The genus Acropora contains at least 150 extant coral species, most of which are found in the Indo-Pacific. Acropora are in the family Acroporidae, which also includes three other genera: Montipora, Anacropora, and Astreopora. Acropora is a genus of small polyp stony coral witnin the phylum Cnidaria. Acropora spp. are the dominant corals throughout the Indo-Pacific, especially in low-nutrient, high-energy reef environments. Notable characteristics of this genus include varied growth forms and strong skeletons. Because of these characteristics, Indo-Pacific acroporids can produce a wide range of architectures/morphologies, including branching, tables and plates. These forms, particularly plate and table forms, allow rapid exploitation of limiting resources—growth rate, substrate coverage and exposure to sunlight can be maximized, while effectively outcompeting most other species. Acropora are also important species in the Caribbean Sea, although Caribbean acroporids have experienced a 95% decline in abundance throughout their range since the 1970s.
Staghorn coral (Acropora cervicornis) and elkhorn coral (Acropora palmata) and a hybrid species (Acropora prolifera) are the only extant acroporids in the Caribbean, and are of considerable ecological importance. Staghorn coral forms a loosely branched colony which can grow up to three meters high and several meters across. Elkhorn can grow up to four meters across and two meters high, and is found on the reef crest and in the shallower waters of the forereef, especially in high energy environments, while staghorn is found in the deeper waters of the forereef and can also be found in lagoonal habitats. Elkhorn coral forms dense aggregations of flat, frond-like branches which resemble the horns of an elk. Elkhorn and staghorn corals are found throughout the Florida Keys, the Bahamas and the West Indies. Acroporids were historically prevalent in the Caribbean due to high growth rates and reproducing largely by fragmentation; evidence shows continuous habitation of the Caribbean by acroporids throughout paleohistory (since 3.6-2.6 million years ago). However, a combination of factors, especially disease has contributed to their decline in recent years. Currently, elkhorn and staghorn corals are listed as threatened species under the United States Endangered Species Act.
Life history traits of Caribbean Acroporids
Acropora cervicornis and A. palmata reproduce both asexually and sexually. Broadcast spawning is a method of sexual reproduction in which large numbers of eggs and sperm are released into the water column and are fertilized after reaching the surface. The embryos develop fairly rapidly, and the coral larvae, called planulae (singular: planula), are competent to settle in 4-5 days. Fragmentation is the process by which new coral grows asexually from broken pieces. Since A. cervicornis and A. palmata both have branching morphologies and live in hurricane-prone waters, this adaptation proves useful. Acropora cervicornis has relatively asymmetric growth patterns, low rates of larval recruitment, but high rates of reproduction by fragmentation. Staghorn coral (A. palmata) has one of the highest growth rates of all the corals, up to 4-8 inches (10-20 cm) yearly. It also reproduces well by fragmentation. Maturation usually takes between three and eight years, with an increase in colony fecundity (through sexual reproduction) with age.
Feeding is accomplished by a combination of nocturnal filter-feeding using the tentacles of the coral polyps and mutualistic associations with photosynthetic zooxanthellae, which provide the corals with needed carbon during the day. Predators in the Caribbean include the polychaete worm Hermodice carunculata and the gastropods Coralliophilia abbreviata and Drucella spp. Competition between other coral species via overgrowth, chemical defenses and ingestion could also cause mortality. Further causes of mortality are discussed below.
Threats to Caribbean Acroporids
Potential causes of decline for Caribbean acroporids include hurricane damage (though some reproduction occurs via fragmentation), coral bleaching, warming ocean temperatures, increased eutrophication and nutrient outputs, and disease (specifically white band disease). Mortality of both species has reduced coral cover significantly, opening space for colonization of the reef by macroalgae and sponges, which make it more difficult for colonization by new recruits of Acropora spp.
Acroporids in general are one of the more susceptible groups of coral to severe mortality due to exposure to ultraviolet light, breakage and stress. This lack of resistance to stress results in higher rates of coral bleaching.
White band disease is specific to A. palmata and A. cervicornis. Symptoms of white band disease include a band of bare skeleton surrounded by disintegrating coral tissue. The band of dying tissue normally starts near the base of the coral and works its way to the branch tips at rates of up to five millimeters per day. White band disease is thought to be a major factor in the decline of A. palmata and A. cervicornis since the 1980s. The cause of white band disease is unknown, but a bacterium is being investigated as a possible pathogen.
Though A. palmata and A. cervicornis have declined in the Caribbean as a whole, recent research has shown a recovery of A. palmata on the Belize Barrier Reef, with healthy new colonies growing atop skeletons of corals killed by white band disease.
- R.B.Aronson, W.F.Precht. 2001. White-band disease and the changing face of Caribbean coral reefs. Hydrobiologia 460:25–38
- C.Wallace. 1999. Staghorn Corals of the World. CSIRO Publishing.