About the Species
Elkhorn coral is one of the most important corals in the Caribbean. It, along with staghorn coral and star corals (boulder, lobed, and mountainous), built Caribbean coral reefs over the last 5,000 years. Elkhorn coral can form dense groups called “thickets” in very shallow water. These provide important habitat for other reef animals, especially fish.
In the early 1980s, a severe disease event caused major mortality throughout its range and now the population is less than 3 percent of its former abundance. The greatest threat to elkhorn coral is ocean warming, which causes the corals to release the algae that live in their tissue and provide them food, usually causing death. Other threats to elkhorn coral are ocean acidification (decrease in water pH caused by increased carbon dioxide in the atmosphere) that makes it harder for them to build their skeleton, unsustainable fishing practices that deplete the herbivores (animals that feed on plants) that keep the reef clean, and land-based sources of pollution that impacts the clear, low nutrient waters in which they thrive.
NOAA Fisheries and our partners are dedicated to conserving and recovering the elkhorn coral population throughout its range. We use a variety of innovative techniques to study, protect, and restore these threatened corals. We engage our partners as we develop regulations and management plans that foster healthy coral reefs and reduce the impacts of climate change, unsustainable fishing, and land-based sources of pollution.
Elkhorn coral used to be a dominant coral on Caribbean reefs and was so abundant that an entire reef zone is named for it. Beginning in the 1980s, the elkhorn coral population declined 97 percent from white band disease. This disease kills the coral’s tissues.
Currently, there are locations such as the U.S. Virgin Islands where populations of elkhorn coral appear stable at low abundance, and some such as the Florida Keys where population numbers are decreasing. Successful reproduction is very rare, so it is hard for elkhorn coral populations to increase.
Elkhorn coral colonies are golden tan or pale brown with white tips, and they get their color from the algae that live within their tissue. Elkhorn corals have frond-like branches, which appear flattened to near round, and typically stem out from a central trunk and angle upward. Branches are up to 15 inches wide and range in thickness from 1–2 inches. Individual colonies can grow to at least 6 feet in height and 12 feet in diameter. Elkhorn coral colonies can grow in dense stands and form an interlocking framework known as thickets. Each elkhorn coral colony is made up of many individual polyps that grow together. Each polyp is an exact copy of all the polyps in the same colony.
Behavior and Diet
Elkhorn coral get food from photosynthetic algae that live inside the coral's cells. They also feed by capturing plankton with their polyps’ tentacles. Coral bleaching is the loss of the algae that live in coral tissue. This loss can lead to coral death through starvation or increased vulnerability to diseases.
Due to their tree-like growth form, elkhorn corals provide complex habitat for fish and other coral reef organisms. When elkhorn corals are abundant, they provide shoreline protection from large waves and storms.
Where They Live
Elkhorn coral is found typically in clear, shallow water (1–15 feet) on coral reefs 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). Elkhorn coral lives in high-energy zones, with a lot of wave action. Too much wave action (major storms) can cause this branching coral to break. However, fragmentation via branch breakage is one method of reproduction for elkhorn coral. NOAA Fisheries has designated four critical areas determined to provide critical recruitment habitat for elkhorn corals off the coast of Florida and off the islands of Puerto Rico and the U.S. Virgin Islands.
Lifespan & Reproduction
Elkhorn coral reaches reproductive maturity at about 2 square feet. Elkhorn coral is a simultaneous hermaphrodite, meaning each colony produces both eggs and sperm but usually does not self-fertilize. Elkhorn coral sexually reproduces once per year after the full moon in late summer by “broadcast spawning” eggs and sperm into the water column. Fertilized eggs develop into larvae that settle on hard surfaces and form new colonies. Elkhorn coral can also form new colonies when broken pieces, called fragments, re-attach to hard surfaces. Elkhorn coral is one of the fastest growing corals—when healthy, it can grow up to 5 inches in branch length per year.
Climate change is the greatest global threat to corals. Scientific evidence now clearly indicates that the Earth's atmosphere and oceans are warming, and that these changes are primarily due to greenhouse gases derived from human activities. As temperatures rise, mass coral bleaching events and infectious disease outbreaks are becoming more frequent. Additionally, carbon dioxide absorbed into the ocean from the atmosphere has already begun to reduce calcification rates in reef-building and reef-associated organisms by altering seawater chemistry through decreases in pH. This process is called ocean acidification.
Diseases can cause adult mortality, reducing sexual and asexual reproductive success, and impairing colony growth. Coral diseases are caused by a complex interplay of factors including the cause or agent (e.g., pathogen, environmental toxicant), the host, and the environment. Coral disease often produces acute tissue loss. Elkhorn coral is particularly susceptible to white band and white plague diseases.
Unsustainable Fishing Pressure
Fishing, particularly unsustainable fishing, can have large-scale, long-term ecosystem-level effects that can change ecosystem structure from coral-dominated reefs to algal-dominated reefs (“phase shifts”). This results from the removal of fish that eat algae and keep the reef clean to allow for space for corals to grow.
Land-Based Sources of Pollution
Impacts from land-based sources of pollution—including coastal development, deforestation (clearing a wide area of trees), agricultural runoff, and oil and chemical spills—can impede coral growth and reproduction, disrupt overall ecological function, and cause disease and mortality in sensitive species. It is now well accepted that many serious coral reef ecosystem stressors originate from land-based sources, most notably toxicants, sediments, and nutrients.
In the Spotlight
Elkhorn corals are protected under the Endangered Species Act. They have been listed as threatened under the ESA since 2006. This means that elkhorn corals are likely to become in danger of extinction throughout all or a significant portion of their range within the foreseeable future. NOAA Fisheries is working to protect this species in many ways, with the goal that its population will increase.
Recovery Planning and Implementation
Under the ESA, NOAA Fisheries is required to develop and implement recovery plans for the conservation and survival of listed species. The ultimate goal of the elkhorn coral recovery plan is to recover the species so it no longer needs the protection of the ESA. We must combat both global and local threats to help protect elkhorn corals.
The major actions recommended in the plan are:
- Improve understanding of population abundance, trends, and structure through monitoring and experimental research.
- Develop and implement appropriate strategies for population enhancement through restocking and active management (PDF, 39 pages).
- Implement ecosystem-level actions to improve habitat quality and restore keystone species and functional processes such as herbivory to sustain adult colonies and promote successful natural recruitment in the long term.
- Curb ocean warming and acidification impacts to health, reproduction, and growth, and possibly curb disease threats, by reducing atmospheric greenhouse gas concentrations.
- Reduce locally-manageable stress and mortality threats (e.g., predation, anthropogenic physical damage, acute sedimentation, nutrients, contaminants).
- Determine coral health risk factors and their inter-relationships and implement mitigation or control strategies to minimize or prevent impacts to coral health.
Species Recovery Contacts
- Jennifer Moore, Southeast Corals Recovery Coordinator
- Alison Moulding, Southeast Corals Recovery Coordinator
NOAA Fisheries appointed an Acropora Recovery Implementation Team (ARIT) to assist in the implementation of the Acropora recovery plan and to advise NOAA Fisheries on issues related to the status and conservation of Acropora corals in the southeast US. ARIT members meet at least annually to evaluate progress of recovery actions and identify priorities for the upcoming year.
Project Implementation Table (MS Excel). This table contains an inventory of projects related to implementation of the recovery plan for staghorn coral (Acropora cervicornis) and elkhorn coral (A. palmata). The worksheet “Recovery Plan Actions” lists the actions from the recovery plan and their action number. The worksheet “Completed” lists all known completed projects that address actions in the recovery plan. Projects in progress are listed either in the "Long-term" worksheet for those that are ongoing or in separate worksheets for each action number for projects that will have an end-date. Please send updates and additions to Alison Moulding (email@example.com), NOAA Fisheries Southeast Regional Office.
Cryobanking Strategy (PDF, 13 pages). This document identifies a strategy and recommendations for range-wide collection of Acropora sperm for cryobanking.
Critical Habitat Designation
Once a species is listed under the ESA, NOAA Fisheries evaluates and identifies whether any areas meet the definition of critical habitat. Those areas may be designated as critical habitat through a rule making process. The designation of an area as critical habitat does not create a closed area, marine protected area, refuge, wilderness reserve, preservation, or other conservation area; nor does the designation affect land ownership. Federal agencies that undertake, fund, or permit activities that may affect these designated critical habitat areas are required to consult with NOAA Fisheries to ensure that their actions do not adversely modify or destroy designated critical habitat.
For elkhorn corals, facilitating increased successful sexual and asexual reproduction is the key objective to the conservation of these species. NOAA Fisheries has designated (73 FR 72210) four critical habitat areas in Florida, Puerto Rico, and the U.S. Virgin Islands to protect substrate of suitable quality and availability to support successful larval settlement and recruitment, and reattachment and recruitment of fragments.
Working to Enhance Populations
Severely reduced successful reproductive recruitment into the population is one of the major things impeding recovery of elkhorn corals. There are many factors that are contributing to this problem. NOAA Fisheries, with many partners, is taking several steps to help, including:
- Establishing a network of coral nurseries throughout the species' range to grow and asexually produce fragments and outplant them to the reef.
- Developing a plan to guide elkhorn coral population enhancement (PDF, 39 pages) which coral restoration partners, including non-governmental organizations, academia, zoos, aquaria, and federal, state, and local agencies, are requested to follow.
- Researching and implementing sexual reproduction techniques such as cryopreservation (preserving through a cooling process) of sperm and collection and fertilization of eggs and sperm for short-term rearing in the lab and outplanting to the reef.
Responding to Physical Impacts
Ship grounding and other physical impacts can break the branching elkhorn corals. If the broken fragments are stabilized quickly after being broken, the corals can survive and continue to grow. NOAA Fisheries supports a program to respond to these events in Puerto Rico and the U.S. Virgin Islands where tens of thousands of corals have been rescued.
Conserving Coral Reefs
NOAA Fisheries is part of the NOAA Coral Reef Conservation Program which brings together expertise from across NOAA for a multidisciplinary approach to understanding and conserving coral reef ecosystems. The Program focuses on implementing projects to address the impacts from the top three recognized global threats to coral reefs: climate change (including ocean acidification), land-based sources of pollution, and unsustainable fishing practices.
Issuing Protective Regulations
NOAA Fisheries issued a protective regulation called a "4(d) rule" to prohibit import, export, commercial activities, and take including killing, harming, and collecting elkhorn coral.
Key Actions and Documents
NOAA Fisheries conducts various research activities on the biology, behavior, ecology, and threats to elkhorn coral. The results of this research are used to inform management decisions and enhance recovery efforts for this threatened species.
Determining the size of the elkhorn coral population—and whether it is increasing or decreasing from year to year—helps resource managers assess the success of the conservation measures enacted. Our scientists and partners collect population information on elkhorn coral using a standard protocol. The population assessments also document the rate of population growth (recruitment) and decline (mortality), as well as the number of clones in a given population. In the Florida Keys, for example, the elkhorn population contains many colonies that are clones, meaning that there are fewer genetic individuals in the population than we would guess from the number of colonies. Also, we have not observed any new genetic individuals (sexual offspring) in the population in over 12 years, though there are new clonal colonies derived from naturally occurring fragments.
Also, NOAA’s national coral reef monitoring program tallies colony size and density for all coral species in reef habitats throughout U.S. jurisdictions. This broad scale monitoring program can give useful information about status and trends for coral species that are abundant enough to be detected in this survey.
Elkhorn Bleaching Assessment
Two major back-to-back bleaching events severely affected elkhorn coral in the Florida Keys in 2014 and 2015. NOAA Fisheries scientists observed bleached elkhorn corals at various sites in the Upper Florida Keys during a period of unusually warm water temperatures (warmer than 87.8°F) in mid-to-late August 2014. During summer 2015, we observed bleaching among the upper Florida Keys elkhorn population for a second consecutive year. Bleaching response varied between sites, but was consistent with the response observed at these sites during the 2014 bleaching event.
Overall, we estimate that 50 percent of the monitored elkhorn coral population died due to the 2014 and 2015 bleaching events. In 2005, the Upper Florida Keys population suffered similar losses due to the 2005 hurricane season. Recovery was only minimal over the decade prior to the recent bleaching events. While other coral species on the Florida Keys reefs have experienced moderate bleaching events in the past decade, this is the first bleaching event to affect elkhorn since the 1998 El Niño-associated bleaching event.
Restoration of Threatened Corals
NOAA Fisheries and partners have ramped up population enhancement of elkhorn corals. This developing field is supported by NOAA Fisheries’ research program to reduce uncertainties and enhance success in coral outplanting restoration activities. Some of our key questions are:
- How do genetically unique individuals of elkhorn coral outplants survive and grow differently in different habitats or sites?
- What factors worsen or mitigate the effects of predators on restored corals?
- What factors can help outplanted coral fragments form important thicket structures?
- Does disease affect restored versus wild corals differently? Are some nursery coral individuals resistant to disease?
NOAA Fisheries conducts controlled experiments at offshore coral nurseries and restored reefs to help answer these questions and improve the design of future coral restoration activities.
NOAA Fisheries coordinates spawning observations and larval culture with a network of researchers working throughout the Caribbean, including academic researchers and professional aquarists from public zoos and aquaria. Broadcast-spawning corals, like elkhorn, release eggs and sperm into the water column for fertilization only over a few nights per year. NOAA Fisheries collects sperm and eggs, fertilizes them and cultures and observes the larvae in the lab to better understand factors that may enhance the likelihood of larvae successfully settling and surviving to adulthood.
We also conduct experiments to understand the impacts of current and future ocean warming and ocean acidification on these vulnerable early life stages of corals. An ongoing goal is also to develop reliable methods to culture the baby corals to adulthood in order to enhance coral recovery on the reef by adding new genetic individuals.
NOAA Fisheries has published a 5-year review of staghorn coral, elkhorn coral, pillar coral, rough…
A recovery plan to identify a strategy for rebuilding and assuring the long-term viability of…
Proceedings of the Caribbean Acropora Workshop: Potential Application of the U.S. Endangered Species Act as a Conservation Strategy, Miami, Florida
NOAA Technical Memorandum NMFS-OPR-24 Workshop Date: April 16-18, 2002
Data & Maps
Tracks the implementation of recovery actions from Endangered Species Act (ESA) recovery plans.