By: Julia Saper
Caribbean and Atlantic corals, such as Staghorn (Acropora cervicornis) and Elkhorn (Acropora palmata) corals, have experienced a rapid decline over the past four decades due to a series of compounding stressors. Restoration of these endangered species of corals requires basic knowledge of genetics and genetic diversity. Genetic diversity commonly refers to the variety of genes within a species. At high levels of genetic diversity, a species is more adaptable to environmental changes and therefore crucial to the survival of a species. Here at the Coral Restoration Foundation, our restoration program focuses on restoring and promoting the genetic diversity of coral species on the reef.
Let’s review the basics. A gene is a basic unit of heredity, made up of DNA, and acts as a template to make molecules known as proteins . A genotype is the entire set of genes in an individual. Variation is important to ensure the persistence of a species. For example, what happens when a natural or human caused disturbance causes a decrease in the genetic diversity of a species population? If all of the corals in the ocean required the exact same water temperatures and that temperature changed drastically, all of those genetically identical corals would no longer be able to persist in a new environment. Variation of the genetic makeup allow different kinds of corals to persist in different types of environments.
Understanding the importance of genetic diversity comes from understanding the two ways these Caribbean and Atlantic Acropora corals reproduce: asexually and sexually. Coral Restoration Foundation takes advantage of asexual coral reproduction. By breaking off a coral fragment from an existing coral, a new colony can grow. This process, known as fragmenting, increases the coral density yet doesn’t increase genetic diversity as each fragment is an identical clone of its parent coral colony. In contrast, sexual reproduction does increase genetic diversity yet only occurs once a year. During this event, known as a broadcast spawn, individual colonies will simultaneously release millions of gametes containing both sperm and eggs. Once the gametes are released into the water column, the goal is to mix with a distinct genotype from another staghorn or elkhorn colony. The biggest issue today is that with limited wild staghorn and elkhorn populations left, they are geographically too far apart for their gametes to effectively mix, limiting the amount of new genotypes (or new genetic material). Our task is to help fill in the gaps between wild Acropora colonies to enhance success of recruitment.
Under the Endangered Species Act, NOAA’s National Marine Fisheries Service (NMFS) is responsible for most marine species such as staghorn and elkhorn corals in US waters. As of 2006, both species are listed as “threatened” and the proposal of a recovery plan falls under the jurisdiction of NMFS. The recovery plan outlines ten major criterion, one of which is “genotypic diversity”. The genotypic diversity criterion aspires to “maintain current overall average genotypic diversity (proportion of unique genotypes per number of colonies sampled) of approximately 0.5 across these species’ range” . A proportion of 1.0 would mean 100% of the coral colonies are products of sexual reproduction. A proportion of 0.5 indicates a balance between asexual and sexually modes of reproduction. The higher the proportion, the higher the genotypic variance and the more likely the population is able to withstand diseases (such as white band disease) and environmental variability (such as changing pH levels and thermal stress).
In each of our five coral nurseries, individual coral trees contain corals with identical genotypes propagated through fragmentation. Collectively, we have over 200 acropora genotypes, some of which are extinct in the wild. When we outplant a cluster of corals, we use tags to denote the genotype of the coral so we can further monitor the success of certain genotypes as well as ensure a variation of corals on the reef. Our methods are directly in line with the “genotypic diversity” criteria for the staghorn and elkhorn coral Endangered Species Act recovery plan.
We live on a planet which is rapidly changing. There are many compounding stressors acting upon reefs today such as warming waters and ocean acidification. The more genotypes we have, the more resilient coral populations are to the plethora of physical and chemical changes in the ocean. Coral genetics is a new resource to help combat further deterioration of the reefs and help restore populations back to past levels health, density and diversity.
Edmunds PJ. Evidence that reef-wide patterns of coral bleaching may be the result of the distribution of bleaching susceptible clones.Mar Biol. 1994;121:137–142.
Hemond, Elizabeth M., and Steven V. Vollmer. “Genetic Diversity and Connectivity in the Threatened Staghorn Coral (Acropora Cervicornis) in Florida.” PloS ONE 1 (2010): n. pag. Web.
 National Marine Fisheries Service, Southeast Regional Office Protected Resource Division. “Recovery Plan Elkhorn Coral (Acropora Palmata) and Staghorn Coral (A. Cervicornia).” (2015)
 “What is a Gene?” Genetics Home Reference. National Institutes of Health, n.d Web. 21 July 2016. https://ghr.nlm.nih.gov/primer/basics/gene>.