Sturm, Alexis B., et al. “Does depth divide? Variable genetic connectivity patterns among shallow and mesophotic Montastraea cavernosa coral populations across the Gulf of Mexico and western Caribbean.” Ecology and Evolution 13.11 (2023): e10622. https://doi.org/10.1002/ece3.10622
When you think of a coral reef, you probably picture a sunny, colorful ecosystem like the image above. These shallow, sunlit surface waters support most coral reefs on Earth. Some corals, however, live deeper, in the so-called “mesophotic zone” between 30-150 meters. Mesophotic means “middle light”, referring to a region of the ocean that is darker than the surface but not completely lightless. These deep coral populations are relatively isolated, and scientists are setting out to learn more about them.
A genetic refuge for corals?
Coral reefs all over the world have been decimated by climate change, disease, and other human-caused stressors such as pollution and overfishing. However, not all coral populations have been affected equally.
Some scientists think that deep, mesophotic corals are protected from population decline because they are less affected by rising temperatures and further away from human impacts like pollution. This could also be good news for shallow corals, because the deep reefs might be able to repopulate the more vulnerable surface reefs. However, this idea has not been directly tested.
To test whether deep coral reefs might support shallow reefs, scientists examined the genetics of coral populations throughout the Gulf of Mexico and the Caribbean. They focused on an important species of coral called the great star coral (Montastraea cavernosa), which is abundant in the Caribbean and can live in both shallow and deep waters.
Biologists use genetic techniques to understand the connections between populations. In this case, “connections” means interbreeding; populations are connected if some individuals from those populations reproduce with each other. Corals can be connected over large distances because they reproduce by releasing larvae into the water, which travel along ocean currents.
In this study, the scientists sequenced DNA from hundreds of individuals across deep and shallow populations, from sites throughout the Gulf of Mexico and the Caribbean Sea. Geneticists can use DNA to understand how closely related animal species are to one another, similar to genetic techniques used by companies like Ancestry.com or 23andMe.
In general, deep populations of great star coral were more genetically similar to other deep populations, compared to deep and shallow populations from the same area. This suggests that depth can be a genetic barrier for corals. However, there were some exceptions, and certain areas (such as the Florida keys) had unusually high connectivity between deep and shallow coral populations.
Considering depth in coral management
This study indicates that certain deep coral populations can serve as genetic refuges, connecting with shallower surface reefs. However, this may not be true in general. Overall, deep M. cavernosa were not major contributors to shallow populations, meaning that deep corals are not a silver bullet that can always help shallow reefs recover.
Whether or not deep corals can provide a genetic refuge for shallow corals depends on the specific location and is probably influenced by things like local geography and ocean currents. For example, the Florida Keys have strong tidal currents that might prevent coral larvae from leaving the area, while also transporting larvae vertically between deep and shallow reefs. This highlights the importance of local expertise in coral management, as each location is a little bit different.
Cover image source: Wikimedia Commons
I am a PhD student at MIT and the Woods Hole Oceanographic Institution, where I study the evolution and physiology of marine invertebrates. I usually work with zooplankton and sea anemones, and I am especially interested in circadian rhythms of these animals. Outside work, I love to play trumpet, listen to music, and watch hockey.