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.
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.
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.
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 1 to 5 meters, 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.
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.
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/St.Thomas (121 square miles of marine habitat), and St.Croix area (126 square miles of marine habitat).
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.
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.
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.
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.
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 2005-2015 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.
The NMFS published a recovery plan in March 2015, which clearly outlined the conservation goals and efforts for Elkhorn coral. This document indicated that the main goals are to increase the abundance of this species and protect the genetic diversity throughout its entire range.
This latter goal is especially important for conservation. Elkhorn coral are capable of both sexual and asexual reproduction; however, asexual reproduction is more common, which has resulted in only about 50% of Elkhorn being genetic individuals. The lack of genetic variation makes the species more susceptible to threats and is, therefore, an important focus for conservation.
