Characterization of larval settlement and stress responses in two resilient Hawaiian coral species

Abstract

Coral reefs are in decline globally as anthropogenic induced climate change effects ravage our oceans. It is estimated that at least 50% of coral reefs have disappeared over the last 40 years and are declining at an alarming rate. While much scientific research focuses on healthy coral reef ecosystems, data suggests that degraded watersheds with high levels of selective pressure may harbor coral species that are well adapted to stress. A thriving population of corals exists in Honolulu Harbor, a highly degraded ocean habitat exposed to multiple anthropogenic stressors. Following the massive molasses spill in 2013, two species of corals have shown remarkable resilience to multiple stressors. Both species were observed to be brooders, with Leptastrea purpurea demonstrating a larval peak in the late summer. L. purpurea planula larvae are induced by a settlement cue originating from other coral colonies. When a coral scent is present, settlement rates are as high as 90-100% on biofilm and other substrates, including plain untreated glass. Field surveys reveal that L. purpurea colonies are found on average 18mm in distance from their nearest neighbor, and modeling suggests a non-random distribution of colonies at our survey sites. A second species, Harbor Porites, is genetically distinct from Porites lobata, though genetics show a similarity in origin. Harbor Porites larvae will settle in the presence of a biofilm cue, and both larvae and recruits show remarkable resilience to multiple chemical and physical stressors, as well as the ability to undergo reversible metamorphosis. Both coral species are in high abundance inside Honolulu harbor, and coral surveys reveal that the two species are found within an average 16mm of each other. To elucidate the high level of survival in these two species, a thermal tolerance exposure was performed to induce a bleaching response in both species. Molecular biomarkers were used to quantify relative stress levels. Molecular expression analyses could give us insights into how these corals are responding to stress, and if the basis for their resilience is tied to up-regulated molecular processes. While corals continue to face stress as a result of climate change, these two harbor coral species serve as excellent models for studying the resilience of corals to stress. Their persistence in a stressful environment makes them candidate species for coral reef restoration.