There was grim news for the world’s coral reefs this October, when the National Oceanic and Atmospheric Administration (NOAA) declared the third global coral bleaching in history. This event signifies major changes in oceanic living conditions and temperatures, some of which are brought upon by our actions. Coral reefs are an endangered species that researchers across the globe are working to protect, including our team at Shedd Aquarium.
When corals are stressed by changes in conditions such as temperature, light or nutrients, they expel the symbiotic algae living in their tissue, which is called zooxanthellae, causing them to turn completely white. This “coral bleaching” can be caused by changes in oceanic temperature, runoff and pollution, overexposure to sunlight and extreme low tides. This year’s El Niño is the largest cause of the global coral bleaching event, and is expected to impact about 35 percent of the ocean’s corals.
Coral bleaching is important, and serious, because it threatens the survival of coral reefs, which are vital ecosystems that 25 percent of the ocean’s plant and animal species depend on. Less known is the way in which we as humans depend upon them. These animals protect tropical ocean-side communities from storms, support the fishing industry and are associated with a biochemical treasure trove now being used for all types of medical advancements. With this in mind, Shedd Aquarium, the SECORE Foundation and other members of the aquatic research community have been working together to find ways to protect and restore corals and coral reefs.
The SECORE Foundation (SExual COral REproduction) is an international initiative to protect, restore, and conserve the world’s coral reefs. SECORE focuses on studying sexual reproduction of corals because it maximizes genetic diversity, which is critical for the survival of remaining coral populations: If a single genetic strain of coral is susceptible to a specific disease or environmental stressor, adequate genetic diversity will prevent illness from sweeping through entire populations.
For thousands of years, Staghorn coral (Acropora cervicornis) and Elkhorn coral (Acropora palmata) have been two of the dominant reef-building coral species in the Caribbean. Their clustered colonies provide food and shelter for thousands of fish and invertebrate species. While these corals evolved to thrive in high-energy reef zones, this resilience does not protect them from newer environmental stressors, including pollution, warming oceans (and the resulting coral bleaching events), acidification and overfishing. In 2006, Staghorn coral and Elkhorn coral were listed under the U.S. Endangered Species Act. In 2008, they became the first critically endangered coral species on the International Union for Conservation of Nature’s (IUCN) Red List, an internationally recognized inventory of species at risk of extinction.
Our group began working in Puerto Rico to find out if Elkhorn coral specifically could be successfully propagated in a location with known populations of healthy corals. Prior to that, different conservation and research groups had collected coral gametes and fertilized them, but no one had figured out how to get the larvae to settle and grow. Our partnership resulted in the first successful settling of these corals.
Since, our team has sought ways to make the settlement of corals long-term and more widespread. While we continue to work on the latter, we published an original research article this October that reported the first successful outplanting and long-term survival of Elkhorn settlers reared from gametes collected in the field. Among the large amount of findings from our research, the most meaningful was that this method is possible.
The study was carried out on the island of Curaçao in the Southern Caribbean. Reproductively active Elkhorn populations are abundant at our study site near the Curaçao Sea Aquarium. In 2010, a land-based facility to rear corals and their larvae was built at this site. With the proper equipment in place, we were able to take advantage of the unique way Elkhorn reproduce.
Elkhorn coral is a hermaphroditic broadcasting coral species that releases gametes once a year in the summer, just after the full moon. With this knowledge, we were able to calculate when spawning would occur.
Three days after the full moon in August of 2012, we collected egg-sperm bundles from four colonies that spawned a few hours after sunset. The gametes were collected using cone-shaped nylon nets, in which the floating egg-sperm bundles concentrated at the top into a removable tube. The bundles we collected in the tube were immediately transported to the lab, where we began the fertilization process. We were able to determine successfully fertilized eggs three hours after fertilization by quantifying the proportion of eggs going through cell divisions. Five or six days later, we transferred all larvae into containers that would allow them to settle and begin to grow. Soon after, we would out-plant these settlers into the reef in Curaçao.Above, you can see the nylon nets we used to collect egg-sperm bundles during spawning.
Photo by P. Selvaggio
A large focus of our study was to test out different outplanting techniques in order to determine which produced the most long-term results. We compared the survival and growth of settlers transferred to the reef at the age of two weeks to that of settlers raised in the land-based nursery over a period of 2.5 years. We also compared the cost-effectiveness of the two restorative methodologies.
We found that Elkhorn settlers that were transferred to the reef shortly after settlement survived 6.8 times better than settlers that were kept in our land-based rearing facility. Outplanting the settlers after two weeks was also significantly more cost effective and economically viable.
We found that the rearing of sexually produced larvae is possible for Caribbean coral species despite the fact that previous attempts to raise and outplant settlers of the critically endangered Elkhorn colonies have been effectively unsuccessful.
While encouraging, outplanting sexual coral recruits will not “restore a reef” by itself and requires that other causes of degradation are minimized at the restoration site prior to outplanting. Nonetheless, our findings, combined with other case studies, show that if applied on larger scales and in combination with other management tools such as fishing quotas, coastal protection and pollution regulations, sexual coral propagation or “assisted recruitment” could contribute to restoring tropical reef communities in the future. Currently, we are testing out measures to apply assisted recruitment on a larger scale.
In short, our pioneering research in this field, coupled with more sustainable environmental and industrial practices in the area, are proving restoration efforts can be possible. Shedd Aquarium, SECORE and our partners are looking forward to sharing more positive news about our research and remaining committed to the preservation and restoration of our oceans’ most delicate ecosystems.
For more information about Shedd’s conservation efforts, visit http://www.sheddaquarium.org/Conservation–Research/. And, to learn more about SECORE, visit http://www.secore.org/site/home.html.
Mark Schick is the Collections Manager of Special Exhibits at John G. Shedd Aquarium. Schick oversees Shedd Aquarium’s special exhibits, as well as the seahorse collections and live foods area. Schick has more than 25 years of experience with marine aquariums and has worked with a wide variety of animals ranging from invertebrates to fishes and lizards.