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

Evaluation of crystalline amino acids as potent stimulatory chemoattractants for the slipper lobster Thenus orientalis

Intensive research on the effectiveness of chemoattractants has been widely explored to improve the feed qualities in expanding crustacean farming. Taste preferences in slipper lobster remained unknown despite their significant contribution to the lobster fisheries. Chemoattractants allow better performance in aquaculture species by increasing food attractiveness and palatability. Amino acids (AA) have been leading in previous research on crustacean feeding behavior. Given that slipper lobster possesses chemoreceptors to detect and orient towards food, this study investigated an approach to identify the AA with the most potent chemoattractant in eliciting a response from slipper lobster. Behavioral assays were performed to evaluate the responses of slipper lobster Thenus orientalis (carapace length, 52.34 ± 1.52 mm) on 15 crystalline AA and three derivatives of AA (DAA) at three concentrations between 10-1 and 10-3 M as test substances (TS). Meretrix sp. extract was used as a positive control and clean filtered seawater as a negative control. The behavioral responses of 14 T. orientalis were evaluated based on their antennular flicking rate, third maxillipeds activity, and substrate probing by the pereiopods. T. orientalis responded to the solutions of single AA down to a concentration of 10-3 M, excluding histidine and serine. The behavioral activity displayed by T. orientalis increased with the TS concentrations. L-glutamic acid monosodium salt monohydrate, betaine, and glycine solutions elicited the most behavioral responses, whereas histidine exhibited the lowest behavioral responses. Conclusively, L-glutamic acid monosodium salt monohydrate, betaine, and glycine can be potential chemoattractants for T. orientalis.

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

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

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.

Chemoreceptor proteins in the Caribbean spiny lobster, Panulirus argus: Expression of Ionotropic Receptors, Gustatory Receptors, and TRP channels in two chemosensory organs and brain

The spiny lobster, Panulirus argus, has two classes of chemosensilla representing “olfaction” and “distributed chemoreception,” as is typical for decapod crustaceans. Olfactory sensilla are found exclusively on antennular lateral flagella and are innervated only by olfactory receptor neurons (ORNs) that project into olfactory lobes organized into glomeruli in the brain. Distributed chemoreceptor sensilla are found on all body surfaces including the antennular lateral flagella (LF) and walking leg dactyls (dactyls), and are innervated by both chemoreceptor neurons (CRNs) and mechanoreceptor neurons that project into somatotopically organized neuropils. Here, we examined expression of three classes of chemosensory genes in transcriptomes of the LF (with ORNs and CRNs), dactyls (with only CRNs), and brain of P. argus: Ionotropic Receptors (IRs), which are related to ionotropic glutamate receptors and found in all protostomes including crustaceans; Gustatory Receptors (GRs), which are ionotropic receptors that are abundantly expressed in insects but more restricted in crustaceans; and Transient Receptor Potential (TRP) channels, a diverse set of sensor-channels that include several chemosensors in diverse animals. We identified 108 IRs, one GR, and 18 homologues representing all seven subfamilies of TRP channels. The number of IRs expressed in the LF is far greater than in dactyls, possibly reflecting the contribution of receptor proteins associated with the ORNs beyond those associated with CRNs. We found co-receptor IRs (IR8a, IR25a, IR76b, IR93a) and conserved IRs (IR21a, IR40a) in addition to the numerous divergent IRs in the LF, dactyl, and brain. Immunocytochemistry showed that IR25a is expressed in ORNs, CRNs, and a specific type of cell located in the brain near the olfactory lobes. While the function of IRs, TRP channels, and the GR was not explored, our results suggest that P. argus has an abundance of diverse putative chemoreceptor proteins that it may use in chemoreception.

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.

Molecular Mechanisms of Reception and Perireception in Crustacean Chemoreception: A Comparative Review

This review summarizes our present knowledge of chemoreceptor proteins in crustaceans, using a comparative perspective to review these molecules in crustaceans relative to other metazoan models of chemoreception including mammals, insects, nematodes, and molluscs. Evolution has resulted in unique expansions of specific gene families and repurposing of them for chemosensation in various clades, including crustaceans. A major class of chemoreceptor proteins across crustaceans is the Ionotropic Receptors, which diversified from ionotropic glutamate receptors in ancient protostomes but which are not present in deuterostomes. Representatives of another major class of chemoreceptor proteins-the Grl/GR/OR family of ionotropic 7-transmembrane receptors-are diversified in insects but to date have been reported in only one crustacean species, Daphnia pulex So far, canonic 7-transmembrane G-protein coupled receptors, the principal chemoreceptors in vertebrates and reported in a few protostome clades, have not been identified in crustaceans. More types of chemoreceptors are known throughout the metazoans and might well be expected to be discovered in crustaceans. Our review also provides a comparative coverage of perireceptor events in crustacean chemoreception, including molecules involved in stimulus acquisition, stimulus delivery, and stimulus removal, though much less is known about these events in crustaceans, particularly at the molecular level.

Grooming as a secondary behavior in the shrimp Macrobrachiumrosenbergii (Crustacea, Decapoda, Caridea)

The giant freshwater prawn, Macrobrachiumrosenbergii, is a large shrimp extensively used in aquaculture whose grooming behaviors were analyzed in this study. Macrobrachiumrosenbergii exhibits three unique male morphotypes that differ in their behavior, morphology and physiology: small-clawed males (SM), orange-clawed males (OC) and blue-clawed males (BC). The largest and most dominant males, BC males, are predicted to have significantly different grooming behaviors compared to females and the other two male morphotypes. These BC males may be too large and bulky to efficiently groom and may dedicate more time to mating and agonistic interactions than grooming behaviors. Observations were conducted to look at the prevalence of grooming behaviors in the absence and presence of conspecifics and to determine if any differences in grooming behavior exist among the sexes and male morphotypes. Significant differences in the grooming behaviors of all individuals (females and male morphotypes) were found. BC males tended to have the highest grooming time budget (percent of time spent grooming) while SM males had a relatively low grooming time budget. The grooming behaviors of the male morphotypes differed, indicating while these males play distinct, separate roles in the social hierarchy, they also have different grooming priorities. The conditions in which Macrobrachiumrosenbergii are cultured may result in increased body fouling, which may vary, depending on the grooming efficiencies and priorities of these male morphotypes. Overall, grooming behaviors were found to be a secondary behavior which only occurred when primary behaviors such as mating, feeding or fighting were not present.

Morphology and histochemistry of the aesthetasc-associated epidermal glands in terrestrial hermit crabs of the genus Coenobita (Decapoda: Paguroidea)

Crustaceans have successfully adapted to a variety of environments including fresh- and saltwater as well as land. Transition from an aquatic to a terrestrial lifestyle required adaptations of the sensory equipment of an animal, particularly in olfaction, where the stimulus itself changes from hydrophilic to mainly hydrophobic, air-borne molecules. Hermit crabs Coenobita spp. (Anomura, Coenobitidae) have adapted to a fully terrestrial lifestyle as adults and have been shown to rely on olfaction in order to detect distant food items. We observed that the specialized olfactory sensilla in Coenobita, named aesthetascs, are immersed in a layer of mucous-like substance. We hypothesized that the mucous is produced by antennal glands and affects functioning of the aesthetascs. Using various microscopic and histochemical techniques we proved that the mucous is produced by aesthetasc-associated epidermal glands, which we consider to be modified rosette-type aesthetasc tegumental glands known from aquatic decapods. These epidermal glands in Coenobita are multicellular exocrine organs of the recto-canal type with tubulo-acinar arrangement of the secretory cells. Two distinct populations of secretory cells were clearly distinguishable with light and electron microscopy. At least part of the secretory cells contains specific enzymes, CUB-serine proteases, which are likely to be secreted on the surface of the aesthetasc pad and take part in antimicrobial defense. Proteomic analysis of the glandular tissue corroborates the idea that the secretions of the aesthetasc-associated epidermal glands are involved in immune responses. We propose that the mucous covering the aesthetascs in Coenobita takes part in antimicrobial defense and at the same time provides the moisture essential for odor perception in terrestrial hermit crabs. We conclude that the morphological modifications of the aesthetasc-associated epidermal glands as well as the functional characteristics of their secretions are important adaptations to a terrestrial lifestyle.

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.

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.

Distribution and function of splash, an achaete-scute homolog in the adult olfactory organ of the Caribbean spiny lobster Panulirus argus

achaete-scute complex (ASC) genes, which encode basic helix-loop-helix transcription factors, regulate embryonic and adult neurogenesis in many animals. In adult arthropods, including crustaceans, ASC homologs have been identified but rarely functionally characterized. We took advantage of the recently identified crustacean homolog, splash (spiny lobster achaete scute homolog), in the olfactory organ of the Caribbean spiny lobster Panulirus argus to examine its role in adult neurogenesis. We tested the hypothesis that splash is associated with but not restricted to sensory neuron formation in the olfactory organ, the antennular lateral flagellum (LF), of adult spiny lobsters. We demonstrated splash labeling in epithelial cells across LF developmental zones (i.e., proliferation and mature zones), in auxiliary cells surrounding dendrites of olfactory receptor neurons (ORNs), and in immature and mature ORNs, but not in granulocytes or chromatophores. Since ORN proliferation varies with molt stage, we examined splash expression across molt stages and found that molt stage affected splash expression in the ORN mature zone but not in the proliferation zone. In vivo incorporation of bromodeoxyuridine (BrdU) showed no correlation in the cellular pattern of splash expression and BrdU labeling. The intensity of splash labeling was dramatically enhanced in the proliferation zones following LF damage, suggesting enhanced splash expression during repair and/or regeneration. We conclude that splash is not closely associated with the formation of sensory neurons under normal physiological conditions, and we propose that splash is involved in repair and regeneration. We also propose that splash has additional roles other than neurogenesis in adult crustaceans.

It costs to be clean and fit: energetics of comfort behavior in breeding-fasting penguins

BACKGROUND

Birds may allocate a significant part of time to comfort behavior (e.g., preening, stretching, shaking, etc.) in order to eliminate parasites, maintain plumage integrity, and possibly reduce muscular ankylosis. Understanding the adaptive value of comfort behavior would benefit from knowledge on the energy costs animals are willing to pay to maintain it, particularly under situations of energy constraints, e.g., during fasting. We determined time and energy devoted to comfort activities in freely breeding king penguins (Aptenodytes patagonicus), seabirds known to fast for up to one month during incubation shifts ashore.

METHODOLOGY/PRINCIPAL FINDINGS

A time budget was estimated from focal and scan sampling field observations and the energy cost of comfort activities was calculated from the associated increase in heart rate (HR) during comfort episodes, using previously determined equations relating HR to energy expenditure. We show that incubating birds spent 22% of their daily time budget in comfort behavior (with no differences between day and night) mainly devoted to preening (73%) and head/body shaking (16%). During comfort behavior, energy expenditure averaged 1.24 times resting metabolic rate (RMR) and the corresponding energy cost (i.e., energy expended in excess to RMR) was 58 kJ/hr. Energy expenditure varied greatly among various types of comfort behavior, ranging from 1.03 (yawning) to 1.78 (stretching) times RMR. Comfort behavior contributed 8.8-9.3% to total daily energy expenditure and 69.4-73.5% to energy expended daily for activity. About half of this energy was expended caring for plumage.

CONCLUSION/SIGNIFICANCE

This study is the first to estimate the contribution of comfort behavior to overall energy budget in a free-living animal. It shows that although breeding on a tight energy budget, king penguins devote a substantial amount of time and energy to comfort behavior. Such findings underline the importance of comfort behavior for the fitness of colonial seabirds.

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.

Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

The olfactory organ of the Caribbean spiny lobster Panulirus argus maintains lifelong proliferation and turnover of olfactory receptor neurons (ORNs). Towards examining the molecular basis of this adult neurogenesis, we search for expression of homologs of proneural, neurogenic, and pre-pattern genes in this olfactory organ. We report here a homolog of the proneural Achaete-Scute family, called splash (spiny lobster achaete-scute homolog), and a homolog of the pre-pattern and neurogenic hairy-enhancer of split family, called splhairy (spiny lobster hairy). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicates a molt stage dependence of the levels of expression of splash and splhairy mRNA in the olfactory organ, with higher expression in premolt than in postmolt or intermolt animals, which is positively correlated with rates of neurogenesis. splash and splhairy mRNA are expressed not only in the olfactory organ but also in other tissues, albeit at lower levels, irrespective of molt stage. We conclude that the expression of achaete-scute and hairy-enhancer of split in the proliferation zone of the olfactory organ of spiny lobsters and their enhanced expression in premolt animals suggest that they play a role in the proliferation of ORNs and that their expression in regions of the olfactory organ populated by mature ORNs and in other tissues suggests that they have additional functions.

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.

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 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.

Role of the olfactory pathway in agonistic behavior of crayfish, Procambarus clarkii

Crayfish establish social dominance hierarchies through agonistic interactions, and these hierarchies are maintained through assessment of social status. Chemical signals influence several aspects of fighting behavior, but the specific chemosensory sensilla involved in detecting these signals in crayfish are unknown. The goal of our study was to examine the importance of aesthetasc sensilla--olfactory sensors on the antennules of decapod crustaceans--in regulating changes in fighting behavior in crayfish, Procambarus clarkii, over the course of repeated pairings. We selectively ablated aesthetascs from pairs of crayfish after the first day of trials and compared the behavior of these ablated animals to that of pairs of intact controls. Results show that unablated crayfish significantly decreased the number and duration of fights over repeated pairings, whereas crayfish lacking aesthetascs continued to engage in similar amounts of fighting across all three trial days. This difference shows that aesthetascs regulate fighting behavior in P. clarkii.

Directional asymmetry in responses of local interneurons in the crayfish deutocerebrum to hydrodynamic stimulation of the lateral antennular flagellum

We have recorded spiking responses from single, bimodally sensitive local interneurons (Type I) in the crayfish deutocerebrum to hydrodynamic and odorant stimuli flowing in two directions past the lateral antennular flagellum. Changing the direction of seamless introductions (meaning, with minimal variations of fluid velocity magnitude) of odorant flow past the flagellum, from proximal→distal to distal→proximal, did not consistently affect the dose-dependent responses of Type I neurons. By contrast, changing the direction of an abruptly initiated flow of water (or odorant) past the flagellum resulted in consistently larger numbers of spikes in response to this hydrodynamic stimulation when the flow direction was proximal→distal. This response asymmetry is discussed in relation to its possible relevance regarding antennular flicking behavior. The putative involvement of flagellar hydrodynamic receptors, the beaked hairs, and the hydrodynamic flow asymmetries they are exposed to, are examined theoretically in the accompanying paper.

Combining dissimilar senses: central processing of hydrodynamic and chemosensory inputs in aquatic crustaceans

Aquatic environments are by their nature dynamic and dominated by fluid movements driven by lunar tides, temperature and salinity density gradients, wind-driven currents, and currents generated by the earth's rotation. Accordingly, animals within the aquatic realm must be able to sense and respond to both large-scale (advection) and small-scale (eddy turbulence) fluid dynamics, for chemical signals critically important for their survival are embedded within such movements. Aquatic crustaceans possess many types of near-field fluid-flow detectors and two general classes of chemoreceptors on their body appendages: high-threshold, near-field receptors that may be somewhat equated with the sense of taste, and low-threshold far-field receptors that can be considered as olfactory. This review briefly summarizes the distribution of hydrodynamic and high-threshold chemoreceptors in aquatic crustaceans and the physiological characteristics of olfactory receptors in lobsters; it also examines recent physiological evidence for the central nervous integration of inputs from olfactory receptors and hydrodynamic detectors, two dissimilar senses that must be combined within the brain for survival. Marine crustaceans have provided valuable insights about mechanisms of primary olfactory sensory physiology; their additional sensitivity to hydrodynamic stimulation makes them a potentially useful model for examining how these two critical sensory inputs are combined within the brain to enhance foraging behavior. Multimodal sensory processing is critically important to all animals, and the principles and concepts derived from these crustacean studies may provide generalities about neuronal processing across taxa.

The olfactory pathway of decapod crustaceans--an invertebrate model for life-long neurogenesis

The first part of this review includes a short description of the cellular and morphological organization of the olfactory pathway of decapod crustaceans, followed by an overview of adult neurogenesis in this pathway focusing on the olfactory lobe (OL), the first synaptic relay in the brain. Adult neurogenesis in the central olfactory pathway has the following characteristics. 1) It is present in all the diverse species of decapod crustaceans so far studied. 2) In all these species, projection neurons (PNs), which have multiglomerular dendritic arborizations, are generated. 3) Neurons are generated by one round of symmetrical cell divisions of a small population of immediate precursor cells that are located in small proliferation zones at the inner margin of the respective soma clusters. 4) The immediate precursor cells in each soma cluster appear to be generated by repeated cell divisions of one or few neuronal stem cells that are located outside of the proliferation zone. 5) These neuronal stem cells are enclosed in a highly structured clump of small glial-like cells, which likely establishes a specific microenvironment and thus can be regarded as a stem cell niche. 6) Diverse internal and external factors, such as presence of olfactory afferents, age, season of the year, and living under constant and deprived conditions modulate the generation and/or survival of new neurons. In the second part of this review, I address the question why in decapod crustaceans adult neurogenesis persists in the visual and olfactory pathways of the brain but is lacking in all other mechanosensory-chemosensory pathways. Due to the indeterminate growth of most adult decapod crustaceans, new sensory neurons of all modalities (olfaction and chemo-, mechano-, and photoreception) are continuously added during adulthood and provide an ever-increasing sensory input to all primary sensory neuropils of the central nervous system. From these facts, I conclude that adult neurogenesis in the brain cannot simply be a mechanism to accommodate increasing sensory input and propose instead that it is causally linked to the specific "topographic logic" of information processing implemented in the sensory neuropils serving different modalities. For the presumptive odotopic type of information processing in the OL, new multiglomerular PNs allow interconnection of novel combinations of spatially unrelated input channels (glomeruli), whose simultaneous activation by specific odorants is the basis of odor coding. Thus, adult neurogenesis could provide a unique way to increase the resolution of odorant quality coding and allow adaptation of the olfactory system of these long-lived animals to ever-changing odor environments.

Ultrastructure and functional organization of mouthpart sensory setae of the spiny lobster Panulirus argus: new features of putative mechanoreceptors

In comparison with other decapods, the Caribbean spiny lobster Panulirus argus has little diversity in the external morphology of the setae on the mouth apparatus. In mouthpart areas that frequently touch food items only two types of setae can be distinguished: simple setae and cuspidate setae. Simple setae are by far more numerous. The ultrastructural data presented here show that both types of seta are bimodal, in that they both contain mechano- and chemosensory cells as indicated by morphological features. The morphological features divide the sensory cells into three types: type 1, which has a mechanosensory appearance; type 2, which has a chemosensory appearance; and type 3, which is believed to be a mechanoreceptor due to desmosomal connections to a scolopale. All three cell types were found in all examined setae. In an earlier study the simple setae were found to contain two types of mechanosensors: bend-sensitive cells and displacement-sensitive cells. The morphological arrangement of the outer dendritic segment described in the present study cannot explain this division. Instead, it is suggested that the difference in sensitivity is caused by a differential arrangement of their stretch-sensitive ion channels. This hypothesis also provides an explanation for the earlier observation that only bend cells respond to changes in osmolarity.

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.

Integration of hydrodynamic and odorant inputs by local interneurons of the crayfish deutocerebrum

Intracellular electrodes were used to record from local interneurons in the olfactory lobes of the midbrain in the crayfish Procambarus clarkii. Cells that resembled previously studied central targets of olfactory receptor neurons on the lateral antennular flagellum were specifically examined for their responses to hydrodynamic stimuli. Initiation of water movement past the antennular flagellum, confined within an olfactometer, evoked a triphasic excitatory-inhibitory-excitatory postsynaptic potential lasting up to 2 s that generated spikes on depolarizing phases of the response sequence. Odorant pulses seamlessly imbedded in the water pulse past the antennule evoked purely excitatory, dose-dependent postsynaptic responses and associated spike trains. The latency of the initial phase of the response to water was approximately half as long as the latency of the response to odorant, suggesting that different afferent pathways are involved in responses to hydrodynamic and odorant stimuli, respectively. In some olfactory lobe interneurons that resembled previously described cells classified as Type I, conjoint stimulation of fluid onset and odorant evoked responses that were twice the amplitude of the summed response to either hydrodynamic or odorant stimulation alone, suggesting that the olfactory responses were potentiated by hydrodynamic input. Individuals of at least one other class of first-order interneuron that responded to both hydrodynamic and odorant stimulation were occasionally recorded from. These results indicate that multimodal integration of chemical and mechanical information occurs at the level of first-order sensory interneurons in the crayfish brain.