The behaviour and ecology of the zebrafish, Danio rerio

Chronic dimethomorph exposure induced behaviors abnormalities and cognitive performance alterations in adult zebrafish (Danio rerio)

Dimethomorph is a systematic fungicide that inhibits sterol synthesis in fungi and unfortunately, there was only scarce data regarding its toxicity. Therefore, considering its extensive application in agriculture and its presence in food residues and the environment, its toxicities in non-target organisms, including aquatic animals, are required to be evaluated since they are sensitive indicators of ecological change. In this study, we evaluated the toxicities of dimethomorph after chronic exposure to adult zebrafish (Danio rerio) by conducting various behavioral assays, a passive avoidance test, and biochemical assays by ELISA. From the results, ∼ 2 weeks exposure to dimethomorph caused lower locomotion, aggressiveness, and conspecific social interaction, and more robust predator avoidance behaviors. Furthermore, alterations in color preferences and short-term memory loss were also observed in the treated fish. In helping to elucidate the mechanism, the expression level of several important neurotransmitters in the brain tissue was measured. Interestingly, increment in several biomarkers, including serotonin, kisspeptin, epinephrine, norepinephrine, and dopamine was observed in the treated group along with a slight increase in other tested neurotransmitters, which were catalase, acetylcholine, and melatonin, which might play a role in the observed behavior alterations. Nevertheless, the results from the current study suggested possible alterations in the central nervous system by dimethomorph, and thus, consideration is required prior to the usage of this fungicide in the agricultural fields surrounding natural freshwater reservoirs.

An appetitive olfactory learning paradigm for zebrafish in their home tanks

Olfaction is a subject of increasing interest in fish biology, but there are few learning paradigms available to investigate olfactory behaviour. In the present study, groups of zebrafish were trained in their home tanks retrofitted for automated conditioning with a microprocessor-controlled syringe pump and feeder to deliver odourant and food, respectively, and responses recorded remotely to minimize researcher interference. Fish were presented with phenylethyl alcohol (PEA), and a food reward was given 15 seconds later for experimental groups or at a variably delayed interval for controls. This schedule continued for 48 trials over four days. Both groups showed initial attraction to PEA, but by the end of Day 2 all fish exhibited reduced interest in the odourant. Further experiments with different fish indicated that this attraction was dependent upon high food motivation, as starved animals reacted more than non-starved animals to the novel stimulus. On Day 3, experimental fish began once again to show attraction associated with the odourant, thus indicating that they formed an association between the PEA and food reward, which increased by the end of Day 4. Conversely, control fish showed little or no response to the odourant on Days 3 and 4. When exposed to a water-only trial, trained fish largely ignored the cue, indicating that odour and not turbulence was the main stimulus for learning. This experiment demonstrated that an appetitive learning paradigm, using olfactory cues presented in home tanks, is both feasible and cost-effective for testing olfactory behaviour in zebrafish.

The importance of understanding the ethology and ecology of the zebrafish, and of other fish species, in experimental research

This short review appears in a special issue assembled to celebrate the 90th birthday of a Hungarian ethologist, Professor Vilmos Csányi. As such, it includes some autobiographical details specific to that scientist and the author of this review. However, these details also serve an important general message. They exemplify how science, i.e., specifically the use of fish in the analysis of behaviour and brain function progressed from the mid-1970s to the current day. They illuminate how scientists choose their study species, and how this choice influences the research questions one may be able to pose. The review discusses why the zebrafish has become a popular research subject of biology, including behavioural neuroscience. It argues that behavioural analysis should be an integral part of research into the analysis of brain function. It considers the dichotomy between the historical effect of North American behaviourism vs. the legacy of European Nobel laureate ethologists. It demonstrates, through a theoretical example, why merging these two "schools" of thoughts is the appropriate way to conduct behavioural research. It provides a few examples for how combining knowledge of ethology and ecology of the species with systematic laboratory studies may be beneficial. And it presents a brief outlook for the future of fish in biology research.

Food sensing controls C. elegans reproductive behavior by neuromodulatory disinhibition

Like many organisms, the roundworm Caenorhabditis elegans incorporates an assessment of environmental quality into its reproductive strategy. C. elegans hermaphrodites release fertilized eggs into food-rich environments but retain them in the absence of food. Here, we report the discovery of a neural circuit required for the modulation of reproductive behavior by food sensing. A mutation that electrically silences the AVK interneurons uncouples egg laying from detection of environmental food cues. We find that AVK activity inhibits egg laying, and AVKs themselves are inhibited by dopamine released from food-sensing neurons. AVKs express a large number of structurally and functionally diverse neuropeptides. Coordination of food-sensing and reproductive behavior requires a subset of AVK neuropeptides that converge on a small ensemble of premotor neurons that coexpress their cognate receptors. Modulation of C. elegans reproductive behavior, therefore, requires a cascade of neuromodulatory signals that uses disinhibition and combinatorial neuropeptide signals to activate reproductive behavior when food is sensed.

Environmentally induced variation in sperm sRNAs is linked to gene expression and transposable elements in zebrafish offspring

Environmental factors affect not only paternal condition but may translate into the following generations where sperm-mediated small RNAs (sRNAs) can contribute to the transmission of paternal effects. sRNAs play a key role in the male germ line in genome maintenance and repair, and particularly in response to environmental stress and the resulting increase in transposable element (TE) activity. Here, we investigated how the social environment (high competition, low competition) of male zebrafish Danio rerio affects sRNAs in sperm and how these are linked to gene expression and TE activity in their offspring. In a first experiment, we collected sperm samples after exposing males to each social environment for 2 weeks to test for differentially expressed sperm micro- (miRNA) and piwi-interacting RNAs (piRNA). In a separate experiment, we performed in vitro fertilisations after one 2-week period using a split-clutch design to control for maternal effects and collected embryos at 24 h to test for differentially expressed genes and TEs. We developed new computational prediction tools to link sperm sRNAs with differentially expressed TEs and genes in the embryos. Our results support the idea that the molecular stress response in the male germ line has significant down-stream effects on the molecular pathways, and we provide a direct link between sRNAs, TEs and gene expression.

Effects of fulvic acid on zebrafish (Danio rerio) growth, immunity and antioxidant status

This experiment aimed to determine the efficacy of fulvic acid (FLA) on growth performance, innate immune system, antioxidant parameters, and expression of immune and antioxidant-related genes in zebrafish (Danio rerio). To this end, 12 tanks (3 per group), each containing 50 zebrafish (with an average weight of 85.7 ± 10.05 mg) in 72 L, were assigned to diets containing FLA at four levels: 0 (control), 0.25 (FLA1), 0.5 (FLA2), and 1 (FLA3) g/kg diet. Following an eight-week culture period, no significant differences in growth performance were observed among the treatment groups (P > 0.05). However, lysozyme activity, total immunoglobulin (Ig), and total protein concentrations in whole-body extracts were significantly enhanced in the 0.5-1 g FLA/kg diet groups compared to the other treatments (P < 0.05). No significant differences were observed among the groups in catalase (CAT), glutathione peroxidase (GPx), or superoxide dismutase (SOD) activities (P > 0.05). The supplementation of FLA significantly upregulated the gene expression of interferon-α (IFN-α) and tumor necrosis factor-alpha (TNF-α), with the highest expression observed in the 0.5 g FLA/kg diet group (P < 0.05). Additionally, interleukin 1 (IL-1) expression was markedly elevated in this group in comparison to the other treatments (P < 0.05). While there was a significant increase in GPx gene expression with dietary FLA (P < 0.05), no notable differences were observed among FLA treatments (P > 0.05). CAT gene expression remained consistent across all groups (P > 0.05). In contrast, SOD gene expression significantly increased in response to all FLA-supplemented diets, with the highest level observed in the 0.5 g FLA/kg group (P < 0.05). These findings suggest that FLA may serve as an effective dietary supplement to enhance the immune response and antioxidant capacity in zebrafish.

Chains of Commerce: A Comprehensive Review of Animal Welfare Impacts in the International Wildlife Trade

The commercial wildlife trade involves billions of animals each year, consumed for various purposes, including food, fashion, entertainment, traditional medicine, and pets. The experiences of the animals involved vary widely, with negative welfare states being commonplace. To highlight the broad scope of animal welfare impacts across the commercial wildlife trade, we present ten case studies featuring a range of species traded globally for different purposes: (1) Ball pythons captured and farmed to serve as pets; (2) Zebrafish captive bred to serve as pets; (3) African Grey Parrots taken from the wild for the pet industry; (4) Sharks de-finned for traditional medicine; (5) Pangolins hunted for traditional medicine; (6) Crickets farmed for food and feed; (7) Frogs wild-caught for the frog-leg trade; (8) Crocodilians killed for their skins; (9) Lions farmed and killed for tourism; and (10) Elephants held captive for tourism. The case studies demonstrate that wild animals commercially traded can suffer from negative welfare states ranging from chronic stress and depression to frustration and extreme hunger. The individuals involved range from hundreds to billions, and their suffering can last a lifetime. Given the welfare issues identified and the growing recognition and scientific evidence for animal sentience, we propose reducing and redirecting consumer demand for these consumptive wildlife practices that negatively impact animals.

Combined Neurotoxic Effects of Commercial Formulations of Pyrethroid (Deltamethrin) and Neonicotinoid (Imidacloprid) Pesticides on Adult Zebrafish (Danio rerio): Behavioral, Molecular, and Histopathological Analysis

The use of different commercial products that involve one or multiple active substances with specific targeted-pests control has become a widespread practice. Because of this, a severe range of significant consequences has been often reported. Among the most used pesticides worldwide are deltamethrin (DM) and imidacloprid (IMI). With a significative effect on the insect's nervous system, DM acts on the voltage-gated sodium channels in nerve cell membranes, while IMI mimics the acetylcholine neurotransmitter by binding irreversibly to the nicotinic acetylcholine receptors. This study investigates the neurotoxic effects of sub-chronic exposure to commercial formulations of deltamethrin (DM) and imidacloprid (IMI) in adult zebrafish, both individually and in combination. The formulations used in this study contain additional ingredients commonly found in commercial pesticide products, which may contribute to overall toxicity. Fish were exposed to environmentally relevant concentrations of these pesticides for 21 days, individually or in combination. Behavioral, molecular, and histopathological analyses were conducted to assess the impact of these pesticides. Zebrafish exhibited dose-dependent behavioral alterations, particularly in the combined exposure groups, including increased erratic swimming and anxiety-like behavior. Gene expression analysis revealed significant changes in neurotrophic factors (BDNF, NGF, ntf-3, ntf-4/5, ntf-6/7) and their receptors (ntrk1, ntrk2a, ntrk2b, ntrk3a, ntrk3b, ngfra, ngfrb), indicating potential neurotoxic effects. Histopathological examination confirmed neuronal degeneration, gliosis, and vacuolization, with more severe impairments observed in pesticide mixture treatments. These findings highlight the neurotoxic potential of pesticide formulations in aquatic environments and emphasize the need for stricter regulations on pesticide mixtures and further research on pesticide interactions. Our findings emphasize that the combination of pesticides could trigger a synergistic effect by maximizing the toxicity of each compound. Thus, it is a well-known practice for pyrethroids and neonicotinoids to be used together in agriculture. Even so, its prevalence in agriculture and the need to investigate its actual impact on human health, biodiversity, and ecosystem mitigates the development of new strategies for assessing the risk and, at the same time, enhancing the effectiveness.

Validating thermal refuge-seeking behaviour in zebrafish (Danio rerio) during acute thermal challenge

Ectotherms, such as fish, are highly dependent on the stability of their environment to regulate body temperature, performance, and metabolism. Increasing temperatures cause behavioural changes in fish which can be observed and used as indices for determining upper thermal limits. The thermal agitation temperature (Tag) is a recent, and ecologically significant, sublethal index for the upper thermal limit. Previous studies have described thermal agitation as the endpoint, prior to the critical thermal maximum (CTmax), where fish start exhibiting apparent refuge-seeking and thermal avoidance behaviour. It is an assumption that fish are seeking thermal refuge at Tag, but evidence for this is lacking. Therefore, this study aimed to validate this assumption by using zebrafish (Danio rerio) and providing them with thermal refuge while increasing their environment's temperature past Tag. The behavioural responses of D. rerio were observed and their spatial movements were tracked using the animal-tracking software, AnimalTA. The analysis from this study indicated that refuge is sought out prior to Tag and distance between shoal members increases after Tag, indicating D. rerio may trade-off the protective value of a shoal to search for thermal refuge. Our study demonstrates that with refuge available, D. rerio can surpass Tag until refuge itself exceeds Tag, validating that agitation is refuge-seeking behaviour, but a mechanism of last resort. This insight improves our understanding of fish responses to thermal stress and emphasizes the value of using Tag as a sublethal metric alongside CTmax in thermal tolerance studies, with potential applications in ecology and conservation contexts.

Toxicity Assessment of Waters from Rio Doce after the Fundão Dam Rupture to Zebrafish Embryos

The collapse of the Fundão Dam in 2015 caused strong alterations of physical-chemical properties of water bodies in the Rio Doce Basin, including the release of large quantities of metals. This study aimed to evaluate whether water samples collected along the Doce River after dam failure could affect the development of Danio rerio embryos. Water samples were collected immediately and 6 months after the rupture at multiple sites distributed across the river basin, and toxicity tests were conducted using recently fertilized eggs, in 24-well plates. The concentrations of As, Cd, Mn, and Ni in water samples collected immediately after the dam rupture exceeded the limits established by federal standards. No significant lethal or sublethal effects were observed in D. rerio embryos exposed to water samples in both sampling surveys. Based on our results, we hypothesized that although the concentrations of some elements were high, metals were likely not or poorly bioavailable, causing no significant changes in the exposed organisms.

The combined effect of environmentally relevant doses of glyphosate and high temperature: An integrated and multibiomarker approach to delineate redox status and behavior in Danio rerio

Glyphosate, a pesticide commonly found in aquatic ecosystems, affects this habitat and nontarget organisms such as fish. The increase in water temperature, linked to factors such as climate change, poses a considerable threat. Despite extensive ecotoxicological research, we still do not know the real individual and specific consequences of continued exposure to glyphosate and high temperatures, simulating a scenario where the aquatic environment remains contaminated and temperatures continue to rise. Therefore, in this study, we examined the effects of exposure to environmentally relevant concentrations of glyphosate, active ingredient glyphosate (GAI), and glyphosate-based herbicide (GBH) in combination with high temperature (34 °C) in adult zebrafish (Danio rerio). The fish were acclimated to 28 or 34 °C for 96 h. The exposure to 225 and 450 μg L-1 (GBH or GAI) at 28 or 34 °C for 7 days. We analyzed behavioral endpoints (anxiety-like response, sociability, and aggressivity) and biochemical biomarkers of the brain and muscle (oxidative stress). Anxiety-like responses and decreased sociability were disrupted by the combination of glyphosate and high temperature. Furthermore, there is a decrease in Acetylcholinesterase activity in the brain, and an increase in Lipid Peroxidation, Protein Carbonylation, Acetylcholinesterase activity, and Glutathione S-Transferase activity in the muscle. These results demonstrated oxidative stress, anxiety-like behavior and decreased sociability caused by glyphosate and high temperature. We concluded that the combined effects of glyphosate and high temperature affected redox homeostasis and behavior, emphasizing that the field of glyphosate pollution should be carefully considered when evaluating the effects of climate change.

Early-life exposure to di(2-ethylhexyl) phthalate impairs reproduction in adult female zebrafish (Danio rerio)

Di(2-ethylhexyl) phthalate (DEHP) is a plasticizer used in various industrial and consumer products. It is not covalently bound within these products and leaches out during repeated use, heating, or cleaning. Main routes of environmental DEHP pollution are through the industrial and municipal wastewaters, which pollute aquatic environments. The reproductive system of adult fish is known to be vulnerable to DEHP exposure; however, the effects of early-life exposure to DEHP on reproductive function in adult zebrafish (Danio rerio) females are less studied. To evaluate the impact of early-life exposure to DEHP on freshwater female fish reproduction, zebrafish embryos were exposed to DEHP at 0, 10, 100, and 1000 nmol/L from 5 h post-fertilization (hpf) to 120 hpf (larval stage) and then raised to adulthood in clean water. DEHP decreased the number of released eggs and the fertilization rate after mating with unexposed males. Bodyweight and length, the weight of the ovaries, and the gonadosomatic index were decreased in adult female zebrafish following early-life exposure to DEHP. Histological analysis of the ovaries revealed that DEHP inhibited oogenesis. Serum 17β-estradiol levels were significantly reduced. DEHP inhibited gene expression of all three nuclear estrogen receptors in the ovaries, namely esr1, esr2a, and esr2b, and two gonadotropin receptors, fshr and lhr. These results suggest that transient early-life exposure to environmentally relevant concentrations of DEHP can inhibit the reproduction of adult female zebrafish.

The number of conspecific alarm substance donors notably influences the behavioural responses of zebrafish subjected to a traumatic stress procedure

Zebrafish (Danio rerio) represents a complementary pre-clinical model in stress and anxiety research. Conspecific alarm substance (CAS), an alarm pheromone secreted by injured fish, acts as a warning signal and modulates fear responses. Given their schooling nature and that injury precedes CAS release, varying fresh CAS concentrations extracted from different numbers of CAS-donating zebrafish may uniquely influence trauma-related behaviours. Thus, we investigated the behaviour of juvenile and adult zebrafish exposed to traumatic stress protocols, in the presence of CAS extracted from varying numbers of donating zebrafish. Juveniles were assessed for anxiety and boldness in the light-dark and open field tests (LDT and OFT), while adults were assessed in the novel tank test (NTT) and novel OFT (nOFT). We found that (1) trauma minimally impacted juvenile behaviour regardless of donor-derived CAS concentrations, (2) trauma-exposed adults displayed reduced exploration and heightened risk-taking behaviours in the NTT and nOFT compared to control-exposed fish, (3) NTT and nOFT freezing behaviours were distinctly emulated in adult fish and (4) post-trauma behaviour in adults was influenced by the number of donors. Therefore, CAS concentration as determined by donor number has age-related effects on anxiety- and risk-taking behaviours in trauma-exposed zebrafish, a valuable finding for studies utilising fresh CAS as a stress trigger. While we did not directly investigate CAS concentration through serial dilution, our data are of significant translational and ethological relevance, highlighting the importance of in-house method standardization in stress-related studies utilizing fresh CAS as an alarm cue.

Lampbrush chromosomes of Danio rerio

Danio rerio, commonly known as zebrafish, is an established model organism for the developmental and cell biology studies. Although significant progress has been made in the analysis of the D. rerio genome, cytogenetic studies face challenges due to the unclear identification of chromosomes. Here, we present a novel approach to the study of the D. rerio karyotype, focusing on the analysis of lampbrush chromosomes isolated from growing oocytes. Lampbrush chromosomes, existing during diplotene, serve as a powerful tool for high-resolution mapping and transcription analysis due to their profound decondensation and remarkable lateral loops decorated by RNA polymerases and ribonucleoprotein (RNP) matrix. In D. rerio, lampbrush chromosomes are about 20 times longer than corresponding metaphase chromosomes. We found that the lampbrush chromosome stage karyotype of D. rerio is generally undifferentiated, except for several bivalents bearing distinct marker structures, including loops with complex RNP matrix and locus-associated nuclear bodies. Locus-associated nuclear bodies were enriched for coilin and snRNAs; the loci where they formed presumably correspond to the histone gene clusters. Further, we observed the accumulation of splicing factors in giant terminal RNP aggregates on one bivalent. DAPI staining of Danio rerio lampbrush chromosomes revealed large and small chromomeres non-uniformly distributed along the axis. For example, D. rerio lampbrush chromosome 4, comprising the sex-determining region, is divided into two halves-with small chromomeres bearing long lateral loops and with large dense chromomeres bearing no or very tiny lateral loops. As centromeres were not distinguishable, we identified centromeric regions in all bivalents by FISH mapping of pericentromeric RFAL1, RFAL2, and RFAM tandem repeats. Through a combination of morphological analysis, immunostaining of marker structures, and centromere mapping, we developed cytological maps of D. rerio lampbrush chromosomes. Finally, by RNA FISH we revealed transcripts of pericentromeric and telomeric tandem repeats at the lampbrush chromosome stage.

Validating the underpinnings of water corticosterone measurement for aquatic amphibians

Background

Good animal welfare is important ethically but also to ensure animals provide valid scientific models. Despite thousands of amphibians in research laboratories there is minimal quantitative evidence pertaining to their management and welfare. This study validated methods to non-invasively measure corticosterone, the amphibian 'stress' hormone, from tank water to provide a robust and reliable welfare assessment tool.

Methods

We report experiments (A) that evaluate parameters linked to the performance of our biochemical extraction methods for waterborne corticosterone and, importantly, associated sampling procedures. We evaluate appropriate sampling water type, sampling vessel, filtration methods, potential degradation of waterborne corticosterone over time and the impact of sampling procedures on animal corticosterone levels. We wanted to determine sampling parameters that yielded the least background corticosterone and had minimum negative impact on the animals. The second series of experiments (B) evaluated parameters linked to the biology of Xenopus, including the influence of circadian rhythm, sex and snout-vent length on waterborne corticosterone levels, since fundamental knowledge of a species' biology is essential for designing robust experiments and in the interpretation of the results.

Results

We propose collecting corticosterone samples in deionised water in either plastic or glass containers. The filtering process does not impact the amount of corticosterone measured in the water sample. Levels of corticosterone collected in the water change over a 48-hr period so we advocate standardising time from hormone collection to storage at - 20 °C. Repeated transfer of frogs to sampling containers does not increase corticosterone, suggesting our methods are not cumulatively stressful. Corticosterone levels were not impacted by circadian phase, sex or snout-vent length.

Conclusion

We have developed and validated robust methods to quantify waterborne corticosterone. We hope they provide a template for researchers wishing to develop methods to measure waterborne corticosterone in aquatic amphibians.

Days-old zebrafish rapidly learn to recognize threatening agents through noradrenergic and forebrain circuits

Animals need to rapidly learn to recognize and avoid predators. This ability may be especially important for young animals due to their increased vulnerability. It is unknown whether, and how, nascent vertebrates are capable of such rapid learning. Here, we used a robotic predator-prey interaction assay to show that 1 week after fertilization-a developmental stage where they have approximately 1% the number of neurons of adults-zebrafish larvae rapidly and robustly learn to recognize a stationary object as a threat after the object pursues the fish for ∼1 min. Larvae continue to avoid the threatening object after it stops moving and can learn to distinguish threatening from non-threatening objects of a different color. Whole-brain functional imaging revealed the multi-timescale activity of noradrenergic neurons and forebrain circuits that encoded the threat. Chemogenetic ablation of those populations prevented the learning. Thus, a noradrenergic and forebrain multiregional network underlies the ability of young vertebrates to rapidly learn to recognize potential predators within their first week of life.

From colours to cravings: Exploring conditioned colour preference to ethanol in zebrafish

Conditioned preference paradigms like conditioned colour preference tests (CCP) can be used to investigate addictive drug seeking in zebrafish (Danio rerio), but many aspects of this procedure require further study. Conditioned preference can be tested with either biased or unbiased conditioning methods, each with their own strengths and weaknesses. The present study used unbiased stimuli to test seeking behaviour in ethanol-exposed zebrafish at different durations of drug withdrawal. Zebrafish were exposed to one of two equally preferred colours (red or yellow) while dosed with 0.8 % vol/vol ethanol or with habitat water (controls) for 1 h each day for 21 days. Next, fish experienced withdrawal for either 2-, 4-, or 8-days then were tested in a two-way red and yellow task for 10 min with their movement recorded via motion-tracking software. Fish conditioned to red showed a main effect of ethanol and a significant preference for red compared to yellow at 8-days of withdrawal but not at 2-days or 4-days of withdrawal. Fish conditioned to yellow did not show any colour preference during the 2-, 4-, or 8-days of withdrawal, but did show a main effect of withdrawal duration. This work expands our understanding of CCP paradigms in zebrafish and highlights the capacity of zebrafish to develop an association to red but not yellow under our experimental conditions.

Polystyrene nanoplastics impact the bioenergetics of developing zebrafish and limit molecular and physiological adaptive responses to acute temperature stress

Plastic pollution is a growing environmental concern due to its ubiquitous impact on aquatic ecosystems. Nanoplastics can be generated from the breakdown of plastic waste and interact with organisms at the cellular level, potentially disrupting cellular physiology. We investigated the effects of 44 nm polystyrene nanoparticles (44 nm NanoPS) on the development and physiology of zebrafish (Danio rerio) in the presence of sublethal heat stress (32 °C vs control, 28 °C). We hypothesized that the simultaneous exposure to nanoplastics and rising temperatures seriously threaten developing fish. This combination could create a critical imbalance: rising temperatures may lead to heightened energy demands, while nanoplastic exposure reduces energy production, threatening animal survival. As expected, 32 °C increased markers associated with animal metabolism and developmental timing, such as growth, hatching, heart rate, and feeding. Changes in apoptosis dynamics, oxygen consumption rates, and a decrease in mitochondrial content were detected as adaptive processes to temperature. 44 nm NanoPS alone did not alter development but decreased mitochondrial efficiency in ATP production and increased apoptosis in the heart. Surprisingly, exposure to 44 nm NanoPS at 32 °C did not cause major implications to survival, developmental success, or morphology. Still, 44 nm NanoPS mitigated the temperature-driven change in heart rate, increased oxidative stress, and decreased the coupling efficiency of the less abundant and highly active mitochondria under heat stress. We highlight the interplay between temperature and nanoplastics exposure and suggest that the combined impact of nanoplastics and temperature stress results in a scenario where physiological adaptations are strained, potentially leading to compromised development. This research underscores the need for further investigation into the metabolic costs of plastic pollution, particularly in the context of global warming, to better understand its long-term implications for aquatic life.

Dynamic colour change in zebrafish (Danio rerio) across multiple contexts

Many animals are capable of rapid dynamic colour change, which is particularly well represented in fishes. The proximate mechanisms of dynamic colour change in fishes are well understood; however, less attention has been given to understanding its ecological relevance. In this study, we investigate dynamic colour change in zebrafish (Danio rerio) across multiple contexts, using a protocol to image the colouration of live fish without anaesthesia under standardized conditions. We show that zebrafish respond to different visual environments by darkening their overall colouration in a dark environment and lightening in a light environment. This is consistent with crypsis through background matching as a function of dynamic colour change. Additionally, we find that zebrafish use dynamic colour change to increase the internal contrast of their striped patterning in the presence of conspecifics. We speculate that this may function in social signalling and/or dazzle colouration. We find no effect of a predator stimulus on dynamic colour change. Finally, we discuss the potential for zebrafish to use multiple colouration strategies simultaneously as distance-dependent effects, considering the typical viewing distances of zebrafish and their predators.

Proto-arithmetic abilities in zebrafish (Danio rerio)

The increasing use of zebrafish (Danio rerio) as a model for studying the neural bases of numerical/quantity abilities pushes toward the development of fast and reliable behavioral tasks for this species. Here, we investigated the spontaneous use of proto-arithmetic in quantity discrimination in zebrafish taking advantage of their shoaling behavior. Male fish underwent preference choice tests in which sets of live female conspecifics sequentially disappeared one by one behind one of two opaque identical panels. Fish spontaneously approached the panel occluding the larger set in a "1 vs. 2" comparison, but failed at "2 vs. 3" and "2 vs. 4". Limited to an overall amount of three elements in the two groups, zebrafish appeared to be able to deal with additions and subtractions, also suggesting the implicit understanding of an "empty set" (zero) concept. The velocity and the sequential/simultaneous presentation of the stimuli affected the spontaneous preference towards the group with the largest quantity.

Minocycline reduces neurobehavioral deficits evoked by chronic unpredictable stress in adult zebrafish

Chronic stress-related brain disorders are widespread and debilitating, and often cause lasting neurobehavioral deficits. Minocycline, a common antibiotic and an established inhibitor of microglia, emerges as potential treatment of these disorders. The zebrafish (Danio rerio) is an important emerging model organism in translational neuroscience and stress research. Here, we evaluated the potential of minocycline to correct microglia-mediated behavioral, genomic and neuroimmune responses induced by chronic unpredictable stress (CUS) in adult zebrafish. We demonstrated that CUS evoked overt behavioral deficits in the novel tank, light-dark box and shoaling tests, paralleled by elevated stress hormones (CRH, ACTH and cortisol), and upregulated brain expression of the 'neurotoxic M1' microglia-specific biomarker gene (MHC-2) and pro-inflammatory cytokine genes (IL-1β, IL-6 and IFN-γ). CUS also elevated peripheral (whole-body) pro-inflammatory (IL-1β, IFN-γ) and lowered anti-inflammatory cytokines (IL-4 and IL-10), as well as reduced whole-brain serotonin, dopamine and norepinephrine levels, and increased brain dopamine and serotonin turnover. In contrast, minocycline attenuated most of these effects, also reducing CUS-elevated peripheral levels of IL-6 and IFN-γ. Collectively, this implicates microglia in zebrafish responses to chronic stress, and suggests glial pathways as potential evolutionarily conserved drug targets for treating stress-evoked neuropathogenesis. Our findings also support the growing translational value of zebrafish models for understanding complex molecular mechanisms of brain pathogenesis and its therapy.

The Effect of the Combination of Two Postbiotics on Anxiety-like Behavior in Animal Models

With increasing evidence showing the connections between the microbiome, neurophysiology, and behavior, our research endeavors to investigate whether the consumption of a combination of two postbiotics with antioxidant effects can affect behavior regulation in model species. Here, we worked with a combination (1:1 ratio) of heat-treated Bifidobacterium longum subsp. longum ES1 (CECT7347) and Lacticaseibacillus rhamnosus BPL15 (CECT8361) as a dietary supplement. To examine the potential benefit of using this formulation to alleviate anxiety-like behavior, we employed two model species, Caenorhabditis elegans and adult Danio rerio. In C. elegans, the postbiotic supplementation reduced the anxiety-related behavior analyzed by means of the octanol avoidance test. In zebrafish, the novel tank test indicated a different swimming pattern 2 and 4 months after the animals were fed with the postbiotic combination. While fish did not exhibit any variance in their locomotion parameters such as pace and speed, they showed a statistically significant preference to spend more time in the upper zone of the water tank, a behavior that is correlated with a lower anxiety-like behavior in these species. Our aim with this study is to present evidence that can be used to develop whole-cell postbiotic-based novel and innovative dietary supplements for anxiety-related conditions.

Unveiling the environmental impact of tire wear particles and the associated contaminants: A comprehensive review of environmental and health risk

This review offers a novel perspective on the environmental fate and ecotoxicological effects of tire wear particles (TWPs), ubiquitous environmental contaminants ranging in size from micrometers to millimeters (averaging 10-100 micrometers). These particles pose a growing threat due to their complex chemical composition and potential toxicity. Human exposure primarily occurs through inhalation, ingesting contaminated food and water, and dermal contact. Our review delves into the dynamic interplay between TWP composition, transformation products (TPs), and ecological impacts, highlighting the importance of considering both individual chemical effects and potential synergistic interactions. Notably, our investigation reveals that degradation products of certain chemicals, such as diphenylguanidine (DPG) and diphenylamine (DPA), can be more toxic than the parent compounds, underscoring the need to fully understand these contaminants' environmental profile. Furthermore, we explore the potential human health implications of TWPs, emphasizing the need for further research on potential respiratory, cardiovascular, and endocrine disturbances. Addressing the challenges in characterizing TWPs, assessing their environmental fate, and understanding their potential health risks requires a multidisciplinary approach. Future research should prioritize standardized TWP characterization and leachate analysis methods, conduct field studies to enhance ecological realism, and utilize advanced analytical techniques to decipher complex mixture interactions and identify key toxicants. By addressing these challenges, we can better mitigate the environmental and health risks associated with TWPs and ensure a more sustainable future.

Zebrafish: A Cost-Effective Model for Enhanced Forensic Toxicology Capabilities in Low- and Middle-Income Countries

Low- and middle-income countries (LMICs) are increasingly challenged by the rising burden of medicolegal cases. Traditional forensic infrastructure and in vivo rodent models often have significant limitations due to high costs and ethical concerns. As a result, zebrafish (Danio rerio) are gaining popularity as an attractive alternative model for LMICs because of their cost-effectiveness and practical advantages. Zebrafish have a lower acquisition cost, require less demanding husbandry, and have rapid development cycles, all of which facilitate faster and more economical toxicological studies, even in limited laboratory space. Additionally, the optical transparency of zebrafish embryos and larvae allows for non-invasive in vivo observations, reducing the need for extra resources. Research has shown that zebrafish can effectively investigate the behavioral, developmental, and cardiotoxic effects of various novel psychoactive substances (NPSs), including synthetic opioids, cathinones, and hallucinogens. They also excel in metabolic profiling, producing a broader range of metabolites than other models, with significant overlap in human metabolism. The presence of mammalian-like metabolic enzymes further positions zebrafish as a valuable tool for understanding human NPS metabolism and predicting potential effects. Notably, they can identify metabolites that traditional models may not detect, underscoring their potential for novel metabolite discovery. Despite these advantages, standardizing data collection protocols and addressing interlaboratory variability are crucial challenges that must be overcome for the widespread adoption of the zebrafish model. However, ongoing global efforts are paving the way to address these limitations and ensure the successful integration of zebrafish models into the field of forensic toxicology. This review highlights the potential of zebrafish as a cost-effective and versatile model for LMICs, emphasizing their growing application in NPS research and forecasting broader adoption in forensic toxicology.

Nature versus laboratory: how to optimize housing conditions for zebrafish neuroscience research

Although zebrafish (Danio rerio) neuroscience research is rapidly expanding, the fundamental question of how these fish should be maintained in research laboratories remains largely unstudied. This may explain the diverse practices and broad range of environmental parameters used in zebrafish facilities. Here, we provide examples of these parameters and practices, including housing density, tank size, and water chemistry. We discuss the principles of stochastic resonance versus homeostasis and provide hypothetical examples to explain why keeping zebrafish outside of their tolerated range of environmental parameters may increase phenotypical variance and reduce replicability. We call for systematic studies to establish the optimal maintenance conditions for zebrafish. Furthermore, we discuss why knowing more about the natural behavior and ecology of this species could be a guiding principle for these studies.

Acute toxicity of trypsin inhibitor from tamarind seeds in embryo and adult zebrafish (Danio rerio)

The trypsin inhibitor isolated from tamarind seeds (TTI) is being investigated for potential applications in the treatment of noncommunicable diseases (NCD), such as hypertension, obesity, and diabetes. This study aimed to assess TTI embryotoxicity and acute toxicity in adult zebrafish (Danio rerio). TTI was extracted and isolated from tamarind seeds. Embryonic and adult zebrafish were exposed for 96 hours to three concentrations of TTI (12.5, 25, and 50 mg/L). Zebrafish embryos (n=60 per group) were evaluated for survival, hatching, malformations, and potential developmental marker alterations, in addition to cardiotoxicity and neurotoxicity tests. For acute toxicity assessment in adults (n=20 per group), survival and locomotor and anxiety-like behaviors were assessed, along with genotoxicity (micronucleus) evaluation. Embryos exposed to TTI showed no significant adverse effects, presented normal heart rates and positive reflex response in the neurotoxicity tests. In adult fish, TTI did not cause mortality or significant behavioral changes, suggesting no neurotoxicity and no genotoxicity. Histopathological analyses of the whole body showed only changes in the liver and spinal cord, similar to those observed in the control group not exposed to TTI. These findings indicate TTI's biosafety and therapeutic potential in complex organisms. Further research is required to evaluate its long-term effects and efficacy in treating non-communicable diseases.

Inferring the metabolic rate of zebrafish from ventilation frequency

Fish schooling has attracted the interest of the scientific community for centuries. Energy savings have been long posited to be a key determinant for the emergence of schooling patterns. Yet, current methodologies do not allow the precise quantification of the metabolic rate of specific individuals within the school, typically leaving researchers with only a single, global measurement of metabolic rate for the collective. In this paper, we demonstrate the feasibility of inferring metabolic rate of swimming fish using the mouth-opening frequency, a simple proxy that can be scored utilizing video recordings in the laboratory or in the field, even for small fish. The mouth-opening frequency is independent of hydrodynamic interactions within the school, thereby mitigating potential confounding factors that arise when using locomotory measures associated with tail-beat motion. We assessed the reliability of mouth-opening frequency as a proxy for metabolic rate by conducting experiments on zebrafish (Danio rerio) using swimming respirometry. We varied the flow speed from 0.8 to 3.2 body lengths per second and extracted tail-beat motion and mouth opening from video recordings. Our results revealed a strong correlation between oxygen uptake and mouth-opening frequency for nonzero flow speeds but not in quiescent water. Contrary to our expectations, we did not find evidence in favor of the use of tail-beat frequency as a proxy for metabolic rate. Overall, our results open the door to the study of individual metabolic rates in fish schools without confounding factors related to hydrodynamic interactions.

Tackling water contamination by oncologic drugs: Supported ionic liquids as sustainable adsorbents for cyclophosphamide removal

Due to the increasing incidence of cancer, the consumption of highly toxic oncological drugs is continuously growing. Given the current lack of efficient technologies to remove/treat these toxic drugs in wastewater treatment plants, the environmental quality is compromised, and aquatic organisms are at risk. To address this critical environmental burden, a new strategy based on supported ionic liquids (SILs) for the simultaneous removal of oncologic drugs and toxicity reduction of aqueous samples is here proposed. Silica-based SILs functionalized with imidazolium-based and quaternary ammonium-based ILs were designed and kinetics and isotherm adsorption studies performed. Aiming to develop an adsorbent able to reduce the toxicity of aqueous samples contaminated with oncological drugs, the toxicity reduction was appraised using the model organism Danio rerio. The obtained results disclose that among the studied SILs, the [Si][N3888]Cl (silica functionalized with propyltrioctylammonium chloride) is the best adsorption material (maximum adsorption capacity, qmax = 67.64 mg g-1), with a fast adsorption rate (<20 min). Furthermore, [Si][N3888]Cl was able to remove the toxicity of the treated aqueous samples towards D. rerio embryos, as assessed by lethal and several sublethal endpoints, demonstrating that this material holds remarkable potential for oncological drugs pollution remediation.

Recent Advances of the Zebrafish Model in the Discovery of Marine Bioactive Molecules

Marine natural products are increasingly utilized in nutrition, cosmetics, and medicine, garnering significant attention from researchers globally. With the expansion of marine resource exploration in recent years, the demand for marine natural products has risen, necessitating rapid and cost-effective activity evaluations using model organisms. Zebrafish, a valuable vertebrate model, has become an efficient tool for screening and identifying safe, active molecules from marine natural products. This review, based on nearly 10 years of literature, summarizes the current status and progress of zebrafish models in evaluating marine natural product bioactivity. It also highlights their potential in exploring marine resources with health benefits, offering a reference for the future development and utilization of marine biological resources.

Zebrafish (Danio rerio) as a Model System to Investigate the Role of the Innate Immune Response in Human Infectious Diseases

The zebrafish (Danio rerio) has emerged as a valuable model for studying host-pathogen interactions due to its unique combination of characteristics. These include extensive sequence and functional conservation with the human genome, optical transparency in larvae that allows for high-resolution visualization of host cell-microbe interactions, a fully sequenced and annotated genome, advanced forward and reverse genetic tools, and suitability for chemical screening studies. Despite anatomical differences with humans, the zebrafish model has proven instrumental in investigating immune responses and human infectious diseases. Notably, zebrafish larvae rely exclusively on innate immune responses during the early stages of development, as the adaptive immune system becomes fully functional only after 4-6 weeks post-fertilization. This window provides a unique opportunity to isolate and examine infection and inflammation mechanisms driven by the innate immune response without the confounding effects of adaptive immunity. In this review, we highlight the strengths and limitations of using zebrafish as a powerful vertebrate model to study innate immune responses in infectious diseases. We will particularly focus on host-pathogen interactions in human infections caused by various bacteria (Clostridioides difficile, Staphylococcus aureus, and Pseudomonas aeruginosa), viruses (herpes simplex virus 1, SARS-CoV-2), and fungi (Aspergillus fumigatus and Candida albicans).

Controlled in vivo intrinsic detrimental effect of d-Limonene channelized by influential proximal interaction through apoptosis and steatosis in embryonic zebrafish (Danio rerio)

Bioaccumulation of d-Limonene in environment due to the aggrandised usage of their natural sources like citrus food wastes and industrial day to day life products has raised concern to their biotoxicity to environment biotic health. Moreover, their after-usage discharge to aquatic system has enhanced the distress of posing threat and needs attention. This study entails mechanistic and molecular evaluation of in-vivo biotoxicity of d-Limonene in zebrafish embryo models. Experimental analysis excavated the controlled concentration-dependent morphological, physiological and cellular in-vivo impact of d-Limonene in zebrafish embryos through significant changes in oxidative stress, steatosis and apoptosis regulated via 6-fold and 5-fold mRNA expression change in p53 and Sod1 genes. Computational evaluation deduced the cellular mechanism of d-limonene biotoxicity as irregularities in oxidative stress, apoptosis and steatosis due of their intrinsic interaction with metabolic proteins like Zhe1a (-4.8 Kcal/mol), Sod1(-5.3 Kcal/mol), p53, caspase3 and apoa1 leading to influential change in structural and functional integrity of the metabolic proteins. The study unravelled the measured in-vivo biotoxicity of d-Limonene at cellular and molecular level to advocate the controlled usage of d-Limonene related natural and industrial product for a sustainable environmental health.

Long-term exposure to environmentally relevant Bisphenol-A levels affects growth, swimming, condition factor, sex ratio and histology of juvenile zebrafish

Bisphenol A (BPA) is an environmental estrogen which perturbs hormone signaling pathways adversely affecting aquatic organisms. To evaluate the impact of developmental exposure to long term yet environmentally relevant low doses of BPA, wild-type juvenile zebrafish of 35 days post fertilization were treated with BPA (1 and 10 µg/L), treatment control (0.5% v/v methanol) and control for 60 days. Both BPA treatments led to significantly increased morality overtime. Length increment and specific growth rates became significantly high in BPA exposed zebrafish overtime. Obesogenic property of BPA was not evident with longexposure to low BPA doses. A significantly high and BPA dose-dependent female-biased sex ratios were observed following the juvenile exposure. Significantly low swimming speed was recorded in the fish of both BPA-treated tanks than that of control. Condition factor was significantly low in BPA exposed fish indicating the poor-wellness. There were numerous histopathological alterations of gonads, liver and kidney indicating impacts of juvenile exposure in zebrafish. Altered growth, swimming, mortality, feminization and histopathological changes in zebrafish induced by BPA indicate the risks associated with developmental exposures. The findings call for more comprehensive studies to comprehend the ecological risks imposed by low concentrations of environmental estrogens in urban aquatic ecosystems.

Integrated cellular response of the zebrafish (Danio rerio) heart to temperature change

A decrease in environmental temperature represents a challenge to the cardiovascular system of ectotherms. To gain insight into the cellular changes that occur during cold exposure and cold acclimation we characterized the cardiac phosphoproteome and proteome of zebrafish following 24 h or 1 week exposure to 20°C from 27°C; or at multiple points during 6 weeks of acclimation to 20°C from 27°C. Our results indicate that cold exposure causes an increase in mitogen-activated protein kinase signalling, the activation of stretch-sensitive pathways, cellular remodelling via ubiquitin-dependent pathways and changes to the phosphorylation state of proteins that regulate myofilament structure and function including desmin and troponin T. Cold acclimation (2-6 weeks) led to a decrease in multiple components of the electron transport chain through time, but an increase in proteins for lipid transport, lipid metabolism, the incorporation of polyunsaturated fatty acids into membranes and protein turnover. For example, there was an increase in the levels of apolipoprotein C, prostaglandin reductase-3 and surfeit locus protein 4, involved in lipid transport, lipid metabolism and lipid membrane remodelling. Gill opercular movements suggest that oxygen utilization during cold acclimation is reduced. Neither the amount of food consumed relative to body mass nor body condition was affected by acclimation. These results suggest that while oxygen uptake was reduced, energy homeostasis was maintained. This study highlights that the response of zebrafish to a decrease in temperature is dynamic through time and that investment in the proteomic response increases with the duration of exposure.

Effects of urban streams on muscle non-protein thiols, gill and liver histopathology in zebrafish (Danio rerio) assessed by active biomonitoring

Aquatic biota are exposed to toxic substances resulting from human activities, reducing environmental quality and can compromise the health of the organisms. This study aimed to employ Danio rerio as an environmental bioindicator, analyzing the effects of water from distinct urban aquatic environments. An active biomonitoring system was set up to compare the temporal dynamics of histological biomarkers for gill and liver and the patterns of non-protein thiols (NPSH) in muscle in specimens exposed for 3, 6, and 12 days. Three large urban basins in the city of Campo Grande (Midwest of Brazil) were selected. Two sites are in a very populous area (i.e Lagoa and Bandeira) and another on in an area with agricultural activities (i.e Anhanduí). All the streams displayed distinct qualitative characteristics. The presence of metals, including Mn, Zn, Fe, and Al, as well as pH, temperature, and dissolved oxygen, accounted for 38% of the variability (PC1), while total solids, conductivity, ammonia, nitrite, and explained 24 % (PC2). Degree tissue changes index (DTC) in gill and the concentration of NPSH increased in all streams during 3, 6 and 12 days of exposure. DTC in liver increases in all exposure times in most populous stream (i.e Lagoa and Bandeira). Histopathological evidence in the gill, including proliferation, desquamation, and necrosis of the primary lamellar epithelium; fusion and aneurysms in the secondary lamellar epithelium were observed after three days of exposure. Degenerative nuclear figures were noted in the liver after three days of exposure, followed by hepatocellular hypertrophy, lipidosis, and necrosis at twelve days. Our findings showing time-dependent effects of urban aquatic environments in histopathological (i.e DTC) and biochemical biomarkers in zebrafish. The biomonitoring model enabled a comparison of the temporal dynamics of various health markers, using zebrafish as bioindicator. Future studies might use this experimental model and biomarkers for environmental biomonitoring program.

Effects of hemodynamic load on cardiac remodeling in fish and mammals: the value of comparative models

The ability of the vertebrate heart to remodel enables the cardiac phenotype to be responsive to changes in physiological conditions and aerobic demand. Examples include exercise-induced cardiac hypertrophy, and the significant remodeling of the trout heart during thermal acclimation. Such changes are thought to occur in response to a change in hemodynamic load (i.e. the forces that the heart must work against to circulate blood). Variations in hemodynamic load are caused by either a volume overload (high volume of blood returning to the heart, impairing contraction) or a pressure overload (elevated afterload pressure that the heart must contract against). The changes observed in the heart during remodeling are regulated by multiple cellular signaling pathways. The cardiac response to these regulatory mechanisms occurs across levels of biological organization, affecting cardiac morphology, tissue composition and contractile function. Importantly, prolonged exposure to pressure overload can cause a physiological response - that improves function - to transition to a pathological response that causes loss of function. This Review explores the role of changes in hemodynamic load in regulating the remodeling response, and considers the cellular signals responsible for regulating remodeling, incorporating knowledge gained from studying biomedical models and comparative animal models. We specifically focus on the renin-angiotensin system, and the role of nitric oxide, oxygen free radicals and transforming growth factor beta. Through this approach, we highlight the strong conservation of the regulatory pathways of cardiac remodeling, and the specific conditions within endotherms that may be conducive to the development of pathological phenotypes.

Parental thermal conditions affect the brain activity response to alarm cue in larval zebrafish

Temperature is a crucial factor affecting the physiology of ectothermic animals, but exposure to elevated temperature during specific life stages and across generations may confer fish resilience through phenotypic plasticity. In this study, we investigate the effects of developmental and parental temperature on brain activity response to an olfactory cue in the larval zebrafish, Danio rerio. We exposed parents during reproduction and their offspring during development to control (28 °C) or elevated temperature (30 °C) and observed the response of the larval telencephalon to an alarm cue using live calcium imaging. Parental exposure to elevated temperature decreased the time till maximum brain activity response regardless of the offspring's developmental temperature, revealing that parental thermal conditions can affect the excitability of the offspring's neural circuitry. Furthermore, brain activity duration was affected by the interaction between parental and offspring thermal conditions, where longer brain activity duration was seen when either parents or offspring were exposed to elevated temperature. Conversely, we found shorter brain activity duration when the offspring were exposed to the same temperature as their parents, in both control and elevated temperature. This could represent an anticipatory parental effect influencing the offspring's brain response to match the parental environment, or an early developmental effect occurring within a susceptible short time window post-fertilization. Overall, our results suggest that warming can alter processes involved in brain transmission and show that parental conditions could aid in the preparation of their offspring to respond to olfactory stimuli in a warming environment.

Effects of housing density on anxiety-like behavior of zebrafish in the plus maze with ramp

Population density in experimental animals is a crucial factor in maintaining the wellbeing of the organisms. Inadequate housing conditions can compromise the validity and reliability of research results, making comparisons between studies difficult. In sociable species such as zebrafish (Danio rerio), which are housed in groups, overcrowding or undercrowding represents a variable that needs to be considered. In this study, we evaluated the effects of housing at different densities for different exposure times on the anxiety response measured in the Plus Maze with Ramp test in zebrafish. The subjects (144) were divided into three large groups according to the housing time (1, 7, and 30 days). Each group was divided into six subgroups based on the density of the fish (0.25, 0.5, 1, 2, 4, 6 fish/liter, n = 8) and housed in a 4-liter aquarium. After the housing conditions, each animal was tested individually in the PMR. Time and housing density altered the exploratory behavior of zebrafish. Increased housing time reduced the time spent in the ramp arms, with groups kept for 30 days spending less time in this compartment. Density increased the time spent in the flat arms in groups with 2 and 6 fish/liter and, conversely, reduced the exploration of the ramp arms. Isolation, on the other hand, increased the exploration of the ramp arms, indicating an anxiolytic effect. In this study, we demonstrate that housing conditions can act as low-intensity chronic stressors that alter anxiety-like behavior in zebrafish when tested in the PMR protocol.

Early-life exposure to five biodegradable plastics impairs eye development and visually-mediated behavior through disturbing hypothalamus-pituitary-thyroid (HPT) axis in zebrafish larvae

Biodegradable plastics have been commonly developed and applied as an alternative to traditional plastics, which cause environmental plastic pollution. However, biodegradable plastics still present limitations such as stringent degradation conditions and slow degradation rate, and may cause harm to the environment and organisms. Consequently, in this study, zebrafish was used to evaluate the effects of five biodegradable microplastics (MPs), polyglycolic acid (PGA), polylactic acid (PLA), polybutylene succinate (PBS), polyhydroxyalkanoate (PHA) and polybutylene adipate terephthalate (PBAT) exposure on the early development, retina morphology, visually-mediated behavior, and thyroid signaling at concentrations of 1 mg/L and 100 mg/L. The results indicated that all MPs induced decreased survival rate, reduced body length, smaller eyes, and smaller heads, affecting the early development of zebrafish larvae. Moreover, the thickness of retinal layers, including inner plexiform layer (IPL), outer nuclear layer (ONL), and retinal ganglion layer (RGL) was decreased, and the expression of key genes related to eye and retinal development was abnormally altered after all MPs exposure. Exposure to PBS and PBAT led to abnormal visually-mediated behavior, indicating likely affected the visual function. All MPs could also cause thyroid system disorders, among which alterations in the thyroid hormone receptors (TRs) genes could affect the retinal development of zebrafish larvae. In summary, biodegradable MPs exhibited eye developmental toxicity and likely impaired the visual function in zebrafish larvae. This provided new evidence for revealing the effects of biodegradable plastics on aquatic organism development and environmental risks to aquatic ecosystems.

Application of machine learning in the study of development, behavior, nerve, and genotoxicity of zebrafish

Machine learning (ML) as a novel model-based approach has been used in studying aquatic toxicology in the environmental field. Zebrafish, as an ideal model organism in aquatic toxicology research, has been widely used to study the toxic effects of various pollutants. However, toxicity testing on organisms may cause significant harm, consume considerable time and resources, and raise ethical concerns. Therefore, ML is used in related research to reduce animal experiments and assist researchers in conducting toxicological research. Although ML techniques have matured in various fields, research on ML-based aquatic toxicology is still in its infancy due to the lack of comprehensive large-scale toxicity databases for environmental pollutants and model organisms. Therefore, to better understand the recent research progress of ML in studying the development, behavior, nerve, and genotoxicity of zebrafish, this review mainly focuses on using ML modeling to assess and predict the toxic effects of zebrafish exposure to different toxic chemicals. Meanwhile, the opportunities and challenges faced by ML in the field of toxicology were analyzed. Finally, suggestions and perspectives were proposed for the toxicity studies of ML on zebrafish in future applications.

Biomarker responses in Danio rerio following an acute exposure (96 h) to e-waste leachate

Electronic waste (e-waste) has been identified as an emerging pollutant and is the fastest growing waste stream at the present time. Significant technological development and modernization within the last decade has led to the rapid accumulation of outdated, broken and unwanted electrical and electronic equipment (EEE). Electronic products mainly consist of a range of metal containing components that, when disposed of improperly, could result in metal constituents leached into the environment and posing a health risk to humans and animals alike. Metal exposure can induce oxidative stress in organisms, which could lead to synergistic, antagonistic and additive effects. The metals found highest in abundance in the simulated e-waste leachate, were nickel (Ni), barium (Ba), zinc (Zn), lithium (Li), iron (Fe), aluminium (Al) and copper (Cu). An acute exposure study was conducted over a 96 h period to determine the potential toxicity of e-waste on the test organism Danio rerio. Biomarker analysis results to assess the biochemical and physiological effects induced by e-waste leachate, showed a statistically significant effect induced on acetylcholinesterase activity, superoxide dismutase, catalase activity, reduced glutathione content, glutathione s-transferase, malondialdehyde and glucose energy available. The Integrated Biomarker Response (IBRv2) analysis revealed a greater biomarker response induced as the exposure concentration of e-waste leachate increased.

Fish 3D Locomotion app: a user-friendly computer application package for automatic data calculation and endpoint extraction for novel tank behavior in fish

This paper introduces the Fish 3D Locomotion app (F3LA), a Python-based, Graphical User Interface (GUI)-equipped tool designed to automate behavioral endpoint extraction in zebrafish locomotion assays. Building on our previous work, which utilized a specialized aquatic tank with a mirror and a single camera for fish movement tracking in three dimensions, F3LA significantly enhances data processing efficiency. Its accuracy was tested by reanalyzing and comprehensively comparing the calculated data with the previously published data from prior publications. From the comparison results, 90% of endpoints showed a similar statistical difference result. These minor differences were due to the different starting points for the dataset and updated calculation formulas that are implemented in F3LA. In addition, shoaling area or shoaling volume calculations are also included in F3LA as a new feature that can serve as sensitive indicators of social cohesion, group dynamics, or stress responses, offering insights into neuropsychological conditions or the effects of pharmacological interventions. Furthermore, F3LA offers a marked improvement over manual operations, being at least five times faster, while maintaining consistent accuracy as it reduces human-induced errors, ensuring a higher degree of reliability in the results. Finally, the potency of F3LA was tested to evaluate the toxicities of 14 rare earth elements (REEs) to the adult zebrafish behaviors. Based on the results, our findings suggested that each tested REE altered fish behaviors in different patterns and magnitudes to each other. However, among the tested light rare earth elements (LREEs), neodymium was demonstrated to cause more relatively severe behavior alterations than other LREEs, indicated by the statistically higher value of entropy (0.2695 ± 0.04977 (mean with a standard deviation)) than the control group (0.2352 ± 0.05896). Meanwhile, in terms of heavy rare earth elements (HREEs), erbium seemed to lead to more distinct behavior toxicities than other HREEs, which was shown by the statistically lower level of fractal dimension (2.022 ± 0.3412) than the untreated group (2.255 ± 0.1661). Taken together, F3LA's development marks a significant advance in high-throughput toxicological and pharmacological assessments in zebrafish, leveraging three-dimensional locomotion data for a more comprehensive analysis of fish behavior performance, providing a significant contribution to research in various fields.

Energetic Costs of Stress in Developing Fishes: Quantifying Allostasis and Allostatic Load

Stress exerts negative effects on fish health through stimulation of the hypothalamic-pituitary-interrenal axis and autonomic nervous system, resulting in heightened neural and neuroendocrine responses. Energetic investment and physiological adaptation are then required to re-establish homeostatic stability or reach a new allostatic state. The cost of the energetic investment is referred to as allostatic load (AL). While determining the sources of stress and assessing their consequences have resulted in estimates of AL, most of this work has been conducted in adult mammals and humans; no ALs exist for developing fish. From a series of experiments on a model species, zebrafish (Danio rerio), whose yolk-sac larvae were exposed to two chronic stressors (high-temperature and hypoxia), ALs were quantified based on biomarkers of ontogenetic changes in growth, morphometrics, and metabolic activities. Results showed that for zebrafish yolk-sac larvae, chronic stress imposed high AL and, thus, high total allostatic energetic costs, (Rt (AL)), because of prolonged energy demand in the face of limited resources (e.g., yolk). Under severe chronic stress, energetic costs were sufficiently large that energy-limited developing fish may not be able to fully compensate, resulting in maladaptive responses from allostatic overload, leading either to death or to novel allostatic states, possibly more resilient to environmental change.

Environmental Temperature Variation Affects Brain Lipid Composition in Adult Zebrafish (Danio rerio)

This study delves deeper into the impact of environmental temperature variations on the nervous system in teleost fish. Previous research has demonstrated that exposing adult zebrafish (Danio rerio) to 18 °C and 34 °C for 4 or 21 days induces behavioural changes compared to fish kept at a control temperature of 26 °C, suggesting alterations in the nervous system. Subsequent studies revealed that these temperature conditions also modify brain protein expression, indicating potential neurotoxic effects. The primary aim of this work was to investigate the effects of prolonged exposure (21 days) to 18 °C or 34 °C on the brain lipidomes of adult zebrafish compared to a control temperature. Analysis of the brain lipidome highlighted significant alteration in the relative abundances of specific lipid molecules at 18 °C and 34 °C, confirming distinct effects induced by both tested temperatures. Exposure to 18 °C resulted in an increase in levels of phospholipids, such as phosphatidylethanolamine, alongside a general reduction in levels of sphingolipids, including sphingomyelin. Conversely, exposure to 34 °C produced more pronounced effects, with increases in levels of phosphatidylethanolamine and those of various sphingolipids such as ceramide, gangliosides, and sphingomyelin, alongside a reduction in levels of ether phospholipids, including lysophosphatidylethanolamine ether, phosphatidylethanolamine ether, and phosphatidylglycerol ether, as well as levels of glycolipids like monogalactosyldiacylglycerol. These results, when integrated with existing proteomic and behavioural data, offer new insights into the effects of thermal variations on the nervous system in teleost fish. Specifically, our proteomic and lipidomic findings suggest that elevated temperatures may disrupt mitochondrial function, increase neuronal susceptibility to oxidative stress and cytotoxicity, alter axonal myelination, impair nerve impulse transmission, hinder synapse function and neurotransmitter release, and potentially lead to increased neuronal death. These findings are particularly relevant in the fields of cell biology, neurobiology, and ecotoxicology, especially in the context of global warming.

The Human Pathogen Mycobacterium tuberculosis and the Fish Pathogen Mycobacterium marinum Trigger a Core Set of Late Innate Immune Response Genes in Zebrafish Larvae

Zebrafish is a natural host of various Mycobacterium species and a surrogate model organism for tuberculosis research. Mycobacterium marinum is evolutionarily one of the closest non-tuberculous species related to M. tuberculosis and shares the majority of virulence genes. Although zebrafish is not a natural host of the human pathogen, we have previously demonstrated successful robotic infection of zebrafish embryos with M. tuberculosis and performed drug treatment of the infected larvae. In the present study, we examined for how long M. tuberculosis can be propagated in zebrafish larvae and tested a time series of infected larvae to study the transcriptional response via Illumina RNA deep sequencing (RNAseq). Bacterial aggregates carrying fluorescently labeled M. tuberculosis could be detected up to 9 days post-infection. The infected larvae showed a clear and specific transcriptional immune response with a high similarity to the inflammatory response of zebrafish larvae infected with the surrogate species M. marinum. We conclude that M. tuberculosis can be propagated in zebrafish larvae for at least one week after infection and provide further evidence that M. marinum is a good surrogate model for M. tuberculosis. The generated extensive transcriptome data sets will be of great use to add translational value to zebrafish as a model for infection of tuberculosis using the M. marinum infection system. In addition, we identify new marker genes such as dusp8 and CD180 that are induced by M. tuberculosis infection in zebrafish and in human macrophages at later stages of infection that can be further investigated.

The impact of predators and vegetation on shoaling in wild zebrafish

In their natural habitats, animals experience multiple ecological factors and regulate their social responses accordingly. To unravel the impact of two ecological factors on the immediate behavioural response of groups, we conducted experiments on wild zebrafish shoals in arenas with vegetation, predator cues, and both factors simultaneously or neither (control treatments). Analysis of 297 trials revealed that while shoals formed significantly larger subgroups in the presence of predator cues, their subgroup size was comparable to control treatments when they faced predator cues and vegetation. Shoals were highly polarized in open arenas, in the absence of either ecological factors and in the presence of predator cues (with/without vegetation). The presence of vegetation alone, however, significantly reduced shoal polarization. Furthermore, food intake was significantly reduced when predator cues and/or vegetation were present. Tracking individuals revealed that (i) individuals within shoals receiving predator cues had a significantly higher probability to continue being in a group compared with control treatments and (ii) individuals occupying the front positions deviated less from their median position within a shoal as compared with other individuals regardless of predator cues. The adaptability of animals depends on behavioural responses to changing environments, making this study significant in the context of environmental changes.

Evaluation of a gel-based versus micro-pellet diet for adult zebrafish (Danio rerio)

Ad libitum feeding of laboratory zebrafish has potential benefits for colony management, but would require a new type of diet, such as a gel that remains in the tank. We hypothesized that adult zebrafish fed a gel diet would have similar body size and reproductive success compared with those fed a standard micro-pellet diet. The gel diet’s impact on water quality was determined to be safe for zebrafish prior to starting a 12-week feeding study. Two hundred adult AB zebrafish of mixed sex were randomly assigned to be fed exclusively either gel or micro-pellet diet. Fish body length and mass were measured every two weeks, and fish were bred within each feed group to assess fecundity. Zebrafish consumed less gel diet than expected. Body length, mass, and breeding success were lower in the gel diet fish than in the micro-pellet diet fish. Low consumption of the gel diet and/or nutritional differences between the two diets may have contributed to reduced growth and fecundity. Though the gel diet could reduce time personnel spend feeding and be safer for fish in static tanks, the tested formulation was not a satisfactory alternative to the control micro-pellet diet in a research zebrafish colony.

Shoals in troubled waters? The impact of rising temperatures on metabolism, foraging, and shoaling behavior in mixed-species shoals

Rising water temperatures across aquatic habitats, in the current global climate change scenario, can directly affect metabolism and food intake in fish species. This can potentially alter their physiological, behavioral, and shoaling properties. In the current study, we examined the effects of high temperatures on metabolism, foraging, and shoaling in tropical fish. Mixed-species (comprising flying barbs, zebrafish, and gambusia) and single-species (flying barbs and zebrafish) shoals were conditioned for 45 days to three kinds of temperature regimes: the current temperature regime (CTR), in which shoals were maintained at water temperature of 24°C (i.e., the current mean temperature of their habitat), the predicted temperature regime (PTR) at 31°C (i.e., simulating conditions projected for their habitat in 2100), and the dynamic temperature regime (DTR), which experienced daily temperature fluctuations between 24 and 31°C (i.e., resembling rapid temperature changes expected in their natural environments). We found species-specific responses to these temperature regimes. Flying barbs exhibited significantly lower body weight at PTR but maintained consistent muscle glycogen content across all temperature regimes. In contrast, zebrafish and gambusia displayed significantly elevated muscle glycogen content at PTR, with similar body weights across all three temperature regimes. Cohesion within flying barb shoals and cohesion/polarization in mixed-species shoals decreased significantly at PTR. Shoals exposed to DTR exhibited intermediate characteristics between those conditioned to CTR and PTR, suggesting that shoals may be less impacted by dynamic temperatures compared to prolonged high temperatures. This study highlights species-specific metabolic responses to temperature changes and their potential implications for larger-scale shoal properties.

Looks like home: numerosity, but not spatial frequency guides preference in zebrafish larvae (Danio rerio)

Despite their young age, zebrafish larvae have a well-developed visual system and can distinguish between different visual stimuli. First, we investigated if the first visual surroundings the larvae experience during the first days after hatching shape their habitat preference. Indeed, these animals seem to "imprint" on the first surroundings they see and select visual stimuli accordingly at 7 days post fertilization (dpf). In particular, if zebrafish larvae experience a bar background just after hatching, they later on prefer bars over white stimuli, and vice versa. We then used this acquired preference for bars to investigate innate numerical abilities. We wanted to specifically test if the zebrafish larvae show real numerical abilities or if they rely on a lower-level mechanism-i.e. spatial frequency-to discriminate between two different numerosities. When we matched the spatial frequency in stimuli with different numbers of bars, the larvae reliably selected the higher numerosity. A previous study has ruled out that 7 dpf zebrafish larvae use convex hull, cumulative surface area and density to choose between two numerosities. Therefore, our results indicate that zebrafish larvae rely on real numerical abilities rather than other cues, including spatial frequency, when spontaneously comparing two sets with different numbers of bars.