Extreme olfactory sensitivity of silver and bighead carp to overlapping suites of 21-carbon steroids suggests that these species, and likely all other Cyprinoidei, employ them as pheromones

Although well established that several fishes including goldfish in the suborder Cypinoidei within the family Cypriniformes use the maturation-inducing steroid 17,20β-dihydroxy-pregn-4-ene-3-one (17,20βP) and its metabolites as a priming pheromone which they detect with sensitivity and specificity, it is unclear whether and how other Cypriniformes might have evolved to do so. This study examined this question in the family Xenocyprididae. Using electro-olfactogram recording we tested the olfactory sensitivity of silver (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) to a range of 213 steroids in 21 mixtures at 10-9M. While silver carp detected 6 of 21 mixtures, bighead carp detected 5 (p< 0.05). Silver carp were sensitive to 13 21-carbon steroids in these mixtures including 17,20βP while bighead carp detected 9, including 8 detected by silver carp. This assortment of steroids overlapped that detected by goldfish (family Cyprinidae) but no non-Cyprinoid, suggesting common evolutionary origin and function with differences characteristic of species-specificity.

Hormonal Prostaglandin F2α Mediates Behavioral Responsiveness to a Species-Specific Multi-component Male Hormonal Sex Pheromone in a Female Fish

Although hormonally-derived female sex pheromones have been well described in approximately a dozen species of teleost fish, only a few male sex pheromones have been characterized and the neuroendocrine underpinnings of behavioral responsiveness to them is not understood. Herein, we describe a study that addresses this question using the goldfish, Carassius auratus, an important model species of how hormones drive behavior in egg-laying teleost fishes. Our study had four components. First, we examined behavioral responsiveness of female goldfish and found that when injected with prostaglandin F2α (PGF2α), a treatment that drives female sexual receptivity, and found that they became strongly and uniquely attracted to the odor of conspecific mature males, while non-PGF2α-treated goldfish did not discern males from females. Next, we characterized the complexity and specificity of the male pheromone by examining the responsiveness of PGF2α-treated females to the odor of either mature male conspecifics or male common carp odor, as well as their nonpolar and polar fractions. We found that the odor of male goldfish was more attractive than that of male common carp, and that its activity was attributable to both its nonpolar and polar fractions with the later conveying information on species-identity. Third, we hypothesized that androstenedione (AD), a 19-carbon sex steroid produced by all male fish might be the nonpolar fraction and tested whether PGF2α-treated goldfish were attracted to either AD alone or as part of a mixture in conspecific water. We found that while AD was inactive on its own, it became highly attractive when added to previously unattractive female conspecific water. Lastly, in a test of whether nonhormonal conspecific odor might determine species-specificity, we added AD to water of three species of fish and found that while AD made goldfish water strongly attractive, its effects on other species holding water were small. We conclude that circulating PGF2α produced at the time of ovulation induces behavioral sensitivity to a male sex pheromone in female goldfish and that this male pheromone is comprised of AD and a mixture of body metabolites. Because PGF2α commonly mediates ovulation and female sexual behavior in egg-laying fishes, and AD is universally produced by male fishes as a precursor to testosterone, we suggest that these two hormones may have similar roles mediating male-female behavior and communication in many species of fish.

Nonlinear relationship between Silver Carp density and their eDNA concentration in a large river

Although environmental DNA (eDNA) is increasingly being used to survey for the presence of rare and/or invasive fishes in aquatic systems, the utility of this technique has been limited by a poor understanding of whether and how eDNA concentrations relate to fish density, especially in rivers. We conducted a field study to systematically test whether the eDNA released by a model invasive fish, Silver Carp (Hypophthalmichthys molitrix), was related to the density of this species in a large river. We quantified fish density throughout the 460 km long Illinois River using hydroacoustic surveys at 23 sites while concurrently collecting 192 surface water samples for eDNA analysis. We found that Silver Carp numerical density and biomass density were positively and non-linearly related to eDNA concentration and detection rate. Both eDNA concentration (copy number) and detection rate increased rapidly as Silver Carp density increased but plateaued at moderate densities. These relationships could prove useful for estimating Silver Carp relative abundance in newly invaded locations where population numbers are low to moderate. Future studies should explore the causes of this nonlinear relationship as it would ultimately benefit aquatic species monitoring and management programs.

High levels of circulating prostaglandin F2α associated with ovulation stimulate female sexual receptivity and spawning behavior in the goldfish (Carassius auratus)

This study tested the hypothesis that blood-borne prostaglandin F_2α (PGF_2α) produced at the time of ovulation by female goldfish, a typical scramble-spawning, egg-laying cyprinid fish, functions as a hormone which stimulates female sexual receptivity, behavior, and pheromone release, thereby synchronizing female mating behavior with egg availability. We conducted 5 experiments. First, we tested whether PGF_2α is found in the blood of female fish and if it increases at the time of ovulation. Using gas chromatography-mass spectrometry, we found that circulating PGF_2α was approximately 1 ng/ml prior to ovulation, increased over 50-fold within 3 h of ovulation and returned to preovulatory values after spawning and egg release. Ovulated fish also released over 2 ng/h of PGF_2α and 800 ng/h of 15-keto-PGF_2α, a metabolite of PGF_2α - both compounds with known pheromonal function. Second, we tested how closely levels of circulating PGF_2α tracked the timing of ovulation by sampling fish at the time of ovulation and discovered that PGF_2α increased within 15 min of ovulation, peaked after 9 h, and fell to basal levels as fish spawned and released their eggs. Third, we tested whether an interaction between eggs and the reproductive tract serves as a source of circulating PGF_2α and its relationship with female sexual receptivity by injecting ovulated eggs (or an egg-substitute) into the reproductive tract of females stripped of ovulated eggs. We found both of these treatments elicited measurable increases in plasma PGF_2α as well as female sexual behavior. A fourth experiment showed that indothemacin, a PG synthase inhibitor, blocked both PGF_2α increase and female sexual behavior in egg-substitute-injected fish. Finally, we tested the relationship between the expression of female behavior and PGF_2α in PGF_2α-injected fish and found that circulating PGF_2α levels closely paralleled behavior, rising within 15 min and peaking at 45 min. Together, these experiments establish that PGF_2α functions as a behavioral blood-borne hormone in the goldfish, suggesting it likely has similar activity in other related, externally-fertilizing fishes.

Attracting Common Carp to a bait site with food reveals strong positive relationships between fish density, feeding activity, environmental DNA, and sex pheromone release that could be used in invasive fish management

Measurement of environmental DNA (eDNA) is becoming a common technique to survey for rare and invasive fish due to its sensitivity and specificity. However, its utility is limited by an incomplete understanding of factors governing its sources and fates. Failure to detect eDNA is especially difficult to interpret so surveillance techniques often collect large numbers of samples across broad regions. If, however, fish could be reliably attracted to a single location where their eDNA could be easily measured that would be useful. We conducted a proof-of-concept study of this idea using invasive Common Carp. We monitored the distribution of radio-tagged Carp and their eDNA across a 67 ha lake focusing at the bait site while a pheromone (Prostaglandin F2α; PGF 2α) was also measured to determine their reproductive condition. Prior to baiting, Carp were patchily distributed and while eDNA was occasionally detectable, it was patchy and only loosely associated with moderately dense groups of fish. Further, neither Carp, nor their eDNA were consistently measurable at the bait site and surrounding region, and the pheromone was not measurable at all. However, once baiting commenced, Carp started visiting the bait site and feeding, especially at night, where eDNA levels increased 500-fold as fish densities doubled and PGF 2α became detectable. Fish presence, eDNA and pheromone concentrations peaked at night after 6 days, strongly suggesting feeding activity was the main driver. While the presence of eDNA precisely coincided with this aggregation, levels had dropped dramatically within 5 m. PGF 2α levels dropped less rapidly and demonstrated the presence of live mature fish. We suggest that food could be used to train fish to come to locations where they otherwise are too scarce to be reliably measured, increasing their eDNA release, making them measurable, and their reproductive condition also discernable by measuring pheromones.

The Chemical Sensitivity and Electrical Activity of Individual Olfactory Sensory Neurons to a Range of Sex Pheromones and Food Odors in the Goldfish

Although it is well established that the olfactory epithelium of teleost fish detects at least 6 classes of biologically relevant odorants using 5 types of olfactory sensory neurons (OSNs), little is understood about the specificity of individual OSNs and thus how they encode identity of natural odors. In this study, we used in vivo extracellular single-unit recording to examine the odor responsiveness and physiological characteristics of 109 individual OSNs in mature male goldfish to a broad range of biological odorants including feeding stimuli (amino acids, polyamines, nucleotides), sex pheromones (sex steroids, prostaglandins [PGs]), and a putative social cue (bile acids). Sixty-one OSNs were chemosensitive, with over half of these (36) responding to amino acids, 7 to polyamines, 7 to nucleotides, 5 to bile acids, 9 to PGs, and 7 to sex steroids. Approximately a quarter of the amino acid-sensitive units also responded to polyamines or nucleotides. Three of 6 amino acid-sensitive units responded to more than 1 amino acid compound, and 5 sex pheromone-sensitive units detected just 1 sex pheromone. While pheromone-sensitive OSNs also responded to the adenylyl cyclase activator, forskolin, amino acid-sensitive OSNs responded to either forskolin or a phospholipase C activator, imipramine. Most OSNs responded to odorants and activators with excitation. Our results suggest that pheromone information is encoded by OSNs specifically tuned to single sex pheromones and employ adenylyl cyclase, suggestive of a labeled-line organization, while food information is encoded by a combination of OSNs that use both adenylyl cyclase and phospholipase C and are often less specifically tuned.

Chemicals released by predation increase the growth rate of yellow perch, Perca flavescens

Water-soluble factors associated with walleye Sander vitreus predation on either yellow perch Perca flavescens or fathead minnows Pimephales promelas markedly increased the growth rate of P. flavescens. The findings suggest that P. flavescens possess an inducible growth-promoting mechanism regulated by water-born chemicals. It may be possible to increase the growth rate of farm-raised P. flavescens by manipulating this system.

Silver, bighead, and common carp orient to acoustic particle motion when avoiding a complex sound

Behavioral responses of silver carp (Hypopthalmichthys molitrix), bighead carp (H. nobilis), and common carp (Cyprinus carpio) to a complex, broadband sound were tested in the absence of visual cues to determine whether these species are negatively phonotaxic and the roles that sound pressure and particle motion might play mediating this response. In a dark featureless square enclosure, groups of 3 fish were tracked and the distance of each fish from speakers and their swimming trajectories relative to sound pressure and particle acceleration were analyzed before, and then while an outboard motor sound was played. All three species exhibited negative phonotaxis during the first two exposures after which they ceased responding. The median percent time fish spent near the active speaker for the first two trials decreased from 7.0% to 1.3% for silver carp, 7.9% to 1.1% for bighead carp, and 9.5% to 3% for common carp. Notably, when close to the active speaker fish swam away from the source and maintained a nearly perfect 0° orientation to the axes of particle acceleration. Fish did not enter sound fields greater than 140 dB (ref. 1 μPa). These results demonstrate that carp avoid complex sounds in darkness and while initial responses may be informed by sound pressure, sustained oriented avoidance behavior is likely mediated by particle motion. This understanding of how invasive carp use particle motion to guide avoidance could be used to design new acoustic deterrents to divert them in dark, turbid river waters.

Chemical Cues which Include Amino Acids Mediate Species-Specific Feeding Behavior in Invasive Filter-Feeding Bigheaded Carps

This study tested whether and how dissolved chemicals might assist food recognition in two filter-feeding fishes, the silver (Hypophthalmichthys molitrix) and the bighead carp (H. nobilis). These species evolved in Asia, are now invasive in the Mississippi River, and feed voraciously on microparticles including plankton. The food habits and biology of these carps are broadly similar to many filter-feeding fish, none of whose chemical ecology has been examined. We conducted five experiments. First, we demonstrated that buccal-pharngeal pumping (BPP), a behavior in which fish pump water into their buccal cavities, is responsible for sampling food: BPP activity in both silver and bighead carps was low and increased nearly 25-fold after exposure to a filtrate of a planktonic food mixture (P < 0.01) and over 35-fold when planktonic food was added (P < 0.001). Next, we showed that of nine food filtrates, the one containing chemicals released by spirulina, a type of cyanobacterium, was the most potent planktonic component for both species. The potency of filtrates varied between species in ways that reflected their different chemical compositions. While L-amino acids could explain about half of the activity of food filtrate, other unknown chemical stimuli were also implicated. Finally, occlusion experiments showed the olfactory sense has a very important, but not exclusive, role in bigheaded carp feeding behaviors and this might be exploited in both their control and culture.

Biological invasion by a benthivorous fish reduced the cover and species richness of aquatic plants in most lakes of a large North American ecoregion

Biological invasions are projected to be the main driver of biodiversity and ecosystem function loss in lakes in the 21st century. However, the extent of these future losses is difficult to quantify because most invasions are recent and confounded by other stressors. In this study, we quantified the outcome of a century-old invasion, the introduction of common carp to North America, to illustrate potential consequences of introducing non-native ecosystem engineers to lakes worldwide. We used the decline in aquatic plant richness and cover as an index of ecological impact across three ecoregions: Great Plains, Eastern Temperate Forests and Northern Forests. Using whole-lake manipulations, we demonstrated that both submersed plant cover and richness declined exponentially as carp biomass increased such that plant cover was reduced to <10% and species richness was halved in lakes in which carp biomass exceeded 190 kg ha^-1 . Using catch rates amassed from 2000+ lakes, we showed that carp exceeded this biomass level in 70.6% of Great Plains lakes and 23.3% of Eastern Temperate Forests lakes, but 0% of Northern Forests lakes. Using model selection analysis, we showed that carp was a key driver of plant species richness along with Secchi depth, lake area and human development of lake watersheds. Model parameters showed that carp reduced species richness to a similar degree across lakes of various Secchi depths and surface areas. In regions dominated by carp (e.g., Great Plains), carp had a stronger impact on plant richness than human watershed development. Overall, our analysis shows that the introduction of common carp played a key role in driving a severe reduction in plant cover and richness in a majority of Great Plains lakes and a large portion of Eastern Temperate Forests lakes in North America.

Environment shapes the fecal microbiome of invasive carp species

BACKGROUND

Although the common, silver, and bighead carps are native and sparsely distributed in Eurasia, these fish have become abundant and invasive in North America. An understanding of the biology of these species may provide insights into sustainable control methods. The animal-associated microbiome plays an important role in host health. Characterization of the carp microbiome and the factors that affect its composition is an important step toward understanding the biology and interrelationships between these species and their environments.

RESULTS

We compared the fecal microbiomes of common, silver, and bighead carps from wild and laboratory environments using Illumina sequencing of bacterial 16S ribosomal RNA (rRNA). The fecal bacterial communities of fish were diverse, with Shannon indices ranging from 2.3 to 4.5. The phyla Proteobacteria, Firmicutes, and Fusobacteria dominated carp guts, comprising 76.7 % of total reads. Environment played a large role in shaping fecal microbial community composition, and microbiomes among captive fishes were more similar than among wild fishes. Although differences among wild fishes could be attributed to feeding preferences, diet did not strongly affect microbial community structure in laboratory-housed fishes. Comparison of wild- and lab-invasive carps revealed five shared OTUs that comprised approximately 40 % of the core fecal microbiome.

CONCLUSIONS

The environment is a dominant factor shaping the fecal bacterial communities of invasive carps. Captivity alters the microbiome community structure relative to wild fish, while species differences are pronounced within habitats. Despite the absence of a true stomach, invasive carp species exhibited a core microbiota that warrants future study.

Theory and Application of Semiochemicals in Nuisance Fish Control

Controlling unwanted, or nuisance, fishes is becoming an increasingly urgent issue with few obvious solutions. Because fish rely heavily on semiochemicals, or chemical compounds that convey information between and within species, to mediate aspects of their life histories, these compounds are increasingly being considered as an option to help control wild fish. Possible uses of semiochemicals include measuring their presence in water to estimate population size, adding them to traps to count or remove specific species of fish, adding them to waterways to manipulate large-scale movement patterns, and saturating the environment with synthesized semiochemicals to disrupt responses to the natural cue. These applications may be especially appropriate for pheromones, chemical signals that pass between members of same species and which also have extreme specificity and potency. Alarm cues, compounds released by injured fish, and cues released by potential predators also could function as repellents and be especially useful if paired with pheromonal attractants in "push-pull" configurations. Approximately half a dozen attractive pheromones now have been partially identified in fish, and those for the sea lamprey and the common carp have been tested in the field with modest success. Alarm and predator cues for sea lamprey also have been tested in the laboratory and field with some success. Success has been hampered by our incomplete understanding of chemical identity, a lack of synthesized compounds, the fact that laboratory bioassays do not always reflect natural environments, and the relative difficulty of conducting trials on wild fishes because of short field seasons and regulatory requirements. Nevertheless, workers continue efforts to identify pheromones because of the great potential elucidated by insect control and the fact that few tools are available to control nuisance fish. Approaches developed for nuisance fish also could be applied to valued fishes, which suffer from a lack of powerful management tools.

Invasive Bighead and Silver Carps Form Different Sized Shoals that Readily Intermix

Two species of congeneric filter-feeding microphagous carps from Asia, the bighead and the silver carp, were recently introduced to North America and have become highly invasive. These species of carp have similar food habits but the silver carp has the unique habit of jumping when disturbed. Both species have complex but poorly understood social behaviors and while both are thought to aggregate (form groups) and shoal (form tight social groups), this possibility has not yet been examined in these species. The present study examined the grouping tendencies of these species in the laboratory and the effects of fish density and species identity on it. Using nearest neighbor distance (NND) as a metric, we showed that both juvenile bighead and juvenile silver carp grouped (aggregate) strongly (P<0.05) but to different extents, and that fish density had no effect (P>0.05) on this behavior. Within aggregations, bighead carp tended to form a single large shoal while silver carp formed shoals of 2–3 individuals. Further, when tested as mixed-species groups, bighead and silver carp readily shoaled with each other but not with the common carp, which is from Eurasia and a member of another feeding guild. Due to their similar feeding strategies, we speculate that the bighead and silver carp tend to aggregate and shoal to facilitate both their foraging efforts and to avoid predation, while the differences in the size of the shoals they form may seemingly reflect their different anti-predation strategies. These complex shoaling behaviors likely influence Asian carp distribution in rivers, and thus how they might be sampled and managed.

Male-typical courtship, spawning behavior, and olfactory sensitivity are induced to different extents by androgens in the goldfish suggesting they are controlled by different neuroendocrine mechanisms

Male-typical reproductive behaviors vary greatly between different species of fishes with androgens playing a variety of roles that appear especially important in the gonochorist cypriniform fishes. The goldfish is an important model for the cypriniformes and while it is clear that male goldfish are fully feminized by prostaglandin F2α(PGF2α), it is not clear whether females will exhibit normal levels of male-typical reproductive behaviors as well as olfactory function when treated with androgens. To answer this question, we exposed sexually-regressed adult female goldfish to several types of androgen and monitored their tendencies to court (inspect females) and mate (spawn, or attempt to release gametes) while monitoring their olfactory sensitivity until changes in these attributes were maximized. Untreated adult males (intact) were included to determine the extent of masculinization. Treatments included the natural androgens, 11-ketotestosterone and testosterone (KT and T), administered via capsules (KT+T-implanted fish); the artificial androgen, methyltestosterone (MT), administered via capsules (MT-C); and MT administered in the fishes' water (MT-B). Male-typical olfactory sensitivity to a pheromone (15keto-PGF2α) increased in all androgen-treated groups and by week 6 was fully equivalent to that of males. Male-typical courtship behavior increased in all androgen-treated groups although slowly, and only MT-B females came to exhibit levels equivalent to those of males after 18weeks. In contrast, male-typical mating activity increased only slightly, with MT-B females reaching levels one-third that of males after 30weeks. We conclude that while androgens fully masculinize olfactory sensitivity and courtship behavior in goldfish, mating behavior is controlled by a different neuroendocrine mechanism(s) that has yet to be fully elucidated.

Effects of Temperature and Trophic State on Degradation of Environmental DNA in Lake Water

Degradation of environmental DNA (eDNA) in aquatic habitats can affect the interpretation of eDNA data and the ability to detect aquatic organisms. The effect of temperature and trophic state on the decay of Common Carp (Cyprinus carpio) eDNA was evaluated using lake water microcosms and quantitative PCR for a Common Carp-specific genetic marker in two experiments. The first experiment tested the effect of temperature on Common Carp eDNA decay. Common Carp eDNA exhibited exponential decay that increased with temperature. The slowest decay rate was observed at 5 °C, with a T90 value (time to 90% reduction from initial concentration) of 6.6 days, as opposed to ∼1 day at higher temperatures. In a second experiment, decay was compared across waters from lakes of different trophic states. In this experiment, Common Carp eDNA exhibited biphasic exponential decay, characterized by rapid decay for 3-8 days followed by slow decay. Decay rate was slowest in dystrophic water and fastest in oligotrophic water, and decay rate was negatively correlated to dissolved organic carbon concentration. The overall rapid decay of eDNA and the effects of temperature and water quality should be considered in protocols for water sample storage and field sampling design.

The relationship between the distribution of common carp and their environmental DNA in a small lake

Although environmental DNA (eDNA) has been used to infer the presence of rare aquatic species, many facets of this technique remain unresolved. In particular, the relationship between eDNA and fish distribution is not known. We examined the relationship between the distribution of fish and their eDNA (detection rate and concentration) in a lake. A quantitative PCR (qPCR) assay for a region within the cytochrome b gene of the common carp (Cyprinus carpio or ‘carp’), an ubiquitous invasive fish, was developed and used to measure eDNA in Lake Staring (MN, USA), in which both the density of carp and their distribution have been closely monitored for several years. Surface water, sub-surface water, and sediment were sampled from 22 locations in the lake, including areas frequently used by carp. In water, areas of high carp use had a higher rate of detection and concentration of eDNA, but there was no effect of fish use on sediment eDNA. The detection rate and concentration of eDNA in surface and sub-surface water were not significantly different (p≥0.5), indicating that eDNA did not accumulate in surface water. The detection rate followed the trend: high-use water > low-use water > sediment. The concentration of eDNA in sediment samples that were above the limit of detection were several orders of magnitude greater than water on a per mass basis, but a poor limit of detection led to low detection rates. The patchy distribution of eDNA in the water of our study lake suggests that the mechanisms that remove eDNA from the water column, such as decay and sedimentation, are rapid. Taken together, these results indicate that effective eDNA sampling methods should be informed by fish distribution, as eDNA concentration was shown to vary dramatically between samples taken less than 100 m apart.

Anatomical and physiological studies of bigheaded carps demonstrate that the epibranchial organ functions as a pharyngeal taste organ

The epibranchial organ (EO) is an enigmatic tubular organ found in the pharyngeal cavity of many filter-feeding fishes. We investigated whether it might function as a taste organ that mediates aggregation and ingestion of planktonic food within the buccal cavity. The EO and associated structures of bighead and silver carps, two successful and invasive planktivorous fishes, were examined using histological and electrophysiological techniques. Both species possess finely structured gill rakers that extend directly via a series of protrusions into each of the four blind canals which are organized as the muscular EO, suggesting that the gill rakers and EO probably function in an integrated manner. Both the interior and exterior surfaces of the EOs of both species are covered with high densities of taste buds and solitary chemosensory cells (SCCs) as well as mucous cells. Conversely, taste buds are scarce in both the buccal cavities and external portions of the head and mouth of both species. Electrophysiological recordings from a caudal branch of the vagus nerve (cranial nerve X) found to innervate the EO showed it to be sensitive to chemicals found in a planktonic diet. l-Amino acids accounted for some, but not all of the neural activity. We conclude that taste buds and SCCs located on the EO and gill rakers probably serve to chemically detect food particles, which the EO then aggregates by mucus secretion before eventually expelling them onto the floor of the pharynx for ingestion. This specialized, pharyngeal chemosensory structure may explain the feeding success of these, and perhaps other planktivorous, filter-feeding fishes.

Polar metabolites synergize the activity of prostaglandin F2α in a species-specific hormonal sex pheromone released by ovulated common carp

Many species of teleost fish detect and release F prostaglandins (PGFs), but the specific identities of these compounds and how they function as species-specific pheromones have yet to be resolved. This study addressed these questions in the common carp. An initial set of experiments established that mature male common carp were attracted to chemicals released by ovulated conspecifics, whereas the odor of female goldfish, a close relative, was less attractive. Tests of fractionated holding water from ovulated carp revealed that only the non-polar fraction was attractive on its own. Mass spectrometry and immunoassay next demonstrated that the non-polar fraction contained large quantities of prostaglandin F(2α) (PGF(2α)), 15keto-prostaglandinF(2α), and 13,14-dihydro-15keto-prostaglandin F(2α) (100 g fish released over 1 μg of all 3 PGFs per h at a ratio of 1.0: 1.7: 0.7). Ovulated goldfish released the same three PGFs but at a slightly greater rate and in a different ratio. Tests of synthetic mixtures of these PGFs revealed that the carp-specific mixture attracted male carp but was no better than the goldfish-specific mixture or PGF(2α) alone and that PGF(2α) was just as attractive as mixture of all three PGFs. A final set of attraction tests revealed that although PGF(2α) could explain all of the activity of the non-polar portion of female carp holding water, it could not explain the entire activity of female water but that a mixture of PGFs and the polar fraction could. We conclude that ovulated female carp release a multi-component sex pheromone complex that is comprised of PGF(2α) and unknown species-specific polar compound(s) that synergize the activity of the former. The pheromone also might be useful in controlling this invasive species. The observation that a fish hormonal sex pheromone incorporates bodily metabolites in addition to relatively common hormonal products demonstrates a mechanism by which species specificity may be conferred to this common type of sex pheromone.

Behavioral responses of adult male and female fathead minnows to a model estrogenic effluent and its effects on exposure regime and reproductive success

Laboratory studies of adult male fathead minnows have shown that when they are exposed to estrogens, they lose their ability to compete for access to females and sire young, suggesting that estrogenic effluents may reduce the genetic fitness of populations of wild fishes. However, it is unknown whether wild fish which are exposed to effluent actually compete with unexposed fishes, how long effects of estrogen exposure last, and whether females are affected by estrogens. This study addressed these issues using the fathead minnow (FHM) and effluent from the Metropolitan Wastewater Treatment Plant (MWTP) a well-studied source of environmental estrogens (EEs) in the Mississippi River. Maze tests found that adult FHMs are neither attracted nor repelled by MWTP effluent while previous studies have shown that minnows are attracted to the warmer waters which characterize effluents; it is realistic that previously unexposed fish enter MWTP effluent in the spring and then compete with exposed individuals. Competitive spawning experiments showed that male FHMs exposed to 44ng E2/l (a high but realistic level) for three weeks failed to compete with unexposed males while males exposed to 4ng E2/l outcompeted and sired more young than unexposed males (p<0.05). The effects of estrogen exposure disappeared within a week of moving fish into uncontaminated water. Female FHM reproductive output and behavior were unaffected by exposure to estrogen. Taken together, these experiments suggest that the behavior of wild fishes likely determines their exposure to EEs and that while the effects of this exposure are likely significant to populations of wild fish, they will be location specific because of factors which determine the duration and intensity of male exposure. We conclude that the role of fish behavior in endocrine disruption strongly warrants additional consideration.

Synthesis and olfactory activity of unnatural, sulfated 5β-bile acid derivatives in the sea lamprey (Petromyzon marinus)

A variety of unnatural bile acid derivatives (9a-9f) was synthesized and used to examine the specificity with which the sea lamprey (Petromyzon marinus) olfactory system detects these compounds. These compounds are analogs of petromyzonol sulfate (PS, 1), a component of the sea lamprey migratory pheromone. Both the stereochemical configuration at C5 (i.e., 5α vs. 5β) and the extent and sites of oxygenation (hydroxylation or ketonization) of the bile acid derived steroid skeleton were evaluated by screening the compounds for olfactory activity using electro-olfactogram recording. 5β-Petromyzonol sulfate (9a) elicited a considerable olfactory response at sub-nanomolar concentration. In addition, less oxygenated systems (i.e., 9b-9e) elicited olfactory responses, albeit with less potency. The sea lamprey sex pheromone mimic 9f (5β-3-ketopetromyzonol sulfate) was also examined and found to produce a much lower olfactory response. Mixture studies conducted with 9a and PS (1) suggest that stimulation is occurring via similar modes of activation, demonstrating a relative lack of specificity for recognition of the allo-configuration (i.e., 5α) in sea lamprey olfaction. This attribute could facilitate design of pheromone analogs to control this invasive species.

A multi-component species identifying pheromone in the goldfish

Although it has been established that sexually-immature goldfish and their relatives recognize members of their own species by using chemicals that they release, the identity of this cue(s) and whether it might be produced and used by other life stages is not yet known. To address this question, this study tested the behavioral responses of sexually immature and mature goldfish to each other's body washings, their sensitivity to this cue, the role of the olfactory sense in detecting it, and whether it is comprised of either polar and/or non-polar compounds. Tests that used two-choice mazes discovered that juvenile, immature, mature male, and mature female goldfish all release and respond to a common chemical cue(s). Dilution studies next demonstrated that this cue is active when diluted over 10 times and thus capable of functioning as a short range attractant/identifier. Olfactory occlusion demonstrated that it is detected by the olfactory sense. Finally, chemical fractionation demonstrated that it is comprised of both polar and non-polar components but likely does not include bile acids. Together, these results suggest that all life stages of goldfish use a complex multicomponent pheromonal odor to discern species identity, and that this odor has the potential to function with hormonal metabolites to identify sexual condition in behaviorally active fish of many species.

Behavioral and genomic impacts of a wastewater effluent on the fathead minnow

Rivers containing effluents from water treatment plants are complex soups of compounds, ranging from pharmaceuticals to natural hormones. Male fathead minnows (Pimephales promelas) were exposed for 3 weeks to effluent waters from the Metropolitan Wastewater Treatment Plant in St. Paul, MN. Fish were tested for their competitive nest holding behavior. Changes in vitellogenin were measured and these were correlated to changes in gene expression using a 22,000 gene microarray developed specifically for fathead minnows. Significant changes in gene expression were observed in both liver and testis, which correlate to phenotypic changes of vitellogenin induction and reduced competitive behavior. We also compared by real-time PCR the expression changes in key genes related to steroid biosynthesis and metabolism in fish exposed to the effluent as well as in fish exposed to a model estrogen and a model androgen. While the gene expression signature from effluent-exposed fish shared some elements with estrogen and androgen signatures, overall it was different, underscoring the complexity of compounds present in sewage and their different modes of action.

Production and fate of the sea lamprey migratory pheromone

Biochemical studies demonstrate that three steroids postulated to function as the sea lamprey migratory pheromone are released in sufficient quantities, and possess adequate stability and binding characteristics, to function as a multi-component pheromone in natural river waters. Mass spectrometric (MS) analyses of the holding water of recently fed larval lamprey demonstrated that each of these compounds is released at rates of 5-25 ng larva(-1) h(-1), adequate to produce picomolar (biologically relevant) concentrations in river waters. Petromyzonamine disulfate (PSDS) was released at about twice the rate of the other two components, petromyzonamine disulfate (PADS) and petromyzonol sulfate (PS). Unfed larvae also released all three steroids but only at about two-thirds the rate of fed larvae and in a different ratio. However, a behavioral test of fed and unfed larval holding waters suggested this change in pheromone ratio does not diminish pheromonal signal function in the winter when larvae are not feeding. A study of steroid degradation found that PADS and PSDS had half-lives of about 3 days, similar to values previously described for PS and sufficiently slow for the entire pheromone to persist in river mouths. Finally, both MS and electro-olfactogram recording found that contrary to previous suggestions, natural levels of natural organic matter found in streams do not bind to these steroids in ways that diminish their natural biological potency. In conclusion, it appears highly likely that a mixture of PADS, PSDS and PS is present at biologically relevant concentrations and ratios in many Great Lakes streams where it functions as a pheromonal attractant.

High-potency olfactory receptor agonists discovered by virtual high-throughput screening: molecular probes for receptor structure and olfactory function

The detection of diverse chemical structures by the vertebrate olfactory system is accomplished by the recognition of odorous ligands by their cognate receptors. In the present study, we used computational screening to discover novel high-affinity agonists of an olfactory G protein-coupled receptor that recognizes amino acid ligands. Functional testing of the top candidates validated several agonists with potencies higher than any of the receptor's known natural ligands. Computational modeling revealed molecular interactions involved in ligand binding and further highlighted interactions that have been conserved in evolutionarily divergent amino acid receptors. Significantly, the top compounds display robust activities as odorants in vivo and include a natural product that may be used to signal the presence of bacteria in the environment. Our virtual screening approach should be applicable to the identification of new bioactive molecules for probing the structure of chemosensory receptors and the function of chemosensory systems in vivo.

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.

Temporal variation in the estrogenicity of a sewage treatment plant effluent and its biological significance

Daily variation in the estrogenic activity of effluent released by a modern sewage treatment plant (STP) was measured and its effects on the physiology, behavior, and reproductive success of male fish were evaluated. As measured by an estrogen receptor binding assay, the daily estrogenic activity of this effluent was both high and extremely variable (42 +/- 25.4 [mean +/- SD] ng 17beta-estradiol (E2) equivalents/L; n = 18). Liver VTG mRNA expression in male fathead minnows (FHM) covaried with the binding assay estimates, suggesting that these fluctuations are biologically relevant. Tests which exposed male FHMs to either fluctuating levels of E2, a constant concentration of E2 (time-weighted to reflect average concentrations), or control (no E2) demonstrated that while the estrogenic activity of this effluent was detrimental to male spawning success, the fact that its concentration varied in a daily manner was without additional influence. The variability of the effluent's estrogenicity suggests that studies concerned with the effects of STP effluents should collect multiple daily samples and then test them on an appropriate time-weighted basis.

A sterol-like odorant in the urine of Mozambique tilapia males likely signals social dominance to females

Many species of freshwater fish with relatively simple mating strategies release hormonally derived sex pheromones in urine. However, it is not known whether species with more complex reproductive strategies use specialized urinary chemical signals. We addressed this by using the Mozambique tilapia (Oreochromis mossambicus Peters 1852), a lek-breeding species in which males establish dominance hierarchies and visiting females mate preferentially with territorial/dominant males. We measured urination frequency of territorial males in social isolation and in the presence of females that were either ready to spawn or had finished spawning. In groups of fish, we monitored the volume of urine stored in subordinate and dominant males to determine if urine volume and olfactory potency (by recording electro-olfactograms, EOG, in females) are related to the male's social rank. Dominant, territorial males stored more urine than subordinates and released it in short pulses, the frequency of which increased in the presence of females ready to spawn but not in the presence of post-spawn females. Urine from subordinate and dominant males was fractionated by liquid chromatography and fractions tested for olfactory potency by using the EOG, with the most potent fraction analyzed by mass spectrometry (MS). The olfactory system of females was sensitive to a urinary compound that was more abundant in the urine of dominant males than in that of subordinates. MS analysis suggested the compound is a sulfated aminosterol-like compound with a formula of C29H40N2O10S. Therefore, we suggest that dominant/territorial tilapia males dramatically increase urination frequency in the presence of females ready to spawn and that the urinary odorant acts as a pheromonal signal of dominance, thereby influencing female spawning.

Details of the structure determination of the sulfated steroids PSDS and PADS: new components of the sea lamprey (petromyzon marinus) migratory pheromone

The discovery of two new components of the migratory pheromone used by sea lamprey to guide adults to spawning grounds was recently reported. These hold promise for use in a pheromone-based control program for this species, an invasive pest in the Great Lakes. Details of the structure determination of these steroidal bis-sulfates [petromyzosterol disulfate (PSDS, 2) and petromyzonamine disulfate (PADS, 3)] are described here. Pattern matching of 1H NMR data was particularly valuable. This involved comparison of spectra of the natural samples of 2 and 3 with those of appropriate steroidal analogues [e.g., petromyzonol sulfate (PS, 1, a previously known sea lamprey bile acid derivative that is a third component of the migratory pheromone), cholesterol sulfate (6), and squalamine (8)] and model compounds containing the unprecedented aminolactam substructure present in 3. The logic underlying the iterative analyses used is presented.

Environmental estrogens suppress hormones, behavior, and reproductive fitness in male fathead minnows

This study explored the possibility that environmental estrogens in sewage effluent may reduce the reproductive fitness of adult male fish by suppressing their reproductive behaviors, including their ability to compete for nests and females. Male fathead minnows (Pimephales promelas) were exposed for three weeks to either blank control, effluent released by a sewage treatment plant (STPE), waterborne estradiol (E2), or a synthetic androgen (methyltestosterone [MT]). Afterward, fish were placed with females and a nest, and their behavior was monitored for 5 d in either the presence or the absence of a competing (unexposed control) male. Males exposed to either the STPE or E2 (approximately 50 ng/L, a level chosen to mimic the estrogenic content of the STPE) had elevated levels of circulating vitellogenin (p < 0.05) and lower levels of 11-ketotestosterone (KT; p < 0.05). Nearly all STPE- and E2-exposed males spawned successfully in the absence of a competing male, but in both cases, exposed males suffered nearly total reproductive failure when they had to compete. Conversely, males exposed to MT (approximately 50 ng/L) outcompeted control males. Behavioral observations suggested that subtle differences in agonistic behaviors, typically associated with circulating androgens (i.e., KT), were responsible. We speculate that male fish exposed to estrogenic effluent in the field are less likely to reproduce successfully within large populations of wild fish, thereby causing abnormal and potentially detrimental patterns of gene flow within those populations.

Mixture of new sulfated steroids functions as a migratory pheromone in the sea lamprey

The sea lamprey is an ancient, parasitic fish that invaded the Great Lakes a century ago, where it triggered the collapse of many fisheries. Like many fishes, this species relies on chemical cues to mediate key aspects of its life, including migration and reproduction. Here we report the discovery of a multicomponent steroidal pheromone that is released by stream-dwelling larval lamprey and guides adults to spawning streams. We isolated three compounds with pheromonal activity (in submilligram quantities from 8,000 l of larval holding water) and deduced their structures. The most important compound contains an unprecedented 1-(3-aminopropyl)pyrrolidin-2-one subunit and is related to squalamine, an antibiotic produced by sharks. We verified its structure by chemical synthesis; it attracts adult lamprey at very low (subpicomolar) concentrations. The second component is another new sulfated steroid and the third is petromyzonol sulfate, a known lamprey-specific bile acid derivative. This mixture is the first migratory pheromone identified in a vertebrate and is being investigated for use in lamprey control.

Biologically Relevant Concentrations of Petromyzonol Sulfate, a Component of the Sea Lamprey Migratory Pheromone, Measured in Stream Water

Adult sea lampreys locate spawning streams in the Great Lakes by using a migratory pheromone that is released by stream-resident larval conspecifics. Behavioral, electrophysiological, and biochemical analyses of larval release water have suggested that this pheromone is composed of several components, one of which is petromyzonol sulfate (PS), a known lamprey-specific bile acid. Its precursor, allocholic acid (ACA), has also been implicated. In this study, we employed high-performance liquid chromatography and mass spectrometry to look for both bile acids in various stream waters, thereby testing whether they might have a role in natural pheromone function. Although PS was measured at picomolar concentrations in streams known to contain larval lampreys and attract migratory adults, ACA was not. Neither compound was measured in streams lacking larvae. This finding indicates that PS is a component of the natural pheromone, and it suggests that ACA has little relevance.

Sexually mature male goldfish release large quantities of androstenedione into the water where it functions as a pheromone

Previous studies have demonstrated that ovulatory female goldfish release a variety of sex steroids into the water where they function as a pheromonal blend dominated by C21 steroids that stimulates male hormone release, sperm production and behavior. This study investigated whether male goldfish might also release sex steroids with pheromonal activity. It found that spermiated male goldfish release substantial quantities of androstenedione (AD; about 50 ng/h) together with smaller (10-20 ng/h) quantities of several other related C19 steroids but only very small quantities (<5 ng/h) of C21 steroids. Further, when sexually aroused by females and/or their pheromones, males released even greater quantities of AD (up to 1 microg/h) while C21 steroid release rate changed little. This created a ratio of C19 to C21 steroids of about 50:1 that was dramatically different from that emitted by females (1:7). The male olfactory system was also found to be extremely sensitive to AD, detecting it to near picomolar concentrations. Together with previous studies that have shown water-borne AD to increase male aggressive behavior while suppressing responsiveness to female pheromones, this study establishes AD as a male pheromone in the goldfish. Because ovulating females also release AD but in the presence of C21 steroids, recognition of the male-derived steroid pheromone is presumably mixture dependent.

Evidence That Petromyzontid Lampreys Employ a Common Migratory Pheromone That Is Partially Comprised of Bile Acids

This study examined whether the larval pheromone employed by adult sea lamprey (Petromyzon marinus) to locate spawning streams and known to be at least partially comprised of bile acids is also employed by other lamprey species. Both production and release of lamprey-specific bile acids, and sensitivity to them were examined in a wide variety of species. High pressure liquid chromatography and electrospray ionization/mass spectrometry (ESI-MS) found gallbladders from 10 species of European and North American lamprey to contain large quantities of petromyzonol sulfate (PS) together with much smaller quantities of allocholic acid (ACA) and petromyzonol (P). Evaluation of holding waters from three of these species using ESI-MS found all to contain large quantities of PS and lesser quantities of ACA in similar ratios. Electro-olfactogram recording from the olfactory systems of three parasitic lamprey species found all to detect PS and ACA with high sensitivity. Behavioral studies using migratory adult sea lamprey found them to be attracted to the odors of heterospecific larvae as well as conspecific larvae, both of which contained similar amounts of PS and ACA. Finally, adult silver lampreys (Ichthyomyzon unicuspis) were also found to be attracted to the odor of larval sea lamprey. Together, these results demonstrate that PS and ACA are commonly produced and released by larval petromyzontid lampreys and likely used as part of a common evolutionarily conserved pheromone. This scenario is reasonable because lampreys share similar larval and spawning habitat requirements, and their larvae derive no apparent benefit from producing compounds that serve as an attractant for adults.

Evidence that 4-pregnen-17,20beta,21-triol-3-one functions as a maturation-inducing hormone and pheromonal precursor in the percid fish, Gymnocephalus cernuus

Behavioral, biochemical, and electrophysiological studies suggest that the trihydroxylated progestin steroid, 4-pregnen-17,20beta,21-triol-3-one (20beta-S) stimulates oocyte maturation and pheromone release in the Eurasian ruffe, a freshwater percid fish. Behavioral observations found that female ruffe undergoing oocyte maturation (OM) release a pheromonal cue that stimulates swimming activity and social interactions among conspecific males. Neither vitellogenic nor ovulated females released the cue. Pheromone production was directly associated with elevated plasma levels of 20beta-S in maturing female ruffe which in vitro incubation suggested to be a possible maturation-inducing hormone (MIH) in this species along with 4-pregnen-17,20beta-diol-3-one (17,20betaP). However, neither of these steroids appear to be the pheromone because electrophysiological and behavioral studies found them to lack olfactory (EOG) and behavioral activity. Instead, studies of the odor of steroid-injected fish suggest the pheromone is a metabolite of 20beta-S. In particular, inter-peritoneal injection of 20beta-S (but not 17,20betaP) consistently induced release of a urinary cue with strong behavioral activity. The pheromone may be a highly polar and novel metabolite because it could not be extracted using octadecylsilane resin (C18) which has proven effective for other teleost hormonal pheromones.

Polyamines as olfactory stimuli in the goldfish Carassius auratus

Electrophysiological responses of goldfish olfactory receptor neurons (ORNs) and goldfish behavioral responses to polyamines were investigated in vivo. Electro-olfactogram (EOG) recordings indicated that polyamines (putrescine, cadaverine and spermine) are potent olfactory stimuli for goldfish with estimated electrophysiological thresholds of 10(-8)-10(-7) mol l(-1), similar to that for L-arginine, the most stimulatory amino acid. Although thresholds were similar, the magnitude of the EOG responses to intermediate (10(-5)-10(-4) mol l(-1)) and high (10(-3) mol l(-1)) concentrations of polyamines dwarfed the responses to amino acids and related single amine containing compounds (amylamine and butylamine). The EOG responses to 0.1 mmol l(-1) putrescine, cadaverine and spermine were, respectively, 4.2x, 4.3x and 10.3x the response of the standard, 0.1 mmol l(-1) L-arginine. Electrophysiological cross-adaptation experiments indicated that polyamine receptor sites are independent from those to L-amino acids (alanine, arginine, glutamate, lysine, methionine and ornithine), bile salts (sodium taurocholate and taurolithocholate), the single amine containing compounds (amylamine and butylamine) and ATP. Further, the cross-adaptation experiments revealed the existence of independent receptor sites for the different polyamines tested. Pharmacological experiments suggested that polyamine odorant transduction does not primarily involve the cyclic AMP and IP(3) second messenger pathways. Behavioral assays indicated that polyamines are attractants that elicit feeding behavior similar to that elicited by L-amino acids.

Ten-week exposure to treated sewage discharge has relatively minor, variable effects on reproductive behavior and sperm production in goldfish

It is well established that effluent from many sewage treatment plants (STPs) possesses estrogenic properties; however, the biological significance of this estrogenicity to fish reproductive behavior and sperm production is unknown. This study tested the hypothesis that 10-week exposure to a STP effluent with well-established estrogenic properties compromises the reproductive vigor of mature goldfish by decreasing the intensity of their reproductive drive and sperm production. Male goldfish were exposed for 10 weeks to one of four treatments: well water, a solvent control, 50 ng/L estradiol (E2), or undiluted effluent from a major metropolitan STP. Two trials were conducted, one in the winter and one in the summer when the effluent was chlorinated. Exposure to effluent in both the summer and winter trials induced small, but nonsignificant, decreases in the frequency of male spawning behaviors but had no effect on sperm production. Exposure to E2 caused a clear reduction in spawning behavior in one trial while causing reduced sperm production in both trials. We conclude that the effect of effluent exposure from this STP on adult fish is likely to be relatively minor and that the effluent's estrogenic properties may vary over the course of the year.

The three steroidal components of the goldfish preovulatory pheromone signal evoke different behaviors in males

The goldfish sex pheromone system is the best understood among the teleost fishes. Pheromones in this species are unspecialized hormonal products, which are released in ratios that vary with reproductive status. This study examined behavioral responses of male goldfish to three steroidal components of the female preovulatory pheromone: 17,20beta-dihydroxy-4-pregnen-3-one (1720betaP); 17,20beta-dihydroxy-4-pregnen-3-one-20-sulfate (1720betaP-S); and androstenedione (AD). Males were observed during exposure to nanomolar concentrations of each steroid over a 2-h period. We observed chasing, nudging (courtship behaviors) and pushing (an aggressive behavior). Each steroid elicited a different set of behaviors. 1720betaP, which is released by ovulatory females, elicited a low level of chasing and nudging that persisted throughout the experiment. Exposure to 1720betaP-S, which is released primarily by ovulatory females, triggered a large increase in nudging and chasing that lasted for only 5 min. In contrast, AD, which is released by females early in the ovulatory cycle and by mature males, elicited increases in aggressive behavior. 1720betaP and 1720betaP-S both caused increases in GtH-II release while AD did not. These results demonstrate that goldfish can discriminate components found in the female pheromone blend, suggesting that goldfish, and likely other fish species, may employ blends of hormonal products as pheromones.

Functional identification of a goldfish odorant receptor

The vertebrate olfactory system utilizes odorant receptors to receive and discriminate thousands of different chemical stimuli. An understanding of how these receptors encode information about an odorant's molecular structure requires a characterization of their ligand specificities. We employed an expression cloning strategy to identify a goldfish odorant receptor that is activated by amino acids-potent odorants for fish. Structure-activity analysis indicates that the receptor is preferentially tuned to recognize basic amino acids. The receptor is a member of a multigene family of G protein-coupled receptors, sharing sequence similarities with the calcium sensing, metabotropic glutamate, and V2R class of vomeronasal receptors. The ligand tuning properties of the goldfish amino acid odorant receptor provide information for unraveling the molecular mechanisms underlying olfactory coding.

Ultrastructure of the olfactory epithelium in intact, axotomized, and bulbectomized goldfish, Carassius auratus

The ultrastructure of the olfactory epithelium in intact, axotomized, and bulbectomized goldfish was studied by scanning and transmission electron microscopy. A total of 58 adult goldfish of various survival times were examined to determine whether the different types of surgery--either olfactory nerve transection or bulbectomy--yielded differences in the extent or time course of cellular degeneration and renewal. Control animals were also examined in detail to elucidate previous controversial findings concerning the types of olfactory receptor neurons present in goldfish. We found that the intact olfactory epithelium of unoperated control goldfish contains the previously observed ciliated and microvillous receptor neurons, and the crypt cell, a cell type not yet seen in the goldfish but recently reported in other species of teleosts. Following either olfactory nerve transection or bulbectomy, the olfactory receptor neurons showed similar signs of degeneration and subsequent cell death, but, surprisingly, the thickness of the olfactory epithelium did not change significantly with either treatment. The time course of receptor cell renewal was different in axotomized and bulbectomized goldfish. In axotomized goldfish, the amount of receptor cells decreased continuously until 8-13 days after surgery, followed by rapid cell renewal. For bulbectomized goldfish, cell replacement began almost immediately after surgery, with degeneration and cell renewal occurring simultaneously. Six weeks after bulbectomy, cell death and cell proliferation reached a "steady state," and the epithelia did not further improve.

Sex pheromones and amino acids evoke distinctly different spatial patterns of electrical activity in the goldfish olfactory bulb

Until now, electrophysiological studies of the vertebrate olfactory bulb have tested only 'generalist' cues. These studies suggest that odorants are discriminated by a broadly distributed spatial map. In this study, we tested for the first time in a vertebrate the hypothesis that 'specialist' cues (pheromones) are discriminated by a more restricted component of the olfactory bulb. Our model is the male goldfish, Carassius auratus, for which five sex pheromones with both behavioral and physiological activity have now been identified. Electrical activity (electroencephalography: EEG) was recorded over a 12-point grid from the surface of the olfactory bulb, while fish were exposed to one of ten stimuli including: five sex pheromones, two amino acids, two bile steroids and a control. Evoked activity was evaluated by time series analysis. Power ratios were calculated by dividing the power of the dominant frequency in the power spectrum before stimulation by the power of the dominant frequency during stimulation. Next, the average magnitudes of odorant responses were compared using analysis of variance (ANOVA). The spatial patterning of these responses was also described using cluster analysis, which grouped odorants based on the similarity of their spatial patterns of activity. Although all odorants elicited EEG responses with similar dominant frequencies, odorant-specific differences were evident in the size and distribution of these responses. Sex pheromones and bile steroids elicited relatively small responses that were spatially restricted in distinctive manners, although some overlap was evident. In contrast, amino acids consistently produced large responses at all positions. These results are consistent with the hypothesis that vertebrate pheromones are discriminated by a distinctive subcomponent of the vertebrate olfactory system comprised of a relatively small number of olfactory neurons.

Discrimination of pheromonal cues in fish: emerging parallels with insects

Many fish species employ hormonal products as sex pheromones, and these cues are often mixtures that are released with a temporal pattern. This behavior is strikingly similar to that of insects, as moths use precise blends of odorants as sex pheromones and are skillful at tracking them in spite of changes in odor intensity associated with aerial dispersal. New studies in both groups of animals suggest many parallels in the functional anatomy of olfactory pathways and the organization of information-coding circuits.

Biological responsiveness to pheromones provides fundamental and unique insight into olfactory function

When exposed to the odor of conspecifics, most organisms exhibit an adaptive behavioral response, particularly if the individuals are sexually mature. Evidence increasingly suggests that behavioral responsiveness to these odors, which are termed 'pheromones', reflects neuroethological mechanisms associated with olfactory function. Reproductive pheromones, which are the best understood, are commonly used by both invertebrates and vertebrates. In both instances they are generally comprised of mixtures of compounds and behavioral responsiveness to them is largely instinctual, sexually-dimorphic, and attributable to a specialized component(s) of the olfactory system. While pheromonal responsiveness in some systems (e.g. moths) appears highly stereotypic and symptomatic of a relatively simple 'labeled line', behavioral responsiveness of other animals (e.g. rodents) can be modified by experience, suggesting a more complex underlying central mechanism. In any case, our understanding of these fascinating systems is progressing only because of an active dialogue between behavioral and neurological investigations. This review briefly examines how behavioral studies have provided fundamental insight into the neuroethology of olfactory function by drawing comparisons between some of the better understood sex pheromone systems which have been described in heliothine moths, the goldfish, and the pig. Many similarities between invertebrate and vertebrate pheromone systems are noted.