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New Shedd Aquarium Study Finds Heat-Tolerant Corals That Will Optimize Reef Restoration Efforts in Florida

Findings Provide Immediate Actions that Benefit the World’s Largest Coral Restoration Program

October 20, 2021

A diver ties dangling coral polyps to a tall propagation frame.

A new, peer-reviewed study published today in the journal Proceedings of the Royal Society B will help optimize the human interventions necessary to help corals survive the impacts of climate change. Led by Shedd Aquarium (Chicago) with support from coral nurseries in south Florida that make up the world’s largest coral restoration program, the first-of-its-kind study identifies strains (or genotypes) of critically endangered staghorn coral (Acropora cervicornis) that can better withstand heatwaves in the ocean. Insights from the study provide a blueprint for the organizations working to restore climate-resilient reefs in Florida and will boost the success of restoration projects globally.

“Coral reef decline due to climate change is accelerating worldwide, but so are active reef restoration efforts, which offer the best chance to boost the resilience of reefs through challenging future environments,” said Dr. Ross Cunning, research biologist at Shedd Aquarium and lead author of the study. “However, restoration and other conservation interventions must be optimized with heat tolerance in mind, in order to favor the survival of future reefs in warmer oceans. In this study, we conducted the first large-scale census of coral heat tolerance to identify thermotolerant individuals, which can be immediately used for various restoration techniques.”

The study was conducted over two research expeditions that took place in 2020 in Florida, where Shedd’s research vessel, the R/V Coral Reef II, enabled a team to test the heat tolerance of 229 genotypes of staghorn coral growing at six nurseries from Broward County to the lower Florida Keys—operated by Nova Southeastern University, Mote Marine Laboratory, the Florida Fish and Wildlife Conservation Commission, Reef Renewal, the Coral Restoration Foundation and the University of Miami.

After taking fragments of each coral aboard the vessel, the team used Coral Bleaching Automated Stress Systems (CBASS) to determine at which temperature each coral showed signs of bleaching—the heat stress response in which corals expel the vital symbiotic algae that live within them. DNA samples were also taken from the fragments for testing in Shedd’s molecular and microbial ecology lab, where they will be analyzed to determine genetic factors underlying heat tolerance. The study’s results showed significant variability in the heat tolerance of the 229 corals, with certain genotypes exhibiting higher heat tolerance that may translate to better survival in warming oceans.

“Mote Marine Laboratory & Aquarium has been maintaining underwater coral nurseries in the Lower Keys since 2007 with the goal of repopulating reefs throughout Florida,” said Mote Senior Scientist Dr. Erinn Muller, a co-author of the study. “At Mote, we're dedicated to ensuring our coral restoration efforts last in the long-term, and these new findings will help guide restoration efforts, ensuring our outplanted corals survive for decades to come. This study provides a regional perspective of heat tolerance of staghorn coral, data that will help Mote make more informed decisions associated with restoration. These types of investigations could be used for other coral species around the world informing restoration globally."

The CBASS used for the study were portable, experimental tanks custom-made at Shedd using Coleman party coolers with aquarium chillers, heaters and controllers that precisely manipulate the seawater temperature in each system. During the research expeditions, the CBASS enabled the team of scientists to rapidly determine each coral’s tolerance to warmer water, which would otherwise only be possible by observing natural bleaching events over the course of years and with immense resources. According to the researchers, these systems have the potential to drastically speed up efforts to identify corals with natural heat tolerance.

“Coral reef decline due to climate change is accelerating worldwide, but so are active reef restoration efforts,”

Dr. Ross Cunning, research biologist at Shedd Aquarium

One restoration method that can be optimized by the study’s findings is coral propagation and outplanting—a process in which scientists grow coral fragments in nurseries and then place them out on reefs. In this method, the corals proven to withstand warm waters through the study could be preferentially propagated and placed on shallow or near-shore reefs that are more likely to reach high temperatures, whereas more sensitive fragments should be placed on reefs less likely to heat up. As greater numbers of tolerant corals are outplanted to reefs, they will spawn and reproduce, helping to boost the heat tolerance of the next generation of corals and increase the climate-resilience of natural populations.

Other restoration methods include crossbreeding specific corals with thermotolerant genotypes to create heat-resistant offspring, as well as transporting thermotolerant corals, or their genetic material, from reefs that are flourishing to reefs where corals are dying.

“While this study was performed in Florida, there is growing interest among scientists and managers in surveying heat tolerance in other coral populations around the world,” said Dr. Andrew Baker, professor in the Department of Marine Biology and Ecology at University of Miami Rosenstiel School, and a co-author of the study. “Our study provides a template for other efforts to identify heat-tolerant corals and comes at a time when such knowledge can help transform approaches to coral conservation under climate change. Population censuses of heat tolerance like this one are not only useful for scientists seeking to understand how and why corals vary in their thermal tolerance, but also to managers and policy makers guiding the future of reef restoration.”

Shedd aims to take the momentum from the study to the Bahamas to expand heat tolerance testing to even larger coral populations and help boost the resilience of Bahamian coral reefs—the primary marine research location for Cunning’s work. They also plan to bring efforts back to Chicago to inspire local teens to help save coral reefs. This fall, students participating in Shedd’s Teen Programs will help develop new models for the CBASS that researchers can use during future expeditions.

Coral reefs support an estimated 25% of all marine life, provide storm protection, support economies and offer a source of food. Staghorn coral in particular is a critical reef-building species, but Caribbean staghorn coral has been decimated regionally since the early 1980s, making them a primary focal species in restoration efforts throughout the region, including Florida. Annually, tens of thousands of staghorn coral fragments are propagated and outplanted onto local reefs by nursery-managing organizations. Since 2006, Shedd has been working with fellow research organizations to outplant staghorn corals and restore reefs across the western Atlantic and Caribbean.

The study, “Census of heat tolerance among Florida’s threatened staghorn corals finds resilient individuals throughout existing nursery populations,” can be read online at:
For more information about Shedd Aquarium’s coral studies, visit Shedd’s website here.

VISUALS: High-resolution photos are available for download:
Photo credit: ©Shedd Aquarium or ©Mote Marine Laboratory & Aquarium (folders labeled accordingly)