Physiological and behavioral responses of seahorse newborns to acute hypoxia

Seahorse populations have been declining globally as a result of intensive anthropogenic activities. However, knowledge about the impacts of climate change on these charismatic organisms is still scarce. Considering their vulnerability and current conservation status, it is crucial to comprehend how these species cope with stress from rapid climate change, particularly in early life stages, known to be particularly vulnerable to environmental stress. Thus, the objective of the present study was to assess the effects of extreme hypoxia (∼27 % dissolved oxygen for 12 h) on swimming behavior, ventilation, and heart rates in newborn seahorses Hippocampus hippocampus. Moreover, seahorses' physiological responses to different chemical cues (conspecific, prey and predators) after exposure to acute hypoxia conditions were also analyzed. Results indicate that under oxygen depletion, seahorses showed a significant decrease in ventilation and heart rates, while their swimming activity tended to decrease, although not significantly. Furthermore, when seahorses were exposed to odor cues from a potential predator, their ventilation and heart rates increased significantly, however, this increment was higher under control than hypoxic conditions. This study shows that early-stage seahorses may lack the physiological coping mechanisms found in adults when exposed to hypoxia, thus facing greater challenges under climate change conditions. Future studies are needed to better understand how these vulnerable species will endure the changing climate conditions.