Alarm reaction in the crucian carp is mediated by the medial bundle of the medial olfactory tract

Experiments were performed to determine which bundles of the olfactory tracts were essential for mediating alarm reaction in crucian carp (Carassius carassius L.). The fish were maintained in physiological saline after surgery to preserve the remaining tracts and postoperative inspections revealed the functionality of the intact tracts. Operations on the tracts were performed symmetrically on both sides. Sham-operated and non-operated fish showed the typical alarm behaviour of fast swimming to the bottom, dashing movements and aggregation when exposed to skin extract which contain alarm substance. Fish with only the medial bundle of the medial olfactory tract intact also displayed the alarm behaviour upon exposure; however, these fish did not react to the amino acid, L-alanine with either feeding response or alarm reaction. Crucian carp which had the medial bundle of the medial olfactory tract cut, leaving both the lateral bundle of the medial olfactory tract and the lateral olfactory tract intact, did not display any alarm reaction to skin extract; however, these fish reacted to exposure to L-alanine with feeding behaviour. There were statistically significant differences between the behaviour scores for the fish subject to different treatments. The present study demonstrates that the medial bundle of the medial olfactory tract appears to be both necessary and sufficient for mediation of the alarm reaction. The results also show that the sensory neurons which respond to alarm substance terminate and make synaptic connections with the secondary neurons that make up the medial bundle of the medial olfactory tract; thereby demonstrating the specificity of the spatial aspect of olfactory processing. The results are discussed with respect to the spatial aspect of organization within the olfactory system, the pattern of generalization across orders of fish, and the functional implications of the spatial arrangement of information transmission between the peripheral olfactory organ and the brain.     

Learn and live: predator experience and feeding history determines prey behaviour and survival

Determining how prey learn the identity of predators and match their vigilance with current levels of threat is central to understanding the dynamics of predator–prey systems and the determinants of fitness. Our study explores how feeding history influences the relative importance of olfactory and visual sensory modes of learning, and how the experience gained through these sensory modes influences behaviour and survival in the field for a juvenile coral reef damselfish. We collected young fish immediately prior to their settlement to benthic habitats. In the laboratory, these predator-naïve fish were exposed to a high- or low-food ration and then conditioned to recognize the olfactory cues (odours) and/or visual cues from two common benthic predators. Fish were then allowed to settle on reefs in the field, and their behaviour and survival over 70 h were recorded. Feeding history strongly influenced their willingness to take risks in the natural environment. Conditioning in the laboratory with visual, olfactory or both cues from predators led fish in the field to display risk-averse behaviour compared with fish conditioned with sea water alone. Well-fed fish that were conditioned with visual, chemical or a combination of predator cues survived eight times better over the first 48 h on reefs than those with no experience of benthic predator cues. This experiment highlights the importance of a flexible and rapid mechanism of learning the identity of predators for survival of young fish during the critical life-history transition between pelagic and benthic habitats.     

Is feeding behaviour in crucian carp mediated by the lateral olfactory tract?

Experiments were performed to investigate which bundle of the olfactory tract was essential for mediating feeding behaviour in crucian carp. Fish were divided in three groups: control fish, fish with only the lateral olfactory tracts (LOTs) intact and fish with the LOTs cut. The fish were maintained in physiological saline after surgery to preserve the remaining tracts and postoperative inspections revealed the functional status of the remaining tracts. With the injection of food odour into the aquaria the scores for various feeding behaviours--biting, snapping, mouth openings and vertical posture--were not significantly different between those of the control fish and the fish with the LOT intact. Those fish that had the LOT cut but the medial and lateral parts of the medial olfactory tract (mMOT, lMOT) intact had significantly lower feeding-related scores than the other two groups of fish. The results of the present study indicate that the LOT is necessary to maintain the full qualitative and quantitative extent of feeding behaviour in crucian carp.     

Infection with an acanthocephalan manipulates an amphipod's reaction to a fish predator's odours

Many parasites with complex life cycles increase the chances of reaching a final host by adapting strategies to manipulate their intermediate host's appearance, condition or behaviour. The acanthocephalan parasite Pomphorhynchus laevis uses freshwater amphipods as intermediate hosts before reaching sexual maturity in predatory fish. We performed a series of choice experiments with infected and uninfected Gammarus pulex in order to distinguish between the effects of visual and olfactory predator cues on parasite-induced changes in host behaviour. When both visual and olfactory cues, as well as only olfactory cues were offered, infected and uninfected G. pulex showed significantly different preferences for the predator or the non-predator side. Uninfected individuals significantly avoided predator odours while infected individuals significantly preferred the side with predator odours. When only visual contact with a predator was allowed, infected and uninfected gammarids behaved similarly and had no significant preference. Thus, we believe we show for the first time that P. laevis increases its chance to reach a final host by olfactory-triggered manipulation of the anti-predator behaviour of its intermediate host.     

Shoal sex composition affects exploration in the Mediterranean killifish

As a consequence of sexual selection, males and females may exhibit wide behavioural differences, for example, spatial behaviour. In fish, the two sexes often show different exploratory tendencies. This sex difference has been usually studied by testing individual fish. As many fish species live in social groups with different sex composition, the aforementioned approach might not picture the natural variation of the exploratory behaviour expressed by males and females. Here, we observed shoals of four Mediterranean killifish, Aphanius fasciatus, with three different sex compositions (4 females, 4 males, or 2 females and 2 males) during the exploration of a novel environment. Sex composition of the shoals did not predict the latency to emerge from a shelter into the novel environment. However, once emerged, shoals composed by four males displayed reduced exploratory behaviour compared to 4‐female and mixed‐sex shoals. These results indicated that sex differences in exploration subsist also at group level and highlighted the importance of sex composition in determining the behaviour of the entire shoal.     

Can Paradise Fish (Macropodus opercularis,Anabantidae) Recognize a Natural Predator? An Ethological Analysis

Predator recognition and avoidance by paradise fish have been studied with allopatric species and model experiments. The effect of sympatric predators has not been investigated. Here I report that reactions of paradise fish towards a sympatric predator (Channa micropeltes) are quantitatively different from those shown towards an allopatric predator or different harmless species of fishes. I investigate the possible cues eliciting this differential response and show that visual as well as olfactory stimuli play roles. Olfactory stimuli from the sympatric predator alone elicit an elevated level of activity from paradise fish; the appearance of the sympatric predator (with or without olfactory stimuli) results in an exploratory and display reaction. I speculate what visual stimuli may play roles in predator recognition in paradise fish and I suggest that previously asserted key stimuli such as the eyes of the encountered heterospecific fish may not differentiate the harmful species from innocuous. I conclude that the antipredatory behavior of paradise fish may be affected by both genetic factors and learning and that the relative importance of the former or latter factor may vary depending on the situation.     

Olfactory navigation during spawning migrations: a review and introduction of the Hierarchical Navigation Hypothesis

Migrations are characterized by periods of movement that typically rely on orientation towards directional cues. Anadromous fish undergo several different forms of oriented movement during their spawning migration and provide some of the most well‐studied examples of migratory behaviour. During the freshwater phase of the migration, fish locate their spawning grounds via olfactory cues. In this review, we synthesize research that explores the role of olfaction during the spawning migration of anadromous fish, most of which focuses on two families: Salmonidae (salmonids) and Petromyzontidae (lampreys). We draw attention to limitations in this research, and highlight potential areas of investigation that will help fill in current knowledge gaps. We also use the information assembled from our review to formulate a new hypothesis for natal homing in salmonids. Our hypothesis posits that migrating adults rely on three types of cues in a hierarchical fashion: imprinted cues (primary), conspecific cues (secondary), and non‐olfactory environmental cues (tertiary). We provide evidence from previous studies that support this hypothesis. We also discuss future directions of research that can test the hypothesis and further our understanding of the spawning migration.     

Ocean acidification affects prey detection by a predatory reef fish

Changes in olfactory-mediated behaviour caused by elevated CO(2) levels in the ocean could affect recruitment to reef fish populations because larval fish become more vulnerable to predation. However, it is currently unclear how elevated CO(2) will impact the other key part of the predator-prey interaction--the predators. We investigated the effects of elevated CO(2) and reduced pH on olfactory preferences, activity levels and feeding behaviour of a common coral reef meso-predator, the brown dottyback (Pseudochromis fuscus). Predators were exposed to either current-day CO(2) levels or one of two elevated CO(2) levels (～600 μatm or ～950 μatm) that may occur by 2100 according to climate change predictions. Exposure to elevated CO(2) and reduced pH caused a shift from preference to avoidance of the smell of injured prey, with CO(2) treated predators spending approximately 20% less time in a water stream containing prey odour compared with controls. Furthermore, activity levels of fish was higher in the high CO(2) treatment and feeding activity was lower for fish in the mid CO(2) treatment; indicating that future conditions may potentially reduce the ability of the fish to respond rapidly to fluctuations in food availability. Elevated activity levels of predators in the high CO(2) treatment, however, may compensate for reduced olfactory ability, as greater movement facilitated visual detection of food. Our findings show that, at least for the species tested to date, both parties in the predator-prey relationship may be affected by ocean acidification. Although impairment of olfactory-mediated behaviour of predators might reduce the risk of predation for larval fishes, the magnitude of the observed effects of elevated CO(2) acidification appear to be more dramatic for prey compared to predators. Thus, it is unlikely that the altered behaviour of predators is sufficient to fully compensate for the effects of ocean acidification on prey mortality.     

The Acclimation of European Sea Bass (Dicentrarchus labrax) to Temperature: Behavioural and Neurochemical Responses

Studies on fish behavioural and neurophysiological responses to water temperature change may contribute to an improved understanding of the ecological consequences of global warming. We investigated behavioural and neurochemical responses to water temperature in European sea bass (Dicentrarchus labrax) acclimated to three temperatures (18, 22 and 28°C). After 21 d of acclimation, three groups of 25 fish each were exposed to four behavioural challenges (foraging, olfactory, aversive and mirror tests). The expression of choline acetyltransferase (ChAT) was then analysed by Western blotting in CNS homogenates (from a subset of the same fish) as a marker for cholinergic system activity. In both foraging and olfactory tests, fish acclimated to 28°C exhibited significantly higher arousal responses than fish acclimated to lower temperatures. All specimens showed fright behaviour in the aversive test, but the latency of the escape response was significantly less in the fish at 28°C. Finally, the highest mirror responsiveness was exhibited by the fish acclimated to 22°C. As in the case of cholinergic neurotransmission, significantly higher ChAT levels were detected in the telencephalon, diencephalon, cerebellum and spinal cord of fish acclimated to 22 or 28°C in comparison with those maintained at 18°C. Lower ChAT levels were detected in the mesencephalon (optic tectum) at 22 and 28°C than at 18°C. These data indicate that neuronal functions are affected by water temperature. Increases or decreases in ChAT expression can be related to the functional modulation of brain and spinal cord centres involved in behavioural responses to temperature change. Overall, the results of this study suggest that the environmental temperature level influences behaviour and CNS neurochemistry in the European sea bass.     

Variation with reproductive status of PGE₂receptor immunoreactivities in the Bostrichthys sinensis olfactory system

The role of PGE(2) as a putative sex pheromone in Chinese black sleeper Bostrichthys sinensis was investigated, using immunocytochemistry and how the immunoreactivities of the prostaglandin E(2) (PGE(2) ) receptor subtypes EP(1) , EP(2) , EP(3) and EP(4) varied with reproductive status in the olfactory system was determined. The results showed that PGE(2) receptors were present in the whole of the olfactory system of B. sinensis, and that the number of receptors was linked to the reproductive status of the fish. The densities of EP(1) immunoreactivity in the olfactory epithelium of mature fish were significantly (P < 0·01) higher than those in immature fish of both sexes, and the densities of EP(2) and EP(3) immunoreactivities in mature fish were higher (but not significantly) than those in immature fish of both sexes. In the olfactory nerve, the density of EP(2) immunoreactivity in mature fish was higher (but not significantly) than that in immature fish in both sexes. In the olfactory bulb, the densities of EP(1-4) immunoreactivities in mature females were significantly (P < 0·05 or <0·01) higher than those in immature females, and the density of EP(4) immunoreactivity in mature males was significantly (P < 0·01) higher than that in immature males. As far as is known, the present study is the first report of the immunoreactivities of PGE(2) receptor subtypes in the olfactory system of a teleost, and offers new findings regarding the role of PGE(2) as sex pheromone and hormone in the reproductive behaviour and pheromonal communication of B. sinensis.     

The effect of the chemical environment on interactions in groups of crayfish

Most of our knowledge of social behaviour in crustaceans stems from observations of pairs of animals engaged in conflict. Less consideration has been given to the dynamics of group behaviour. We investigated whether chemical signals affect the dynamic of groups of Cherax destructor. Animals were exposed to odours collected from male, female, moulted or dominant crayfish, or from fish. We observed agonistic encounters in the group during a 15 min period after the introduction of the odour. There was a decrease in threat behaviours when the male odour was added. We conclude from this that an olfactory stimulus can affect the dynamic of group interactions and the results suggest that the outcome is likely to be different from that obtained with paired or single individuals.     

Role of olfaction and vision in homing behaviour of black rockfish Sebastes inermis

How fish find their original habitat and natal home remains an unsolved riddle of animal behaviour. Despite extensive efforts to study the homing behaviour of diadromous fish, relatively little attention has been paid to that of non-diadromous marine fish. Among these, most rockfish of the genus Sebastes exhibit homing ability and/or a strong fidelity to their habitats. However, how these rockfish detect the homeward direction has not been clarified. The goal of the present research was to investigate the sensory mechanisms involved in the homing behaviour of the black rockfish Sebastes inermis, using acoustic telemetry. Vision-blocked or olfactory-ablated rockfish were released in natural waters and their homing behaviours compared with those of intact or control individuals. Blind rockfish showed homing from both inside and outside their habitat. The time taken by blind fish to reach their home habitat was not significantly different from that of the control fish. In contrast, most olfactory-ablated fish did not successfully reach their original habitat. Our results indicate that black rockfish predominantly use the olfactory sense in their homing behaviour.     

Rising CO2 concentrations affect settlement behaviour of larval damselfishes

Reef fish larvae actively select preferred benthic habitat, relying on olfactory, visual and acoustic cues to discriminate between microhabitats at settlement. Recent studies show exposure to elevated carbon dioxide (CO₂) impairs olfactory cue recognition in larval reef fishes. However, whether this alters the behaviour of settling fish or disrupts habitat selection is unknown. Here, the effect of elevated CO₂ on larval behaviour and habitat selection at settlement was tested in three species of damselfishes (family Pomacentridae) that differ in their pattern of habitat use: Pomacentrus amboinensis (a habitat generalist), Pomacentrus chrysurus (a rubble specialist) and Pomacentrus moluccensis (a live coral specialist). Settlement-stage larvae were exposed to current-day CO₂ levels or CO₂ concentrations that could occur by 2100 (700 and 850 ppm) based on IPCC emission scenarios. First, pair-wise choice tests were performed using a two-channel flume chamber to test olfactory discrimination between hard coral, soft coral and coral rubble habitats. The habitat selected by settling fish was then compared among treatments using a multi-choice settlement experiment conducted overnight. Finally, settlement timing between treatments was compared across two lunar cycles for one of the species, P. chrysurus. Exposure to elevated CO₂ disrupted the ability of larvae to discriminate between habitat odours in olfactory trials. However, this had no effect on the habitats selected at settlement when all sensory cues were available. The timing of settlement was dramatically altered by CO₂ exposure, with control fish exhibiting peak settlement around the new moon, whereas fish exposed to 850 ppm CO₂ displaying highest settlement rates around the full moon. These results suggest larvae can rely on other sensory information, such as visual cues, to compensate for impaired olfactory ability when selecting settlement habitat at small spatial scales. However, rising CO₂ could cause larvae to settle at unfavourable times, with potential consequences for larval survival and population replenishment.     

Organization of olfactory system of the Indian major carp Labeo rohita (Ham.): a study using scanning and transmission microscopy

Catla catla, Labeo rohita, and Cirrhinus mrigala are important alimentary fish in India. Their reproduction (breeding) depends on season. The fish perceive external factors-stimuli and chemical signals through the olfactory system that plays the key role in the central regulation of reproduction. However, in the available literature, any electron microscopy data on organization of olfactory elements in these fish are absent. We have studied ultrastructure of the olfactory organ in male L. rohita by using scanning (SEM) and transmission electron microscopy (TEM). The olfactory organ consists of olfactory epithelium, a short nerve, and olfactory bulb. The organ has oval shape and consists of approximately 47-52 lamellae in adult fish and of 14-20 lamellae in fish at the stage of fingerling. These lamellae originate from the midline raphe. By using SEM, the presence of microvillar sensory and ciliated non-sensory cells in these lamellae is shown. By using TEM, a microvillar receptor cell is revealed, which has rough endoplasmic reticulum and Golgi apparatus towards the apical end. Basal cells are found at the base of the receptor cell; supporting cells are located adjacent to olfactory receptor neurons, while epithelial cells--in the non-sensory part of olfactory epithelium. Mast, blastema and macrophages cells are also found in the basal lamina. This work is the first publication on structural organization of olfactory system of the Indian major carp, which provides information about morphological and ultrastructural organization of olfactory system and opens new opportunities for study of chemical neuroanatomy, sensory signal processing, and nervous regulation of reproduction of the Indian major carp.     

UV matters in shoaling decisions

Shoaling behaviour in fish is influenced by numerous factors, such as familiarity, kinship, group size and shoal composition. Grouping decisions are based on both olfactory and visual cues. The visual system of many vertebrates is extended into the ultraviolet (UV) wave range as in three-spined sticklebacks (Gasterosteus aculeatus, L.). We investigated whether the presence or absence of UV wavelengths has an influence on shoaling behaviour in this species. Reproductively non-active three-spined sticklebacks were given the choice between two shoals, equal in numbers of individuals, which could be seen either through a UV-transmitting [UV(+)] or a UV-blocking [UV(-)] filter. Test fish preferred to join the shoal seen under UV(+) conditions. Due to differences in quantal flux between the UV(+) and UV(-) filters used, control experiments with neutral-density optical filters were performed in order to clarify the role of luminance. Here, test fish spent significantly more time near shoals that were seen in a darker environment, suggesting a potential trade-off between UV radiation and lower brightness during shoal choice.To our knowledge, these results demonstrate for the first time that shoaling decisions are influenced by UV wavelengths.     

Response of seaward-migrating European eel (Anguilla anguilla) to manipulated flow fields

Anthropogenic structures (e.g. weirs and dams) fragment river networks and restrict the movement of migratory fish. Poor understanding of behavioural response to hydrodynamic cues at structures currently limits the development of effective barrier mitigation measures. This study aimed to assess the effect of flow constriction and associated flow patterns on eel behaviour during downstream migration. In a field experiment, we tracked the movements of 40 tagged adult European eels (Anguilla anguilla) through the forebay of a redundant hydropower intake under two manipulated hydrodynamic treatments. Interrogation of fish trajectories in relation to measured and modelled water velocities provided new insights into behaviour, fundamental for developing passage technologies for this endangered species. Eels rarely followed direct routes through the site. Initially, fish aligned with streamlines near the channel banks and approached the intake semi-passively. A switch to more energetically costly avoidance behaviours occurred on encountering constricted flow, prior to physical contact with structures. Under high water velocity gradients, fish then tended to escape rapidly back upstream, whereas exploratory ‘search’ behaviour was common when acceleration was low. This study highlights the importance of hydrodynamics in informing eel behaviour. This offers potential to develop behavioural guidance, improve fish passage solutions and enhance traditional physical screening.     

Environmental rearing conditions produce forebrain differences in wild Chinook salmon Oncorhynchus tshawytscha

Recent studies suggest that hatchery-reared fish can have smaller brain-to-body size ratios than wild fish. It is unclear, however, whether these differences are due to artificial selection or instead reflect differences in rearing environment during development. Here we explore how rearing conditions influence the development of two forebrain structures, the olfactory bulb and the telencephalon, in juvenile Chinook salmon (Oncorhynchus tshawytscha) spawned from wild-caught adults. First, we compared the sizes of the olfactory bulb and telencephalon between salmon reared in a wild stream vs. a conventional hatchery. We next compared the sizes of forebrain structures between fish reared in an enriched NATURES hatchery and fish reared in a conventional hatchery. All fish were size-matched and from the same genetic cohort. We found that olfactory bulb and telencephalon volumes relative to body size were significantly larger in wild fish compared to hatchery-reared fish. However, we found no differences between fish reared in enriched and conventional hatchery treatments. Our results suggest that significant differences in the volume of the olfactory bulb and telencephalon between hatchery and wild-reared fish can occur within a single generation.     

Study of bioengineered zebra fish olfactory receptor 131-2: receptor purification and secondary structure analysis

How fishes are able to detect trace molecules in large bodies of water is not understood. It is plausible that they use olfactory receptors to detect water-soluble compounds. How the zebra fish Danio Rerio, an organism with only 98 functional olfactory receptors, is able to selectively detect and recognize numerous compounds in water remains a puzzling phenomenon. We are interested in studying the biochemical and molecular mechanisms of olfaction in fish. Here, we report on the study of a bioengineered zebra fish olfactory receptor OR131-2, affinity-purified from a HEK293S tetracycline-inducible system. This receptor was expressed and translocated to the cell plasma membrane as revealed by confocal microscopy. Circular dichroism spectroscopy showed that the purified zebra fish receptor folded into an α-helical structure, as observed for other G-protein coupled receptors (GPCRs). Our study shows that it is possible to produce viable quantities of the zebra fish olfactory receptor. This will not only enable detailed structural and functional analyses, but also aid in the design of biosensor devices in order to detect water-soluble metabolites or its intermediates, which are associated with human health.     

Effect of light intensity on the shoaling behaviour of the guppy (Poecilia reticulata)

It has been shown that the tendency of fish to shoal decreases as night falls. Much is known about shoaling in the daytime, however, little is known about the social behaviour of fish at night. Although the nocturnal disintegration of shoal structure is the conventional expectation for most diurnal marine fish, it has not yet been investigated for diurnal freshwater fish. This possibility has been investigated using guppies (Poecilia reticulata), collected from the wild, as an experimental model. Three preference tanks were used, one of which permitted only visual cues, another only olfactory cues and the other both visual and olfactory cues. Shoaling tendency was observed at four different light intensities (8 wt/m, 0·05 wt/m, 0·025 wt/m, 0·003 wt/m). These light intensities were chosen to mimic daylight, dawn/dusk, clear night and cloudy night conditions, respectively. Trials were carried out on randomly selected male guppies. Results indicated that with both modalities present fish significantly preferred the stimulus shoal at all light intensities. However with only one modality to indicate the presence of the shoal, fish showed no significant shoaling tendency at any of the diminished light intensities. A test of shoal cohesion at the four different light intensities was carried out on freely interacting fish. This test condition was chosen to mimic the situation of guppies in the wild. The results to date suggest that guppies continue to shoal during dusk (at low light intensities) but not during the night. These findings make an important contribution to our understanding of the social behaviour of fish at night and deserve further investigation.