Coastal deoxygenation is poorly documented in the tropics. When the Isthmus of Panama separated the Caribbean from the Pacific, sister lineages diverged and adapted to changing oxy-thermal conditions along both coasts. This provides unique insight into the ecological consequences of ocean warming and deoxygenation. We find deoxygenated, or hypoxic, waters shoal to the shallow depths of 10 m on both sides of the Isthmus, with Caribbean waters generally warmer than those in the Pacific. We tested the performance of two Caribbean Echinometra sea urchin species and their Pacific sister species under different warming and oxygen scenarios. Performance, measured as righting ability, was reduced by 50–100% under hypoxia compared to normoxia in one species from each coast. Only one Caribbean species performed well under hypoxia and did so at ambient temperatures (≤ 29 °C) but not under warming. This tolerant species, E. viridis, appears to be specialized for living on protected Caribbean reefs, unlike its two sister species that occur on well-oxygenated reefs. Our results emphasize the danger of shoaling hypoxia compressing well-oxygenated habitat from beneath and the importance of evolved hypoxia tolerance. This highlights the underappreciated risk deoxygenation poses for shallow tropical ecosystems.
