All of life’s diversity on earth is a result of random mutations being selected for across varying environments. The more that environments vary, the greater the potential for organisms to become locally adapted. Local adaptation refers to the simple idea that an organism will thrive best in the environment where it comes from. We used to think of the ocean as a relatively homogenous environment on small scales, but coral reef research has routinely demonstrated that this may not be the case: even a few kilometers can be the difference between high or low light, stable or volatile temperatures, and extreme to negligible wave activity.
Today, the local environments where corals live are changing more quickly than ever. By looking at how corals today vary across spatial and environmental gradients (i.e. how they are locally adapted), we can also begin to tease apart which aspects of resilience may represent short-term changes in physiology or long-term changes via genetic adaptation. The balance of these two processes will govern the makeup and function of future reefs.
Our lab currently studies local adaptation by seeking to answer three main questions:
- How does coral resilience to thermal stress vary across environments, and why?
- What role do the mutualistic algae (Symbiodiniaceae) living inside coral play in the animal’s local fitness?
- In what ways will local adaptation influence how reefs respond to climate change?
Thermal Variability Regimes
In the next century, many environments are not only expected to become hotter on average, but are also expected to experience more variable temperatures. This interplay between shifting means and shifting extremes presents a major challenge in understanding how reefs will cope with climate change.
Corals may already be locally adapted to different levels of thermal variability due to the spatial gradients that exist in environmental conditions on the reef. Whether this type of local adaptation can help or hinder survival depends on several factors, including whether differences in resilience are genetic or phenotypic and whether corals from one environment can disperse to and repopulate degraded environments.
A current project seeks to understand how different levels of daily thermal variability influence coral (i) fitness, (ii) response to stress, and (iii) recovery from stress. In 2016, a 90-day common garden experiment was conducted on 54 colonies of the massive reef-building coral Siderastrea siderea from the Bocas del Toro reef complex in Panama. Pending analysis includes pulse amplitude modulated (PAM) fluorometry, calcification, protein and carbohydrate reserves, transcriptomic responses of the host and genomic analysis of Symbiodiniaceae assemblages.
Inshore-Offshore Population Structures
A coral reef is not identical from beginning to end. Rather, it consists of many different reef zones, including the inshore reef zone (the area nearest to shore) and the offshore reef zone (nearest to open ocean). These areas differ in their local environment, where the inshore corals face more changes in light, salinity, and temperature due to the tides and the offshore corals experience more stability. Currently, it is poorly understood exactly how corals of the same species cope with the vastly different conditions of these two reef zones.
In 2013, Dr. Sarah Davies collected samples of the brush coral, Acropora hyacinthus, from three reefs in French Polynesia (two adjacent to the island of Moorea and one adjacent to Tahiti). At each the three reefs, corals were collected from an inshore and offshore reef zone. From these samples, we were able to extract DNA and analyze parts of the genomes of both the coral and their symbiotic algae, Symbiodiniaceae. We hypothesize that there will be a significant difference in genetic structure of the coral host and symbiont according to their reef zone. This result would indicate that these corals have undergone local adaptation to the conditions of each reef zone.