Acropora cervicornis (Staghorn Coral): Endangered Species Spotlight

Acropora cervicornis is a species of staghorn coral that is predominantly found in Florida, the Bahamas, the Gulf of Mexico and the Caribbean. Comprised of approximately 400 different species of varying shapes and colours, staghorn corals are branching, stony corals that typically inhabit shallow tropical reefs and lagoons. As well as being some of the fastest growing corals in the world, staghorn corals are incredibly important for their contribution to reef growth and their role in providing habitats for marine life. In the Caribbean, Acropora cervicornis has played a fundamental part in the construction of coral reefs over the past 5,000 years and is widely regarded as one of the most important species in the region. However, an unprecedented disease incident in the early 1980s resulted in the loss of approximately 97% of the species’ cover, abundance, and occupied range. Remaining populations are generally isolated and display low colony abundance, thus increasing their susceptibility to threats of climate change, pollution, unsustainable fishing practices, and disease. With a population trend in punctuated decline, immediate restoration, conservation and monitoring efforts are needed to prevent the extinction of Acropora cervicornis.

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1. Appearance

Acropora cervicornis colonies are typically tan or light brown with white tips, deriving their colour from the zooxanthellae (algae) residing within their tissue. Stemming out from a central trunk at an upwards angle, the cylindrical branches of A. cervicornis are typically two to eight centimetres thick and can exceed two metres in length. Colonies often grow to form interlocking frameworks known as thickets, however A. cervicornis colonies tend to be more open and loosely packed than other species of Acroporidae. Although often confused for plants or rocks, largely due to their sessile nature (permanently fixed in one place; immobile) corals are, in fact, animals.

Corals are made up of hundreds of soft-bodied organisms known as polyps, which attach to a solid substrate (such as a rock or the dead skeletons of other polyps) and begin secreting calcium carbonate to create hard external skeletons, or corallites. As these polyp conglomerates continuously grow and reproduce, they begin to create these incredible hard, stony coral structures.

2. Diet

Most species of coral, particularly those inhabiting shallow, tropical waters, have two sources of food. Firstly, polyps have stinging cells, known as nematocysts, which they extend from out of their corallite to capture prey with, typically plankton. However, corals in shallow, warm environments derive most of their nutrition from the mutually beneficial, or symbiotic, relationship they have with the zooxanthellae (algae) that reside within the tissue of polyps. Zooxanthellae are plant-like organisms that utilise a coral’s metabolic waste products for photosynthesis. Having received these nutrients from the coral, the zooxanthellae then pass on some of the food they make to the coral. This symbiotic relationship, whereby corals receive food and oxygen in exchange for providing zooxanthellae with nutrients and shelter, plays a fundamental part in the rapid growth rate of tropical, shallow water corals. 

3. Habitat & Behaviour

The distribution of Acropora cervicornis spans the western Atlantic, from Mexico (Veracruz), southern Florida, and the northern Bahamas, down south across the Caribbean Sea to Trinidad and Tobago, including the insular and coastal reefs of Barbados, Venezuela, Aruba, Curaçao, Bonaire and Colombia. The species is unlikely to occur further north than Palm Beach County, Florida, or further south than Trinidad and Tobago. 

Acropora cervicornis requires clear, oxygenated, warm waters in order to thrive, and is therefore typically found in tropical, shallow reef ecosystems. Although the species displays a preference for upper to mid-reef slopes and lagoons in regions with low or moderate wave exposure, A. cervicornis has been observed in a range of coral reef habitats, including spur and groove formations, bank reefs, patch reefs, transitional reef habitats, limestone ridges, terraces and hard bottom habitats. Despite a depth range of one to 60 metres, the species is rarely found beyond 25 metres from the surface of the water.

Staghorn corals are simultaneous hermaphrodites, producing both eggs and sperm but not self-fertilising. Upon reaching sexual maturity, typically at around 18 centimetres tall or three to eight years, a staghorn coral will reproduce once a year by broadcast spawning eggs and sperm into the water column. Fertilised eggs will then develop into larvae, settling on hard substrates in the region or drifting hundreds of kilometres away to form new colonies. Studies have indicated that low rates of larval recruitment are typical for this species in the Caribbean, and that recruitment by sexual reproduction is relatively rare despite high levels of gamete production and release. Nevertheless, staghorn corals can also form new colonies when fragments of a coral branch fall off, reattach themselves to a hard substrate, and continue to grow.

More on the topic: Unpacking Florida’s Coral Reef Restoration Agenda

4. Ecological Importance

As mentioned, Acropora cervicornis is regarded as one of the most important species of coral in the Caribbean due to its extensive contribution to reef growth and its role in providing a complex habitat for marine life, thus safeguarding the biodiversity of marine ecosystems in the region. In abundance, staghorn corals further provide shoreline protection from waves and storms.

A. cervicornis, and coral reefs in general, also act as environmental indicators; their sensitivity to changes in the temperature, salinity, pollution levels, clarity, and pH levels of the waters they inhabit can inform scientists of any declines in the quality and health of ocean habitats. 

5. Threats

Although once found in high abundance across its endemic range, studies conducted on Acropora cervicornis have indicated a population decline of at least 80% over the past 30 years, with a current population trend in punctuated decline. As a result, the species has been classified as Critically Endangered under the International Union for Conservation of Nature (IUCN) Red List since 2008. Having suffered a staggering 98% decrease in cover, abundance and occupied range in the 1980s due to disease, and given its acute sensitivity to changes in environmental quality, remaining populations of A. cervicornis are currently isolated, display low colony abundance, and face a high risk of extinction. Major threats, which present themselves in a complex interplay, include climate change, pollution, and disease.

Perhaps posing the greatest threat to coral reefs across the world is the phenomenon of climate change, as rising ocean temperatures, disrupted pH levels, increases in the severity of storms, and possible shifts in ocean circulation patterns have had disastrous effects on ocean habitats over the past decades.

Coral bleaching occurs when ocean temperatures rise at least 1C above the normal seasonal maximum, subjecting coral reefs to increased levels of stress. As a result, coral evict the symbiotic algae (zooxanthellae) from their tissue, causing the coral to turn white. Although the coral remains alive in this bleached state, it has lost one of its primary sources of nutrition and is thus rendered highly susceptible to disease. If exposed to prolonged heat, the coral will eventually die from starvation or disease. Staghorn corals, and Acropora cervicornis in particular, appear to have an especially low resistance and tolerance to bleaching, taking longer to recover than other species.

In La Parguera National Reserve along the southwest coast of Puerto Rico, Acropora cervicornis displayed higher mortality rates due to temperature shifts and disease when compared to Acropora prolifera. After the occurrence of two global bleaching events in 1998 and 2010, the first mass, multi-year coral bleaching event took place between 2014 and 2017, where 30% of coral reefs experienced mortality-level stress. In April 2024, the National Oceanic and Atmospheric Administration (NOAA) detected evidence of a fourth, ongoing global bleaching event that commenced in February 2023. As bleaching events continue to occur with increasing frequency, coral reefs are prevented from ever fully recovering.

In addition to absorbing heat, the ocean absorbs approximately 30% of atmospheric carbon dioxide, acting as a carbon sink. As carbon dioxide dissolves in seawater, the water becomes more acidic, causing a drop in its pH level. With carbon emissions steadily increasing over the past 200 years, reaching a record annual emission of 37.4 billion tonnes of carbon dioxide in 2023, oceans across the globe have become 30% more acidic as a result of absorbing this excess carbon dioxide. Commonly referred to as ocean acidification, this change in the ocean’s pH has reduced calcification rates in reef-building organisms since calcium carbonate only forms when the ocean’s pH level sits within a specific range. Perhaps of greater concern is the possibility that this increasing acidity could also prompt existing corallites and sediment platforms to dissolve away, causing entire reefs to disappear. In a study published in 2018, researchers determined that there is a specific low point in oceanic calcium carbonate levels, below which coral reefs dissolve faster than they can build.

Compounding the vulnerability of coral reefs to disease and mortality is pollution. Runoff from agriculture, gardening, sewage, and costal development projects often contains toxins that affect the feeding habits, growth, reproduction, and ecological function of corals. This is particularly true of chemical and oil spills that occur in close proximity to coastal areas. Certain types of sunscreen can also cause extensive damage to coral reefs as they contain chemicals that induce coral bleaching, particularly in locations popular with snorkelling and diving. Excessive quantities of nutrients that are often found in fertilisers, such as nitrogen and phosphorus, further cause algal blooms that smother corals and affect the clarity of water. Deforestation and human development also typically intensify the process of soil erosion, which results in reefs becoming covered in silt. Since corals rely heavily on zooxanthellae to photosynthesise sunlight and supply them with nutrition, instances of prolonged declines in water clarity can expose coral reefs to the risk of starvation and disease. Additionally, pathogens found in untreated sewage can infect entire coral reefs, spreading into significant outbreaks.

Plastic pollution also poses a significant threat to oceanic habitats across the world due to the myriad of detrimental consequences it has on marine ecosystems. Large pieces of trash that wash into coral reefs from shorelines can damage coral branches or block sunlight from reaching the zooxanthellae within their tissue. Microplastics, often mistaken for food particles, are regularly ingested by corals as the smell of plastic is masked by bacteria found on the plastic. This bacteria, which is introduced to reef habitats from land or the ocean’s surface, may also carry pathogens that can cause widespread infection or mortality. Once ingested, most pieces of plastic are expelled after 48 hours, however some may become embedded within the corallite. These embedded pieces can then begin leaking toxic chemicals, affecting the health of the coral.

With issues of warming ocean temperatures, acidification, and pollution having deteriorated the health and vitality of coral reefs over the past few decades, coral disease has increasingly become a major threat to species worldwide. According to a 2018 study, the rising prevalence of coral disease and mortality can be linked not only to thermal stress, but also reduced water quality and clarity, nutrient enrichment, plastic pollution, and sedimentation due to dredging. The results of a survey conducted on 159 reefs across the Asia-Pacific region indicated that the likelihood of coral disease increases 20-fold once reefs are exposed to plastic. White-band disease (WBD) is thought to be the primary cause for the aforementioned Acroporid disease event that has affected Caribbean reefs since the 1980s, although the prevalence of WBD in A. cervicornis is currently low due to the limited distribution and abundance of the species. Regardless, a mere 6% of remaining A. cervicornis populations have proven resistant to WBD thus far.

Other major threats to A. cervicornis include: overfishing; unsustainable fishing practices, including dynamite fishing, chemical fishing, and dredging; changes in native species dynamics; human recreation and tourism; changes in the frequency and intensity of storms and hurricanes; as well as increased predation by Stegastes planifrons (Three-spot Damselfish), Hermodice carunculata (Bearded Fireworm), and Coralliophyllia spp. (coralivorous snail). Unsustainable fishing practices can have immense, long-term consequences on marine ecosystems, quickly altering the structure of a habitat from coral-dominated reefs to algal-dominated reefs (“phase shifts”) since the fish that consume algae are no longer around to maintain reefs clean and provide space for corals to grow. A. cervicornis is also among the most popular species of coral harvested for aquariums, with legal imports in the United States doubling from 2003 to 2009. Further hindered by restricted gene flow and low larval recruitment, A. cervicornis is unlikely to fully recover and regrow viable populations across its endemic range unless effective, holistic conservation, monitoring and repopulation efforts are put into place.

6. Conservation Efforts

When given the opportunity to recover in an ideal environment, coral have proven to be incredibly resilient. In addition to being listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), Acropora cervicornis has been further classified as ‘Threatened’ under the U.S. Endangered Species Act, as ‘Vulnerable’ under the Venezuelan national Red List, and as “Under Special Protection” under Mexico’s endangered species list. The species is also found in various marine protected areas (MPAs), such as the Florida Keys National Marine Sanctuary, Biscayne National Park, Dry Tortugas National Park, Buck Island Reef National Monument, Hol Chan Marine Reserve and Exuma Cays Land and Sea Park. Legislative measures such as these are of critical importance for safeguarding coral habitats, as they aim to reduce fishing pressures, prohibit trawling and dredging, limit tourism, and maintain clean, balanced ecosystems within which reefs can thrive.

Complimenting legislative measures are localised efforts to propagate, reintroduce and restore Acropora cervicornis within its endemic range, such as in Florida, Mexico, Puerto Rico, the Dominican Republic, Jamaica, and Honduras. Regions regularly affected by ship groundings and hurricanes have attempted to salvage damaged reefs by reattaching corals in Acrpoprid habitats, which accelerates the growth of new coral. In the Caribbean, what began as 60 Acropora restoration programmes in 2012, focusing primarily on asexual propagation or ‘coral gardening’ methods, has since grown significantly both in the number and size of programmes, now implementing larval propagation techniques in the interest of genetic diversity and adaptive capacity. In collaboration with scientists and restoration practitioners worldwide, the Coral Restoration Consortium (CRC) has developed a geo-referenced database to monitor data collected from restoration programmes across the globe, ensuring that coral conservation efforts are in sync and as effective as possible. In the Dominican Republic, the Dominican Consortium of Costal Restoration has implemented in situ nurseries, coral gardens, and sexual propagation programmes, primarily led by Fundación Dominicana de Estudios Marinos (FUNDEMAR) and Fundación Grupo Punta Cana (FGPC). In Bayahibe, novel advances in sexual propagation strategies have resulted in the seeding of hundreds of thousands of sexual recruits.

Due to the ever-growing threats of climate change, ocean acidification and disease, scientists have also shifted their focus towards more drastic interventions that aim at increasing the resistance of restored coral populations to thermal stress and disease. In Northwestern Australia, scientists have been studying a species of staghorn coral (Acropora aspera) that inhabit tidal areas and experience temperature swings of 7C as the tide goes in and out, enduring a maximum temperature of 32C. By studying the genetic profile of coral species that display an incredible resistance towards environmental stressors, researchers hope to transplant this coral globally to restore reefs in afflicted regions, or breed them with other species of coral to create offspring with better resistance to thermal stress. In addition to the genetic makeup of coral, another factor that appears to affect resilience to thermal stress and bleaching is the specific type of symbiotic bacteria that reside within the tissue of coral. As a result, scientists are attempting to apply topical probiotics that contain specific, beneficial strains of bacteria to protect coral species that are particularly susceptible to bleaching. 

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An artificial propagation technique known as cyrofreezing, or cyropreservation, has also gained traction as a potentially important tool for conservation. Pioneered by researchers at the Smithsonian National Zoo, technology typically utilised for human sperm banks has been applied to the preservation of coral sperm and stem cells. By retaining genetic material that can remain viable for years, conservationists hope that frozen gametes can be used to breed new coral colonies in the future, restoring populations with low abundance and genetic diversity. Continued advancements in stem cell research further present the possibility of regenerating frozen stem cells into mature individuals. The sperm of Acropora cervicornis has been preserved within coral bio-repositories in the United States.

To further improve the efficiency and efficacy of conservation actions, continued research into Acropora cervicornis is of critical importance. Areas of research include: taxonomy; biology, behaviour and ecology; population; abundance and trends; habitat status; threats and resilience; restoration efforts; methods of identification; establishment and management of protected areas; recovery management; disease, pathogen and parasite management; and the success of current conservation strategies. In the US, the NOAA conducts research projects that track individuals to better comprehend population trends and causes of death, as well as testing the effects of temperature and acidification on eggs, sperm, larvae and newly settled colonies. With the continued collaboration of scientists and conservationists across the world, efforts to prevent the extinction of Acropora cervicornis have the potential to restore the species’ once immense abundance, cover, and occupied range.

How To Help

• Practice ethical tourism. When snorkelling or scuba diving, take care when wearing flippers or swimming near to coral reefs as coral branches may break easily when kicked or stepped on. Do not touch any marine life, as you may be carrying foreign bacteria, and do not take anything from the ocean (aside from rubbish).

• Use reef-friendly sunscreen. Next time you go swimming in the ocean, make sure to apply a “reef-safe” or “reef-friendly” sunscreen which does not contain any oxybenzone and octinoxate, as these are two common UV-blocking chemicals. When coral come into contact with such chemicals, symbiotic bacteria are unable to photosynthesise, and the coral suffers from bleaching.

• Donate your time with beach and ocean clean ups. Plastic pollution in oceans and along beach fronts is incredibly harmful to the health and vitality of coral reefs. Donate your time to beach and ocean clean ups in your local area, or volunteer abroad to discover the various species of coral across the world.