Reinforcing depleted Caribbean Coral Species through targeted Larvae Exchange

Caribbean coral reefs are on the brink of collapse. Disease and fatal bleaching, particularly in 2023, have claimed many victims, and with parent colonies becoming scarce, there is a lack of offspring. Even human-assisted breeding faces a major problem: the threat of inbreeding and the resulting weakening of generations to come. To address this, SECORE has launched a new program to establish a network that facilitates the exchange of genetic material between regions: the Caribbean Coral Larvae Exchange Program.

The last few years have been particularly hard on Caribbean corals: diseases such as Stony Coral Tissue Loss Disease (SCTLD) and the catastrophic bleaching event of 2023/4 among others have had devastating impacts, leading to the death of many corals.

What makes the reef’s self-recovery difficult, if not practically impossible, is the fact that in many places corals can’t successfully reproduce on their own anymore. Promising breeding programs for reef restoration have been established within SECORE’s implementation network across the Caribbean Basin. However, for some species, there are fewer and fewer healthy parent colonies left to spawn. Coral larvae naturally travel on ocean currents over long distances, often settling on reefs far away from their parents. As the corals’ successful reproduction ceases, their populations become smaller and more isolated.

As fewer parents pass on their genetic material, the risk of damage to certain populations due to inbreeding increases. This is caused by the potential accumulation of deleterious alleles (i.e.: variants of a specific gene), which, although initially rare, can become disproportionately widespread in small populations, when breeding among related individuals becomes more likely.

Dr. Margaret Miller, SECORE’s Chief Science Officer, emphasizes: “Coral populations—when reproducing successfully on their own—used to exchange larvae naturally across reefs and islands. As these populations have become so much smaller, this natural exchange is not happening, and our assistive breeding efforts are hampered by having too few parent colonies to work with in individual locations.”

SECORE has now launched the Caribbean Coral Larvae Exchange Program with the support of Revive & Restore. The goal is to exchange larvae of endangered Caribbean coral species across their native range to increase the genetic diversity of local populations and thereby minimize the risk of inbreeding. The transferred larvae will eventually be included in local coral breeding programs for sustainable reef restoration. The iconic and critically endangered elkhorn coral (Acropora palmata) is a prime example of a target species for this program. Historically, it dominated the shallow reef areas throughout the Caribbean and served as an important wave breaker, thereby protecting the coastline. Today, this species is threatened with extinction in many places, while it is already described as functionally extinct in Florida, meaning it can neither reproduce on its own nor fulfill its (protective) role in the ecosystem anymore.

"In a region as small and interconnected as the Caribbean, coral populations and the threats they face cut across international borders, and so should our interventions,” explains Dr. Liv Liberman, Revive & Restore’s Director for Ocean and Climate. “Revive & Restore advances genetic rescue for endangered species by developing new biotechnologies and integrating them into conservation practice, but new tools are only as powerful as the trust,  partnerships, and regulatory pathways that allow them to be responsibly implemented. The Caribbean Coral Larvae Exchange Program is a strategic investment in this kind of regional infrastructure, building relationships among restoration practitioners and establishing the legal and institutional frameworks to advance coordinated coral population management across the region.”

Funding by Revive & Restore for the first year serves as a jumpstart for the overall endeavor. The kick-off activity was a workshop held this March in Punta Cana, Dominican Republic, bringing together representatives from several of SECORE’s partner organizations that apply Coral Seeding across the Caribbean. 

A key focus of the workshop was to discuss and establish procedures needed to effectively minimize risks of such an undertaking. Exchanging coral larvae rather than whole adult corals is a primary strategy to reduce the risk of inadvertently introducing harmful organisms that may ‘hitchhike’ on whole colonies; other agreed strategies include disinfecting imported larvae and implementing required quarantine measures.

Another important aspect is having a good understanding of the genetic landscape in which we operate, such as examining the degree of relatedness across and within individual populations to track the actual genetic exchange taking place and document potential genetic shifts.

Science pioneer for coral population genomics, Dr. Iliana Baums (Alfred-Wegener-Institute), is part of the team to evaluate how connected populations were in the past, as well as what genetic risks and benefits may result from a larvae exchange. “My team and I aim to improve local capacity by developing fieldable DNA extraction and analysis methods that will shorten the time from sample collection to colony genotype identification. At the first workshop, participants were already trained on extracting coral DNA using the portable BENTO lab.”

Within the planned network of coral larvae exchange, every participating organization or institution is intended to function as a node for both sending and receiving coral larvae. The representatives of the first nodes have now met to discuss a comprehensive agenda for the successful planning and establishment of the program this March: FUNDEMAR, Dominican Republic; The Nature Conservancy, St. Croix, U.S. Virgin Islands; Roatan Marine Park, Honduras; Reef Renewal Foundation Bonaire, ABC Islands; Perry Institute of Marine Sciences, Colombia; The Florida Aquarium, United States.

The locations are spread across the Caribbean basin, which offer welcome geographical diversity but also present logistical challenges. Shipping live coral larvae that are expected to arrive healthy entails certain requirements, so logistics must be carefully planned.

And there are other challenges: international treaties control the movement and uses of biodiversity, including CITES and the Nagoya Protocol. To help participants navigate the maze of related legislative and regulatory hurdles safely, Steven Broad, former CEO of TRAFFIC and an international wildlife trade specialist, is lending his expertise to the program's development. Broad explains: “In line with international agreements, governments in the Caribbean region have adopted important legal safeguards aimed to prevent excessive and unfair exploitation of corals and other wild species. The planned coral exchanges have a clear conservation purpose, but we still need to secure approval under relevant laws and regulations, which vary from country to country in the region.”

Population management efforts take time, especially when implemented across borders and with long-lived species. Nevertheless, all participants agree that, though the process will be challenging and take time, larvae exchange is an important strategy worth committing to. The spawning season in the Caribbean begins in August, and depending on how paperwork progresses, the individual ‘nodes’ hope to pursue initial pilot exchanges, potentially starting between locations that are several hundred kilometers apart but within the same country.

“I'm hopeful this project will create the collaborations, permissions, and trust on the ground needed for genetic rescue to make a real-world difference for Caribbean reefs, and that the precedents we set together will benefit the coral restoration field broadly,” says Dr. Liberman.

Dr. Miller adds: “Coming out of our workshop discussions, there was a strong consensus that implementing larvae exchange will be challenging but definitely not impossible. And we think it is important to start as soon as possible, since it will take time for the successfully exchanged larvae to become part of the local brood population. In five years, we aim to have new parent corals interbreeding in many places throughout the Caribbean.”

The first year of this program is funded by Revive & Restore, the on-site workshop was supported by the Punta Cana Foundation, and the overall work is supported by our partners involved: FUNDEMAR, The Nature Conservancy in St. Croix, Roatan Marine Park, Reef Renewal Foundation Bonaire, Perry Institute of Marine Sciences, and The Florida Aquarium, as well as by our partners at SECORE’s field locations in Mexico, UNAM (the National Autonomous University of Mexico) and Coralium, and Curaçao, CARMABI.