Physiological response of corals of the genus Pocillopora (Scleractinia: Pocilloporidae) during the 2023–2024 heat wave in La Paz, Baja California Sur

Introduction: Coral reefs are essential ecosystems for the functioning of the planet and human well-being, providing three-dimensional habitats for a great diversity of marine life and multiple ecological, economic, and cultural services. Coral reefs are constructed by hermatypic corals, which maintain a symbiotic relationship with dinoflagellates of the Symbiodiniaceae family, which provide them with up to 90-95% of their energy in exchange for shelter and nutrients. However, this relationship has been compromised by stressors associated with climate change, human activities, and natural phenomena such as El Niño-Southern Oscillation (ENSO), which can cause the expulsion of symbionts and the loss of their energy source, thereby endangering their survival. Objective: To evaluate physiological indicators (i.e., symbiont density, chlorophyll, lipids, and proteins) as tools for assessing coral health at the Isla Gaviota and Punta Galeras sites during the 2023-2024 marine heat wave period. Methods: A total of 44 colony fragments, 3 to 5 cm in length, were collected from the Pocillopora verrucosa and Pocillopora damicornis morphotypes during two sampling campaigns, one in July 2023 and the other one in July 2024 at the two sampling sites. The samples were processed to determine symbiont density, chlorophyll a, lipids, and proteins. Results: We found evidence that P. verrucosa and P. damicornis exhibit different adaptive strategies. On one hand, P. verrucosa showed variable responses, with temporary loss of symbionts, and compensatory energetic mechanisms that maintained photosynthetic functionality, demonstrating a gradual recovery by 2024. Pocillopora damicornis, on the other hand, showed greater physiological stability, maintaining active symbiosis and stable photosynthetic pigments, as well as protein and lipid reserves usage. Although both morphotypes have developed effective survival strategies, the observed metabolic and energetic costs highlight their long-term vulnerability. This susceptibility is particularly critical in the context of the increasing frequency and intensity of thermal stress events driven by global climate change. Conclusions: This study demonstrates that the 2023–2024 marine heat wave affected the analyzed corals, with P. damicornis showing the greatest resilience potential, based on a symbiotic stability, while P. verrucosa relies on more complex, but equally functional, adaptive mechanisms.