Sensory ecology of ostariophysan alarm substances

Identification of olfactory alarm substances in zebrafish

Escaping from danger is one of the most fundamental survival behaviors for animals. Most freshwater fishes display olfactory alarm reactions in which an injured fish releases putative alarm substances from the skin to notify its shoaling company about the presence of danger. Here, we identified two small compounds in zebrafish skin extract, designated as ostariopterin and daniol sulfate. Ostariopterin is a pterin derivative commonly produced in many freshwater fishes belonging to the Ostariophysi superorder. Daniol sulfate is a novel sulfated bile alcohol specifically present in the Danio species, including zebrafish. Ostariopterin and daniol sulfate activate distinct glomeruli in the olfactory bulb. Zebrafish display robust alarm reactions, composed of darting, freezing, and bottom dwelling, only when they are concomitantly stimulated with ostariopterin and daniol sulfate. These results demonstrate that the fish alarm reaction is driven through a coincidence detection mechanism of the two compounds along the olfactory neural circuitry.

Sensory environment affects Icelandic threespine stickleback's anti-predator escape behaviour

Human-induced changes in climate and habitats push populations to adapt to novel environments, including new sensory conditions, such as reduced visibility. We studied how colonizing newly formed glacial lakes with turbidity-induced low-visibility affects anti-predator behaviour in Icelandic threespine sticklebacks. We tested nearly 400 fish from 15 populations and four habitat types varying in visibility and colonization history in their reaction to two predator cues (mechano-visual versus olfactory) in high versus low-visibility light treatments. Fish reacted differently to the cues and were affected by lighting environment, confirming that cue modality and light levels are important for predator detection and evasion. Fish from spring-fed lakes, especially from the highlands (likely more diverged from marine fish than lowland fish), reacted fastest to mechano-visual cues and were generally most active. Highland glacial fish showed strong responses to olfactory cues and, counter to predictions from the flexible stem hypothesis, the greatest plasticity in response to light levels. This study, leveraging natural, repeated invasions of novel sensory habitats, (i) illustrates rapid changes in anti-predator behaviour that follow due to adaptation, early life experience, or both, and (ii) suggests an additional role for behavioural plasticity enabling population persistence in the face of frequent changes in environmental conditions.

Predictable chronic stress modulates behavioral and neuroendocrine phenotypes of zebrafish: Influence of two homotypic stressors on stress-mediated responses

The zebrafish (Danio rerio) has been considered a suitable model organism to assess the evolutionarily conserved bases of behavioral and neuroendocrine responses to stress. Depending on the nature of the stressor, prolonged stress may elicit habituation or evoke long-term changes in the central nervous systems (CNS) often associated with various neuropsychiatric disorders. Conspecific alarm substance (CAS) and net chasing (NC) constitute chemical and physical stressors, respectively, which cause aversive behaviors and physiological changes in fishes. Here, we investigate whether predictable chronic stress (PCS) using two homotypic stressors differently modulates behavioral and physiological responses in zebrafish. PCS-CAS or PCS-NC were performed for 14 days, 2-times daily, while locomotion, exploratory activity, anxiety-like behaviors, and whole-body cortisol levels were measured on day 15. PCS-CAS reduced distance traveled, the number of transitions and time in top area, as well as increased the latency to enter the top in the novel tank test. In the light/dark test, CAS-exposed fish showed decreased time spent in lit area, shorter latency to enter the dark area, and increased risk assessments. PCS-CAS also increased whole-body cortisol levels in zebrafish. Although PCS-NC reduced the latency to enter the dark area, whole-body cortisol levels did not change. Moreover, acute experiments revealed that both CAS and NC promoted anxiogenesis and increased cortisol levels, suggesting habituation to stress following PCS-NC. Overall, our novel findings demonstrate that PCS induces behavioral and physiological changes in zebrafish depending on the nature of the stressor.