Hydrodynamic contributions to multimodal guidance of prey capture behavior in fish

Physiological responses of mechanosensory systems in the head of larval zebrafish (Danio rerio)

The lateral line system of zebrafish consists of the anterior lateral line, with neuromasts distributed on the head, and the posterior lateral line, with neuromasts distributed on the trunk. The sensory afferent neurons are contained in the anterior and posterior lateral line ganglia, respectively. So far, the vast majority of physiological and developmental studies have focused on the posterior lateral line. However, studies that focus on the anterior lateral line, especially on its physiology, are very rare. The anterior lateral line involves different neuromast patterning processes, specific distribution of synapses, and a unique role in behavior. Here, we report our observations regarding the development of the lateral line and analyze the physiological responses of the anterior lateral line to mechanical and water jet stimuli. Sensing in the fish head may be crucial to avoid obstacles, catch prey, and orient in water current, especially in the absence of visual cues. Alongside the lateral line, the trigeminal system, with its fine nerve endings innervating the skin, could contribute to perceiving mechanosensory stimulation. Therefore, we compare the physiological responses of the lateral line afferent neurons to responses of trigeminal neurons and responsiveness of auditory neurons. We show that anterior lateral line neurons are tuned to the velocity of mechanosensory ramp stimulation, while trigeminal neurons either only respond to mechanical step stimuli or fast ramp and step stimuli. Auditory neurons did not respond to mechanical or water jet stimuli. These results may prove to be essential in designing underwater robots and artificial lateral lines, with respect to the spectra of stimuli that the different mechanosensory systems in the larval head are tuned to, and underline the importance and functionality of the anterior lateral line system in the larval fish head.

A multisensory perspective on near-field detection and localization of hydroacoustic sourcesa)

This paper gives a brief synopsis of the research career of S.C. in fish bioacoustics with an emphasis on dipole near fields. The hydroacoustic nature of the dipole near field and the effective stimuli to lateral line and auditory systems combine to produce a multisensory, range-fractionated region that is critically important to many fish behaviors. The mottled sculpin and goldfish lateral lines encode the spatial complexities of the near field as spatial excitation patterns along the body surface to provide instantaneous snapshots of various source features such as distance, orientation, and direction of movement. In contrast, the pressure-sensitive channel of the goldfish auditory system [the anterior swim bladder (SB)-saccule complex] encodes the spatial complexities in a temporal fashion whenever the position or orientation of the source changes with respect to the anterior SB. A full appreciation for how these somatotopic and egocentric representations guide fish behavior requires an understanding of how multisensory information, including vision, is combined in sensorimotor regions of the brain to effect behavior. A brief overview of vertebrate brain organization indicates that behaviors directed to or away from hydroacoustic sources likely involve a variety of mechanisms, behavioral strategies, and brain regions.

Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish

All animals need to differentiate between exafferent stimuli, which are caused by the environment, and reafferent stimuli, which are caused by their own movement. In the case of mechanosensation in aquatic animals, the exafferent inputs are water vibrations in the animal's proximity, which need to be distinguishable from the reafferent inputs arising from fluid drag due to locomotion. Both of these inputs are detected by the lateral line, a collection of mechanosensory organs distributed along the surface of the body. In this study, we characterize in detail how hair cells-the receptor cells of the lateral line-in zebrafish larvae discriminate between such reafferent and exafferent signals. Using dye labeling of the lateral line nerve, we visualize two parallel descending inputs that can influence lateral line sensitivity. We combine functional imaging with ultra-structural EM circuit reconstruction to show that cholinergic signals originating from the hindbrain transmit efference copies (copies of the motor command that cancel out self-generated reafferent stimulation during locomotion) and that dopaminergic signals from the hypothalamus may have a role in threshold modulation, both in response to locomotion and salient stimuli. We further gain direct mechanistic insight into the core components of this circuit by loss-of-function perturbations using targeted ablations and gene knockouts. We propose that this simple circuit is the core implementation of mechanosensory reafferent suppression in these young animals and that it might form the first instantiation of state-dependent modulation found at later stages in development.

Optimal Sensor Placement of the Artificial Lateral Line for Flow Parametric Identification

The multi-sensor artificial lateral line system (ALLS) can identify the flow-field’s parameters to realize the closed-loop control of the underwater robotic fish. An inappropriate sensor placement of ALLS may result in inaccurate flow-field parametric identification. Therefore, this paper proposes a method to optimize the sensor placement configuration of the ALLS, which mainly included three algorithms, the feature importance algorithm based on mean and variance (MVF), the feature importance algorithm based on distance evaluation (DF), and the information redundancy (IR) algorithm. The optimal sensor placement performance selected by this method is verified by simulation. In addition, further experimental verification was conducted using the ALLS. Compared with the uniform sensor placement configuration mentioned in recent studies, the experimental results suggest that the optimal sensor placement method can achieve a more effective prediction of the flow-field parameters, therefore strengthening the underwater robotic fish’s perception and control function.

Canal neuromasts enhance foraging in zebrafish (Danio rerio)

Aquatic animals commonly sense flow using superficial neuromasts (SNs), which are receptors that extend from the body's surface. The lateral line of fishes is unique among these systems because it additionally possesses receptors, the canal neuromasts (CNs), that are recessed within a channel. The lateral line has inspired the development of engineered sensors and concepts in the analysis of flow fields for submersible navigation. The biophysics of CNs are known to be different from the SNs and thereby offer a distinct submodality. However, it is generally unclear whether CNs play a distinct role in behavior. We therefore tested whether CNs enhance foraging in the dark by zebrafish (Danio rerio), a behavior that we elicited with a vibrating rod. We found that juvenile fish, which have only SNs, bite at this rod at about one-third the rate and from as little as one-third the distance of adults for a high-frequency stimulus (50  <  f   <  100 Hz). We used novel techniques for manipulating the lateral line in adults to find that CNs offered only a modest benefit at a lower frequency (20 Hz) and that foraging was mediated entirely by cranial neuromasts. Consistent with our behavioral results, biophysical models predicted CNs to be more than an order of magnitude more sensitive than SNs at high frequencies. This enhancement helps to overcome the rapid spatial decay in high-frequency components in the flow around the stimulus. These findings contrast what has been previously established for fishes that are at least ten-times the length of zebrafish, which use trunk CNs to localize prey. Therefore, CNs generally enhance foraging, but in a manner that varies with the size of the fish and its prey. These results have the potential to improve our understanding of flow sensing in aquatic animals and engineered systems.

Push and pull of downstream moving juvenile sea lamprey (Petromyzon marinus) exposed to chemosensory and light cues

Visual and olfactory stimuli induce behavioural responses in fishes when applied independently, but little is known about how simultaneous exposure influences behaviour, especially in downstream migrating fishes. Here, downstream moving juvenile sea lamprey (Petromyzon marinus) were exposed to light and a conspecific chemosensory alarm cue in a flume and movement were monitored with overhead cameras and nets. When exposed to light, sea lamprey were more likely to be captured in a net closest to the light array. When exposed to the alarm cue, sea lamprey transit rate through the flume increased, but sea lamprey did not avoid the alarm cue plume by moving perpendicular to flow. When the alarm cue and light were applied simultaneously in a push and pull configuration, the alarm cue still triggered enhanced downstream movement (push downstream) and more sea lamprey was still captured in the net nearest the light (pull to the side), resulting in twice as many sea lamprey being captured in the lighted net relative to controls. To our knowledge, this is the first study using multiple sensory cues in a push-pull configuration to modulate fish outmigration. Push and pull of juvenile sea lamprey with sensory cues could be useful to reduce turbine entrainment where native and enhance trap catch where invasive.

Functional diversity of the lateral line system among populations of a native Australian freshwater fish

Fishes use their mechanoreceptive lateral line system to sense nearby objects by detecting slight fluctuations in hydrodynamic motion within their immediate environment. Species of fish from different habitats often display specialisations of the lateral line system, in particular the distribution and abundance of neuromasts, but the lateral line can also exhibit considerable diversity within a species. Here, we provide the first investigation of the lateral line system of the Australian western rainbowfish (Melanotaenia australis), a species that occupies a diversity of freshwater habitats across semi-arid northwest Australia. We collected 155 individuals from eight populations and surveyed each habitat for environmental factors that may contribute to lateral line specialisation, including water flow, predation risk, habitat structure and prey availability. Scanning electron microscopy and fluorescent dye labelling were used to describe the lateral line system in M. australis, and to examine whether the abundance and arrangement of superficial neuromasts (SNs) varied within and among populations. We found that the SNs of M. australis were present in distinct body regions rather than lines. The abundance of SNs within each body region was highly variable, and also differed among populations and individuals. Variation in SN abundance among populations was best explained by habitat structure and the availability of invertebrate prey. Our finding that specific environmental factors explain among-population variation in a key sensory system suggests that the ability to acquire sensory information is specialised for the particular behavioural needs of the animal.

Analysis of cilia structure and function in zebrafish

Cilia are microtubule-based protrusions on the surface of most eukaryotic cells. They are found in most, if not all, vertebrate organs. Prominent cilia form in sensory structures, the eye, the ear, and the nose, where they are crucial for the detection of environmental stimuli, such as light and odors. Cilia are also involved in developmental processes, including left-right asymmetry formation, limb morphogenesis, and the patterning of neurons in the neural tube. Some cilia, such as those found in nephric ducts, are thought to have mechanosensory roles. Zebrafish proved very useful in genetic analysis and imaging of cilia-related processes, and in the modeling of mechanisms behind human cilia abnormalities, known as ciliopathies. A number of zebrafish defects resemble those seen in human ciliopathies. Forward and reverse genetic strategies generated a wide range of cilia mutants in zebrafish, which can be studied using sophisticated genetic and imaging approaches. In this chapter, we provide a set of protocols to examine cilia morphology, motility, and cilia-related defects in a variety of organs, focusing on the embryo and early postembryonic development.

Habitat-related specialization of lateral-line system morphology in a habitat-generalist and a habitat-specialist New Zealand eleotrid

An investigation of intraspecific habitat-related patterns of variation in oculoscapular lateral-line superficial neuromasts (SN) identified a decrease in the ratio of total SNs to pores, and a trend towards decreased asymmetry in SNs in the habitat-generalist common bully Gobiomorphus cotidianus from fluvial habitats compared to lacustrine habitats, suggesting habitat-related phenotypic variability. A greater ratio of pores to SNs, as well as less variation in the total number and asymmetry of SNs observed in the fluvial habitat-specialist redfin bully Gobiomorphus huttoni may provide further evidence of variations in the oculoscapular lateral-line morphology of fluvial habitat G. cotidianus individuals serving as adaptations to more turbulent environments.

Effects of Hatchery Rearing on the Structure and Function of Salmonid Mechanosensory Systems

This paper reviews recent studies on the effects of hatchery rearing on the auditory and lateral line systems of salmonid fishes. Major conclusions are that (1) hatchery-reared juveniles exhibit abnormal lateral line morphology (relative to wild-origin conspecifics), suggesting that the hatchery environment affects lateral line structure, perhaps due to differences in the hydrodynamic conditions of hatcheries versus natural rearing environments, and (2) hatchery-reared salmonids have a high proportion of abnormal otoliths, a condition associated with reduced auditory sensitivity and suggestive of inner ear dysfunction.

Flow sensing in developing Xenopus laevis is disrupted by visual cues and ototoxin exposure

We explored how lateral line cues interact with visual cues to mediate flow sensing behaviors in the nocturnal developing frog, Xenopus laevis, by exposing animals to current flows under different lighting conditions and after exposure to the ototoxin gentamicin. Under dark conditions, Xenopus tadpoles move downstream at the onset of current flow, then turn, and orient toward the direction of the flow with high accuracy. Postmetamorphic froglets also exhibit positive rheotaxis but with less accuracy and longer latency. The addition of discrete light cues to an otherwise dark environment disrupts rheotaxis and positioning. Orientation is less accurate, latency to orient is longer, and animals do not move as far downstream in the presence of light. Compared with untreated tadpoles tested in the dark, tadpoles exposed to gentamicin show less accurate rheotaxis with longer latency and do not move as far downstream in response to flow. These effects are compounded by the presence of light cues. The disruptive effects of light on flow sensing in Xenopus emphasize the disturbances to natural behaviors that may be produced by anthropogenic illumination in nocturnal habitats.

The lateral line receptor array of cyprinids from different habitats

The lateral line system of teleost fishes consists of an array of superficial and canal neuromasts (CN). Number and distribution of neuromasts and the morphology of the lateral line canals vary across species. We investigated the morphology of the lateral line system in four diurnal European cyprinids, the limnophilic bitterling (Rhodeus sericeus), the indifferent gudgeon (Gobio gobio), and ide (Leuciscus idus), and the rheophilic minnow (Phoxinus phoxinus). All fish had lateral line canals on head and trunk. The total number of both, CN and superficial neuromasts (SN), was comparable in minnow and ide but was greater than in gudgeon and bitterling. The ratio of SNs to CNs for the head was comparable in minnow and bitterling but was greater in gudgeon and ide. The SN-to-CN ratio for the trunk was greatest in bitterling. Polarization of hair cells in CNs was in the direction of the canal. Polarization of hair cells in SNs depended on body area. In cephalic SNs, hair cell polarization was dorso-ventral or rostro-caudal. In trunk SNs, it was rostro-caudal on lateral line scales and dorso-ventral on other trunk scales. On the caudal fin, hair cell polarization was rostro-caudal. The data show that, in the four species studied here, number, distribution, and orientation of CNs and SNs cannot be unequivocally related to habitat.

Fusion of locomotor maneuvers, and improving sensory capabilities, give rise to the flexible homing strikes of juvenile zebrafish

At 5 days post-fertilization and 4 mm in length, zebrafish larvae are successful predators of mobile prey items. The tracking and capture of 200 μm long Paramecia requires efficient sensorimotor transformations and precise neural controls that activate axial musculature for orientation and propulsion, while coordinating jaw muscle activity to engulf them. Using high-speed imaging, we report striking changes across ontogeny in the kinematics, structure and efficacy of zebrafish feeding episodes. Most notably, the discrete tracking maneuvers used by larval fish (turns, forward swims) become fused with prey capture swims to form the continuous, fluid homing strikes of juvenile and adult zebrafish. Across this same developmental time frame, the duration of feeding episodes become much shorter, with strikes occurring at broader angles and from much greater distances than seen with larval zebrafish. Moreover, juveniles use a surprisingly diverse array of motor patterns that constitute a flexible predatory strategy. This enhances the ability of zebrafish to capture more mobile prey items such as Artemia. Visually-guided tracking is complemented by the mechanosensory lateral line system. Neomycin ablation of lateral line hair cells reduced the accuracy of strikes and overall feeding rates, especially when neomycin-treated larvae and juveniles were placed in the dark. Darkness by itself reduced the distance from which strikes were launched, as visualized by infrared imaging. Rapid growth and changing morphology, including ossification of skeletal elements and differentiation of control musculature, present challenges for sustaining and enhancing predatory capabilities. The concurrent expansion of the cerebellum and subpallium (an ancestral basal ganglia) may contribute to the emergence of juvenile homing strikes, whose ontogeny possibly mirrors a phylogenetic expansion of motor capabilities.

Differences in lateral line morphology between hatchery- and wild-origin steelhead

Despite identification of multiple factors mediating salmon survival, significant disparities in survival-to-adulthood among hatchery- versus wild-origin juveniles persist. In the present report, we explore the hypothesis that hatchery-reared juveniles might exhibit morphological defects in vulnerable mechanosensory systems prior to release from the hatchery, potentiating reduced survival after release. Juvenile steelhead (Oncorhynchus mykiss) from two different hatcheries were compared to wild-origin juveniles on several morphological traits including lateral line structure, otolith composition (a proxy for auditory function), and brain weight. Wild juveniles were found to possess significantly more superficial lateral line neuromasts than hatchery-reared juveniles, although the number of hair cells within individual neuromasts was not significantly different across groups. Wild juveniles were also found to possess primarily normal, aragonite-containing otoliths, while hatchery-reared juveniles possessed a high proportion of crystallized (vaterite) otoliths. Finally, wild juveniles were found to have significantly larger brains than hatchery-reared juveniles. These differences together predict reduced sensitivity to biologically important hydrodynamic and acoustic signals from natural biotic (predator, prey, conspecific) and abiotic (turbulent flow, current) sources among hatchery-reared steelhead, in turn predicting reduced survival fitness after release. Physiological and behavioral studies are required to establish the functional significance of these morphological differences.

Morphological development of larval cobia Rachycentron canadum and the influence of dietary taurine supplementation

The morphological development of larval cobia Rachycentron canadum from 3 days post hatch (dph) until weaning (27 dph) was examined using S.E.M. Two groups of fish were studied: a control group (CF), reared under standard feeding protocol, and a group in which prey items were enriched with supplemental taurine (4 g l(-1) day(-1) ; TF). TF fish grew faster (P < 0·001), attained greater size (mean ±s.e. 55·1 ± 1·5 v. 33·9 ± 1·0 mm total length) and had better survival (mean ±s.e. 29·3 ± 0·4 v. 7·1 ± 1·2 %) than CF fish. Canonical variance analysis confirmed findings with respect to differences in growth between the treatment groups with separation being explained by two cranial measurements. S.E.M. revealed that 3 dph larvae of R. canadum (in both groups) possess preopercular spines, superficial neuromasts on the head and body, taste buds in the mouth, an olfactory epithelium which takes the form of simple concave depressions, and primordial gill arches. Gill filaments start to form as early as 6 dph and lamellae buds are visible at 8 dph in both groups. In CF fish, the cephalic lateral line system continues its development at 12-14 dph with invagination of both supra- and infraorbital canals. At the same time, a thorn-like or acanthoid crest forms above the eye. At 14 dph, invaginations of the mandibular and preopercular canals are visible and around 22 dph enclosure of all cranial canals nears completion. In CF larvae, however, completely enclosed cranial canals were not observed within the course of the trial, i.e. 27 dph. In TF larvae, grooves of the cephalic lateral line system form 4 days earlier than observed in CF larvae of R. canadum (i.e. at 8 dph), with enclosure commencing at 16 dph, and completed by 27 dph. Along the flanks of 6 dph larvae of either treatment, four to five equally spaced neuromasts delineate the future position of the trunk lateral line. As myomeres are added to the growing larvae, new neuromasts appear such that at 16 dph a neuromast is associated with each myomere. By 27 dph, the trunk lateral line starts to invaginate in CF larvae, while it initiates closure in TF larvae. These findings elucidate important features of the larval development of R. canadum and show that dietary taurine supplementation benefits larval development, growth and survival in this species. Moreover, they suggest a conditional requirement for taurine in larval R. canadum.

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

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

Sensory evolution: picking up good vibrations

A new study has identified the sensory basis for behavioral adaptation in cavefish, providing insight into the process of nervous system evolution.

Hydrodynamic object recognition using pressure sensing

Hydrodynamic sensing is instrumental to fish and some amphibians. It also represents, for underwater vehicles, an alternative way of sensing the fluid environment when visual and acoustic sensing are limited. To assess the effectiveness of hydrodynamic sensing and gain insight into its capabilities and limitations, we investigated the forward and inverse problem of detection and identification, using the hydrodynamic pressure in the neighbourhood, of a stationary obstacle described using a general shape representation. Based on conformal mapping and a general normalization procedure, our obstacle representation accounts for all specific features of progressive perceptual hydrodynamic imaging reported experimentally. Size, location and shape are encoded separately. The shape representation rests upon an asymptotic series which embodies the progressive character of hydrodynamic imaging through pressure sensing. A dynamic filtering method is used to invert noisy nonlinear pressure signals for the shape parameters. The results highlight the dependence of the sensitivity of hydrodynamic sensing not only on the relative distance to the disturbance but also its bearing.

ED. Behavioral avoidance of injured conspecific and predatory chemical stimuli by larvae of the aquatic caddisfly Hesperophylax occidentalis

Prey animals often encounter situations that hinder their ability to conduct normal fitness-enhancing behaviors. Mating and foraging are frequently interrupted by predator vigilance and avoidance, and antipredator behavior. Many caddisfly larvae build protective cases that are carried with them throughout the aquatic life cycle. However, they are still vulnerable to predation, yet it is unknown the extent caddisflies use chemical cues for predator recognition and avoidance. We exposed larval caddisfly Hesperophylax occidentalis (Banks, 1908) to predatory, conspecific, and heterospecific chemical cues to determine if caddisfly larvae can use chemical stimuli alone for predator recognition and avoidance. Exposure to predator and injured conspecific chemicals elicited significant decreases in activity, while exposure to injured and uninjured heterospecific chemicals yielded no significant change in activity. The extended latency to move following exposure to predator kairomones indicates larval caddisflies utilize chemical cues for predator recognition and avoidance, and a similar decrease in movement associated with exposure to injured conspecifics suggests the presence of a chemical alarm cue.

Escape responses in juvenile Atlantic cod Gadus morhua L.: the effects of turbidity and predator speed

We examined the effect of turbidity (0.5-14 beam attenuation m(-1)) and predator attack speed (150 and 296 cm s(-1)) on escape responses of juvenile cod Gadus morhua in the laboratory. We triggered escape responses using a predator model and measured escape timing, direction and locomotor performance. We also measured responsiveness and estimated the likelihood of fish escaping the ;predator attack' (putative escape success, PES). Turbidity affected both PES and the type of escape response used by the fish, but these effects depended on predator speed. PES for the fast predator attack declined from 73% in clear water to 21% in highly turbid water, due to decreased responsiveness and poorly timed escapes. Intermediate turbidity enhanced PES and responsiveness to the slow predator attack. Locomotor performance was reduced by turbidity, whereas predator speed had the opposite effect. Our results suggest that both predator attack speed and turbidity have important roles in determining the vulnerability of fish attacked by piscivorous predators.

Fiber connections of the diencephalic nucleus tuberis anterior in the weakly electric fish,Gymnotus cf. carapo: An in vivo tract-tracing study

Transport of biotinylated dextran amine shows the spatial segregation of mechanosensory afferents in the nucleus tuberis anterior (TA) of a gymnotiform fish, Gymnotus cf. carapo. Only the intermediate subdivision of this nucleus receives projections from the lateral region of the ventral torus semicircularis (TSv), which represents the principal midbrain center for mechanosensory information processing, and from the ventral nucleus praeeminentialis, which receives collaterals of ascending second order mechanosensory fibers that emerge from the mechanosensory lateral line lobe. Considering this aspect, a rostrocaudal subdivision of the TA is proposed. The TA also receives input from regions subserving other sensory modalities, suggesting a role in multisensory interaction. Another important finding of this work consisted in the demonstration of reciprocal connections between the TA and the inferior lobe of the hypothalamus, which is known to receive gustatory, visual, and electrosensory input and is therefore considered a multisensory integration center involved in feeding and aggressive behavior. Furthermore, reciprocal connections between the TA and the preelectromotor central-posterior/prepacemaker complex may provide an access for the processed mechanosensory information to interact with the transient modulations of the electric organ discharge that accompany different behaviors.

Distant touch hydrodynamic imaging with an artificial lateral line

Nearly all underwater vehicles and surface ships today use sonar and vision for imaging and navigation. However, sonar and vision systems face various limitations, e.g., sonar blind zones, dark or murky environments, etc. Evolved over millions of years, fish use the lateral line, a distributed linear array of flow sensing organs, for underwater hydrodynamic imaging and information extraction. We demonstrate here a proof-of-concept artificial lateral line system. It enables a distant touch hydrodynamic imaging capability to critically augment sonar and vision systems. We show that the artificial lateral line can successfully perform dipole source localization and hydrodynamic wake detection. The development of the artificial lateral line is aimed at fundamentally enhancing human ability to detect, navigate, and survive in the underwater environment.

Second-order projection from the posterior lateral line in the early zebrafish brain

Background

Mechanosensory information gathered by hair cells of the fish lateral-line system is collected by sensory neurons and sent to the ipsilateral hindbrain. The information is then conveyed to other brain structures through a second-order projection. In the adult, part of the second-order projection extends to the contralateral hindbrain, while another part connects to a midbrain structure, the torus semicircularis.

Results

In this paper we examine the second-order projection from the posterior lateral-line system in late embryonic/early larval zebrafish. At four days after fertilization the synaptic field of the sensory neurons can be accurately targeted, allowing a very reproducible labeling of second-order neurons. We show that second-order projections are highly stereotyped, that they vary according to rhombomeric identity, and that they are almost completely lateralized. We also show that the projections extend not only to the contralateral hindbrain and torus semicircularis but to many other brain centers as well, including gaze- and posture-controlling nuclei in the midbrain, and presumptive thalamic nuclei.

Conclusion

We propose that the extensive connectivity observed in early brain development reveals a basic scaffold common to most vertebrates, from which different subsets are later reinforced in various vertebrate groups. The large repertoire of projection targets provides a promising system to study the genetic encoding of this differential projection capacity.

Hunting archer fish match their take-off speed to distance from the future point of catch

Archer fish can shoot down insect prey with a sharp jet of water. Fish usually fire from positions that are not directly below their target so that a dislodged insect falls ballistically with a horizontal velocity component. Only 100 ms after the insect is on its path both the shooter and other school members can initiate a rapid turn and then head straight in the direction of the later point of impact of their falling prey. The quick turn and subsequent take-off are performed ;open-loop', based on the initial values of the falling insect's motion. We report here that archer fish can not only take off in the direction of the later point of impact but also predict its distance. Distance information allows the fish to adjust their take-off speed so that they would arrive within a narrow time slot slightly (about 50 ms) after their prey's impact, despite large differences in the size of the aligning turn and in the distance to be covered. Selecting a constant speed program with matched speed and catching the insect on the move minimizes frictional losses. The initial speed of starting fish is slightly but systematically too slow and is increased later so that the fish arrive 20 ms earlier than expected and often make the catch on a higher than take-off speed. The variability of later speed changes suggests a systematic ;error' in the take-off, as if the fish underestimated distance. However, this apparent deficiency seems well adapted to the fish catching their prey at a high speed: if later the fish had no possibility to correct an initial error then it is better to start slightly too slow in order to minimize the risk of overshooting the point of catch.

Prey-capture in the African clawed toad (Xenopus laevis): comparison of turning to visual and lateral line stimuli

Separately delivered visual and lateral line stimuli elicit similar but not identical orientation and approach by intact, sighted Xenopus. Response frequencies for visual stimuli declined sharply for distant or caudal stimuli while those for lateral line stimuli changed little. Turn angles correlated highly with stimulus angles but were smaller on average, so regression slopes were less than one. Regression slopes were smaller for visual than for lateral line stimuli, but this apparent difference was due to different distributions of stimulus distance interacting with the toad's rotation center. Errors in final headings, most often under-rotations, did not differ by modality. Frequencies of lunges and arm capture movements were higher for visual stimuli both overall and especially for rostral proximal stimuli. The results demonstrate accurate orientation by sighted Xenopus to visual and lateral line stimuli; they are consistent with expectations based on in-register tectal maps. Orientation to lateral line stimuli is similar to previous results with blinded animals, revealing no heightened acuity in the latter. Modality differences indicate that the lateral line system is better for omnidirectional orientation and approach to distant stimuli whereas the visual system is more attuned to nearby rostral stimuli and more apt to mediate strikes.

Sensitivity of the anterior lateral line to natural stimuli in the oyster toadfish, Opsanus tau (Linnaeus)

Inductive neural telemetry was used to record from microwire electrodes chronically implanted into the anterior lateral line nerve of the oyster toadfish, Opsanus tau (L.). The lateral lines of free-ranging toadfish were stimulated by the swimming movements of a prey fish (Fundulus heteroclitus), and the corresponding neural activity was quantified. Both spontaneously active and silent afferent fibers experienced an increase in neural firing as the prey approached the lateral line. Activity was evoked when the prey fish approached to within 8-12 cm of the neuromast, with increases in nerve firing rates directly correlated with diminishing distance. Thus, adult toadfish (28 cm standard length; 33 cm total length) were only able to detect mobile prey that approached within approximately 40% of their body length. Both spontaneously active and silent afferent fibers also experienced a dramatic increase in firing during predatory strikes, indicating that the fibers were not inhibited during rapid body movement. This study investigates, for the first time, the neural response of the anterior lateral line to prey stimuli in free-ranging fish.

Coordination of feeding, locomotor and visual systems in parrotfishes(Teleostei: Labridae)

Fishes require complex coordinated motions of the jaws, body and fins during feeding in order to successfully execute the strike or bite and then move away from the predation site. In conjunction with locomotor systems,sensory modalities guide coordinated feeding behavior, with vision playing an important role in many fishes. Although often studied separately, the locomotor, feeding and visual systems have not previously been examined together during fish feeding. To explore feeding coordination, we examined the kinematics of feeding behavior in two species of herbivorous parrotfish, Sparisoma radians and Scarus quoyi, which exhibit different single bite and repetitive bite strategies. Kinematic data on pectoral fin movements and body position show distinctive differences in strategies for the approach and post-strike motion between these species. Sparisoma and Scarus exhibited significant differences in the magnitude of jaw protrusion, time to maximum jaw protrusion, cranial elevation, and order of events in the feeding sequence. Oculomotor data show that both species orient the pupil forward and downward directed at the site of jaw contact until 100 ms before the bite, at which point the visual field is rotated laterally. Combinations of kinematic variables show repeated patterns of synchrony (onset and duration) for the approach to the food (distance, velocity, eye movement),prey capture (eye movement, jaw movement, fin movement) and post-capture maneuvering (fin movement, distance). Kinematic analyses of multiple functional systems reveal coordination mechanisms for detecting and approaching prey and executing the rapid opening and closing of the jaws during acquisition of food. Comparison of the coordination of feeding,swimming and sensory systems among fish species can elucidate alternative coordination strategies involved in herbivory in coral reef fishes.

Prey tracking by larval zebrafish: axial kinematics and visual control

High-speed imaging was used to record the prey-tracking behavior of larval zebrafish as they fed upon paramecium. Prey tracking is comprised of a variable set of discrete locomotor movements that together align the larva with the paramecium and bring it into close proximity, usually within one body length. These tracking behaviors are followed by a brief capture swim bout that was previously described [Borla et al., 2002]. Tracking movements were classified as either swimming or turning bouts. The swimming bouts were similar to a previously characterized larval slow swim [Budick and O'Malley, 2000], but the turning movements consisted of unique J-shaped bends which appear to minimize forward hydrodynamic disturbance when approaching the paramecium. Such J-turn tracking bouts consisted of multiple unilateral contractions to one side of the body. J-turns slowly and moderately alter the orientation of the larva - this is in contrast to previously described escape and routine turns. Tracking behaviors appear to be entirely visually guided. Infra-red (IR) imaging of locomotor behaviors in a dark environment revealed a complete absence of tracking behaviors, even though the normal repertoire of other locomotive behaviors was recorded. Concomitantly, such larvae were greatly impaired in consuming paramecia. The tracking behavior is of interest because it indicates the presence of sophisticated locomotor control circuitry in this relatively simple model organism. Such locomotor strategies may be conserved and elaborated upon by other larval and adult fishes.

Sensory integration in the hydrodynamic world of rainbow trout

Water movements, of both abiotic and biotic origin, provide a wealth of information for fishes. They detect these water movements by arrays of hydrodynamic sensors located on the surface of the body as superficial neuromasts and embedded in subdermal lateral line canals. Recently, the anatomical dichotomy between superficial and canal neuromasts has been matched by demonstrations of a corresponding functional dichotomy. Superficial neuromasts are sensitive to water flows over the surface of the fish and are the sub-modality that participates in orientation to water currents, a behaviour known as rheotaxis. The canal neuromasts are sensitive to water vibration and it is this sub-modality that determines the localization of artificial prey. Recently, however, it has been shown that the complex behaviour of natural prey capture in the dark requires input from both lateral line sensory submodalities and here we show that the ability of trout to hold station behind a stationary object in fast flowing water also requires integration of information from both sub-modalities.