New Research Illustrates the Complexity of Reef Restoration
By Shannon McGraw
Coral reefs are one of nature’s most beautiful and diverse ecosystems, but how much longer will they naturally persist?
Coral Reefs: A Source of Ecological and Economic Wealth
The world’s coral reefs are declining much faster than they can recolonize. Over the course of tens of thousands of years, coral and their symbiotic algae form expansive colonies called reefs (Basic Facts About Corals). Reefs create cavernous structures along the ocean floor that provide habitat, shelter, and protection from disturbances such as tropical storms. Nearly one-third of all marine organisms will spend time in or near coral reefs (Basic Facts About Corals).
Beyond their ecological importance, coral reefs also have economic value. The fishing industry is dependent on coral reefs functioning as nursery habitat for spawning and juvenile fish (Human Impact on Reefs). Without these nurseries fish populations would plummet, in turn crippling fisheries everywhere. Reefs also bring in revenue via ecotourism. Every year, tourism to the Great Barrier Reefs generates $1.5 billion dollars of revenue for Australia alone (Human Impact on Reefs).
Climate Change Threatens Reefs Globally
Despite their ecological and economic importance, coral reefs are rapidly disappearing. An effect of climate change, sea temperature rise is a major contributor to reef destruction. When coral colonies become overheated, the stress forces their algae symbionts to evacuate. This then results in coral polyps appearing white or bleached, a phenomenon known widely as coral bleaching. Without algae, coral polyps starve and die, thus destroying the structure of the reef.
The most threatened portion of coral reefs are shallow reefs, up to 12 meters in depth. These reefs are in closer proximity to the sea surface than deep reefs. Due to this factor, shallow reefs experience greater effects of sea temperature rise. Similarly, studies have shown that the deeper a coral reef is, the more protected it becomes from the dangers of bleaching. Reefs of 30–40 meters in depth, known as mesophotic or deep reefs, exist in relatively cooler temperatures that buffer them from the bleaching effects of climate change (Bongaerts et al., 2017).
The “Deep Reef Refuge” Hypothesis and What It Could Mean for Restoration
More recently, scientists have wondered how shallow and deep coral reef ecosystems interact. The “Deep Reef Refuge” Hypothesis proposes that deep reefs provide coral larva to shallow reefs through dispersal, and thus have the potential to restore the shallow coral reef populations by “reseeding” them or distributing new larva to bleached areas (Bongaerts et al., 2017). By providing shallow reefs with coral larva, deep reefs would effectively both decrease the probability of local extinction and increase recruitment of new coral polyps to bleached areas.
But is it True?
Despite its popularity among ecologists, the “Deep Reef Refuge” Hypothesis has remained largely untested — until now. A team of researchers from Queensland, Australia challenged this hypothesis by designing an experiment to test it. In their experiment, the team looked at the genomes of two coral species, with different reproductive behaviors, and compared the genomes of shallow and deep coral individuals within a species.
If deep coral reefs are indeed acting as refuges, then the genomes of corals living in shallow and deep reefs would be identical. Likewise, if deep coral reefs are not acting as refuges, then the genomes would be very different.
The team also accounted for differences in reproductive behaviors. There are two main reproductive behaviors for corals: brooding and broadcasting. Brooding corals release relatively few yet large larvae that settle quickly. In contrast, broadcasting corals release large clouds of sperm and eggs, which drift longer and farther before fertilization. They believed these differing reproductive behaviors might be the determining factor in which coral species were able to disperse from deep to shallow reefs.
To begin, the team looked at the genome-wide variation of a broadcasting coral, Stephanocoenia intersepta, and of a brooding coral, Agaricia fragilis, from shallow and deep reef populations in Bermuda. Using new sequencing technology, they discovered that reproductive behavior did in fact influence dispersal and reseed potential.
Complexities of the “Deep Reef Refuge” Hypothesis Revealed
As predicted, brooding corals with large larvae and weaker dispersal were not genetically similar between shallow and deep reefs, indicating that the brooding coral was not able to disperse larva from one to the other. In contrast, the broadcasting corals were genetically indistinguishable between shallow and deep reefs, meaning that the deep reef was indeed dispersing larva to and “reseeding” the shallow reef.
Ultimately, the team found that the potential for deep coral reefs to act as refuges was unique to each species of coral and more specifically determined by the reproductive behavior of each species. The ability of deep reefs to naturally restore and “reseed” shallow reefs is limited to broadcasting coral species. Brooding species, like Agaricia fragilis, are therefore much more threatened by bleaching than previously thought.
The Future of Coral Restoration
While it may be grim to know the “Deep Reef Refuge” Hypothesis is not valid for all corals, it’s a good step in the right direction. From this information, more effective coral restoration strategies can be created. In the future, environmental scientists can focus their restoration efforts by prioritizing reefs with less broadcasting and more brooding corals.
As one of our most prized ecosystems, it’s important to understand the complexities of coral reefs so we might better protect and restore them for our future generations and the future generations of other creatures to enjoy and utilize.
References
Basic Facts About Coral Reefs. 2018. Defenders of Wildlife. Webpage. https://defenders.org
Biodiscovery and the Great Barrier Reef. 2018. Human Impact on Reefs. Queensland Museum. Webpage. http://www.qm.qld.gov.au/
Bongaerts, P., C. Riginos, R. Brunner, N. Englebert, S. R. Smith, & O. Hoegh-Guldberg. 2017. Deep reefs are not universal refuges: Reseeding potential varies among coral species. Science Advances. Marine Ecology. 3: 1602373.
Coral Reproduction. Secore International. Webpage. http://www.secore.org
