Activity of FoxP2-positive neurons correlate with tadpole begging behavior.

Motor function is a critical aspect of social signaling in a wide range of taxa. The transcription factor FoxP2 is well studied in the context of vocal communication in humans, mice, and songbirds, but its role in regulating social signaling in other vertebrate taxa is unclear. We examined the distribution and activity of FoxP2-positive neurons in tadpoles of the mimetic poison frog (Ranitomeya imitator). In this species, tadpoles are reared in isolated plant nurseries and are aggressive to other tadpoles. Mothers provide unfertilized egg meals to tadpoles that perform a begging display by vigorously vibrating back and forth. We found that FoxP2 is widely distributed in the tadpole brain and parallels the brain distribution in mammals, birds, and fishes. We then tested the hypothesis that FoxP2-positive neurons would have differential activity levels in begging or aggression contexts compared to non-social controls. We found that FoxP2-positive neurons showed increased activation in the striatum and cerebellum only during begging. Overall, this work suggests a generalizable role for FoxP2 in social signaling across terrestrial vertebrates.

Neural activation patterns associated with mouthbrooding, maternal care, infanticide and fry release in an African cichlid fish.

Parental care has evolved several times and is present across taxa. Parental care behaviors, such as food provisioning and protection, are critical for offspring success. However, infanticide can co-exist with parental care in the same species. The mechanisms underlying the switch from care to consumption and from offspring dependence to independence are relatively unknown, especially in fishes, the oldest and largest group of vertebrates. Mouthbrooding, an extreme example of parental care present in dozens of genera of fishes, provides an excellent opportunity to investigate the brain regions important for parental care. The maternal mouthbrooding African cichlid fish Astatotilapia burtoni broods developing young inside the mouth for approximately 14 days, then provides post-release maternal care by protecting fry inside the mouth when threatened. Following the post-release maternal care phase, females can exhibit infanticide and consume their own offspring. We used immunohistochemistry for the neural activation marker pS6 to identify differences in neural activation among mouthbrooding, maternal-care-providing and infanticide-exhibiting females, and between pre- and post-release fry. We identified five brain regions (Dc-5, ATn, nPPa, Vd-c and Dl-g) that are differentially activated among mouthbrooding, maternal care and infanticide females as well as six regions (Dm, Vv, Vd, Vs-m, TPp, PGZ and INL of retina) differentially activated between pre- and post-release fry. This study identifies both shared and distinct circuitry that may support transitions between parental care states and from care to infanticide, as well as regions in developed fry that support the transition from pre- to post-release.

Albino Xenopus laevis tadpoles prefer dark environments compared to wild type.

Tadpoles display preferences for different environments but the sensory modalities that govern these choices are not well understood. Here, we examined light preferences and associated sensory mechanisms of albino and wild-type Xenopus laevis tadpoles. We found that albino tadpoles spent more time in darker environments compared to the wild type, although they showed no differences in overall activity. This preference persisted when the tadpoles had their optic nerve severed or pineal glands removed, suggesting these sensory systems alone are not necessary for phototaxis. These experiments were conducted by an undergraduate laboratory course, highlighting how X. laevis tadpole behavior assays in a classroom setting can reveal new insights into animal behavior.

Reproductive- and Social-State Plasticity of Multiple Sensory Systems in a Cichlid Fish.

Intra- and inter-sexual communications are vital to the survival and reproductive success of animals. In species that cycle in and out of breeding or other physiological condition, sensory function can be modulated to optimize communication at crucial times. Little is known, however, about how widespread this sensory plasticity is across taxa, whether it occurs in multiple senses or both sexes within a species, and what potential modulatory substances and substrates are involved. Thus, studying modulation of sensory communication in a single species can provide valuable insights for understanding how sensory abilities can be altered to optimize detection of salient signals in different sensory channels and social contexts. The African cichlid fish Astatotilapia burtoni uses multimodal communication in social contexts such as courtship, territoriality, and parental care and shows plasticity in sensory abilities. In this review, we synthesize what is known about how visual, acoustic, and chemosensory communication is used in A. burtoni in inter- and intra-specific social contexts, how sensory funtion is modulated by an individual's reproductive, metabolic, and social state, and discuss evidence for plasticity in potential modulators that may contribute to changes in sensory abilities and behaviors. Sensory plasticity in females is primarily associated with the natural reproductive cycle and functions to improve detection of courtship signals (visual, auditory, chemosensory, and likely mechanosensory) from high-quality males for reproduction. Plasticity in male sensory abilities seems to function in altering their ability to detect the status of other males in the service of territory ownership and future reproductive opportunities. Changes in different classes of potential modulators or their receptors (steroids, neuropeptides, and biogenic amines) occur at both peripheral sensory organs (eye, inner ear, and olfactory epithelium) and central visual, olfactory, and auditory processing regions, suggesting complex mechanisms contributing to plasticity of sensory function. This type of sensory plasticity revealed in males and females of A. burtoni is likely more widespread among diverse animals than currently realized, and future studies should take an integrative and comparative approach to better understand the proximate and ultimate mechanisms modulating communication abilities across taxa.

Opsin Expression Varies with Reproductive State in the Cichlid Fish Astatotilapia burtoni.

Animals use visual communication to convey crucial information about their identity, reproductive status, and sex. Plasticity in the auditory and olfactory systems has been well-documented, however, fewer studies have tested for plasticity in the visual system, a surprising detail since courtship and mate choice are largely dependent on visual signals across taxa. We previously found reproductive state-dependent plasticity in the eye of the highly social cichlid fish Astatotilapia burtoni. Male A. burtoni increase their courtship, including multicomponent visual displays, when around ovulated females, and ovulated females are more responsive to male visual courtship displays than non-ovulated females. Based on this, we hypothesized that ovulation status impacts visual capabilities in A. burtoni females. Using electroretinograms, we found that ovulated females had greater visual sensitivity at wavelengths corresponding to male courtship coloration compared with non-reproductively-receptive females. In addition, ovulated females had higher neural activation in the retina and higher mRNA expression levels of neuromodulatory receptors (e.g., sex-steroids; gonadotropins) in the eye than non-ovulated females. Here, we add to this body of work by testing the hypothesis that cone opsin expression changes with female reproductive state. Ovulated females had higher expression of short wavelength sensitive opsins (sws1, sws2a, sws2b) compared with mouthbrooding females. Further, expression of sws2a, the most abundant opsin in the A. burtoni eye, positively correlated with levels of circulating 11-ketotestosterone and estradiol and estrogen, androgen, and gonadotropin system receptor expression in the eye in females. These data indicate that reproductive state-dependent plasticity also occurs at the level of photoreceptors, not just through modulation of visual signals at downstream retinal layers. Collectively, these data provide crucial evidence linking endocrine modulation of visual plasticity to mate choice behaviors in females.

Expression patterns and evolution of urocortin and corticotropin-releasing hormone genes in a cichlid fish.

The corticotropin-releasing hormone and urocortin family of peptides consists of five members in many vertebrates: CRH (crha/crhb in teleosts), CRH2, UCN/UTS1, UCN2, and UCN3. These genes differ in expression pattern, as well as receptor affinity, allowing them to serve a wide range of functions in a variety of species. To better understand the roles of these genes in a single species, we examined their expression patterns in the cichlid fish Astatotilapia burtoni. In situ hybridization to map mRNA expression patterns of crhb, uts1, ucn2, and ucn3 in the brain revealed conserved and distinct spatial features of expression. crhb- and uts1-expressing cells were the most broadly distributed, with several areas of co-regionalization. ucn3 was less abundant but was found in discrete regions throughout the extent of the brain, with high expression in the cerebellum, while ucn2 was restricted to only a few areas. RT-PCR showed that while crhb, uts1, and ucn3 are found in several body tissues and widespread throughout the brain, ucn2 is quite restricted in the brain, and crha is only expressed in the eye. Bayesian phylogenetic analyses identified detailed relationships and novel orthologs in the urocortin family. We found evidence for a UCN2 gene loss in some reptiles. Our detailed description of the complete family of genes in the central nervous system of a model organism will inform future studies on the function of these genes in A. burtoni and provides a foundation for comparative studies with teleosts and other vertebrates.

Female reproductive state is associated with changes in distinct arginine vasotocin cell types in the preoptic area of Astatotilapia burtoni.

Nonapeptides play a crucial role in mediating reproduction, aggression, and parental care across taxa. In fishes, arginine vasotocin (AVT) expression is related to social and/or reproductive status in most male fishes studied to date, and is linked to territorial defense, paternal care, and courtship. Despite a plethora of studies examining AVT in male fishes, relatively little is known about how AVT expression varies with female reproductive state or its role in female social behaviors. We used multiple methods for examining the AVT system in female African cichlid fish Astatotilapia burtoni, including immunohistochemistry for AVT, in situ hybridization for avt-mRNA, and quantitative PCR. Ovulated and mouthbrooding females had similar numbers of parvocellular, magnocellular, and gigantocellular AVT cells in the preoptic area. However, ovulated females had larger magnocellular and gigantocellular cells compared to mouthbrooding females, and gigantocellular AVT cell size correlated with the number of days brooding, such that late-stage brooding females had larger AVT cells than mid-stage brooding females. In addition, we found that ventral hypothalamic cells were more prominent in females compared to males, and were larger in mouthbrooding compared to ovulated females, suggesting a role in maternal care. Together, these data indicate that AVT neurons change across the reproductive cycle in female fishes, similar to that seen in males. These data on females complement studies in male A. burtoni, providing a comprehensive picture of the regulation and potential function of different AVT cell types in reproduction and social behaviors in both sexes.

Noise during mouthbrooding impairs maternal care behaviors and juvenile development and alters brain transcriptomes in the African cichlid fish Astatotilapia burtoni.

Anthropogenic noise has increased underwater ambient sound levels in the range in which most fishes detect and produce acoustic signals. Although the impacts of increased background noise on fish development have been studied in a variety of species, there is a paucity of information on how noise affects parental care. Mouthbrooding is an energetically costly form of parental care in which the brooding fish carries developing larvae in the buccal cavity for the duration of development. In the African cichlid Astatotilapia burtoni, females carry their brood for ~2 weeks during which time they do not eat. To test the hypothesis that increased background noise impacts maternal care behaviors and brood development, we exposed brooding females to a 3-h period of excess noise (~140 dB) played through an underwater speaker. Over half of noise-exposed brooding females cannibalized or pre-maturely released their brood, but 90% of control females exhibited normal brooding behaviors. RNA-seq analysis revealed that transcripts related to feeding and parental care were differentially expressed in the brains of noise-exposed females. Juveniles that were exposed to noise during their brood period within the mother's mouth had lower body condition factors, higher mortality and altered head transcriptomes compared with control broods. Furthermore, onset of adult-typical coloration and behaviors was delayed compared with control fish. Together, these data indicate that noise has severe impacts on reproductive fitness in mouthbrooding females. Our results, combined with past studies, indicate that parental care stages are extremely susceptible to noise-induced perturbations with detrimental effects on species persistence.

Galanin neuron activation in feeding, parental care, and infanticide in a mouthbrooding African cichlid fish.

Galanin is a conserved neuropeptide involved in parental care and feeding. While galanin is known to mediate parental care and infanticide in rodents, its role in parental care and feeding behaviors in other taxa, particularly fishes, remains poorly understood. Mouthbrooding is an extreme form of parental care common in fishes in which caregivers carry offspring in their buccal cavity for the duration of development, resulting in obligatory starvation. In the cichlid fish Astatotilapia burtoni, females brood their young for ~2 wks and perform maternal care after release by collecting them into their mouth when threatened. However, females will cannibalize their brood after ~5 days. To examine the role of gal in feeding and maternal care, we collected mouthbrooding, fed, and starved females, as well as those displaying post-release maternal care and infanticide behaviors. Activation of gal neurons in the preoptic area (POA) was associated with parental care, providing the first link between gal and offspring-promoting behaviors in fishes. In contrast, activation of gal neurons in the lateral tuberal nucleus (NLT), the Arcuate homolog, was associated with feeding and infanticide. Overall, these data suggest gal is functionally conserved across vertebrate taxa with POA gal neurons promoting maternal care and Arc/NLT gal neurons promoting feeding.

Neural Activation Patterns Associated with Maternal Mouthbrooding and Energetic State in an African Cichlid Fish.

Parental care is widespread in the animal kingdom, but for many species, provisioning energetic resources must be balanced with trade-offs between self-promoting and offspring-promoting behaviors. However, little is known about the neural mechanisms underlying these motivational decisions. Mouthbrooding is an extreme form of parental care most common in fishes that provides an ideal opportunity to examine which brain regions are involved in parenting and energetics. The African cichlid fish Astatotilapia burtoni is a maternal mouthbrooder in which females hold developing young inside their mouths for 2 weeks. This brood care makes feeding impossible, so females undergo obligatory starvation. We used immunohistochemistry for the neural activation marker pS6 to examine which brain regions were involved in processing salient information in mouthbrooding, starved, and fed females. We identified brain regions more associated with maternal brood care (TPp, Dc-4/-5), and others reflective of energetic state (Dl-v, NLTi). Most nuclei examined, however, were involved in both maternal care and energetic status. Placement of each of the 16 examined nuclei into these functional categories was supported by node by node comparisons, co-activity networks, hierarchical clustering, and discriminant function analysis. These results reveal which brain regions are involved in parental care and food intake in a species where provisioning is skewed towards the offspring when parental feeding is not possible. This study provides support for both distinct and shared circuitry involved in regulation of maternal care, food intake, and energy balance, and helps put the extreme parental case of mouthbrooding into a comparative and evolutionary context.

Distribution of aromatase in the brain of the African cichlid fish Astatotilapia burtoni: Aromatase expression, but not estrogen receptors, varies with female reproductive-state.

Estrogen synthesis and signaling in the brains of vertebrates has pleotropic effects ranging from neurogenesis to modulation of behaviors. The majority of studies on brain-derived estrogens focus on males, but estrogenic signaling in females likely plays important roles in regulation of reproductive cycling and social behaviors. We used females of the mouth brooding African cichlid fish, Astatotilapia burtoni, to test for reproductive state-dependent changes in estrogenic signaling capacity within microdissected brain nuclei that are important for social behaviors. Expression levels of the rate-limiting enzyme aromatase, but not estrogen receptors, measured by qPCR changes across the reproductive cycle. Gravid females that are close to spawning had higher aromatase levels in all brain regions compared to females with lower reproductive potential. This brain aromatase expression was positively correlated with circulating estradiol levels and ovarian readiness. Using chromogenic in situ hybridization we localized aromatase-expressing cells to ependymal regions bordering the ventricles from the forebrain to the hindbrain, and observed more abundant staining in gravid compared to mouth brooding females in most regions. Staining was most prominent in subpallial telencephalic regions, and diencephalic regions of the preoptic area, thalamus, and hypothalamus, but was also observed in sensory and sensorimotor areas of the midbrain and hindbrain. Aromatase expression was observed in radial glial cells, revealed by co-localization with the glial marker GFAP and absence of co-localization with the neuronal marker HuC/D. Collectively these results support the idea that brain-derived estradiol in females may serve important functions in reproductive state-dependent physiological and behavioral processes across vertebrates.

A Comparison of Four Presumptive Tests for the Detection of Blood on Dark Materials.

Detection of blood on dark materials is difficult for crime scene examiners so presumptive tests are used to assist. This study compared the ability of luminol, leuko crystal violet, tetramethylbenzidine, and Combur Test®E to detect whole, diluted blood (1:100) and a key-shaped blood transfer stain (1:10), on dark cotton sheeting, tea towel, socks, synthetic carpet, and car mats. Powdered bleach was used to evaluate specificity of the blood detection tests. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall misclassification rate (OMR) assessed the quality of the blood tests. Luminol was the preferred test for diluted blood having the highest sensitivity (79%-96%), NPV (66%-93%), and the lowest OMR (3%-15%). Luminol was also found to be most efficient with a testing time on 25 items of 2 h 50 min compared with up to 8 h. Overall, luminol was the most effective method, also providing information on bloodstain patterns.

Reproductive state-dependent plasticity in the visual system of an African cichlid fish.

Visual communication is used widely across the animal kingdom to convey crucial information about an animals' identity, reproductive status, and sex. Although it is well-demonstrated that auditory and olfactory sensitivity can change with reproductive state, fewer studies have tested for plasticity in the visual system, a surprising detail since courtship and mate choice behaviors in many species are largely dependent on visual signals. Here, we tested for reproductive state-dependent plasticity in the eye of the cichlid fish Astatotilapia burtoni using behavioral, gene expression, neural activation, and electrophysiology techniques. Males court ovulated females more intensely than gravid females, and ovulated females were more responsive to male courtship behaviors than gravid females. Using electroretinography to measure visual sensitivity in dark-adapted fish, we revealed that gravid, reproductively-ready females have increased visual sensitivity at wavelengths associated with male courtship coloration compared to non-gravid females. After ovulation was hormonally induced, female's spectral sensitivity further increased compared to pre-injection measurements. This increased sensitivity after hormone injection was absent in non-gravid females and in males, suggesting an ovulation-triggered increase in visual sensitivity. Ovulated females had higher mRNA expression levels of reproductive neuromodulatory receptors (sex-steroids; gonadotropins) in the eye than nonovulated females, whereas males had similar expression levels independent of reproductive/social state. In addition, female mate choice-like behaviors positively correlated with expression of gonadotropin system receptors in the eye. Collectively, these data provide crucial evidence linking endocrine modulation of visual plasticity to mate choice behaviors in females.

Expression of tachykinin3 and related reproductive markers in the brain of the African cichlid fish Astatotilapia burtoni.

Neurokinin B, encoded by the tachykinin3 gene, plays a crucial role in regulating reproduction in mammals via KNDy neurons and interaction with GnRH. Previous work in teleost fishes has focused on hypothalamic tac3 expression for its role in reproduction, but detailed studies on extra-hypothalamic tac3 expression are limited. Here, we identified two tac3 genes in the social African cichlid fish Astatotilapia burtoni, only one of which produces a functional protein containing the signature tachykinin motif. In situ hybridization for tac3a mRNA identified cell populations throughout the brain. Numerous tac3a cells lie in several thalamic and hypothalamic nuclei, including periventricular nucleus of posterior tuberculum, lateral tuberal nucleus (NLT), and nucleus of the lateral recess (NRL). Scattered tac3-expressing cells are also present in telencephalic parts, such as ventral (Vv) and supracomissural (Vs) part of ventral telencephalon. In contrast to other teleosts, tac3 expression was absent from the pituitary. Using double-fluorescent staining, we localized tac3a-expressing cells in relation to GnRH and kisspeptin cells. Although no GnRH-tac3a colabeled cells were observed, dense GnRH fibers surround and potentially synapse with tac3a cells in the preoptic area. Only minimal (<5%) colabeling of tac3a was observed in kiss2 cells. Despite tac3a expression in many nodes of the mesolimbic reward system, it was absent from tyrosine hydroxylase (TH)-expressing cells, but tac3a cells were located in areas with dense TH fibers. The presence of tac3a-expressing cells throughout the brain, including in socially relevant brain regions, suggest more diverse functions beyond regulation of reproductive physiology that may be conserved across vertebrates.

Neural and behavioural correlates of repeated social defeat.

Dominance hierarchies are common across the animal kingdom and have important consequences for reproduction and survival. Animals of lower social status cope with repeated social defeat using proactive and reactive behaviours. However, there remains a paucity of information on how an individual's coping behaviours changes over time or what neural mechanisms are involved. We used a resident-intruder paradigm in the African cichlid fish Astatotilapia burtoni to investigate the neural correlates of these two opposing behaviour groups. Fish initially used both proactive and reactive behaviours, but had a dramatic increase in use of proactive behaviours during the third interaction, and this was followed by cessation of proactive behaviours and exclusive use of reactive coping. By quantifying neural activation in socially-relevant brain regions, we identify a subset of brain nuclei, including those homologous to the mammalian amygdala, showing higher activation in fish displaying proactive but not reactive behaviours. Fish displaying reactive behaviours had greater neural activation in the superior raphe, suggesting a possible conserved function during social defeat across vertebrates. These data provide the first evidence on the involvement of specific brain regions underlying proactive and reactive coping in fishes, indicating that these nuclei have conserved functions during social defeat across taxa.

Swim bladder morphology changes with female reproductive state in the mouth-brooding African cichlid Astatotilapia burtoni.

Mouth brooding is an extreme form of parental care in which the brooding parent carries the developing young in their buccal cavity for the duration of development. Brooding fish need to compensate for the brood weight on the anterior portion of their body. For fishes with a compartmentalized swim bladder, gas distribution between the chambers may aid in regulating buoyancy during brooding. To test this hypothesis, we took radiographs of Astatotilapia burtoni to compare the swim bladder morphology of gravid, mouth-brooding and recovering females. Following spawning, females carry developing fish in their buccal cavity for ∼2 weeks, resulting in a larger and rounder anterior swim bladder compartment. Comparatively, the swim bladder of gravid females is long and cylindrical. Using small beads to mimic brood weight and its effects on female buoyancy, swim bladder changes were induced that resembled those observed during brooding. Immediately after releasing their fry, brooding females swim at a positive angle of attack but correct their swimming posture to normal within 5 min, suggesting a rapid change in swim bladder gas distribution. These data provide new insights into how swim bladder morphology and swimming behavior change during mouth brooding, and suggest a compartmentalized swim bladder may be a morphological adaptation for mouth brooding.

Reproductive and metabolic state differences in olfactory responses to amino acids in a mouth brooding African cichlid fish.

Olfaction mediates many crucial life-history behaviors such as prey detection, predator avoidance, migration and reproduction. Olfactory function can also be modulated by an animal's internal physiological and metabolic states. While this is relatively well studied in mammals, little is known about how internal state impacts olfaction in fishes, the largest and most diverse group of vertebrates. Here we apply electro-olfactograms (EOGs) in the African cichlid fish Astatotilapia burtoni to test the hypothesis that olfactory responses to food-related cues (i.e. l-amino acids; alanine and arginine) vary with metabolic, social and reproductive state. Dominant males (reproductively active, reduced feeding) had greater EOG magnitudes in response to amino acids at the same tested concentration than subordinate males (reproductively suppressed, greater feeding and growth rates). Mouth brooding females, which are in a period of starvation while they brood fry in their mouths, had greater EOG magnitudes in response to amino acids at the same tested concentration than both recovering and gravid females that are feeding. Discriminant function analysis on EOG magnitudes also grouped the male (subordinate) and female (recovering, gravid) phenotypes with higher food intake together and distinguished them from brooding females and dominant males. The slope of the initial negative phase of the EOG also showed intra-sexual differences in both sexes. Our results demonstrate that the relationship between olfaction and metabolic state observed in other taxa is conserved to fishes. For the first time, we provide evidence for intra-sexual plasticity in the olfactory response to amino acids that is influenced by fish reproductive, social and metabolic state.

Localization of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish, Astatotilapia burtoni.

Neural communication depends on release and reception of different neurotransmitters within complex circuits that ultimately mediate basic biological functions. We mapped the distribution of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish Astatotilapia burtoni using in situ hybridization to label vesicular glutamate transporters (vglut1, vglut2.1, vglut3), glutamate decarboxylases (gad1, gad2), and choline acetyltransferase (chat). Cells expressing the glutamatergic markers vgluts 1-3 show primarily nonoverlapping distribution patterns, with the most widespread expression observed for vglut2.1, and more restricted expression of vglut1 and vglut3. vglut1 is prominent in granular layers of the cerebellum, habenula, preglomerular nuclei, and several other diencephalic, mesencephalic, and rhombencephalic regions. vglut2.1 is widely expressed in many nuclei from the olfactory bulbs to the hindbrain, while vglut3 is restricted to the hypothalamus and hindbrain. GABAergic cells show largely overlapping gad1 and gad2 expression in most brain regions. GABAergic expression dominates nuclei of the subpallial ventral telencephalon, while glutamatergic expression dominates nuclei of the pallial dorsal telencephalon. chat-expressing cells are prominent in motor cranial nerve nuclei, and some scattered cells lie in the preoptic area and ventral part of the ventral telencephalon. A localization summary of these markers within regions of the conserved social decision-making network reveals a predominance of either GABAergic or glutamatergic cells within individual nuclei. The neurotransmitter distributions described here in the brain of a single fish species provide an important resource for identification of brain nuclei in other fishes, as well as future comparative studies on circuit organization and function. J. Comp. Neurol. 525:610-638, 2017. © 2016 Wiley Periodicals, Inc.

Mechanosensory signaling as a potential mode of communication during social interactions in fishes.

Signals produced during social interactions convey crucial information about the sender's identity, quality, reproductive state and social status. Fishes can detect near-body water movements via the mechanosensory lateral line system, and this sense is used during several common fish behaviors, such as schooling, rheotaxis and predator-prey interactions. In addition, many fish behaviors, such as aggressive lateral displays and reproductive body quivers, involve fin and body motions that generate water movements that can be detected by the lateral line system of nearby fish. This mechanosensory system is well studied for its role in obstacle avoidance and detection of inadvertent hydrodynamic cues generated during schooling and predator-prey interactions; however, little research has focused on the role of mechanosensory communication during social interactions. Here, we summarize the current literature on the use of mechanosensation-mediated behaviors during agonistic and reproductive encounters, as well as during parental care. Based on these studies, we hypothesize that mechanosensory signaling is an important but often overlooked mode of communication during conspecific social interactions in many fish species, and we highlight its importance during multimodal communication. Finally, we suggest potential avenues of future research that would allow us to better understand the role of mechanosensation in fish communication.

Cobalt Chloride Treatment Used to Ablate the Lateral Line System Also Impairs the Olfactory System in Three Freshwater Fishes.

Fishes use multimodal signals during both inter- and intra-sexual displays to convey information about their sex, reproductive state, and social status. These complex behavioral displays can include visual, auditory, olfactory, tactile, and hydrodynamic signals, and the relative role of each sensory channel in these complex multi-sensory interactions is a common focus of neuroethology. The mechanosensory lateral line system of fishes detects near-body water movements and is implicated in a variety of behaviors including schooling, rheotaxis, social communication, and prey detection. Cobalt chloride is commonly used to chemically ablate lateral line neuromasts, thereby eliminating water-movement cues to test for mechanosensory-mediated behavioral functions. However, cobalt acts as a nonspecific calcium channel antagonist and could potentially disrupt function of all superficially located sensory receptor cells, including those for chemosensing. Here, we examined whether CoCl2 treatment used to ablate the lateral line system also impairs olfaction in three freshwater fishes, the African cichlid fish Astatotilapia burtoni, goldfish Carassius auratus, and the Mexican blind cavefish Astyanax mexicanus. To examine the impact of CoCl2 on the activity of peripheral receptors, we quantified DASPEI fluorescence intensity of the olfactory epithelium from fish exposed to control and CoCl2 solutions. In addition, we examined brain activation in olfactory processing regions of A. burtoni immersed in either control or cobalt solutions. All three species exposed to CoCl2 had decreased DASPEI staining of the olfactory epithelium, and in A. burtoni, cobalt treatment caused reduced neural activation in olfactory processing regions of the brain. To our knowledge this is the first empirical evidence demonstrating that the same CoCl2 treatment used to ablate the lateral line system also impairs olfactory function. These data have important implications for the use of CoCl2 in future research and suggest that previous studies using CoCl2 should be reinterpreted in the context of both impaired mechanoreception and olfaction.