http://sanctuaries.noaa.gov

Once Abundant Coral Listed as Threatened Species

Elkhorn coral, Acropora palmata, was once the most abundant stony 

coral on shallow reef crests and fore-reefs of the Caribbean and 

Florida reef tract. In the Florida Keys, this fast-growing coral with its 

thick antler-like branches formed extensive habitat for fish and marine 

invertebrates. By the early 1990s, elkhorn coral had experienced 

widespread losses throughout its range. A similar trend was seen in 

a close relative, staghorn coral, Acropora cervicornis. Multiple factors 

are thought to have contributed to coral declines, including impacts 

from hurricanes, coral disease, mass coral bleaching, climate change, 

coastal pollution, overfishing, and damage from boaters and divers. 

In 2006, the two species were listed as threatened under the 

Endangered Species Act. The recovery strategy, developed by NOAA 

Fisheries, included designating critical habitats in Florida, Puerto Rico 

and the U.S. Virgin Islands and called for further investigations into the 

factors that prevent natural recovery of the species.

NOAA Scientists Investigate Causes of Coral Declines

In 2004, scientists from NOAA’s Southeast Fisheries Science Center (Miami) began a longterm monitoring program in Florida Keys National Marine Sanctuary to determine the current 

population status of elkhorn coral and to better understand the primary threats and stressors 

affecting its recovery. Understanding basic population statistics (coral recruitment and 

mortality) and the nature of stressors is critical to the species recovery plan and management 

of elkhorn coral within the sanctuary. 

The study area is focused in the Upper Keys and contains 25 permanent study 

plots located on eight different fore-reefs. All elkhorn coral colonies in each study 

plot are mapped and assessed annually. A randomly selected subset of colonies is 

assessed more comprehensively three times per year. During each of these more in-depth 

surveys, scientists record the sources and severity of coral tissue loss, which includes 

prevalence and impact of disease, physical damage, and predation by the coral-eating snail, 

Coralliophila abbreviata. From these surveys, a measure of live coral known as a “live area 

index” is calculated for each colony. This index is used to track the changes in the total live 

tissue for each coral over time and to determine the relative importance of each cause of 

tissue loss.

Several Factors Affect Coral Recovery

Overall, according to this study, the top three reasons for elkhorn tissue loss have been 

fragmentation or breakage of coral branches, “white” disease, and predation by the coraleating snail (see graph on back). However, the proportion of loss due to fragmentation was 

skewed somewhat because of the large degree of physical damage done by hurricanes in 

2005. Sixty-three percent of coral tissue loss that year was due to breakage. Since then, 

colonies within the study plots have only shown modest signs of recovery and most hurricanegenerated fragments did not survive. 

Elkhorn Coral: Science of Recovery 

Florida Keys National Marine Sanctuary

http://floridakeys.noaa.gov/

Coral-eating snails covered in algae feed on elkhorn coral tissue.

Photo: NOAA Fisheries

In response to population 

declines, the Center for 

Biological Diversity 

petitioned NOAA 

Fisheries to list elkhorn 

coral, along with its close 

relative, staghorn coral, 

under the Endangered 

Species Act (ESA). After 

a thorough review 

process that involved 

opportunities for public 

comment, NOAA 

Fisheries determined that 

these two corals met the 

criteria to be listed as 

threatened under the 

ESA. ESA protection is 

aimed at reducing threats 

and stressors to the 

focal species in order to 

encourage its natural 

recovery. 
To learn more about these and other accomplishments, 

visit sanctuaries.noaa.gov

Low Population Density Reduces Spawning and Fragmentation Success

Elkhorn populations depend upon both fragmentation and spawning as methods of reproduction for long-term viability. Spawning 

results in a new, genetically unique coral, while fragmentation produces genetic copies of the parent coral. Fragmented corals 

have the potential to grow into new colonies under the right conditions, but scientists have noted that relatively few fragments are 

managing to do so. At the reef crest, elkhorn coral colony density is so 

low that when fragments break off, they tend to land in nearby sandy 

areas where they usually die, as opposed to being trapped by dense 

elkhorn thickets where they are held in place, allowing them to flourish 

and grow. 

Low population density can also reduce the chances for successful 

spawning. During spawning, coral gametes (egg and sperm) do not 

survive long, and if individual colonies are located too far apart on the 

reef, the chance of successful reproduction decreases. 

Predation Revealed as Factor Affecting Tissue Loss

To reveal the threats facing elkhorn coral when hurricanes are not a 

factor, the live area index data were re-analyzed excluding the 2005 

data. Analysis of the remaining survey years showed that the proportion 

of tissue loss attributed to fragmentation (20%) was less important 

than the proportion attributed to either disease (36%) or predation 

from snails (27%). These results indicate that white disease and coraleating snails represent more chronic threats to elkhorn corals than 

breakage from hurricanes. 

About one-third of the elkhorn colonies surveyed had snails feeding on them, with an average of four snails per affected colony. 

Yet, coral-eating snails accounted for about 25% of all tissue loss recorded in the survey. Thus, a relatively small number of snails 

were capable of causing substantial tissue loss. Scientists observed that snails often feed on a colony until it is dead or nearly 

dead. Studies have also shown that as coral colonies die, snails will move to nearby “healthier” colonies to find food. In turn, the 

remaining corals succumb to the higher densities of snails feeding on them. Physically removing snails from corals might reduce 

live tissue loss, which could promote colony survival. Because snails are also a known carrier of coral disease, their removal might 

indirectly reduce the rate of live tissue loss to “white” disease. Continued research into the factors that compromise recovery in 

elkhorn corals is underway. More information about the impact of snail removal is needed before this is deemed an effective and 

appropriate management response. 

Restoration Efforts Using Coral Fragments are Underway 

While the issues facing elkhorn and staghorn coral recovery are complex, the capacity for these species to asexually reproduce 

through fragmentation and their fast growth rates make them good candidates for coral restoration. Coral restoration has been 

underway in the Florida Keys since local marine life expert Ken Nedimyer started the first offshore coral nursery in 2000. Nedimyer 

is a long-time sanctuary advisory council member who has worked closely with sanctuary staff and volunteers on this project. Since 

2003, more than 1700 staghorn coral fragments grown at the Coral Restoration Foundation nursery have been transplanted to 20 

restoration sites where they have shown high survival and growth rates. The foundation has also been growing elkhorn coral for 

future restoration projects. 

In 2009, the Coral Restoration Foundation became one of several nursery partners participating in a regional restoration project 

coordinated by The Nature Conservancy and supported by NOAA using American Reinvestment and Recovery Act (ARRA) funds. 

The project seeks to restore reef areas that once supported large thickets of acroporid corals by out-planting 5,000 nurseryreared staghorn corals throughout south Florida and the U.S. Virgin Islands. Out-planted corals are being grown from at least 144 

genetically distinct colonies. The seeding of different genotypes helps ensure genetic diversity and sustainability in the growing 

population. Increased coral density can improve the chances for successful fertilization during spawning, as well as improve 

retention and growth of fragmented corals.

Scientists from NOAA and partner agencies are continuing to evaluate potential actions to enhance coral recovery, carefully monitor 

coral population dynamics and investigate the causes of coral disease. For more information on the status of elkhorn and staghorn 

corals, visit http://www.sefsc.noaa.gov/species/corals/acropora.htm.

Without 2005 data, fragmentation accounts for 20% of tissue loss. 

Figure: NOAA Fisheries

Principle Investigator: Dr. Dana Williams, NOAA Southeast Fisheries Science Center 

Funding Source: NOAA Coral Reef Conservation Program

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary

Publication Date: October 2011