Physiological tolerance of marine animals in a changing world: how the sea urchin Echinometra viridis responds to shoaling hypoxia in shallow Caribbean coral reefs.
Supervisor: Rachel Collin (Smithsonian Tropical Research Centre)
Coral reef ecosystems support more than 25% of described marine species. Over the last decades, an increase in mass mortality events associated with low oxygen led to a decline of 60% of the live coral abundance in the Caribbean, suggesting that hypoxia is an important but under-looked stressor on reefs. Located in Panama on the Caribbean Coast, Almirante Bay is a semi-enclosed lagoon representing half of the reef diversity in the country. It experiences seasonal hypoxia at depth. Hypoxia is defined when concentrations of seawater dissolved oxygen are below 2mgL-1. Within the bay, on shallow coral reefs, severe hypoxic conditions have also been recorded. Seawater warming can increase the impacts of hypoxia, so it is essential to consider temperature when conducting studies on hypoxic conditions. The dissolved oxygen concentrations regulate the coral reef-associated invertebrates through their metabolic activities and their movements towards more oxygenated waters. Reef urchins are the most abundant mobile invertebrates in Almirante Bay. It contributes to reef resilience by grazing algae and providing settlement space for corals, thereby helping to maintain conditions necessary for coral communities to recover after acute disturbances. This report describes how the sea urchin Echinometra viridis reacts to different dissolved oxygen concentrations by identifying key in situ behavioral responses. Photographs of the reef were taken throughout the hypoxic season from September to November 2019 at 5 m, and physical parameters were measured at 3, 5, and 7 m depths on a reef. Dissolved oxygen and temperature time series data showed that hypoxia is most likely to occur under warmer conditions at 3 m depth compared to 5. Oxygen concentrations were correlated with key behaviors of the animals under normoxia, moderate and severe hypoxia. Under severe hypoxia, sea urchins were less mobile than under normoxic conditions. However, a slight increase in mobility is observed under severe hypoxia compared to moderate

oxygen conditions. This might be the description of an “escape” behavior where animals try to move towards more oxygenated water. Understanding how ecologically important reef grazers respond to low oxygen is necessary to determine the fate of coral reefs in a warming, deoxygenating ocean.

Keywords: climate change, ocean deoxygenation, tropical ecosystems, coral reefs, sea urchin