Ghost Knifefish Research Articles

The effect of extracellular potassium concentration on the oscillation frequency of the pacemaker nucleus in the weakly electric fish Apteronotus leptorhynchus.

The weakly electric brown ghost knifefish (Apteronotus leptorhynchus) exhibits a pronounced sexual dimorphism in its electric behavior-males discharge at higher frequencies than females, with little overlap between the sexes. The frequency of these electric organ discharges is controlled by the frequency of the synchronized oscillations of the medullary pacemaker nucleus. Previous studies have suggested that sex-specific differences in the morphology and gene expression pattern of the astrocytic syncytium that envelopes the pacemaking neural network cause differences in its capacity to buffer the extracellular concentration of K+. This change in the K+ buffering capacity affects the K+ equilibrium potential of the neurons constituting the neural network, which in turn modulates the frequency of the pacemaker nucleus. In the present study, we have tested a critical element of this hypothesis by examining whether, and how, changes in the extracellular K+ concentration influence the frequency of the pacemaker nucleus oscillations. By using an in vitro preparation of the pacemaker nucleus, the results of this investigation demonstrate that exposure of this nucleus to acutely increased/decreased concentrations of K+ in the perfusate (while maintaining osmolarity) leads to concentration-dependent increases/decreases in the frequency of the synchronized oscillations generated by the pacemaker nucleus.

Effect of Dexamethasone on Tail Regeneration in the Electric Black Ghost Knifefish Apteronotus albifrons (Linnaeus, 1766)

The study was conducted to find out the impact of dexamethasone of different doses on tail regeneration in the electric black ghost knifefish Apteronotus albifrons (Linnaeus, 1766) after two weeks of amputation. Fish were distributed into five equal groups; Dexamethasone drug was applied at concentrations of 0, 1, 2, 3 and 4 mg.l-1. Water was exchanging every 48 hours with new prepared drug to ensure its stability in treatments. Concentrations 1, 2 and 3 mg.l-1 were not lethal , while the concentration 4 mg.l-1 was lethal, It was observed that fish were dead after four days for concentration 4 mg.l-1.The study showed that the group of control was increased considerably (P < 0.05) (4.5mm± 0.866) in the tail after 14 days of amputation in comparing with treated groups. Treated groups (1, 2 and 3 mg.l-1) showed slowly increase with an average 2.83mm ±0.763, 2.50mm ±0.500, and 2.33mm ±0.577 respectively. The mean of relative gene expression of the junb was 1.109 in control group that is significant (P < 0.05) compared with treated groups that showed a decrease in junb expression (0.074 for 1 mg.l-1, 0.050 for 2 mg.l-1 and 0.006 for 3 mg.l-1). Current study concludes that dexamethasone inhibit tissues regeneration after amputation via suppress stem cells growth, and junb gene specific for stem cells enhancing and the drug reduced its expression.

A CPG-based framework for flexible locomotion control and propulsion performance evaluation of underwater undulating fin platform

The black ghost knifefish is considered to possess central pattern generators (CPGs), which generates rhythms in neural circuits, coordinating the deformation of its elongated fin to achieve efficient and agile locomotion. Current biomimetic robots imitating the locomotion of the black ghost knifefish and using an undulating propulsion control method face significant challenges in adapting their swimming gaits to different environments and tasks. To overcome this limitation, this study conducted biomimetic robot research that extended from morphology to neurobiology and mimicked the CPG to construct a unified framework based on coupled Hopf oscillators. Meanwhile, an amplitude mapping function and a novel coupling method for the CPG-based control framework are proposed. The advantages of the proposed control framework are the ability to modulate different control parameters and replicate different swimming gaits, including forward, hovering, and backward swimming, realising seamless gait transitions. The control framework was tested on a specially designed undulating fin platform to evaluate the propulsion performance by modulating the control parameters, including amplitude, frequency, and phase difference. The experimental results demonstrate that the proposed CPG-based control framework achieves multimodal locomotion, enabling rapid and smooth transitions between swimming gaits, thus enhancing the robot's adaptability and stability in variable swimming environments.

Hydrodynamic performance of a biomimetic undulating fin robot under different water conditions

Inspired by the agile swimming behavior of the Black Ghost Knife Fish utilizing undulating propulsion in complex aquatic environments, this study investigates the locomotion principles and hydrodynamic performance of an undulating propulsion system under varying hydrodynamic conditions through numerical simulations and experiments. The influence of the fin aspect ratio on the thrust performance of the robot was explored, revealing a linear correlation between the aspect ratio and thrust generation. Furthermore, based on an aspect ratio of 0.3, the effects of frequency, amplitude, and wavenumber on the robot’s swimming performance were examined. The results indicated a quadratic relationship between frequency and thrust, a linear relationship between amplitude and thrust, and a trend where thrust initially increased and then decreased with wavenumber, reaching an optimal level around wavenumber 1. Using numerical simulations and experimental investigations, the thrust performance of the robot under wave conditions demonstrated a noticeable reduction in hydrodynamic performance compared to still water conditions. The effects of frequency, amplitude, and wavelength on thrust remained consistent with still water conditions, but the instantaneous thrust became unstable. The current conclusions provide valuable insights for designing and optimizing undulating propulsion robots, offering systematic guidance for selecting appropriate motion parameters to accomplish different tasks.

Inspired by the Black Ghost Knifefish: Bionic Design of Undulatory Fin with 2-DOF Rays and Its Propulsion Performance

The demand for high-performance underwater thrusters in marine engineering is increasing. The concealed, mobile, and efficient underwater ability of fish provides many directions for research. The black ghost knifefish uses only wavy ventral fins to swim and can hover and roll in the water. Based on the physiological and morphological characteristics of the black ghost knifefish, we explored the structure and movement mode of the ventral fin, so as to establish a two-degree of freedom (2-DOF) structural model and kinematic model. We reveal the motion mechanism of the undulating fin propulsion through the constructed model and computational fluid dynamics. It is found that when the fin surface fluctuates, a pair of vortices with opposite directions will be formed on the concave side of the fin surface. These vortices will produce a central jet on the fin surface, provide a reverse impulse for the ventral fin, and make the fin obtain power. In addition, we found that the propulsive force of the ribbon fin along the body direction is positively correlated with the swing amplitude and frequency of the fin movement, and the propulsive torque of the ribbon fin to realize the maneuvering movement increases first and then decreases with the increase of the torsion angle. The research on the structure and motion mechanism of the ribbon fin of the black ghost knifefish provides a basis for the development of a bionic prototype of multi-DOF motion and the control strategy of high-mobility motion.

Knifefish turning control and hydrodynamics during forward swimming.

Rapid turning and swimming contribute to ecologically important behaviors in fishes such as predator avoidance, prey capture, mating and the navigation of complex environments. For riverine species, such as knifefishes, turning behaviors may also be important for navigating locomotive perturbations caused by turbulent flows. Most research on fish maneuvering focuses on fish with traditional fin and body morphologies, which primarily use body bending and the pectoral fins during turning. However, it is uncertain how fishes with uncommon morphologies are able to achieve sudden and controllable turns. Here, we studied the turning performance and the turning hydrodynamics of the black ghost knifefish (Apteronotus albifrons, N=6) which has an atypical elongated ribbon fin. Fish were filmed while swimming forward at ∼2 body lengthss-1 and feeding from a fixed feeder (control) and an oscillating feeder (75 Hz) at two different amplitudes. 3D kinematic analysis of the body revealed the highest pitch angles and lowest body bending coefficients during steady swimming. Low pitch angle, high maximum yaw angles and large body bending coefficients were characteristic of small and large turns. Asynchrony in pectoral fin use was low during turning; however, ribbon fin wavelength, frequency and wave speed were greatest during large turns. Digital particle image velocimetry (DPIV) showed larger counter-rotating vortex pairs produced during turning by the ribbon fin in comparison to vortices rotating in the same direction during steady swimming. Our results highlight the ribbon fin's role in controlled rapid turning through modulation of wavelength, frequency and wave speed.

Systematics of Neotropical electric knifefish Tembeassu (Gymnotiformes, Apteronotidae)

A new species of the poorly known and critically endangered ghost knifefish Tembeassu is described from the upper Paraná and Araguari rivers, Brazil, using external anatomy and X-ray microcomputed tomography (µCT scan). Tembeassu titanicus sp. nov. is distinguished from its sole congener, T. marauna, by a unique set of morphometric and meristic characters, in addition to the absence of a tooth patch at the anterior portion of the roof of the oral cavity and the external corner of the mouth slightly passing the vertical through the posterior margin of the posterior nare. To test the monophyly of Tembeassu and reassess its phylogenetic position, a total-evidence approach was performed through a Bayesian inference (BI) and maximum-parsimony analysis (MP). Our results indicate that Tembeassu is monophyletic and the sister taxon of a clade formed by Apteronotus s.s., Megadontognathus, and Parapteronotus (BI); or as part of a large polytomy at the base of Apteronotidae (MP). Species of Tembeassu co-occur in the Paraná River basin, and the absence of the patch of accessory teeth in T. titanicus sp. nov. may indicate that this species accesses a different food resource, and also putatively occupies a different habitat than T. marauna. Comments on the evolution of the mandibular lobe in Gymnotiformes, dentition pattern in Tembeassu, and apteronotid diversity in the Paraná River are provided. http://zoobank.org/urn:lsid:zoobank.org:act:691CB9A5-5CDB-4BF9-8A6FEAE877563C12

The Description of a Rare and Critically Endangered Species of Ghost Knifefish from the Amazon Basin (Ostariophysi: Gymnotiformes: Apteronotidae)

A new species of ghost knifefish is described from the Rio Uatumã at Cachoeira do Miriti, Amazon basin, Brazil. It is distinguished from all species of Apteronotus by the absence of a clear stripe from the chin to the dorsal portions of the head and/or dorsum, a reduced number of anal-fin rays (118–122), and by having four branchiostegal rays. More specifically, the new species resembles Apteronotus quilombola by the low number of anal-fin rays; however, the number of branchiostegal rays and several additional counts and measurements differentiate the species. Furthermore, the new species co-occurs with A. lindalvae, and it can be distinguished by the number of premaxillary teeth. The occurrence area of the new species was highly impacted after the construction of Balbina dam. Thus, considering the threats to the species and the restricted area of distribution, its conservation status was assessed as critically endangered.

SPAWNING OF BLACK GHOST KNIFEFISH, Apteronotus albifrons WITH DIFFERENT SEX RATIOS

Black ghost knifefish, Apteronotus albifrons, is a South America-introduced ornamental fish species that has been widely cultivated in Indonesia. Some farmers breed this fish with different sex ratios, but the optimum sex ratio remains unclear. This study aimed to evaluate the spawning behavior and reproductive performance of black ghost knifefish with different sex ratios. The treatments in this study were arranged in a completely randomized design consisting of different sex ratios between males and females, namely: A (one male : three females), B (two males : three females), and C (three males : three females). Each treatment was done in triplicate. The broodstock were maintained in an aquarium (80 cm x 40 cm x 40 cm) and fed with bloodworm twice a day. The water was changed every day as much as 60% of the total volume. During the experiment, the parameters of spawning behavior, number of fish spawning, number of eggs, fertilization rate, hatching rate, and daily spawning frequencies were observed. The observation was done for seven days. The study results showed that black ghost knifefish spawned at night (11 pm - 2 am). The mating and spawning occurred between one male and one female. Competition between males was observed in the treatments indicated by aggressive movements of a male toward the others, such as sudden approaching, chasing, and driving away the others. Fish in treatment-B spawned consistently from day-1 to day-5. Fish in treatment-A spawned from day-2 to day-4, while fish in treatment-C spawned only on day-7. The reproductive performance parameters showed no significant difference in all treatments except the hatching rate parameter of treatment-B. The study concludes that better reproductive performance of black ghost knifefish, A. albifrons can be achieved with a spawning ratio of two males and three females. Further research on individual and mass spawning methods with the best spawning ratio of the fish is required.

Beyond the Kármán gait: knifefish swimming in periodic and irregular vortex streets.

Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over the pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U∞≈1-5 BL s-1) and in periodic vortex streets (U∞≈2-4 BL s-1). Two different cylinders (∼2 and ∼3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (∼2 cm diameter; U∞≈2 BL s-1). In laminar flows, knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U∞. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins, which resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending and ribbon and pectoral fin movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder, but when placed one cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

The weakly electric fish, Apteronotus albifrons, actively avoids experimentally induced hypoxia

Anthropogenic environmental degradation has led to an increase in the frequency and prevalence of aquatic hypoxia (low dissolved oxygen concentration, DO), which may affect habitat quality for water-breathing fishes. The weakly electric black ghost knifefish, Apteronotus albifrons, is typically found in well-oxygenated freshwater habitats in South America. Using a shuttle-box design, we exposed juvenile A. albifrons to a stepwise decline in DO from normoxia (> 95% air saturation) to extreme hypoxia (10% air saturation) in one compartment and chronic normoxia in the other. On average, A. albifrons actively avoided the hypoxic compartment below 22% air saturation. Hypoxia avoidance was correlated with upregulated swimming activity. Following avoidance, fish regularly ventured back briefly into deep hypoxia. Hypoxia did not affect the frequency of their electric organ discharges. Our results show that A. albifrons is able to sense hypoxia at non-lethal levels and uses active avoidance to mitigate its adverse effects.

A new species of sexually dimorphic and rheophilic ghost knifefish (Apteronotidae: Gymnotiformes) from the Amazon basin.

A new species of ghost knifefish, Apteronotus, is described from high-energy environments in the Rios Mapuera and Trombetas (at Cachoeira Porteira waterfalls), Brazil. X-ray microcomputed tomography (μCT scan) was used to access the internal anatomy of the type series. The new species is distinguished from all congeners by the anteriormost position of the anus, with its posterior margin extending less than one eye diameter beyond the vertical through the caudal limit of the posterior nostril, the low number of anal-fin rays (117-125) and the reduced number of branchiostegal rays (three). A series of modifications associated with secondary sexual dimorphism on the preorbital region of mature males are depicted and discussed. In addition, comments on homologies of the branchiostegal rays in Apteronotidae are provided.

Simultaneous spike-time locking to multiple frequencies.

Locking of neural firing is ubiquitously observed in the brain and occurs when neurons fire at a particular phase or in synchronization with an external signal. Here we study in detail the locking of single neurons to multiple distinct frequencies at the example of p-type electroreceptor afferents in the electrosensory system of the weakly electric fish Apteronotus leptorhynchus (brown ghost knifefish). We find that electrosensory afferents and pyramidal cells in the electrosensory lateral line lobe (ELL) lock to multiple frequencies, including the electric organ discharge (EOD) frequency, beat, and stimulus itself. We identify key elements necessary for locking to multiple frequencies, study its limits, and provide concise mathematical models reproducing our main findings. Our findings provide another example of how rate and temporal codes can coexist and complement each other in single neurons and demonstrate that sensory coding in p-type electroreceptor afferents provides a much richer representation of the sensory environment than commonly assumed. Since the underlying mechanisms are not specific to the electrosensory system, our results could provide the basis for studying multiple frequency locking in other systems.NEW & NOTEWORTHY Locking of neuronal spikes to external and internal signals is a ubiquitous neurophysiological mechanism that has been extensively studied in several brain areas and species. Using experimental data from the electrosensory system and concise mathematical models, we analyze how a single neuron can simultaneously lock to multiple frequencies. Our findings demonstrate how temporal and rate codes can complement each other and lead to rich neuronal representations of sensory signals.

A redescription of deep-channel ghost knifefish, Sternarchogiton preto (Gymnotiformes: Apteronotidae), with assignment to a new genus

ABSTRACT From a study of morphological and molecular datasets we determine that a species originally described as Sternarchogiton preto does not form a monophyletic group with the other valid species of Sternarchogiton including the type species, S. nattereri. Previously-published phylogenetic analyses indicate that this species is sister to a diverse clade comprised of six described apteronotid genera. We therefore place it into a new genus diagnosed by the presence of three cranial fontanels, first and second infraorbital bones independent (not fused), the absence of an ascending process on the endopterygoid, and dark brown to black pigments over the body surface and fins membranes. We additionally provide a redescription of this enigmatic species with an emphasis on its osteology, and provide the first documentation of secondary sexual dimorphism in this species.

Anatomical, taxonomic, and phylogenetic reappraisal of a poorly known ghost knifefish, Tembeassu marauna (Ostariophysi: Gymnotiformes), using X-ray microcomputed tomography.

A detailed osteological study of the poorly known and critical endangered ghost knifefish, Tembeassu marauna, from the rio Paraná, Brazil, was conducted using X-ray microcomputed tomography (μCT scan). A redescription of the external anatomy was performed, including the unusual presence of a rostral patch of extra teeth on the region of the upper lip anterior to the premaxilla and the prominent anterior fleshy expansions in both upper and lower lips. The newly surveyed characters were included and analyzed in light of a recent morphological data matrix for Gymnotiformes. In spite of some uncertainties that remains as to phylogenetic allocation of the genus, the most probable hypothesis is that Tembeassu is the sister group of a clade that includes Megadontognathus and Apteronotus sensu stricto. The phylogenetic analysis also supports that Tembeassu is considered a valid genus of Apteronotidae. An amended diagnosis for the genus is also provided.

Dominance in Habitat Preference and Diurnal Explorative Behavior of the Weakly Electric Fish Apteronotus leptorhynchus.

Electrocommunication and -localization behaviors of weakly electric fish have been studied extensively in the lab, mostly by means of short-term observations on constrained fish. Far less is known about their behaviors in more natural-like settings, where fish are less constrained in space and time. We tracked individual fish in a population of fourteen brown ghost knifefish (Apteronotus leptorhynchus) housed in a large 2 m3 indoor tank based on their electric organ discharges (EOD). The tank contained four different natural-like microhabitats (gravel, plants, isolated stones, stacked stones). In particular during the day individual fish showed preferences for specific habitats which provided appropriate shelter. Male fish with higher EOD frequencies spent more time in their preferred habitat during the day, moved more often between habitats during the night, and less often during the day in comparison to low-frequency males. Our data thus revealed a link between dominance indicated by higher EOD frequency, territoriality, and a more explorative personality in male A. leptorhynchus. In females, movement activity during both day and night correlated positively with EOD frequency. In the night, fish of either sex moved to another habitat after about 6 s on average. During the day, the average transition time was also very short at about 20 s. However, these activity phases were interrupted by phases of inactivity that lasted on average about 20 min during the day, but only 3 min in the night. The individual preference for daytime retreat sites did not reflect the frequent explorative movements at night.

Calbindin-D28k expression in spinal electromotoneurons of the weakly electric fish Apteronotus leptorhynchus during adult development and regeneration.

Additive neurogenesis, the net increase in neuronal numbers by addition of new nerve cells to existing tissue, forms the basis for indeterminate spinal cord growth in brown ghost knifefish (Apteronotus leptorhynchus). Among the cells generated through the activity of adult neural stem cells are electromotoneurons, whose axons constitute the electric organ of this weakly electric fish. Electromotoneuron development is organized along a caudo-rostral gradient, with the youngest and smallest of these cells located near the caudal end of the spinal cord. Electromotoneurons start expressing calbindin-D28k when their somata have reached diameters of approximately 10μm, and they continue expression after they have grown to a final size of about 50μm. Calbindin-D28k expression is significantly increased in young neurons generated in response to injury. Immunohistochemical staining against caspase-3 revealed that electromotoneurons in both intact and regenerating spinal cord are significantly less likely to undergo apoptosis than the average spinal cord cell. We hypothesize that expression of calbindin-D28k protects electromotoneurons from cell death; and that the evolutionary development of such a neuroprotective mechanism has been driven by the indispensability of electromotoneurons in the fish's electric behavior, and by the high size-dependent costs associated with their production or removal upon cell death.

The subtle art of electro-flirting in wild knifefish

![Graphic][1] What do James Brown and ghost knifefish ( Apteronotus ) have in common? They've both harnessed the power of electric organs in order to deliver declarations of adoration. As close relatives of the electric eel ( Electrophorus electricus ), it's probably not too shocking that

How the ghost knifefish became the fastest electrical discharger in the animal kingdom

How the ghost knifefish became the fastest electrical discharger in the animal kingdom

Being bullied reduces brain cell birth in harried weakly electric fish

South American knifefish are remarkably adaptable. While the loss of a tail would be a major concern for most fish, some knifefish can replace the lost appendage entirely in only 3 weeks – which is just as well in an environment where hungry catfish continually nibble at them. Despite the knifefish's extraordinary ability to recover from the most extreme injury, Kent Dunlap from Trinity College, USA, noticed that Brachyhypopomus occidentalis survivors that had lost their tails naturally to predators in the streams of Panama also had lower rates of cell birth in the part of the brain that controls responses to predators: the telencephalic region. Could this brain deficit be increasing the fish's vulnerability to a catfish attack, raising the number of wounded fish in the predator's vicinity? Or had living in the constant shadow of a predator affected cell birth in the region of the brain that could protect the fish from assault? And, if catfish attacks were the cause of the reduced brain growth, were the knifefish investing more of their resources in tail regrowth at the expense of their own brains?Intrigued by the possibility that living under the constant threat of predation was affecting the brains of knifefish, Dunlap and his colleagues decided to simulate the impact of the catfish's attentions in the laboratory. Dunlap admits, ‘Coming up with an experimental “chase” protocol for the lab that might mimic the experience of living among dense predators was hard’; it was not clear how closely tapping the fish lightly on the tail and amputating the rear portion of the body under anaesthetic reproduces the experience of a harried B. gauderio. However, when Vielka Salazar from Cape Breton University, Canada, measured the levels of the stress hormone cortisol in the blood of B. gauderio, it was apparent that losing the tail was stressful, although the fish didn't seem too bothered when they were harassed with a plastic rod as their cortisol levels did not rise.Next, Geoffrey Keane, Elise Lasky and Michael Ragazzi checked the amount of brain cell birth in the fish that had been disturbed with a rod but kept their tails, and found it was reduced to half of that of the undisturbed fish. Living under the constant threat of attack had altered cell birth in the agitated fish's brains. And when the team checked how losing a tail affected brain cell birth after 17 days, it was clear that the fish suffered a similar reduction. In addition, when they tested whether the fish had reduced brain cell birth in favour of regrowing their tails, it was clear that there was no trade-off. ‘The reduction of brain cell proliferation following tail amputation was… greatest even before the fish were adding a lot of tail cells and lower during the period of fastest tail generation’, says Dunlap, adding that brain cell birth plummeted by 80% within 24 h of the loss of the tail, which is before regrowth has began in earnest.So, B. gauderio fish reduce cell birth in the telencephalic region of the brain when they are bullied, and when Dunlap and his team tried to terrorise brown ghost knifefish (Apteronotus leptorhynchus), which are popular in the aquarium trade and more easily available, the fish suffered the same brain impairment. Speculating about possible reasons for the fish to scale back the region of the brain that might provide them with the best protection from lurking predators, Dunlap says, ‘We suspect that fish with a lower cell birth rate might be less exploratory and more timid. This could reduce their opportunity to forage and find mates, but might also reduce the probability of falling victim to a predator’.