Seahorse Hippocampus reidi is a vulnerable species, inhabiting estuarine and coastal waters. The safety of acidic environments for fish has been considered in terms of ocean acidification in nature and decreasing pH in intensive aquaculture systems. This study aimed to investigate the effects of acute exposition (96 h) of juvenile seahorses to different pH (5, 6, 7, and 8) in brackish (BW - salinity 11) or seawater (SW - salinity 33). For that, we studied the responses of cortisol, oxidative stress, and survival, thus covering primary, secondary, and tertiary stress responses. In SW, cortisol levels were not altered for fish maintained at pH 5 and 8. However, in BW, cortisol was higher for fish kept at pH 5. Regarding secondary stress responses, only GST activity increased with acidification in SW. However, acidification in BW caused biochemical alterations at enzymatic level (SOD, GST, GPx) and glutathione metabolism, accompanied by reduction of antioxidant capacity (TEAC) and increased lipid peroxidation (TBARS). Survival was always above 90% and it did not differ significantly among pH levels. Our results suggest that H. reidi juveniles are more vulnerable to acidic exposure in BW than in SW.

海马海马reidi是一种脆弱的物种，栖息在河口和沿海水域。鱼类的酸性环境的安全性已被考虑到自然界中的海洋酸化和集约化水产养殖系统中降低的 pH 值。本研究旨在调查幼海马急性暴露（96 小时）对咸水（BW - 盐度 11）或海水（SW - 盐度 33）中不同 pH（5、6、7 和 8）的影响。为此，我们研究了皮质醇、氧化应激和存活的反应，从而涵盖了初级、次级和三级应激反应。在 SW 中，保持在 pH 5 和 8 的鱼的皮质醇水平没有改变。然而，在 BW 中，保持在 pH 5 的鱼的皮质醇更高。关于二次应激反应，只有 GST 活性随着 SW 的酸化而增加。然而，BW 中的酸化导致酶水平（SOD、GST、GPx）和谷胱甘肽代谢的生化改变，伴随着抗氧化能力（TEAC）的降低和脂质过氧化（TBARS）的增加。存活率始终高于 90%，并且在 pH 水平之间没有显着差异。我们的结果表明H. reidi幼鱼在 BW 中比在 SW 中更容易受到酸性暴露。