Beyond the behavioural phenotype: Uncovering mechanistic foundations in aquatic eco-neurotoxicology

Generalized additive modeling as a tool for the analysis of the time course of tail coiling behavior in zebrafish (Danio rerio) embryos - A proof-of-concept study with nicotine, a known developmental neurotoxicant

The early detectable tail coiling behavior of zebrafish (Danio rerio) embryos is receiving increasing attention in the context of (developmental) neurotoxicity testing and may be used as a rapid screening tool for compounds with unknown or suspected neurotoxic potential. The observation of this behavior over a longer period of time already offered advantages such as the possibility of detecting effects that only occur after a few hours of development. The two major parameters, duration and frequency of coiling, allow a detailed characterization of the movements. However, this approach usually leads to complex data sets, which are often heavily simplified to allow for simpler analysis of the effects on an hourly basis. In this study, the suitability of generalized additive modeling (GAM) for the analysis of coiling behavior was tested in order to obtain an integrated impression of the trends in movement patterns. To this end, nicotine, a known potent developmental neurotoxicant, was used in a proof-of-concept study. The main advantage of GAM for biological data lies in the relaxation of assumptions, such as effect monotony, data distribution and homogeneity of variances and is, therefore, more flexible in describing different trends over time. The possibility to consider replicates and individuals as additional sources of (biological) variance is a further benefit, as highly variable data are common in behavioral studies. Here, the modeling approach demonstrates a monotone reduction of movement duration as a direct consequence of nicotine exposure. Additional pathomorphological studies revealed structural damage in secondary motoneurons and skeletal muscles as potential underlying mechanisms of changes in movement patterns. The GAM proved well-suited to illustrate and analyze complex non-linear behavioral data with high natural variability. The model also allows to reliably extract no observed effect (NOEC) and lowest observed effect concentrations (LOEC) from complex data sets, which may be of relevance in a regulatory context.

Seizure-like behavior and hyperactivity in napb knockout zebrafish as a model for autism and epilepsy

We identified N-ethylmaleimide-sensitive factor attachment protein beta (NAPB) as a potential risk gene for autism and epilepsy. Notably, Qatari monozygotic triplets with loss of function mutations in NAPB exhibit early onset epileptic encephalopathy and varying degrees of autism. In this study, we generated NAPB zebrafish model using CRISPR-Cas9-sgRNAs technology for gene editing of the two orthologs napba and napbb. We observed that napb crispants (CR) show shorter motor neuron axons length together with altered locomotion behavior, including significant increases in larvae total distance traveled, swimming velocity, and rotation frequency, indicating that these behavioral changes effectively mimic the human epileptic phenotype. We applied microelectrode array (MEA) technology to monitor neural activity and hyperactivity in the zebrafish model. The napb CR shows hyperexcitability in the brain region. By combining behavioral tests with electrophysiological MEA assays, the established NAPB zebrafish model can be employed to study the pathophysiological mechanisms of ASD and epilepsy to screen potential therapeutic drugs.

Caging Gammarus roeseli to track pesticide contamination: How agricultural practices shape water quality in small waterbodies?

Contaminant monitoring in agroecosystems is increasingly revealing overlooked molecules, particularly within complex pesticide mixtures. This study assessed the effectiveness of chemical and ecotoxicological methods for evaluating contamination and biological responses in Gammarus roeseli exposed to pesticides and transformation products (TPs) in lentic small water bodies (LSWBs) near agricultural zones. We examined 7 LSWBs, finding variable contamination levels shaped by watershed composition differences. Analysis of 136 compounds identified key TPs, including chlorothalonil R471811, metazachlor ESA, and OXA, which collectively represented 86.2 % of the total quantified contaminants. These results underscore the persistence of both current and banned pesticides in the ponds studied. While G. roeseli showed favorable survival rates, significant reductions in locomotion and ventilation were observed at heavily contaminated sites, with biochemical analyses suggesting neurotoxic effects and activation of detoxification mechanisms in response to contaminants. Multivariate analyses revealed site-specific variations, highlighting the complex interactions between contamination levels and environmental conditions. Biomarker responses in gammarids served as sensitive indicators of residual toxicity in LSWBs, with frequent associations with historical contamination or current pesticide applications. This in situ caging approach across a contamination gradient demonstrates strong potential for biomonitoring and ecotoxicological assessments in agroecosystems. Extending exposure durations and including more heavily contaminated ponds could further enhance risk evaluation, thereby improving biomonitoring accuracy in headwater aquatic ecosystems. By integrating site-specific environmental conditions, contamination profiles, and biological responses, this study provides valuable insights into the influence of agricultural practices on LSWBs contamination and underscores the critical need to incorporate TPs into future risk assessment frameworks.

Bioimaging marine crustacean brain: quantitative comparison of micro-CT preparations in an Alpheid snapping shrimp

Non-invasive bioimaging techniques like X-ray micro-computed tomography (μCT), combined with contrast-enhancing techniques, allow the 3D visualization of the central nervous system in situ, without the destruction of the sample. However, quantitative comparisons of the most common fixation and contrast-enhancing protocols are rare, especially in marine invertebrates. Using the snapping shrimp (Alpheus richardsoni) as a model, we test three common fixation and staining agents combinations to prepare specimens prior to μCT scanning. The contrast ratios of the resulting images are then quantitatively compared. Our results show that a buffered iodine solution on a specimen fixed with 10% formalin offers the best nervous tissue discriminability. This optimal combination allows a semi-automated segmentation of the central nervous system organs from the μCT images. We thus provide general guidance for μCT applications, particularly suitable for marine crustaceans. Species-specific morphological adaptations can then be characterized and studied in the context of evolution and behavioral ecology.

Behavioral Responses of Chironomus aprilinus Larvae as Proxies for Cyanobacterial Metabolite Interactions: Insights from Ternary Combinations

This study aimed to assess the behavioral responses (immobilization, horizontal and vertical motility, and response to light) of Chironomus aprilinus larvae exposed to individual cyanobacterial metabolites aeruginosin 98B (AER-B), anabaenopeptin-B (ANA-B), and cylindrospermopsin (CYL), and their binary and ternary mixtures. The investigation revealed that single metabolites ANA-B and CYL exhibited the highest potency in immobilizing the larvae. Notably, the binary mixture AER-B+CYL induced a remarkably strong synergistic interaction, while other tested binary and ternary mixtures demonstrated antagonistic effects. Both individual metabolites and their mixtures led to a decrease in larval movement speed, with the AER-B+CYL combination showing a very synergistic effect, and strong antagonistic interactions between the oligopeptides in the ternary mixture. Conversely, while AER-B and the binary mixture ANA-B+CYL stimulated vertical movement, other single metabolites and binary and ternary mixtures decreased this parameter. Antagonistic interactions were observed in all mixtures. ANA-B emerged as the most potent inhibitor, yet all tested metabolites and their mixtures decreased larval response to light, displaying synergistic interactions, except for the AER-B+ANA-B mixture at 250 μg L-1 + 250 μg L-1. These findings underscore the sensitivity of Chironomus larvae behavioral parameters as indicators of environmental stressors and mixtures. Consequently, they are recommended for assessing toxic effects induced by cyanobacterial products and other bioactive chemicals.

Local thermal adaption mediates the sensitivity of Daphnia magna to nanoplastics under global warming scenarios

The toxicity of nanoplastics at environmentally relevant concentrations has received widespread attention in the context of global warming. Despite numerous studies on the impact of mean temperature (MT), the effects of daily temperature fluctuations (DTFs) on the ecotoxicity of nanoplastics remains largely unexplored. Moreover, the role of evolutionary adaptation in assessing long-term ecological risks is unclear. Here, we investigated the effects of polystyrene nanoplastics (5 μg L-1) on Daphnia magna under varying MT (20 °C and 24 °C) and DTFs (0 °C, 5 °C, and 10 °C). Capitalizing on a space-for-time substitution approach, we further assessed how local thermal adaptation affect the sensitivity of Daphnia to nanoplastics under global warming. Our results indicated that nanoplastics exposure in general reduced heartbeat rate, thoracic limb activity and feeding rate, and increased CytP450, ETS activity and Hgb concentrations. Higher MT and DTFs enhanced these effects. Notably, clones originating from their respective sites performed better under their native temperature conditions, indicating local thermal adaptation. Warm-adapted low-latitude D. magna showed stronger nanoplastics-induced increases in CytP450, ETS activity and Hgb concentrations under local MT 24 °C, while cold-adapted high-latitude D. magna showed stronger nanoplastics-induced decreases in heartbeat rate, thoracic limb activity and feeding rate under high MT than under low MT.

Effects of ambient UVB light on Pacific oyster Crassostrea gigas mantle tissue based on multivariate data

Ambient ultraviolet radiation (UVB) from solar and artificial light presents serious environmental risks to aquatic ecosystems. The Pacific oyster, Crassostrea gigas, perceives changes in the external environment primarily through its mantle tissue, which contains many nerve fibers and tentacles. Changes within the mantles can typically illustrate the injury of ambient UVB. In this study, a comprehensive analysis of phenotypic, behavioral, and physiological changes demonstrated that extreme UVB radiation (10 W/m²) directly suppressed the behavioral activities of C. gigas. Conversely, under ambient UVB radiation (5 W/m²), various physiological processes exhibited significant alterations in C. gigas, despite the behavior remaining relatively unaffected. Using mathematical model analysis, the integrated analysis of the full-length transcriptome, proteome, and metabolome showed that ambient UVB significantly affected the metabolic processes (saccharide, lipid, and protein metabolism) and cellular biology processes (autophagy, apoptosis, oxidative stress) of the C. gigas mantle. Subsequently, using Procrustes analysis and Pearson correlation analysis, the association between multi-omics data and physiological changes, as well as their biomarkers, revealed the effect of UVB on three crucial biological processes: activation of autophagy signaling (key factors: Ca2+, LC3B, BECN1, caspase-7), response to oxidative stress (reactive oxygen species, heat shock 70, cytochrome c oxidase), and recalibration of energy metabolism (saccharide, succinic acid, translation initiation factor IF-2). These findings offer a fresh perspective on the integration of multi-data from non-model animals in ambient UVB risk assessment.

Toward Real-Time Animal Tracking with Integrated Stimulus Control for Automated Conditioning in Aquatic Eco-Neurotoxicology

Aquatic eco-neurotoxicology is an emerging field that requires new analytical systems to study the effects of pollutants on animal behaviors. This is especially true if we are to gain insights into one of the least studied aspects: the potential perturbations that neurotoxicants can have on cognitive behaviors. The paucity of experimental data is partly caused by a lack of low-cost technologies for the analysis of higher-level neurological functions (e.g., associative learning) in small aquatic organisms. Here, we present a proof-of-concept prototype that utilizes a new real-time animal tracking software for on-the-fly video analysis and closed-loop, external hardware communications to deliver stimuli based on specific behaviors in aquatic organisms, spanning three animal phyla: chordates (fish, frog), platyhelminthes (flatworm), and arthropods (crustacean). The system's open-source software features an intuitive graphical user interface and advanced adaptive threshold-based image segmentation for precise animal detection. We demonstrate the precision of animal tracking across multiple aquatic species with varying modes of locomotion. The presented technology interfaces easily with low-cost and open-source hardware such as the Arduino microcontroller family for closed-loop stimuli control. The new system has potential future applications in eco-neurotoxicology, where it could enable new opportunities for cognitive research in diverse small aquatic model organisms.

Cyanobacterial metabolites: aeruginosin 98A, microginin-FR1, anabaenopeptin-A, cylindrospermopsin and their mixtures affect behavioral and physiological responses of Daphnia magna

We determined the effects influence of cyanobacterial products metabolites: aeruginosin-A (AER-A), microginin-FR1 (MG-FR1), anabaenopeptin-A (ANA-A), cylindrospermopsin (CYL) and their binary and quadruple mixtures on swimming behavior, heart rate, thoracic limb activity, oxygen consumption and in vivo cell health of Daphnia magna. The study showed that CYL induced mortality of daphnids at the highest concentrations, however three oligopeptides had no lethal effect. All the tested Each single metabolites inhibited swimming speed. The mixtures AER+MG-FR1 and AER-A+ANA-A induced antagonistic and the quadruple mixture synergistic effects. Physiological endpoints were depressed by CYL, however they were simulated by the oligopeptides and their binary mixtures. The quadruple mixture inhibited the physiological parameters with antagonistic interactions between the components were antagonistic. Single CYL, MG-FR1 and ANA-A induced cytotoxicity with synergistic interactions and the metabolites in mixtures showed. The study suggests that swimming behavior and physiological parameters may be affected by single cyanobacterial oligopeptides, however their mixtures may induce different total effects.

The sensitivity of the zebrafish embryo coiling assay for the detection of neurotoxicity by compounds with diverse modes of action

In the aim to determine neurotoxicity, new methods are being validated, including tests and test batteries comprising in vitro and in vivo approaches. Alternative test models such as the zebrafish (Danio rerio) embryo have received increasing attention, with minor modifications of the fish embryo toxicity test (FET; OECD TG 236) as a tool to assess behavioral endpoints related to neurotoxicity during early developmental stages. The spontaneous tail movement assay, also known as coiling assay, assesses the development of random movement into complex behavioral patterns and has proven sensitive to acetylcholine esterase inhibitors at sublethal concentrations. The present study explored the sensitivity of the assay to neurotoxicants with other modes of action (MoAs). Here, five compounds with diverse MoAs were tested at sublethal concentrations: acrylamide, carbaryl, hexachlorophene, ibuprofen, and rotenone. While carbaryl, hexachlorophene, and rotenone consistently induced severe behavioral alterations by ~ 30 h post fertilization (hpf), acrylamide and ibuprofen expressed time- and/or concentration-dependent effects. At 37-38 hpf, additional observations revealed behavioral changes during dark phases with a strict concentration-dependency. The study documented the applicability of the coiling assay to MoA-dependent behavioral alterations at sublethal concentrations, underlining its potential as a component of a neurotoxicity test battery.

Neurotoxicity of polystyrene nanoplastics with different particle sizes at environment-related concentrations on early zebrafish embryos

Nanoplastics (NPs) have received global attention due to their wide application and detection in various environmental or biological media. NPs can penetrate physical barriers and accumulate in organisms after being ingested, producing a variety of toxic effects and possessing particle size-dependent effects, distinguishing them from traditional contaminants. This paper explored the neurotoxicity of polystyrene (PS)-NPs of different particle sizes on zebrafish (Danio rerio) embryos at environmental concentrations at the tissue and molecular levels using visualized transgenic zebrafish. Results showed that all particle sizes of PS-NPs produced developmental toxicity in zebrafish embryos and induced neuronal loss, axonal deletion/shortening/hybridization, and developmental and apoptotic-related genetic alterations, ultimately leading to behavioral abnormalities. PS-NPs with smaller sizes may have more severe neurotoxicity due to their entry into the embryo and brain through the chorionic pore before hatching. In addition, PS-NPs at 100 nm and 1000 nm can specifically interfere with GABAergic, cholinergic or serotonergic system and affect neuronal signaling. Our results reveal the neurotoxic risk of NPs, and smaller particle-size NPs may have a greater ecological risk. We anticipate that our study can provide a basis for exploring the toxicity mechanisms of NPs and the environmental risk assessment of NPs.

Adapting classic paradigms to analyze alterations of shoal-wide behavior in early-life stages of zebrafish (Danio rerio) - A case study with fluoxetine

Given the strong increase in prescription of neuroactive pharmaceuticals, neurotoxicity has received growing concern in science and the public. Regulatory requirements stimulated the development of new methods to evaluate the risk of neurotoxic substances for humans and the environment, and, with respect to potential damage to aquatic ecosystems, a variety of behavior-based assays have been proposed for neurotoxicity testing, most of which, however, are restricted to changes in the behavior of individual fish. Since many fish species form shoals under natural conditions, this may cause important aspects of behavior to be overlooked and there is a need for behavior assays integrating individual behavior with behavior of the entire swarm. In order to combine more environmentally realistic sub-chronic exposure scenarios with undistorted social behavior and animal welfare considerations, two behavioral assays are proposed that might be integrated into early-life stage toxicity studies according to OECD TG 210, which are commonly run for a multitude of regulations: To this end, protocols for a novel tank test and a predator response assay were adapted to also record the behavior of free-swimming zebrafish (Danio rerio) juveniles within shoals. Comparisons of the diving response (novel tank) or the shoal's coherence and position relative to the stimulus (predator) with control groups allow conclusions about the anxiety state of the fish, which might well have an impact on survival chances in the wild. As a model substance, the antidepressant fluoxetine ((RS)-N-Methyl-3-phenyl-3-(4-trifluoromethylphenoxy)propylamine) produced adverse effects down to concentrations three orders of magnitude below the EC₁₀ from acute fish embryo toxicity tests according to OECD TG 236. With the integration of such behavior tests into OECD TG 210, important population-relevant information on potential neurotoxicity can be collected without increasing the number of experimental animals.

High-Throughput Phototactic Ecotoxicity Biotests with Nauplii of Artemia franciscana

Analysis of sensorimotor behavioral responses to stimuli such as light can provide an enhanced relevance during rapid prioritisation of chemical risk. Due to technical limitations, there have been, however, only minimal studies on using invertebrate phototactic behaviors in aquatic ecotoxicity testing. In this work, we demonstrate an innovative, purpose-built analytical system for a high-throughput phototactic biotest with nauplii of euryhaline brine shrimp Artemia franciscana. We also, for the first time, present a novel and dedicated bioinformatic approach that facilitates high-throughput analysis of phototactic behaviors at scale with great fidelity. The nauplii exhibited consistent light-seeking behaviors upon extinguishing a brief programmable light stimulus (5500K, 400 lux) without habituation. A proof-of-concept validation involving the short-term exposure of eggs (24 h) and instar I larval stages (6 h) to sub-lethal concentrations of insecticides organophosphate chlorpyrifos (10 µg/L) and neonicotinoid imidacloprid (50 µg/L) showed perturbation in light seeking behaviors in the absence of or minimal alteration in general mobility. Our preliminary data further support the notion that phototactic bioassays can represent an attractive new avenue in behavioral ecotoxicology because of their potential sensitivity, responsiveness, and low cost.

Sensory-Motor Perturbations in Larval Zebrafish (Danio rerio) Induced by Exposure to Low Levels of Neuroactive Micropollutants during Development

Due to increasing numbers of anthropogenic chemicals with unknown neurotoxic properties, there is an increasing need for a paradigm shift toward rapid and higher throughput behavioral bioassays. In this work, we demonstrate application of a purpose-built high throughput multidimensional behavioral test battery on larval stages of Danio rerio (zebrafish) at 5 days post fertilization (dpf). The automated battery comprised of the established spontaneous swimming (SS), simulated predator response (SPR), larval photomotor response (LPR) assays as well as a new thermotaxis (TX) assay. We applied the novel system to characterize environmentally relevant concentrations of emerging pharmaceutical micropollutants including anticonvulsants (gabapentin: 400 ng/L; carbamazepine: 3000 ng/L), inflammatory drugs (ibuprofen: 9800 ng/L), and antidepressants (fluoxetine: 300 ng/L; venlafaxine: 2200 ng/L). The successful integration of the thermal preference assay into a multidimensional behavioral test battery provided means to reveal ibuprofen-induced perturbations of thermal preference behaviors upon exposure during embryogenesis. Moreover, we discovered that photomotor responses in larval stages of fish are also altered by the as yet understudied anticonvulsant gabapentin. Collectively our results demonstrate the utility of high-throughput multidimensional behavioral ecotoxicity test batteries in prioritizing emerging risks associated with neuroactive drugs that can perturb neurodevelopment. Moreover, we showcase the added value of thermotaxis bioassays for preliminary screening of emerging contaminants.