Mangrove fishes rely on emersion behavior and physiological tolerance to persist in sulfidic environments

Hydrogen sulfide (H2S) is a potent respiratory toxin that makes sulfidic environments tolerable to only a few organisms. We report the presence of fishes (Kryptolebias marmoratus, Poecilia orri, Gambusia sp., and Dormitator maculatus) in Belizean mangrove pools with extremely high H2S concentrations (up to 1,166 mM) that would be lethal for most fishes. Thus, we asked whether the three most prevalent species (Kryptolebias, Poecilia, and Gambusia) persist in sulfidic pools because they are exceptionally H2S tolerant and/or because they can leave water (emerse) and completely avoid H2S. We show that both physiological tolerance and emersion behavior are important. Kryptolebias demonstrated high H2S tolerance, as they lost equilibrium significantly later than Poecilia and Gambusia during H2S exposure (1,188 +/- 21 mM H2S). However, the fact that all species lost equilibrium at an ecologically relevant [H2S] suggests that physiological tolerance may suffice at moderate H2S concentrations but that another strategy is required to endure higher concentrations. In support of the avoidance behavior hypothesis, H2S elicited an emersion response in all species. Kryptolebias was most sensitive to H2S and emersed at H2S concentrations 52% and 34% lower than Poecilia and Gambusia, respectively. Moreover, H2 Sexposure caused Kryptolebias to emerse more frequently and spend more time out ofwater compared to control conditions. We suggest that physiological H2S tolerance and emersion behavior are complementary strategies. The superior H2S tolerance and amphibious capability of Kryptolebias may explain why this species was more prevalent in H2S-rich environments than other local fishes.硫化氢（Hydrogen sulfide, H₂S）是一种强效呼吸毒素，仅少数生物能够耐受富含硫化物的环境。本研究报道了在伯利兹红树林水塘中发现的鱼类类群：斑纹鳉（Kryptolebias marmoratus）、普氏花鳉（Poecilia orri）、食蚊鱼属（Gambusia sp.）及斑尾鲈（Dormitator maculatus），这些水塘的硫化氢浓度极高（最高达1166 mM），足以致死绝大多数鱼类。因此，我们针对三种最优势物种（斑纹鳉、普氏花鳉及食蚊鱼）为何能在硫化物环境水塘中存活展开探究，其潜在生存策略分为两类：一是具备极强的硫化氢耐受性，二是能够离水（emerse）以完全规避硫化氢暴露。 研究结果表明，生理耐受性与离水行为均为关键生存策略。在1188±21 mM的硫化氢暴露实验中，斑纹鳉的平衡失调时间显著晚于普氏花鳉与食蚊鱼，证实其具备较高的硫化氢耐受性。但所有受试物种均在具有生态相关性的硫化氢浓度下出现平衡失调，这提示生理耐受性仅能在中等硫化氢浓度下满足生存需求，而应对更高浓度的硫化氢则需要其他生存策略。 为验证规避行为假说，我们发现硫化氢可诱导所有受试物种产生离水反应。其中斑纹鳉对硫化氢的敏感性最高，其触发离水行为所需的硫化氢浓度分别比普氏花鳉与食蚊鱼低52%和34%。此外，与对照组相比，硫化氢暴露会使斑纹鳉的离水频率更高，且脱离水体的时长更长。 综上，我们认为生理硫化氢耐受性与离水行为是两类互补的生存策略。斑纹鳉所具备的优异硫化氢耐受性与两栖生存能力，或可解释为何该物种在富硫化氢环境中的种群占比高于其他本地鱼类。