Swim Bladder Inflation in Fish: A Sensitive Endpoint for Risk Assessment and Mechanistic Research

The swim bladder is a vital organ in many bony fish, playing an essential role in buoyancy regulation, position control in water and consequently survival. Ultimately, its potential malfunctioning may affect foraging efficiency, predator avoidance, and therefore population fitness. Nevertheless, proper swim bladder inflation is rarely systematically recorded as an endpoint in fish toxicity studies, regardless of the chemical class or mode of action under investigation. This represents a missed opportunity to detect sensitive sublethal effects and accumulate mechanistic evidence across studies. This paper therefore argues for the systematic inclusion of swim bladder inflation measurements in experimental designs that assess chemical exposure or manipulate neural and endocrine function. We highlight the biological basis of swim bladder development and inflation, summarize the growing body of evidence indicating its sensitivity to various chemical contaminants and pharmacological agents, and discuss its potential as an integrative indicator linking endocrine, metabolic, and neurobehavioral processes. We also identify key practical and methodological considerations for incorporating this endpoint into standardized tests, including timing, observation methods and species applicability. Incorporating this endpoint in studies ranging from ecotoxicological testing to neurodevelopmental and behavioral experiments could enhance the mechanistic interpretation of results, improve the detection of subtle sublethal effects and strengthen the ecological and physiological relevance of fish-based research.