reaction not evoked by stimulating the gills or the oral regions. Our results lend support to the idea that chemoreceptors located in different parts of the body play different functional roles in decapod crustaceans.

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.

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.

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.

The peripheral and central antennular pathway of the Caribbean stomatopod crustacean Neogonodactylus oerstedii

Although stomatopod crustaceans use their chemical senses in many facets of behavior, little is known about their chemosensory neural pathways, especially in comparison to the better-studied decapod crustaceans. We examined the stomatopod Neogonodactylus oerstedii to determine organizational aspects of peripheral and central neural pathway of antennules, which is a major chemosensory organ. We describe the three flagella of the triramous antennule as the medial, dorsolateral, and ventrolateral flagella. The primary branch point is between the medial flagellum and lateral flagella, and the secondary branch point is at the junction of the dorsolateral and ventrolateral flagella. The antennule bears at least three types of setae, based on their external morphology. Simple setae are present only on the medial flagellum and ventrolateral flagellum, organized as a tuft of 10-15 setae on each flagellar annulus. Aesthetasc setae and asymmetric setae occur only on the distal annuli of the dorsolateral flagellum, with each annulus bearing a row of three aesthetascs and one asymmetric seta. DiI fills of the antennular nerve near the junction of the flagella show that sensory neurons in the antennular flagella project to two neuropils in the ipsilateral midbrain-the olfactory lobe (OL) and lateral antennular neuropil (LAN). The OL is glomerular and has rich serotonergic innervation, a characteristic of the OL in decapods. The LAN is bi-lobed and stratified as it is in decapods. However, the LAN of stomatopods differs from that of decapods in being relatively large and containing extensive serotonergic innervation. The median antennular neuropil of stomatopods has sparse serotonergic innervation, and it is more diffusely organized compared to decapods. No accessory lobes were found in N. oerstedii. Thus, the stomatopod antennular flagella have the same two, highly organized parallel pathways common to decapods-the OL pathway and the LAN pathway.

Two sniffing strategies in palinurid lobsters

Most studies of lobster chemoreception have focused on the model systems of Panulirus argus (Palinuridae) and Homarus americanus(Nephropidae). We compare antennule morphology across lobsters and conduct the first kinematic study of antennule flicking in a palinurid species other than P. argus. High-speed video analysis shows that Palinurus elephas flicks at a rate more than an order of magnitude higher than in P. argus. However, both species flick their antennular flagella at a Reynolds number (Re) of approximately one, such that an asymmetry in the speed of the flick phases causes both species to have a leaky closing flick phase and a non-leaky opening phase. The antennular flagella of P. argus are nearly seven times longer than those of P. elephas,and, when compared across palinurid genera, Panulirus species sample far greater areas of water over greater spatial and time scales than do any other palinurid genera. Palinurid lobsters appear to have two sniffing strategies: low flick rates over a large area of water (e.g. P. argus) or high flick rates over a small area of water (e.g. P. elephas). P. argus is a highly informative model system in which to study aquatic chemoreception; however, its antennule anatomy and kinematics suggest a separate strategy, unique to Panulirus species, for sensing chemical plumes in fluid environments.