Elkhorn coral (Acropora palmata) is an important reef-building coral in the Caribbean. The species has a complex structure with many branches which resemble that of elk antlers; hence, the common name. The branching structure creates habitat and shelter for many other reef species. Elkhorn coral is known to grow quickly with an average growth rate of 5 to 10 cm (2.0 to 3.9 in) per year. They can reproduce both sexually and asexually, though asexual reproduction is much more common and occurs through a process called fragmentation.
Although Elkhorn coral dominated the Caribbean in the early 1980s, the species has since dramatically declined in numbers. Scientists have estimated that between 1980 and 2006, when it was listed in the Endangered Species Act (ESA), the population declined by around 97%. This decline was due to a variety of factors, including disease, algae growth, climate change, ocean acidification, and human activity. In May 2006, Elkhorn coral was officially listed as threatened in the ESA along with another species in the Acropora genus, staghorn coral. Some conservation efforts are now in place to protect the species and promote increased genetic variability among the species.
Species overview
Appearance Elkhorn coral produce hard antler-like structures composed of calcium carbonate. These structures can be over 2 (> 6 ft.) meters high and 13 meters (43 ft) wide and are a dull, brownish-yellow.
Diet Elkhorn coral's primary source of nutrients comes from photosynthetic algae, zooxanthellae, that live inside the coral's cells. Therefore, Elkhorn coral are highly dependent on the sunlight for sustenance, leaving Elkhorn coral vulnerable to increased turbidity or water clarity. During bleaching events, or long-term periods of increased turbidity, Elkhorn coral can obtain nourishment from alternative food sources through filter feeding.
Zooxanthellae As mentioned, Elkhorn coral have special algae living in their tissues known as zooxanthellae. Zooxanthellae meet their nutritional requirements using photosynthesis, a process that converts carbon dioxide and water into sugars and oxygen. Elkhorn coral provide habitat for these algae and, in turn, receive an abundant source of oxygen, enabling them to grow quickly.Crabtree, R. E (2014). Endangered Species Act - Section 7 Consultation Biological Opinion. United States Corps of Engineers, 17-28.
Filter feeding Elkhorn coral can also use filter feeding techniques to obtain food. At night, Elkhorn coral use their tentacles to snatch free-swimming zooplankton from the water. Zooplankton complete daily diel migrations. In the morning, zooplankton sink to the depths of the ocean where predators are scarce, and then come nightfall, they rise back towards the surface. By concentrating their predation activities at night, Elkhorn coral expend less energy catching large amounts of prey. However, because filter feeding is more energy intensive than relying on zooxanthellae, Elkhorn coral will only resort to this feeding method when water conditions are poor and block out sunlight.
Habitat Elkhorn coral live in shallow habitats, which give them abundant access to light. Like all animals, Elkhorn coral requires oxygen to survive, most of which they obtain through special, photosynthetic algae living in their tissues called zooxanthellae. Therefore, in order to help zooxanthellae produce as much oxygen as possible, Elkhorn coral live in shallow regions between 1 and 5 meters deep to ensure that abundant light reaches the zooxanthellae, maximizing photosynthesis.
Elkhorn niche
Niche habitat These corals are found in clear, shallow water throughout the Bahamas, Florida, the Caribbean, and beyond to the northern shores of Venezuela. The most northern region occupied by Elkhorn coral is off the coast of Broward County, Florida. Characteristically found in shallow, turbulent water ranging from , Elkhorn coral thrive best in high-energy zones where wave action is frequent. This success is because wave action increases fragmentation, which allows more new colonies to form.
Niche evolution Due to fragmentation and rapid growth rates, Elkhorn coral were the primary foundation in the development of the Caribbean coral reefs. Over the last 5,000 years, Elkhorn coral, Staghorn coral, and Star coral comprised the southeastern reefs. Research has shown that the Elkhorn coral evolved during the Mid-Pliocene epoch, 5.2–2.58 million years ago. The unstable climate during this period allowed Elkhorn coral to thrive during glacial and interglacial events. During glacial events, sea levels rise and cause an increase in wave energy. As previously mentioned, Elkhorn coral does best in environments that are shallow and experience high energy changes, which aid in the reproductive process.
Ecosystem services Through their ecosystem services, Elkhorn coral increase the commercial value of coral reefs.
Reef-building service Like the foundation of a house, Elkhorn coral support coral reef communities. Their interwoven lattice-like branches create a variety of habitats varying in topography, which in turn, support diverse fish populations. These habitats support fish nurseries and protect fish from predators, increasing reef diversity. This elevated diversity makes the reefs more appealing to tourists, thereby increasing their economic value.
Storm protection Elkhorn coral provide insulation from storms to beaches and reefs. Their thick branches absorb the energy of storm waves, in turn, dissipating the force that crashes into beach shorelines. With less force, beachfront properties experience less damage, reducing the financial demand needed to restore the homes and livelihoods of many.
Life history
Age and growth Elkhorn coral colonies live for centuries. These colonies grow 5–10 cm (2–4 inches) each year, reaching maximum size in 10 to 12 years. Healthy Elkhorn coral can grow up to 13 cm (5 inches) in branch length a year, making them one of the fastest growing coral species. Elkhorn coral can span a diameter of 4 m (12 feet) wide and 2 m (6 feet) tall. Under stressful conditions, Elkhorn coral colonies and fragment growth slow down substantially. For example, the Fortuna Reefer Vessel grounding site showed no measurable growth over the last ten years due to stress. Elkhorn coral create thick, interlocking groups known as thickets. Thickets provide shelter to other marine life such as fish, crustaceans, and marine invertebrates, and consist of many identical polyps that have grown together. These dense groups provide protection to shorelines against large waves and storms.
Reproductive biology
Sexual reproduction The Elkhorn coral is a simultaneous hermaphrodite, meaning that in each coral colony both egg and sperm are produced. Despite this, self-fertilization usually does not occur. In order for successful fertilization to take place, two genetically distinct parents are needed. Successful reproduction rates are low in Elkhorn coral which limit the growth of new colonies. The probability of successful reproduction rates are low in Elkhorn coral because only about 50% of Elkhorn corals are genetically unique individuals. The Elkhorn coral reproduction cycle occurs once per year, after a full moon in the late summer, during the months of either July, August, or September. The gametes are released for only a few nights by broadcast spawning eggs and sperm into a water column. Once fertilized, the eggs develop into larvae that settle on hard surfaces and begin the growth of a new colony.
Asexual reproduction Most commonly, Elkhorn coral reproduce asexually, their most common type of reproduction. During storms, strong waves, or ship disturbances, fragments of coral break off and are transported to other areas where new colonies can begin. The new colonies are genetically identical to their parent colonies, while sexual reproduction results in new genotypes. Also, as opposed to sexual reproduction, fragmentation can occur at any time of the year.
Historical and present range Quantitative data available on the distribution and abundance of Elkhorn coral is scarce. Present locations of the Elkhorn coral include the southern coasts of Florida and the northern regions of the Gulf of Mexico. Elkhorn coral are also located in the Bay Islands of Honduras.
Critical habitat There are four critical habitats for the elkhorn coral that are essential to the conservation of this species. The specific regions designated and protected by the Endangered Species Act as critical habitats include regions in Florida (1,329 square miles of marine habitat), Puerto Rico (3,582 square miles of marine habitat), St. John and St. Thomas (121 square miles of marine habitat), and the St. Croix area (126 square miles of marine habitat).
Historical and present population size Elkhorn coral populations once dominated the southeastern region of the Northern hemisphere. Due to bleaching events, disease, and climate change, Elkhorn population has significantly decreased since the 1980s. In total, 80–98% of the Caribbean coral reef population has been lost. A longitudinal study done from 2005 to 2015 shows that the Elkhorn coral population had a proportional decline of 0.4–0.7 times in colony density across the entirety of its occupied range. This decline indicates that despite conservation efforts, the population is still decreasing. Overall, there are no concrete estimates of current or historical population sizes due to technological limitations.
Life history limitations Several factors limit the amount of information regarding the life history of Elkhorn coral. These factors include the necessity for additional tools to assess future population changes, sexual reproduction, and environmental disturbances.
Endangered Species Act listing
Elkhorn coral decline In Endangered Species Act (ESA) listings, Elkhorn coral is listed alongside Staghorn coral, A. cervicornia, because the corals share the same genus. While recognized as two separate species in scientific literature, they inhabit similar ranges and share many of the same characteristics.
Since the 1980s, Elkhorn coral populations have rapidly declined by an estimated 97%. Elkhorn coral was initially recognized as a candidate for ESA listing in June 1991, but was removed from the list of candidates in 1997 for a lack of sufficient evidence in regard to their status and threats. However, they were re-added to this list in 1999 due to concrete evidence of their population decline. In April 2004, they were transferred from the candidate species list to the species of concern list.
Petition to list Elkhorn coral In 2004, the National Marine Fisheries Service (NMFS) received a petition to list the Elkhorn coral, Staghorn coral, and a hybrid species, A. prolifera, as either threatened or endangered. The NMFS determined that the petition contained substantial information and established the Atlantic Acropora Biological Review Team (BRT) to review the status of these coral species. The BRT included a diverse group of experts from many different scientific fields, including coral conservationists. The BRT published a status review on March 3, 2005, which summarized the relevant, published literature on these species, comments from the public, and an assessment regarding current conservation efforts.
ESA listing as threatened