Inhibition of Fin Regeneration in Fathead Minnow (Pimephales promelas) by a Potent Synthetic Glucocorticoid and Development of Adverse Outcome Pathway 334

Despite structural and functional conservation across vertebrate species, the glucocorticoid receptor has been minimally studied in comparison to other biological targets for endocrine-disrupting compounds in aquatic systems. Because prolonged use of pharmaceutical glucocorticoids in humans has been linked to osteoporosis and impaired bone growth, we hypothesized that the ability of teleost fish to regenerate fins following damage may be inhibited by exposure to synthetic glucocorticoids in the environment. In the present study, we examined fin regeneration following a 7 days waterborne exposure of juvenile fathead minnows (Pimephales promelas) to the synthetic glucocorticoids, fluticasone propionate and dexamethasone. Expression of several biologically relevant gene products (sgk1, tdgf1, runx2a, lef1, shha, and tsc22d3) was measured in paired caudal fin and whole-body tissues. Fluticasone propionate and dexamethasone significantly impaired fin regeneration at measured water concentrations of 2.62 μg/L and 4.62 mg/L, respectively. Changes in gene expression indicated disruption of intercellular communication in the Wnt/β-catenin and bone morphogenetic protein (BMP) signaling pathways after exposure to 4.86 μg/L fluticasone propionate. Upregulation of tsc22d3, a transcription factor responsible for suppression of anti-inflammatory response, may be the plausible cause of repressed cellular signaling. These findings advance the development of adverse outcome pathway 334─Glucocorticoid Receptor Activation Leads to Impaired Fin Regeneration─and elucidate both the mechanistic relationship between activation of the glucocorticoid receptor by fluticasone propionate and inhibition of fin regeneration, which could plausibly reduce individual fitness in aquatic systems.

Axial muscle-fibre orientations in larval zebrafish

Most teleost fish propel themselves with lateral body waves powered by their axial muscles. These muscles also power suction feeding through rapid expansion of the mouth cavity. They consist of muscle segments (myomeres), separated by connective tissue sheets (myosepts). In adult teleosts, the fast axial muscle fibres follow pseudo-helical trajectories, which are thought to distribute strain (relative fibre length change) approximately evenly across transverse sections during swimming, thereby optimizing power generation. To achieve strain equalization, a significant angle to the longitudinal axis on the frontal plane (azimuth) is necessary near the medial plane, increasing strain. Additionally, a deviation from longitudinal orientation on the sagittal plane (elevation) is required laterally to decrease strain. Despite several detailed morphological studies, our understanding of muscle-fibre orientations in the entire axial musculature of fish remains incomplete. Furthermore, most research has been done in post-larval stages, leaving a knowledge gap regarding the changing axial muscle architecture during larval development. Larval fish exhibit different body size, body shape and swimming kinematics compared to adults. They experience relatively high viscous forces, requiring higher tail-beat amplitudes to overcome increased drag. Additionally, larval fish swim with higher tail-beat frequencies. Histological studies have shown that in larval fish, muscle fibres in the anal region transition from an almost longitudinal orientation to a pseudo-helical pattern by 3 dpf (days post-fertilization). However, these studies were limited to a few sections of the body and were prone to shrinkage and tissue damage. Here, we introduce a novel methodology for quantifying muscle-fibre orientations along the entire axial muscles. We selected 4 dpf larval zebrafish for our analyses, a stage where larvae are actively swimming but not yet free-feeding. High-resolution confocal 3D scans were obtained from four genetically modified zebrafish expressing green fluorescent protein in fast muscle fibres. Fluorescence variation allowed segmentation of individual muscle fibres, which were then converted to fish-bound coordinates by correcting for the fish's position and orientation in the scan, and normalized to pool results across individuals. We show that at 4 dpf, muscle-fibre trajectories exhibit a helical pattern tapering towards the tail. Average fibre angles decrease from anterior to posterior, with azimuth varying over the dorsoventral axis and elevation varying over the mediolateral axis. Notably, only the anteriormost 20% of the body displayed higher azimuth angles near the medial plane. Angles between neighbouring fibres were substantial, particularly at the rim of the epaxial and hypaxial muscles. The revealed muscle-fibre architecture at this age presumably contributes to the swimming performance of these larvae, but that swimming performance is probably not the only driving factor for the fibre pattern. Our methodology offers a promising avenue for exploring muscle-fibre orientations across ontogenetic series and provides a foundation for in-depth functional studies on the role of muscle architecture in facilitating swimming performance of larval fish.

Modeling zebrafish escape swim reveals maximum neuromuscular power output and efficient body movement adaptation to increased water viscosity

Under evolutionary pressure, the kinematic and energetic characteristics of animal locomotion have been optimized for survival. We investigated the kinematics and energetic performance of zebrafish eleutheroembryo escape swims triggered by electrical stimuli in fluids of increasing viscosity. Eleutheroembryos exhibited a decrease in both tail movement frequency and swimming velocity in more viscous environments, while the amplitude of body curvature remains constant. We then combined experimental imaging of freely swimming eleutheroembryos with Navier-Stokes numerical simulations. The results showed that the mechanical power output was initially maximal and remained essentially stable with increasing viscosity, while the cost of transport was linearly correlated with viscosity. Eleutheroembryos maximize neuromuscular power output during the fast-start escape response, enabling them to potentially escape predators under all circumstances in a natural environment. This model may be used to identify genetic and toxicological factors that reduce the mechanical power developed by the neuromuscular system or induce a loss of efficiency in its use.

Swimming kinematics of deep-sea fishes

Although the deep oceans represent Earth's largest habitat, the challenges of studying deep-sea organisms in situ have limited our understanding of adaptation, ecology, and behaviour in these important ecosystems. One fundamental trait of fishes that remains largely unexplored in the deep ocean is swimming, a vital process for movement, migration, and dispersal in marine habitats. Deep-sea conditions such as temperature, pressure, and food availability could each impact the speed and efficiency of swimming in fishes. To investigate swimming kinematics of fishes with increasing depth, we analysed in situ video of bony fishes across a 6000-m depth gradient. We compared open-source videos of fishes from National Oceanic and Atmospheric Administration (NOAA) Ocean Exploration with tank-based recordings of shallow-water relatives from Puget Sound, Washington, USA to understand how both habitat depth and phylogeny influence swimming in fishes. We analysed kinematics in four dominant demersal fish groups, the orders Anguilliformes, Gadiformes, Ophidiiformes, and Perciformes. Deep-sea fishes swam consistently slowly. Swimming kinematics varied across temperature, oxygen, body elongation, and depth. These results suggest that swimming kinematics do not change linearly with increasing habitat depth in fishes and that the impacts of deep-sea conditions such as low temperatures, high pressures, and low nutrient availability on swimming behaviour need to be considered independently of one another. These findings provide insight into the evolution of fish form and function in the deep ocean.

The effects of diatom polyunsaturated aldehydes on embryonic and larval zebrafish (Danio rerio)

Marine diatoms are pervasive in many planktonic and benthic environments and represent an important food source for a wide range of species. Some diatoms produce polyunsaturated aldehydes (PUAs) as defensive toxins. PUA exposure is known to reduce the fecundity of invertebrate grazers like copepods and echinoderm larvae, but little is known about the effects of PUAs on vertebrates. Many fish species are likely to come into close contact with diatoms. Many may deposit eggs on diatom-coated substrates, consume diatoms as larvae, and/or feed heavily on zooplankters that may be gut-loaded with diatoms. The purpose of this study was to test whether dissolved diatom PUAs affect the early life stages of a model fish, Danio rerio (zebrafish). To test this, zebrafish embryos and larvae were exposed to proportionally increasing mixtures of the dissolved diatom PUAs 2E,4E-decadienal, 2E,4E-octadienal and 2E,4E-heptadienal. Under PUA exposure, three metrics of fitness were assessed: embryo heart rate, larval size at hatch, and pre-feeding mortality rate. Zebrafish embryos exposed at 24 h post fertilization (hpf) experienced decreased average heart rate after 2 days of PUA exposure. Embryos 24 hpf exposed to PUA mixtures for 6 days showed a reduction in size in comparison to embryos from controls. Embryos exposed to PUAs from 2 hpf until death showed lower survivorship compared to larvae in controls. The results of this study suggest that larval fish that are sympatric with PUA producing diatoms during their embryonic and larval stages may be susceptible to detrimental effects from PUA exposure.

The combined metabolism and transcriptome of tail muscles reveal the effects of antimony pulse exposure on swimming behavior of Pelophylax nigromaculatus tadpoles

Due to the periodic emission of pollutants, the exposure mode of contaminants in water bodies is mostly pulse exposure, and the toxic effects of fluctuating exposure on aquatic animals are not consistent with traditional toxicological experiments of constant exposure. The toxic effects of heavy metal antimony (Sb) on the swimming behavior of Pelophylax nigromaculatus tadpoles after pulse exposure (PESb) and continuous exposure (CESb) for 28 days were explored. The mechanisms were analyzed from the perspectives of tail muscle metabolism and transcriptomics. Compared to the control group, PESb and CESb decreased the average speed of P. nigromaculatus tadpoles by 25.72 % and 18.08 %, respectively. PESb and CESb led to changes in 70 and 24 metabolites of tail muscle, respectively. PESb led to alterations in metabolic pathways related to pyrimidine metabolism, arginine biosynthesis, and glycerophospholipid metabolism. In contrast, CESb altered metabolic pathways such as alanine, aspartate, and glutamate metabolism. Compared to the control, 1225 and 1139 DEGs were identified for PESb and CESb, respectively. These DEGs were mainly associated with functions such as immune response, DNA replication, protein digestion, and absorption. It can be seen that PESb and CESb can alter the metabolism and transcriptome of the tail muscle of P. nigromaculatus tadpoles, leading to differential expression of individual movements.

Modeling Musculoskeletal Disorders in Zebrafish: Advancements in Muscle and Bone Research

Zebrafish (Danio rerio) have emerged as a valuable model organism for investigating musculoskeletal development and the pathophysiology of associated diseases. Key genes and biological processes in zebrafish that closely mirror those in humans, rapid development, and transparent embryos make zebrafish ideal for the in vivo studies of bone and muscle formation, as well as the molecular mechanisms underlying musculoskeletal disorders. This review focuses on the utility of zebrafish in modeling various musculoskeletal conditions, with an emphasis on bone diseases such as osteoporosis and osteogenesis imperfecta, as well as muscle disorders like Duchenne muscular dystrophy. These models have provided significant insights into the molecular pathways involved in these diseases, helping to identify the key genetic and biochemical factors that contribute to their progression. These findings have also advanced our understanding of disease mechanisms and facilitated the development of potential therapeutic strategies for musculoskeletal disorders.

External morphology and growth patterns of larvae and juveniles of Bryconops gracilis (Characiformes, Iguanodectidae) from Amazon basin

During early development, fishes undergo significant changes that influence external morphology and the functioning of internal organs and systems. This often results in gradual variation of the morphological traits of individuals across developmental stages. The investigation of larval and juvenile fish development and growth patterns has pertinent implications for the systematic and ecological elucidation of species. Bryconops gracilis is a medium-sized fish, omnivorous that inhabits lotic and lentic environments with acidic and transparent waters in the Amazon basin. In this study, the early development of B. gracilis is described, until recently a practically unknown species. In terms of development, we used morphological, meristic, and morphometric data to characterize the larvae and juveniles. The individuals were collected in the Curuá-Una River, Amazon basin, Brazil. Fifty-four specimens were examined. Samples include individuals with 3.39-21.79 mm SL. Yolk-sac larvae have two attachment organs on the dorsal surface of head and body. The larvae of B. gracilis are considered altricial, with a fusiform body, and the intestine reaches the median region of the body. Initially, the mouth is subterminal and becomes isognathic from the postflexion stage on. During the postflexion stage, the most relevant morphological changes occur (e.g., presence of all fins, mouth position similar to adults, increased body pigmentation), making individuals more specialized to explore new habitats and diets and maximize their chances of survival. Furthermore, vertebrae and myomeres are compared and assist with differentiating some Bryconops species at early life stages that occur in sympatry in the Amazon basin. Our results contribute to knowledge about the external morphology of neotropical freshwater fishes, enabling the identification of larvae and juveniles through traditional taxonomy and broadening the perspective on the ontogenetic study of the adipose fin in Characoidei.

Recent advances in neurotechnology-based biohybrid robots

Biohybrid robots retain the innate biological characteristics and behavioral traits of animals, making them valuable in applications such as disaster relief, exploration of unknown terrains, and medical care. This review aims to comprehensively discuss the evolution of biohybrid robots, their key technologies and applications, and the challenges they face. By analyzing studies conducted on terrestrial, aquatic, and aerial biohybrid robots, we gain a deeper understanding of how these technologies have made significant progress in simulating natural organisms, improving mechanical performance, and intelligent control. Additionally, we address challenges associated with the application of electrical stimulation technology, the precision of neural signal monitoring, and the ethical considerations for biohybrid robots. We highlight the importance of future research focusing on developing more sophisticated and biocompatible control methods while prioritizing animal welfare. We believe that exploring multimodal monitoring and stimulation technologies holds the potential to enhance the performance of biohybrid robots. These efforts are expected to pave the way for biohybrid robotics technology to introduce greater innovation and well-being to human society in the future.

Integrated organismal responses induced by projected levels of CO2 and temperature exposures in the early life stages of lake sturgeon

Atmospheric CO2 and temperature are rising concurrently, and may have profound impacts on the transcriptional, physiological and behavioural responses of aquatic organisms. Further, spring snowmelt may cause transient increases of pCO2 in many freshwater systems. We examined the behavioural, physiological and transcriptomic responses of an ancient fish, the lake sturgeon (Acipenser fulvescens) to projected levels of warming and pCO2 during its most vulnerable period of life, the first year. Specifically, larval fish were raised in either low (16°C) or high (22°C) temperature, and/or low (1000 μatm) or high (2500 μatm) pCO2 in a crossed experimental design over approximately 8 months. Following overwintering, lake sturgeon were exposed to a transient increase in pCO2 of 10,000 μatm, simulating a spring melt based on data in freshwater systems. Transcriptional analyses revealed potential connections to otolith formation and reduced growth in fish exposed to high pCO2 and temperature in combination. Network analyses of differential gene expression revealed different biological processes among the different treatments on the edges of transcriptional networks. Na+/K+-ATPase activity increased in fish not exposed to elevated pCO2 during development, and mRNA abundance of the β subunit was most strongly predictive of enzyme activity. Behavioural assays revealed a decrease in total activity following an acute CO2 exposure. These results demonstrate compensatory and compounding mechanisms of pCO2 and warming dependent on developmental conditions in lake sturgeon. Conserved elements of the cellular stress response across all organisms provide key information for how other freshwater organisms may respond to future climate change.

Mitochondrial DNA Stress-Mediated Health Risk to Dibutyl Phthalate Contamination on Zebrafish (Danio rerio) at Early Life Stage

Plastics contaminations are found globally and fit the exposure profile of the planetary boundary threat. The plasticizer of dibutyl phthalate (DBP) leaching has occurred and poses a great threat to human health and the ecosystem for decades, and its toxic mechanism needs further comprehensive elucidation. In this study, environmentally relevant levels of DBP were used for exposure, and the developmental process, oxidative stress, mitochondrial ultrastructure and function, mitochondrial DNA (mtDNA) instability and release, and mtDNA-cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling pathway with inflammatory responses were measured in zebrafish at early life stage. Results showed that DBP exposure caused developmental impairments of heart rate, hatching rate, body length, and mortality in zebrafish embryo. Additionally, the elevated oxidative stress damaged mitochondrial ultrastructure and function and induced oxidative damage to the mtDNA with mutations and instability of replication, transcription, and DNA methylation. The stressed mtDNA leaked into the cytosol and activated the cGAS-STING signaling pathway and inflammation, which were ameliorated by co-treatment with DBP and mitochondrial reactive oxygen species (ROS) scavenger, inhibitors of cGAS or STING. Furthermore, the larval results suggest that DBP-induced mitochondrial toxicity of energy disorder and inflammation were involved in the developmental defects of impaired swimming capability. These results enhance the interpretation of mtDNA stress-mediated health risk to environmental contaminants and contribute to the scrutiny of mitochondrial toxicants.

Transmembrane Channel-Like (Tmc) Subunits Contribute to Frequency Sensitivity in the Zebrafish Utricle

Information about dynamic head motion is conveyed by a central "striolar" zone of vestibular hair cells and afferent neurons in the inner ear. How vestibular hair cells are tuned to transduce dynamic stimuli at the molecular level is not well understood. Here we take advantage of the differential expression pattern of tmc1, tmc2a, and tmc2b, which encode channel subunits of the mechanotransduction complex in zebrafish vestibular hair cells. To test the role of various combinations of Tmc subunits in transducing dynamic head movements, we measured reflexive eye movements induced by high-frequency stimuli in single versus double tmc mutants. We found that Tmc2a function correlates with the broadest range of frequency sensitivity, whereas Tmc2b mainly contributes to lower-frequency responses. Tmc1, which is largely excluded from the striolar zone, plays a minor role in sensing lower-frequency stimuli. Our study suggests that the Tmc subunits impart functional differences to the mechanotransduction of dynamic stimuli.Significance Statement Information about dynamic head movements is transmitted by sensory receptors, known as hair cells, in the labyrinth of the inner ear. The sensitivity of hair cells to fast or slow movements of the head differs according to cell type. Whether the mechanotransduction complex that converts mechanical stimuli into electrical signals in hair cells participates in conveying frequency information is not clear. Here we find that the transmembrane channel-like 1/2 genes, which encode a central component of the complex, are differentially expressed in the utricle and contribute to frequency sensitivity in zebrafish.

A novel gnotobiotic experimental system for Atlantic salmon (Salmo salar L.) reveals a microbial influence on mucosal barrier function and adipose tissue accumulation during the yolk sac stage

Gnotobiotic models have had a crucial role in studying the effect that commensal microbiota has on the health of their animal hosts. Despite their physiological and ecological diversity, teleost fishes are still underrepresented in gnotobiotic research. Moreover, a better understanding of host-microbe interactions in farmed fish has the potential to contribute to sustainable global food supply. We have developed a novel gnotobiotic experimental system that includes the derivation of fertilized eggs of farmed and wild Atlantic salmon, and gnotobiotic husbandry of fry during the yolk sac stage. We used a microscopy-based approach to estimate the barrier function of the skin mucus layer and used this measurement to select the derivation procedure that minimized adverse effects on the skin mucosa. We also used this method to demonstrate that the mucus barrier was reduced in germ-free fry when compared to fry colonized with two different bacterial communities. This alteration in the mucus barrier was preceded by an increase in the number of cells containing neutral mucosubstances in the anterior segment of the body, but without changes in the number of cells containing acidic substances in any of the other segments studied along the body axis. In addition, we showed how the microbial status of the fry temporarily affected body size and the utilization of internal yolk stores during the yolk sac stage. Finally, we showed that the presence of bacterial communities associated with the fry, as well as their composition, affected the size of adipose tissue. Fry colonized with water from a lake had a larger visceral adipose tissue depot than both conventionally raised and germ-free fry. Together, our results show that this novel gnotobiotic experimental system is a useful tool for the study of host-microbe interactions in this species of aquacultural importance.

Evaluating Evasion Strategies in Zebrafish Larvae

An effective evasion strategy allows prey to survive encounters with predators. Prey are generally thought to escape in a direction that is either random or serves to maximize the minimum distance from the predator. Here we introduce a comprehensive approach to determine the most likely evasion strategy among multiple hypotheses and the role of biomechanical constraints on the escape response of prey fish. Through a consideration of six strategies with sensorimotor noise and previous kinematic measurements, our analysis shows that zebrafish larvae generally escape in a direction orthogonal to the predator's heading. By sensing only the predator's heading, this orthogonal strategy maximizes the distance from fast-moving predators, and, when operating within the biomechanical constraints of the escape response, it provides the best predictions of prey behavior among all alternatives. This work demonstrates a framework for resolving the strategic basis of evastion in predator-prey interactions, which could be applied to a broad diversity of animals.

Gap junction Delta-2b (gjd2b/Cx35.1) depletion causes hyperopia and visual-motor deficiencies in the zebrafish

The zebrafish is a powerful model to investigate the developmental roles of electrical synapses because many signaling pathways that regulate the development of the nervous system are highly conserved from fish to humans. Here, we provide evidence linking the mammalian connexin-36 (Cx36) ortholog gjd2b/Cx35.1, a major component of electrical synapses in the zebrafish, with a refractive error in the context of morphological, molecular, and behavioral changes of zebrafish larvae. Two abnormalities were identified. The optical coherence tomography analysis of the adult retina confirmed changes to the refractive properties caused by eye axial length reduction, leading to hyperopic shifts. The gjd2b/Cx35.1 depletion was also correlated with morphological changes to the head and body ratios in larvae. The differential expression of Wnt/ß-catenin signaling genes, connexins, and dopamine receptors suggested a contribution to the observed phenotypic differences. The alteration of visual-motor behavioral responses to abrupt light transitions was aggravated in larvae, providing evidence that cone photoreceptor cell activity was enhanced when gjd2b/Cx35.1 was depleted. The visual disturbances were reversed under low light conditions in gjd2b ^-/- /Cx35.1^-/- larvae. Since qRT-PCR data demonstrated that two rhodopsin genes were downregulated, we speculated that rod photoreceptor cells in gjd2b/Cx35.1^-/- larvae were less sensitive to bright light transitions, thus providing additional evidence that a cone-mediated process caused the VMR light-ON hyperactivity after losing Cx35.1 expression. Together, this study provides evidence for the role of gjd2b/Cx35.1 in the development of the visual system and visually guided behaviors.

Swimming behavior and hydrodynamics of the Chinese cavefish Sinocyclocheilus rhinocerous and a possible role of its head horn structure

The blind troglobite cavefish Sinocyclocheilus rhinocerous lives in oligotrophic, phreatic subterranean waters and possesses a unique cranial morphology including a pronounced supra-occipital horn. We used a combined approach of laboratory observations and Computational Fluid Dynamics modeling to characterize the swimming behavior and other hydrodynamic aspects, i.e., drag coefficients and lateral line sensing distance of S. rhinocerous. Motion capture and tracking based on an Artificial Neural Network, complemented by a Particle Image Velocimetry system to map out water velocity fields, were utilized to analyze the motion of a live specimen in a laboratory aquarium. Computational Fluid Dynamics simulations on flow fields and pressure fields, based on digital models of S. rhinocerous, were also performed. These simulations were compared to analogous simulations employing models of the sympatric, large-eyed troglophile cavefish S. angustiporus. Features of the cavefish swimming behavior deduced from the both live-specimen experiments and simulations included average swimming velocities and three dimensional trajectories, estimates for drag coefficients and potential lateral line sensing distances, and mapping of the flow field around the fish. As expected, typical S. rhinocerous swimming speeds were relatively slow. The lateral line sensing distance was approximately 0.25 body lengths, which may explain the observation that specimen introduced to a new environment tend to swim parallel and near to the walls. Three-dimensional simulations demonstrate that just upstream from the region under the supra-occipital horn the equipotential of the water pressure and velocity fields are nearly vertical. Results support the hypothesis that the conspicuous cranial horn of S. rhinocerous may lead to greater stimulus of the lateral line compared to fish that do not possess such morphology.

Detailed Analysis of Zebrafish Larval Behaviour in the Light Dark Challenge Assay Shows That Diel Hatching Time Determines Individual Variation

Research on stress coping style, i.e., the response of an organism to adverse conditions, which is constant over time and context, gained momentum in recent years, to better understand behavioural patterns in animal welfare. However, knowledge about the ontogeny of stress coping style is still limited. Here, we performed a detailed analysis of the light dark challenge behavioural assay in zebrafish larvae, where after acclimation in ambient light sudden alternating dark and light phases elicit an anxiety-like response. A principal component analysis on parameters related to locomotion (distance moved, swimming velocity, acceleration, mobility) and directionality (angular velocity, meandering of swimming path) revealed independence between the parameters determined in the light and the dark phases of the assay, indicating unrelated generalised behaviours per phase. However, high collinearity was observed between behavioural parameters within the same phase, indicating a robust response to the stimulus within behavioural phenotypes. Subsequently, this assay was used to determine the correlation between individual hatching time and the behavioural phenotype. The results show that fish that had hatched during daytime have a stronger behavioural response to the dark phase at 5 days post-fertilisation in locomotion related parameters and a weaker response in directionality related parameters, than fish that had hatched during nighttime. These results show that behavioural responses to the light dark challenge assay are robust and can be generalised for the light and the dark phase, and that diel hatching time may determine the behavioural phenotype of an individual.

Decoupling and Reprogramming the Wiggling Motion of Midge Larvae Using a Soft Robotic Platform

The efficient motility of invertebrates helps them survive under evolutionary pressures. Reconstructing the locomotion of invertebrates and decoupling the influence of individual basic motion are crucial for understanding their underlying mechanisms, which, however, generally remain a challenge due to the complexity of locomotion gaits. Herein, a magnetic soft robot to reproduce midge larva's key natural swimming gaits is developed, and the coupling effect between body curling and rotation on motility is investigated. Through the authors' systematically decoupling studies using programmed magnetic field inputs, the soft robot (named LarvaBot) experiences various coupled gaits, including biomimetic side-to-side flexures, and unveils that the optimal rotation amplitude and the synchronization of curling and rotation greatly enhance its motility. The LarvaBot achieves fast locomotion and upstream capability at the moderate Reynolds number regime. The soft robotics-based platform provides new insight to decouple complex biological locomotion, and design programmed swimming gaits for the fast locomotion of soft-bodied swimmers.

Comparison of Aminoglycoside Antibiotics and Cobalt Chloride for Ablation of the Lateral Line System in Giant Danios

The mechanoreceptive lateral line system in fish is composed of neuromasts containing hair cells, which can be temporarily ablated by aminoglycoside antibiotics and heavy metal ions. These chemicals have been used for some time in studies exploring the functional role of the lateral line system in many fish species. However, little information on the relative effectiveness and rate of action of these chemicals for ablation is available. In particular, aminoglycoside antibiotics are thought to affect canal neuromasts, which sit in bony or trunk canals, differently from superficial neuromasts, which sit directly on the skin. This assumed ablation pattern has not been fully quantified for commonly used lateral line ablation agents. This study provides a detailed characterization of the effects of two aminoglycoside antibiotics, streptomycin sulfate and neomycin sulfate, and a heavy metal salt, cobalt (II) chloride hexahydrate (CoCl2), on the ablation of hair cells in canal and superficial neuromasts in the giant danio (Devario aequipinnatus) lateral line system, as a model for adult teleost fishes. We also quantified the regeneration of hair cells after ablation using CoCl2 and gentamycin sulfate to verify the time course to full recovery, and whether the ablation method affects the recovery time. Using a fluorescence stain, 4-Di-2-ASP, we verified the effectiveness of each chemical by counting the number of fluorescing canal and superficial neuromasts present throughout the time course of ablation and regeneration of hair cells. We found that streptomycin and neomycin were comparably effective at ablating all neuromasts in less than 12 h using a 250 μM dosage and in less than 8 h using a 500 μM dosage. The 500 μM dosage of either streptomycin or neomycin can ablate hair cells in superficial neuromasts within 2–4 h, while leaving those in canal neuromasts mostly intact. CoCl2 (0.1 mM) worked the fastest, ablating all of the hair cells in less than 6 h. Complete regeneration of the neuromasts in the lateral line system took 7 days regardless of chemicals used to ablate the hair cells. This study adds to the growing knowledge in hearing research about how effective specific chemicals are at ablating hair cells in the acoustic system of vertebrates.

Fishes regulate tail-beat kinematics to minimize speed-specific cost of transport

Energetic expenditure is an important factor in animal locomotion. Here we test the hypothesis that fishes control tail-beat kinematics to optimize energetic expenditure during undulatory swimming. We focus on two energetic indices used in swimming hydrodynamics, cost of transport and Froude efficiency. To rule out one index in favour of another, we use computational-fluid dynamics models to compare experimentally observed fish kinematics with predicted performance landscapes and identify energy-optimized kinematics for a carangiform swimmer, an anguilliform swimmer and larval fishes. By locating the areas in the predicted performance landscapes that are occupied by actual fishes, we found that fishes use combinations of tail-beat frequency and amplitude that minimize cost of transport. This energy-optimizing strategy also explains why fishes increase frequency rather than amplitude to swim faster, and why fishes swim within a narrow range of Strouhal numbers. By quantifying how undulatory-wave kinematics affect thrust, drag, and power, we explain why amplitude and frequency are not equivalent in speed control, and why Froude efficiency is not a reliable energetic indicator. These insights may inspire future research in aquatic organisms and bioinspired robotics using undulatory propulsion.

Developmental exposure to non-dioxin-like polychlorinated biphenyls promotes sensory deficits and disrupts dopaminergic and GABAergic signaling in zebrafish

The most abundant polychlorinated biphenyl (PCB) congeners found in the environment and in humans are neurotoxic. This is of particular concern for early life stages because the exposure of the more vulnerable developing nervous system to neurotoxic chemicals can result in neurobehavioral disorders. In this study, we uncover currently unknown links between PCB target mechanisms and neurobehavioral deficits using zebrafish as a vertebrate model. We investigated the effects of the abundant non-dioxin-like (NDL) congener PCB153 on neuronal morphology and synaptic transmission linked to the proper execution of a sensorimotor response. Zebrafish that were exposed during development to concentrations similar to those found in human cord blood and PCB contaminated sites showed a delay in startle response. Morphological and biochemical data demonstrate that even though PCB153-induced swelling of afferent sensory neurons, the disruption of dopaminergic and GABAergic signaling appears to contribute to PCB-induced motor deficits. A similar delay was observed for other NDL congeners but not for the potent dioxin-like congener PCB126. The effects on important and broadly conserved signaling mechanisms in vertebrates suggest that NDL PCBs may contribute to neurodevelopmental abnormalities in humans and increased selection pressures in vertebrate wildlife.

Direct ingestion, trophic transfer, and physiological effects of microplastics in the early life stages of Centropristis striata, a commercially and recreationally valuable fishery species

Microplastics are ubiquitous in marine and estuarine ecosystems, and thus there is increasing concern regarding exposure and potential effects in commercial species. To address this knowledge gap, we investigated the effects of microplastics on larval and early juvenile life stages of the Black Sea Bass (Centropristis striata), a North American fishery. Larvae (13-14 days post hatch, dph) were exposed to 1.0 × 10⁴, 1.0 × 10⁵, and 1.0 × 10⁶ particles L^-1 of low-density polyethylene (LDPE) microspheres (10-20 μm) directly in seawater and via trophic transfer from microzooplankton prey (tintinnid ciliates, Favella spp.). We also compared the ingestion of virgin and chemically-treated microspheres incubated with either phenanthrene, a polycyclic aromatic hydrocarbon, or 2,4-di-tert-butylphenol (2,4-DTBP), a plastic additive. Larval fish did not discriminate between virgin or chemically-treated microspheres. However, larvae did ingest higher numbers of microspheres through ingestion of microzooplankton prey than directly from the seawater. Early juveniles (50-60 dph) were directly exposed to the virgin and chemically-treated LDPE microspheres, as well as virgin LDPE microfibers for 96 h to determine physiological effects (i.e., oxygen consumption and immune response). There was a significant positive relationship between oxygen consumption and increasing microfiber concentration, as well as a significant negative relationship between immune response and increasing virgin microsphere concentration. This first assessment of microplastic pollution effects in the early life stages of a commercial finfish species demonstrates that trophic transfer from microzooplankton can be a significant route of microplastic exposure to larval stages of C. striata, and that multi-day exposure to some microplastics in early juveniles can result in physiological stress.

Microplastics alone or co-exposed with copper induce neurotoxicity and behavioral alterations on zebrafish larvae after a subchronic exposure

Microplastics (MPs, <5 mm) have been frequently detected in aquatic ecosystems, representing both health and ecological concerns. However data about the combined effects of MPs and other contaminants is still limited. This study aimed to evaluate the impact of MPs and the heavy metal copper (Cu) on zebrafish (Danio rerio) larvae development and behavior. Zebrafish embryos were subchronically exposed to MPs (2 mg/L), two sub-lethal concentrations of Cu (60 and 125 µg/L) and binary mixtures of MPs and Cu using the same concentrations, from 2-h post fertilization until 14 days post fertilization. Lethal and sub-lethal responses (mortality, hatching, body length) were evaluated during the embryogenesis period, and locomotor, avoidance, anxiety and shoaling behaviors, and acetylcholinesterase (AChE) activity were measured at 14 dpf. The results showed that survival of larvae was reduced in groups exposed to MPs, Cu and Cu+MPs. Regarding the behavioral patterns, the higher Cu concentration and mixtures decreased significantly the mean speed, the total distance traveled and the absolute turn angle, demonstrating an adverse effect on swimming competence of zebrafish larvae. Exposure to MPs and Cu, alone or combined, also affected avoidance behavior of zebrafish, with larvae not reacting to the aversive stimulus. There was a significant inhibition of AChE activity in larvae exposed to all experimental groups, compared to the control group. Moreover, a higher inhibition of AChE was noticed in larvae exposed to MPs and both Cu+MPs groups, comparatively to the Cu alone groups. Our findings demonstrate the adverse effects of MPs, alone or co-exposed with Cu, on fish early life stages behavior. This study highlights that MPs and heavy metals may have significant impacts on fish population fitness by disrupting locomotor and avoidance behaviors.

Effect of body stiffness distribution on larval fish-like efficient undulatory swimming

Energy-efficient propulsion is a critical design target for robotic swimmers. Although previous studies have pointed out the importance of nonuniform body bending stiffness distribution (k) in improving the undulatory swimming efficiency of adult fish-like robots in the inertial flow regime, whether such an elastic mechanism is beneficial in the intermediate flow regime remains elusive. Hence, we develop a class of untethered soft milliswimmers consisting of a magnetic composite head and a passive elastic body with different k These robots realize larval zebrafish-like undulatory swimming at the same scale. Investigations reveal that uniform k and high swimming frequency (60 to 100 Hz) are favorable to improve their efficiency. A shape memory polymer-based milliswimmer with tunable k on the fly confirms such findings. Such acquired knowledge can guide the design of energy-efficient leading edge-driven soft undulatory milliswimmers for future environmental and biomedical applications in the same flow regime.

Growth, survivorship, and predator avoidance capability of larval shortnose sturgeon (Acipenser brevirostrum) in response to delayed feeding

Larval shortnose sturgeon, reared at 17°C, were subjected to delayed feeding treatments of 0, 5, 10, 15, 18, and 23 days post-yolk absorption to examine effects of food deprivation on growth, survival, swimming activity, and escape capabilities. Starvation affected growth and survival but despite degree of starvation, larvae were able to resume growth and experience high survivorship following feeding. Specific growth rate based on larval dry weight for the period directly following first feeding was highest for the day 15 and 18 delayed feeding treatments. There were no differences in survival between the 0 and 5 day treatments, however survival was reduced to 71.2%, 45.4%, and 28.8% for 10, 15, and 18 day delayed feeding treatments, respectively. Shortnose sturgeon had a point-of-no-return (PNR; 55.7% initiated feeding) at ~19 days (or 42 days post-fertilization) following the full absorption of yolk. Mean percent swimming activity and swimming speeds showed an interaction between delayed feeding treatment and larval age, such that no differences were detected at 1 and 6 days post-yolk absorption, while these swimming behaviors generally increased or spiked as feeding was delayed for 10, 15, and 18 days post-yolk absorption. At 23 days post-yolk absorption, only swimming speed increased for larvae that were denied food for 18 days. While there was an interaction between delayed feeding treatments and age for proportion of larvae exhibiting an escape response, generally, larvae from all feeding treatments exhibited a positive escape response. There were also interactions between delayed feeding treatments and age post-yolk absorption for mean and maximum escape speeds, such that less aggressive escape responses were typically detected the longer larvae were denied food. Our research suggests that larval shortnose sturgeon increase physical activity during periods of starvation to find a food patch while remaining vigilant but maybe not as capable to defend against a predatory attack as fed individuals.

Survival, Growth, and Development in the Early Stages of the Tropical Gar Atractosteus tropicus: Developmental Critical Windows and the Influence of Temperature, Salinity, and Oxygen Availability

Alterations in fish developmental trajectories occur in response to genetic and environmental changes, especially during sensitive periods of development (critical windows). Embryos and larvae of Atractosteus tropicus were used as a model to study fish survival, growth, and development as a function of temperature (28 °C control, 33 °C, and 36 °C), salinity (0.0 ppt control, 4.0 ppt, and 6.0 ppt), and air saturation (control ~95% air saturation, hypoxia ~30% air saturation, and hyperoxia ~117% air saturation) during three developmental periods: (1) fertilization to hatch, (2) day 1 to day 6 post hatch (dph), and (3) 7 to 12 dph. Elevated temperature, hypoxia, and hyperoxia decreased survival during incubation, and salinity at 2 and 3 dph. Growth increased in embryos incubated at elevated temperature, at higher salinity, and in hyperoxia but decreased in hypoxia. Changes in development occurred as alterations in the timing of hatching, yolk depletion, acceptance of exogenous feeding, free swimming, and snout shape change, especially at high temperature and hypoxia. Our results suggest identifiable critical windows of development in the early ontogeny of A. tropicus and contribute to the knowledge of fish larval ecology and the interactions of individuals × stressors × time of exposure.

Hydrodynamic stress maps on the surface of a flexible fin-like foil

We determine the time dependence of pressure and shear stress distributions on the surface of a pitching and deforming hydrofoil from measurements of the three dimensional flow field. Period-averaged stress maps are obtained both in the presence and absence of steady flow around the foil. The velocity vector field is determined via volumetric three-component particle tracking velocimetry and subsequently inserted into the Navier-Stokes equation to calculate the total hydrodynamic stress tensor. In addition, we also present a careful error analysis of such measurements, showing that local evaluations of stress distributions are possible. The consistency of the force time-dependence is verified using a control volume analysis. The flapping foil used in the experiments is designed to allow comparison with a small trapezoidal fish fin, in terms of the scaling laws that govern the oscillatory flow regime. As a complementary approach, unsteady Euler-Bernoulli beam theory is employed to derive instantaneous transversal force distributions on the flexible hydrofoil from its deflection and the results are compared to the spatial distributions of hydrodynamic stresses obtained from the fluid velocity field.

Effects of crude oil vapors on the cardiovascular flow of embryonic Gulf killifish

Direct contact with toxicants in crude oil during embryogenesis causes cardiovascular defects, but the effects of exposure to airborne volatile organic compounds released from spilled oil are not well understood. The effects of crude oil-derived airborne toxicants on peripheral blood flow were examined in Gulf killifish (Fundulus grandis) since this model completes embryogenesis in the air. Particle image velocimetry was used to measure in vivo blood flow in intersegmental arteries of control and oil-exposed embryos. Significant effects in oil-exposed embryos included increased pulse rate, reduced mean blood flow speed and volumetric flow rate, and decreased pulsatility, demonstrating that normal-appearing oil-exposed embryos retain underlying cardiovascular defects. Further, hematocrit moderately increased in oil-exposed embryos. This study highlights the potential for fine-scale physiological measurement techniques to better understand the sub-lethal effects of oil exposure and demonstrates the efficacy of Gulf killifish as a unique teleost model for aerial toxicant exposure studies.

Quantification of computational fluid dynamics simulation assists the evaluation of protection by Gypenosides in a zebrafish pain model

In recent years, due to its rapid reproduction rate and the similarity of its genetic structure to that of human, the zebrafish has been widely used as a pain model to study chemical influences on behavior. Swimming behaviors are mediated by motoneurons in the spinal cord that drive muscle contractions, therefore a knowledge of internal muscle mechanics can assist the understanding of the effects of drugs on swimming activity. To demonstrate that the technique used in our study can supplement biological observations by quantifying the contribution of muscle effects to altered swimming behaviours, we have evaluated the pain/damage caused by 0.1% acetic acid to the muscle of 5 dpf zebrafish larvae and the effect of protection from this pain/damage with the saponin Gypenosides (GYP) extracted from Gynostemma pentaphyllum. We have quantified the parameters related to muscle such as muscle power and the resultant hydrodynamic force, proving that GYP could alleviate the detrimental effect of acetic acid on zebrafish larvae, in the form of alleviation from swimming debility, and that the muscle status could be quantified to represent the degree of muscle damage due to the acetic acid and the recovery due to GYP. We have also linked the behavioral changes to alteration of antioxidant and inflammation gene expression. The above results provide novel insights into the reasons for pain-related behavioral changes in fish larvae, especially from an internal muscle perspective, and have quantified these changes to help understand the protection of swimming behaviors and internal muscle by GYP from acetic acid-induced damage.

Mechanical Characteristics of Ultrafast Zebrafish Larval Swimming Muscles

Zebrafish (Danio rerio) swim within days of fertilization, powered by muscles of the axial myotomes. Forces generated by these muscles can be measured rapidly in whole, intact larval tails by adapting protocols developed for ex vivo muscle mechanics. But it is not known how well these measurements reflect the function of the underlying muscle fibers and sarcomeres. Here, we consider the anatomy of the 5-day-old, wild-type larval tail, and implement technical modifications to measuring muscle physiology in intact tails. Specifically, we quantify fundamental relationships between force, length, and shortening velocity, and capture the extreme contractile speeds required to swim with tail-beat frequencies of 80-100 Hz. Therefore, we analyze 1000 frames/s videos to track the movement of structures, visible in the transparent tail, which correlate with sarcomere length. We also characterize the passive viscoelastic properties of the preparation to isolate forces contributed by nonmuscle structures within the tail. Myotomal muscles generate more than 95% of their maximal isometric stress (76 ± 3 mN/mm2) over the range of muscle lengths used in vivo. They have rapid twitch kinetics (full width at half-maximal stress: 11 ± 1 ms) and a high twitch/tetanus ratio (0.91 ± 0.05), indicating adaptations for fast excitation-contraction coupling. Although contractile stress is relatively low, myotomal muscles develop high net power (134 ± 20 W/kg at 80 Hz) in cyclical work loop experiments designed to simulate the in vivo dynamics of muscle fibers during swimming. When shortening at a constant speed of 7 ± 1 muscle lengths/s, muscles develop 86 ± 2% of isometric stress, whereas peak instantaneous power during 100 Hz work loops occurs at 18 ± 2 muscle lengths/s. These approaches can improve the usefulness of zebrafish as a model system for muscle research by providing a rapid and sensitive functional readout for experimental interventions.

Experimental-numerical method for calculating bending moments in swimming fish shows that fish larvae control undulatory swimming with simple actuation

Most fish swim with body undulations that result from fluid-structure interactions between the fish's internal tissues and the surrounding water. Gaining insight into these complex fluid-structure interactions is essential to understand how fish swim. To this end, we developed a dedicated experimental-numerical inverse dynamics approach to calculate the lateral bending moment distributions for a large-amplitude undulatory swimmer that moves freely in three-dimensional space. We combined automated motion tracking from multiple synchronised high-speed video sequences, computation of fluid dynamic stresses on the swimmer's body from computational fluid dynamics, and bending moment calculations using these stresses as input for a novel beam model of the body. The bending moment, which represent the system's net actuation, varies over time and along the fish's central axis due to muscle actions, passive tissues, inertia, and fluid dynamics. Our three-dimensional analysis of 113 swimming events of zebrafish larvae ranging in age from 3 to 12 days after fertilisation shows that these bending moment patterns are not only relatively simple but also strikingly similar throughout early development and from fast starts to periodic swimming. This suggests that fish larvae may produce and adjust swimming movements relatively simply, yet effectively, while restructuring their neuromuscular control system throughout their rapid development.

Cell fusion is differentially regulated in zebrafish post-embryonic slow and fast muscle

Skeletal muscle fusion occurs during development, growth, and regeneration. To investigate how muscle fusion compares among different muscle cell types and developmental stages, we studied muscle cell fusion over time in wild-type, myomaker (mymk), and jam2a mutant zebrafish. Using live imaging, we show that embryonic myoblast elongation and fusion correlate tightly with slow muscle cell migration. In wild-type embryos, only fast muscle fibers are multinucleate, consistent with previous work showing that the cell fusion regulator gene mymk is specifically expressed throughout the embryonic fast muscle domain. However, by 3 weeks post-fertilization, slow muscle fibers also become multinucleate. At this late-larval stage, mymk is not expressed in muscle fibers, but is expressed in small cells near muscle fibers. Although previous work showed that both mymk and jam2a are required for embryonic fast muscle cell fusion, we observe that muscle force and function is almost normal in mymk and jam2a mutant embryos, despite the lack of fast muscle multinucleation. We show that genetic requirements change post-embryonically, with jam2a becoming much less important by late-larval stages and mymk now required for muscle fusion and growth in both fast and slow muscle cell types. Correspondingly, adult mymk mutants perform poorly in sprint and endurance tests compared to wild-type and jam2a mutants. We show that adult mymk mutant muscle contains small mononucleate myofibers with average myonuclear domain size equivalent to that in wild type adults. The mymk mutant fibers have decreased Laminin expression and increased numbers of Pax7-positive cells, suggesting that impaired fiber growth and active regeneration contribute to the muscle phenotype. Our findings identify several aspects of muscle fusion that change with time in slow and fast fibers as zebrafish develop beyond embryonic stages.

Suction Flows Generated by the Carnivorous Bladderwort Utricularia—Comparing Experiments with Mechanical and Mathematical Models

Suction feeding is a well-understood feeding mode among macroscopic aquatic organisms. The little we know about small suction feeders from larval fish suggests that small suction feeders are not effective. Yet bladderworts, an aquatic carnivorous plant with microscopic underwater traps, have strong suction performances despite having the same mouth size as that of fish larvae. Previous experimental studies of bladderwort suction feeding have focused on the solid mechanics of the trap door’s opening mechanism rather than the mechanics of fluid flow. As flows are difficult to study in small suction feeders due to their small size and brief event durations, we combine flow visualization on bladderwort traps with measurements on a mechanical, dynamically scaled model of a suction feeder. We find that bladderwort traps generate flows that are more similar to the inertia-dominated flows of adult fish than the viscosity-dominated flows of larval fish. Our data further suggest that axial flow transects through suction flow fields, often used in biological studies to characterize suction flows, are less diagnostic of the relative contribution of inertia versus viscosity than transverse transects.

Butylated hydroxytoluene induces hyperactivity and alters dopamine-related gene expression in larval zebrafish (Danio rerio)

Butylated hydroxytoluene (BHT) is one of the most frequently used synthetic phenolic antioxidants added to food and consumer products such as plastics as a preservative. Due to its high production volume, BHT has been detected in aquatic environments, raising concerns about sub-lethal toxicity. However, there are limited toxicological data for BHT, especially in fish. In this study, zebrafish embryos were exposed to BHT at concentrations ranging 0.01-100 μM for up to 6 days post fertilization (dpf). Acute toxicity was assessed, and experiments revealed that BHT had a 96 h LC50 value of 57.61 μM. At sub-lethal doses (0.1-60 μM), BHT markedly decreased heart rates of zebrafish embryos at 48 h and 72 h by ∼25-30%. Basal and maximal respiration of zebrafish embryos at 24 hpf were decreased by 59.3% and 41.4% respectively following exposure to 100 μM BHT. Behavior in zebrafish was measured at 6 dpf following exposures to 0.01-10 μM BHT. Locomotor behaviors (e.g. total distance moved and velocity) were significantly increased in larvae at doses higher than 0.1 μM BHT. In addition, dark-avoidance behavior was decreased following exposure to 0.01 μM BHT, while conversely, it was increased in zebrafish exposed to 0.1 μM BHT. To investigate potential underlying mechanisms that could explain behavioral changes, transcripts involved in dopamine signaling were measured. Relative expression of dat mRNA was increased in larval fish from the 0.01 μM BHT treatment, while there were no effects on dat mRNA levels at higher concentrations. The mRNA levels of drd3 were decreased in zebrafish from the 1 μM BHT treatment. Taken together, BHT can affect the expression of the dopamine system, which is hypothesized to be related to the abnormal anxiety-associated behavior of larval zebrafish.

Matrix Metalloproteinase 13 Activity is Required for Normal and Hypoxia-Induced Precocious Hatching in Zebrafish Embryos

Hypoxia induces precocious hatching in zebrafish, but we do not have a clear understanding of the molecular mechanisms regulating the activation of the hatching enzyme or how these mechanisms trigger precocious hatching under unfavorable environmental conditions. Using immunohistochemistry, pharmacological inhibition of matrix metalloproteinase 13 (Mmp13), and in vivo zymography, we show that Mmp13a is present in the hatching gland just as embryos become hatching competent and that Mmp13a activity is required for both normal hatching and hypoxia-induced precocious hatching. We conclude that Mmp13a likely functions in activating the hatching enzyme zymogen and that Mmp13a activity is necessary but not sufficient for hatching in zebrafish. This study highlights the broad nature of MMP function in development and provides a non-mammalian example of extra-embryonic processes mediated by MMP activity.

Quantification of the influence of drugs on zebrafish larvae swimming kinematics and energetics

The use of zebrafish larvae has aroused wide interest in the medical field for its potential role in the development of new therapies. The larvae grow extremely quickly and the embryos are nearly transparent which allows easy examination of its internal structures using fluorescent imaging techniques. Medical treatment of zebrafish larvae can directly influence its swimming behaviours. These behaviour changes are related to functional changes of central nervous system and transformations of the zebrafish body such as muscle mechanical power and force variation, which cannot be measured directly by pure experiment observation. To quantify the influence of drugs on zebrafish larvae swimming behaviours and energetics, we have developed a novel methodology to exploit intravital changes based on observed zebrafish locomotion. Specifically, by using an in-house MATLAB code to process the recorded live zebrafish swimming video, the kinematic locomotion equation of a 3D zebrafish larvae was obtained, and a customised Computational Fluid Dynamics tool was used to solve the fluid flow around the fish model which was geometrically the same as experimentally tested zebrafish. The developed methodology was firstly verified against experiment, and further applied to quantify the fish internal body force, torque and power consumption associated with a group of normal zebrafish larvae vs. those immersed in acetic acid and two neuroactive drugs. As indicated by our results, zebrafish larvae immersed in 0.01% acetic acid display approximately 30% higher hydrodynamic power and 10% higher cost of transport than control group. In addition, 500 μM diphenylhydantoin significantly decreases the locomotion activity for approximately 50% lower hydrodynamic power, whereas 100 mg/L yohimbine has not caused any significant influences on 5 dpf zebrafish larvae locomotion. The approach has potential to evaluate the influence of drugs on the aquatic animal’s behaviour changes and thus support the development of new analgesic and neuroactive drugs.

Larval morphology predicts geographical dispersal range of Eastern Pacific eels

The geographical range of many marine species is strongly influenced by the dispersal potential of propagules such as eggs and larvae. Here, we investigate morphological diversity and the effect of body shape on geographical range of leptocephali, the unique, laterally compressed larvae of eels (order Anguilliformes). We used phylogenetically informed analyses to examine the morphological variation of larvae for 17 Eastern Pacific eel species from three adult habitats. We also investigated whether morphological traits of leptocephali could predict larval latitudinal range, hypothesizing that body shape may influence passive dispersal via currents. We found that no two species shared the same multivariate growth trajectories, with the size and scaling of pectoral fin length and snout-to-anus length being particularly variable. Larvae with longer relative predorsal and snout-to-anus lengths at median sizes exhibited wider larval geographical ranges. Body aspect ratio and maximum body length at metamorphosis, two traits we hypothesized to be important for passive transport, were not significant predictors of maximal larval range. We discovered an increase in phylogenetic signal over larval development as eels approach metamorphosis, potentially due to similar selective pressures between related species (such as juvenile habitat or adult morphology). Lastly, we conclude that larval body shape is probably influenced by adult habitat and adult morphology.

Reorientation and propulsion in fast-starting zebrafish larvae: an inverse dynamics analysis

Most fish species use fast starts to escape from predators. Zebrafish larvae perform effective fast starts immediately after hatching. They use a C-start, where the body curls into a C-shape, and then unfolds to accelerate. These escape responses need to fulfil a number of functional demands, under the constraints of the fluid environment and the larva's body shape. Primarily, the larvae need to generate sufficient escape speed in a wide range of possible directions, in a short-enough time. In this study, we examined how the larvae meet these demands. We filmed fast starts of zebrafish larvae with a unique five-camera setup with high spatiotemporal resolution. From these videos, we reconstructed the 3D swimming motion with an automated method and from these data calculated resultant hydrodynamic forces and, for the first time, 3D torques. We show that zebrafish larvae reorient mostly in the first stage of the start by producing a strong yaw torque, often without using the pectoral fins. This reorientation is expressed as the body angle, a measure that represents the rotation of the complete body, rather than the commonly used head angle. The fish accelerates its centre of mass mostly in stage 2 by generating a considerable force peak while the fish 'unfolds'. The escape direction of the fish correlates strongly with the amount of body curvature in stage 1, while the escape speed correlates strongly with the duration of the start. This may allow the fish to independently control the direction and speed of the escape.

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

Predatory posture and performance in a precocious larval fish targeting evasive copepods

Predatory fishes avoid detection by prey through a stealthy approach, followed by a rapid and precise fast-start strike. Although many first-feeding fish larvae strike at non-evasive prey using an S-start, the clownfish Amphiprion ocellaris feeds on highly evasive calanoid copepods from a J-shaped position, beginning 1 day post-hatch (dph). We quantified this unique strike posture by observing successful predatory interactions between larval clownfish (1 to 14 dph) and three developmental stages of the calanoid copepod Bestiolina similis The J-shaped posture of clownfish became less tightly curled (more L-shaped) during larval development. Larvae were also less tightly curled when targeting adult copepods, which are more evasive than younger copepod stages. Strike performance measured as time to capture and as peak speed improved only slightly with larval age. Therefore, the J-posture may allow first-feeding larvae to minimize disturbance during their approach of sensitive prey, and may represent an alternative predatory strategy to the prototypical S-start.

The Expression of agrp1, A Hypothalamic Appetite-Stimulating Neuropeptide, Reveals Hydrodynamic-Induced Starvation in a Larval Fish

Larval fish suffer dramatic mortality in the days following transition to autonomous feeding, with over 90% of larvae being eliminated within a period of few weeks. Recent work has shown that the hydrodynamic environment experienced by recently-hatched larvae impedes their feeding rates even under high prey densities. Here, we quantified starvation through early ontogeny in Sparus aurata larvae (8–18 days post-hatching; DPH) and tested whether the emerging ontogenetic pattern is consistent with that expected one based on the hydrodynamic environment that these larvae experience. We screened three candidate genes agrp1, npy, and hsp70, whose expression was previously shown to respond to starvation in fish. Of the three genes, agrp1 was identified as a suitable indicator for starvation. Localization of agrp1 mRNA by whole-mount in-situ hybridization confirmed that, in S. aurata larvae, agrp1 is expressed only in the hypothalamus. Quantification of agrp1 mRNA using real-time PCR revealed that the expression of this gene is elevated in starved compared to fed larvae, and in younger (8 DPH) compared to older larvae (18 DPH). Manipulating the water viscosity to simulate the hydrodynamic conditions during the onset of the critical period led to increased agrp1 expression. These findings suggest that the hydrodynamic constraints on larval feeding lead to the starvation of small larvae. Further, they provide a mechanistic explanation for the “safe harbor” hypothesis, which postulates that larvae should allocate resources toward rapid linear growth to escape detrimental effects of dwelling in an environment where viscous fluid forces dominate.

Development of vestibular behaviors in zebrafish

Most animals orient their bodies with respect to gravity to facilitate locomotion and perception. The neural circuits responsible for these orienting movements have long served as a model to address fundamental questions in systems neuroscience. Though postural control is vital, we know little about development of either balance reflexes or the neural circuitry that produces them. Recent work in a genetically and optically accessible vertebrate, the larval zebrafish, has begun to reveal the mechanisms by which such vestibular behaviors and circuits come to function. Here we highlight recent work that leverages the particular advantages of the larval zebrafish to illuminate mechanisms of postural development, the role of sensation for balance circuit development, and the organization of developing vestibular circuits. Further, we frame open questions regarding the developmental mechanisms for functional circuit assembly and maturation where studying the zebrafish vestibular system is likely to open new frontiers.

Environmental calcium and variation in yolk sac size influence swimming performance in larval lake sturgeon (Acipenser fulvescens)

In many animal species, performance in the early life stages strongly affects recruitment to the adult population; however, factors that influence early life history stages are often the least understood. This is particularly relevant for lake sturgeon, Acipenser fulvescens, living in areas where environmental calcium concentrations are declining, partly due to anthropogenic activity. As calcium is important for muscle contraction and fatigue resistance, declining calcium levels could constrain swimming performance. Similarly, swimming performance could be influenced by variation in yolk sac volume, because the yolk sac is likely to affect drag forces during swimming. Testing swimming performance of larval A. fulvescens reared in four different calcium treatments spanning the range of 4-132 mg l^-1 [Ca²⁺], this study found no treatment effects on the sprint swimming speed. A novel test of volitional swimming performance, however, revealed reduced swimming performance in the low calcium environment. Specifically, volitionally swimming larvae covered a shorter distance before swimming cessation in the low calcium environment compared with the other treatments. Moreover, sprint swimming speed in larvae with a large yolk sac was significantly slower than in larvae with a small yolk sac, regardless of body length variation. Thus, elevated maternal allocation (i.e. more yolk) was associated with reduced swimming performance. Data suggest that larvae in low calcium environments or with a large yolk sac exhibit reduced swimming performance and could be more susceptible to predation or premature downstream drift. Our study reveals how environmental factors and phenotypic variation influence locomotor performance in a larval fish.

Three-dimensional analysis of the fast-start escape response of the least killifish, Heterandria formosa

Fish make C-starts to evade predator strikes. Double-bend (DB) C-starts consist of three stages: Stage 1, in which the fish rapidly bends into a C-shape; Stage 2, in which the fish bends in the opposite direction; and a variable Stage 3. In single-bend (SB) C-starts, the fish immediately straightens after Stage 1. Despite fish moving in 3D space, fast-start responses of adult fish have mainly been studied in a horizontal plane. Using automated 3D tracking of multi-camera high-speed video sequences, we show that both SB and DB fast-starts by adult female least killifish (Heterandria formosa) often contain a significant vertical velocity component, and large changes in pitch (DB: up to 43 deg) and roll (DB: up to 77 deg) angles. Upwards and downwards elevation changes are correlated with changes in pitch angle of the head; movement in the horizontal plane is correlated with changes in yaw angle of the head. With respect to the stimulus, escape heading correlates with the elevation of the fish at the onset of motion. Irrespective of the initial orientation, fish can escape in any horizontal direction. In many cases, the centre of mass barely accelerates during Stage 1. However, it does accelerate in the final direction of the escape in other instances, indicating that Stage 1 can serve a propulsive role in addition to its preparatory role for Stage 2. Our findings highlight the importance of large-scale 3D analyses of fast-start manoeuvres of adult fish in uncovering the versatility of fish escape repertoire.