Impacts of marine heatwaves and acute hypoxia on the short-snouted seahorse metabolism and behaviour

Tese de mestrado, Ecologia Marinha , 2022, Universidade de Lisboa, Faculdade de CiênciasSeahorses are teleost fish with unusual anatomical and behavioural characteristics, that make them one of the most unique and enigmatic animals. They are recognized as flagship species for several conservation issues. Unfortunately, seahorses’ populations have been declining worldwide, mainly due to anthropogenic pressure. Their unique lifestyle, namely, their reduced swimming ability, small distribution, high site fidelity and monogamy, constrains their ability to migrate, adapt and evolve in future climate scenarios, especially when exposed to short-term extreme events such as marine heatwaves and acute hypoxia. Yet, seahorses inhabit shallow coastal waters that display daily or seasonal fluctuations of temperature and oxygen, which suggests that these animals may show some phenotypic plasticity to these environmental changes. Within this context, the objective of this dissertation was to test the individual and combined effects of a category II (strong) marine heatwave (21.5°C) and an extreme hypoxia exposure (~27% dissolved oxygen) on the metabolism, behaviour and food intake of the temperate seahorse Hippocampus hippocampus. Concomitantly, the impact of the marine heatwave on the number and size of seahorses oocytes was also examined. Regarding metabolism, hypoxia exposure, alone or accompanied by the marine heatwave, led to a significant reduction in metabolic and ventilation rates. The metabolic rates under the combined treatment were slightly higher than that of those exposed to hypoxia, which may indicate a temperature compensation in relation to oxygen depletion. The marine heatwave treatment led to a significant increase in ventilation rates and food intake, but did not change seahorse behavioural patterns. In contrast, seahorses showed signs of movement lethargy whenever there was oxygen depletion. The number and size of the oocytes increased significantly with increasing temperature. The results show that, although seahorses may have some adaptation elasticity to heat stress, an extreme decrease in dissolved oxygen results in metabolic and behavioural changes that may jeopardize the development and survival of these iconic organisms. The two stressors together have similar impacts to those observed in the hypoxia treatment, which suggests that oxygen depletion elicits greater biological effects than strong marine heatwaves