Common name: Elkhorn Coral

Scientific name: Acropora palmata 

Range: Throughout the Bahamas, Florida, and the Caribbean. The northern extent of the range in the Atlantic Ocean is Broward County, Florida, where it is relatively rare (only a few known colonies).

Conservation status: Listed as critically endangered on the IUCN Red List of Threatened Species.

Introduction 

Coral reefs are extensive structures created by limestone that has been produced by coral polyps that grow together in a colony (National Marine Sanctuaries, 2022). These polys are individual organisms that have a structurally simple body plan and limited degree of organ development (NOAA Coral Reef Information System, 2023). Elkhorn corals are typically found in clumps called “thickets” in shallow clear waters with much wave action (NOAA Fisheries, 2022).  Elkhorn coral is considered a keystone species because its complex structure provides crucial habitat for wildlife, making it one of the most important corals in the Caribbean (Miller et. al., 2022).  Elkhorn Coral colonies are golden tan or pale brown with white tips—their color comes from the symbiotic algae that live within their tissues and provide the animal with food (NOAA Fisheries, 2022). 

The Elkhorn Coral is one of the top ten coral species considered both Evolutionarily Distinct and Globally Endangered (Miller et. al., 2022).  In the United States, this species can be found in several marine protected areas,  including the Florida Keys National Marine Sanctuary, Biscayne National Park, Dry Tortugas National Park, Buck Island Reef National Monument and Flower Garden Banks National Marine Sanctuary (Id.). It is also present in Hol Chan Marine Reserve (Belize), Exuma Cays Land and Sea Park (Id). 

The Elkhorn Coral is one of the fastest growing corals when healthy and can grow up to five inches per year. (NOAA Fisheries, 2022). They reproduce once per year after a full moon phase in the late summer by “broadcast spawning,” in which they emit sperm and egg into the water. (Id.). The fertilized eggs become embryos and then settle on a hard surface to start new colonies (Id.).  New colonies can also happen asexually by fragmentation, when fragments break off from the colony and reattach to hard surfaces (Id.).

A severe disease event in the early 1980’s caused major mortality throughout the species’ range, reducing the population to less than 3 percent of its former abundance (NOAA Fisheries, 2022). There are some populations (in locations such as the U.S. Virgin Islands) where populations of Elkhorn Coral appear stable at low abundance, however, in the Florida Keys the population numbers are decreasing (Id.).

Threats

Climate change presents the greatest threat to corals worldwide, primarily through the occurrence of bleaching events (NOAA Fisheries, 2022). Bleaching is when corals expel the symbiotic algae that live inside their tissues, causing them to turn white.  This happens when corals are stressed by changes in conditions such as warmer temperatures (National Ocean Service, 2022). Mass coral bleaching events are more frequent as a result of the warming of the Earth's atmosphere and oceans, accelerated by greenhouse gasses derived from human activities (NOAA Fisheries, 2022).  Additionally, as more carbon is emitted into the atmosphere by humans, more carbon dioxide is absorbed into the ocean from the atmosphere, causing the ocean to have a higher level of acidity (Id.). Higher acidity levels in the seawater, which is known as ocean acidification, threaten to reduce calcification rates in reef-building and reef-associated organisms (Id.)

Diseases can also cause adult mortality in corals, reducing sexual and asexual reproductive success, and impairing colony growth. (NOAA Fisheries, 2022). Various factors may combine to make coral diseases worse, including the composition of pathogens and environmental toxicants (Id.). Coral disease can also cause acute tissue loss—the Elkhorn Coral is particularly susceptible to white band and white plague diseases, which affect coral tissues (Id.).

Unsustainable fishing can have significant negative long-term effects on coral reefs and can alter ecosystem structures from coral-dominated reefs to algal-dominated reefs. (NOAA Fisheries, 2022). Many species of fish that eat algae and keep the reef clean to allow space for corals to grow, maintaining coral species diversity in a coral reef ecosystem (Id.). Removing these fish can result in overgrowth of algae, reducing the diversity of corals (Id.).

Impacts from land-based sources of pollution can also impede coral growth and reproduction, disrupt ecosystem function, and cause coral disease (NOAA Fisheries, 2022). These impacts can result from coastal development, deforestation (clearing a wide area of trees), agricultural runoff, and oil and chemical spills (Id.).

In order to save coral species like the Elkhorn Coral, policies that reduce land-based sources of pollution, eliminate unsustainable fishing practices, and mitigate climate change impacts must be implemented and aggressively enforced (NOAA Fisheries, 2021).  There are also efforts to restore coral reefs, in which corals are grown in nurseries and replanted (Id.). Restoration projects have also involved building resistance of Elkhorn corals to high temperatures and removing invasive algae species that negatively impact coral growth. (NOAA Fisheries, 2021), (NOAA Fisheries, 2022).

references

1. Miller, M., Crabbe, J., Villamizar, E., Croquer, A. & Banaszak, A. 2022. Acropora palmata. The IUCN Red List of Threatened Species 2022: e.T133006A165672016.  https://www.iucnredlist.org/species/133006/165672016 

2. National Geographic, 2022. Keystone Species. https://education.nationalgeographic.org/resource/keystone-species

3. National Ocean Service. 2022. What is Coral Bleaching? https://oceanservice.noaa.gov/facts/coral_bleach.html

4. National Marine Sanctuaries, 2022. Coral reefs. https://floridakeys.noaa.gov/corals/coralreef.html

5. NOAA Coral Reef Information System, 2023. What Are Coral Reefs? https://www.coris.noaa.gov/about/what_are/#Anchor-From-63388

6. NOAA Fisheries. 2022. Elkhorn Coral. https://www.fisheries.noaa.gov/species/elkhorn-coral

7. NOAA Fisheries. 2021. Restoring Coral Reefs. https://www.fisheries.noaa.gov/national/habitat-conservation/restoring-coral-reefs