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While soundscapes shape the structure and function of auditory systems over evolutionary timescales, there is limited information regarding the adaptation of wild fish populations to their natural acoustic environments. This is particularly relevant for freshwater ecosystems, which are extremely diverse and face escalating pressures from human activities and associated noise pollution. The Siamese fighting fish Betta splendens is one of the most important cultured species in the global ornamental fish market and is increasingly recognized as a model organism for genetics and behavioural studies. This air-breathing species (Anabantoidei), characterized by the presence of a suprabranchial labyrinth organ that enhances auditory sensitivity, is native to Southeast Asia and inhabits low flow freshwater ecosystems that are increasingly threatened due to habitat destruction and pollution. We characterized the underwater soundscape, along with various ecological parameters, across five marshland habitats of B. splendens, from lentic waterbodies to small canals near a lake in Chiang Rai province (Thailand). All habitats exhibited common traits of low dissolved oxygen and dense herbaceous vegetation. Soundscapes were relatively quiet with Sound Pressure Level (SPL) around 102-105 dB re 1 mu Pa and most spectral energy below 1,000 Hz. Sound recordings captured diverse biological sounds, including potential fish vocalizations, but primarily insect sounds. Hearing thresholds were determined using auditory evoked potential (AEP) recordings, revealing best hearing range within 100-400 Hz. Males exhibited lower hearing thresholds than females at 400 and 600 Hz. This low-frequency tuning highlights the potential susceptibility of B. splendens to anthropogenic noise activities. This study provides first characterization of the auditory sensitivity and natural soundscape of B. splendens, establishing an important ground for future hearing research in this species. The information provided on the auditory sensory adaptation of B. splendens emphasizes the importance of preserving quiet soundscapes from lentic freshwater ecosystems.
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Aggressive behavior is an adaptive trait present across all taxa. However, the neuroendocrine mechanisms regulating it, particularly in fish, are not well understood. Oxytocin (OXT) and arginine vasotocin (VT) are known modulators of aggression, but their actions remain controversial. This study tested the possible modulation of endocrine and behavioral responses to an aggression challenge by these nonapeptides in Siamese fighting fish, Betta splendens, a species known for its intrinsic aggressiveness. Male B. splendens were injected with different dosages of either Manning compound or L-368,899, VT and OXT receptor antagonists respectively, and were exposed to a mirror challenge for 30 min. While all fish displayed high levels of aggression toward their mirror image, no differences were observed between control-injected and treatment fish. However, blocking VT inhibited the post-fight increase in plasma levels of the androgen 11-ketotestosterone (KT). To further investigate this result, testis tissue from males was incubated with and without VT and Manning compound, and KT levels were measured after 180 min. Results showed a direct effect of VT on in vitro KT secretion, indicating the presence of VT receptors in the testes of this species. Overall, the study does not support a modulatory role of VT or OXT in aggressive behavior, although VT might be implicated in the regulation of peripheral androgen response to aggression in B. splendens.