About the Blog Author
Dr. Jennifer Hoey is an evolutionary ecologist and Postdoctoral Researcher in the Reefscape Genomics Lab at the California Academy of Sciences. She studies how marine organisms adapt to changing conditions by investigating the evolutionary processes that contribute to genomic and morphological patterns of variation in the sea. Follow Jennifer on Twitter @jahoey13 or on her website.
Coral reefs are lively and dynamic places. When on a coral reef, you first might notice all the things that move – fish schooling around you, darting about for an invisible but tasty nibble; a moray eel leering at you from a stony crevice. But if you’ve been to the reef before, you might also notice how the corals themselves have changed – grown, fragmented, disappeared, or changed color.
In mid-December 2023, I returned from 2.5 weeks in Curaçao, a small island in the southern Caribbean where I’ve been studying a genus of corals called Madracis. This was my second time in Curaçao, and the reef looked very different from when I first visited in May 2022. So many corals were white due to coral bleaching (Figure 1). Corals are small colonial animals related to jellyfish and sea anemones, but they secrete calcium carbonate skeletons and have a symbiotic relationship with a special algae that lives within their translucent tissues. These algae photosynthesize, converting sunlight into sugars and nourishing their coral hosts. Healthy corals look various shades of brown, yellow, green, and purple due to the algae they host inside themselves. However, when corals become stressed, they may kick out their algal symbionts, revealing their underlying skeleton and causing them to appear white or ‘bleached.’ If the stress event subsides, corals can uptake algal symbionts from their environment, regain their color, and recover. But if the stress event is too acute or too long, the corals cannot feed themselves properly and are quickly overtaken by filamentous turf algae.
In the summer of 2023, the Caribbean Sea experienced an extreme heat event. Ocean temperatures reached 90°F – more than two degrees warmer than average – and remained higher than normal through the rest of the year (Figure 2). Warm ocean temperatures are caused in part by warm atmospheric temperatures. The temperature of our atmosphere has been increasing over time due to excess carbon dioxide (CO2) that is released when we burn fossil fuels as an energy source. The excess CO2 acts like an insulating blanket around our planet, raising atmospheric and ocean temperatures alike. The extreme heat in the Caribbean Sea created a stressful environment for corals throughout the Caribbean, including those in Curaçao, contributing to the widespread coral bleaching I saw while in the field.
The Reefscape Genomics Lab at the California Academy of Sciences has been monitoring patches of reef at different sites around Curaçao at depths between ~16-197 feet since 2019. For each of these reef patches, we generate 3D digital representations of the reef. These 3D models are snapshots of what the reef looked like at a particular moment in time. We also take fingernail-sized fragments of coral colonies within the patches for genomic analyses, assess colony morphology, and measure abiotic factors, such as temperature or light availability. During our December 2023 field trip, we were able to capture dramatic imagery of the bleached reef. The reefs in Curaçao have bleached before – most recently in 2020. From this observation, as well as others, we know that some species are more susceptible to bleaching and that some colonies can recover. But as I dove on the reef observing the first bleaching event that I have personally seen, I couldn’t help but wonder if some of these colonies would be lost forever. We will only be able to know with time.
In 2022, I sampled 700+ Madracis within the reef patches to understand genetic diversity and hybridization within the genus. Some Madracis species are visually distinct, but others can be ambiguous and are often found growing in cryptic parts of the reef, such as small caves or overhanging areas, which makes them hard to see in the 3D models. Armed with underwater maps and plenty of anxiety over what I might find, my major objective on this recent field trip was to revisit many of my sampled Madracis colonies and assess their bleaching status (Figure 3). I’m sure I looked ridiculous to recreational SCUBA divers as I hovered above the reef staring at maps on a phone in an underwater case, but it gave me hope that many of my sampled colonies were still around and looked relatively healthy. However, some were bleached, pale or had otherwise disappeared since 2022. So, while this variation in bleaching status raises interesting ecological and evolutionary questions, I was relieved that most were seemingly okay. It’s a small win, but I’ll take any positive news about corals that I can and I’m excited to delve into the genomic data to understand if there’s a genomic-basis to stress-tolerance in the corals that I’m studying.