Acropora palmata, (Lamarck, 1816)
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. Following the 2023 marine heatwave, Elkhorn corals are now considered functionally extinct within Florida’s Coral Reef.
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 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 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.
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.
The BRT's status review concluded that the ESA should list the Elkhorn coral as a threatened species, given the importance of its ecosystem contributions and the severity of the numerous threats impacting the species. As a formerly abundant species, its high growth rate allowed it to keep up with sea level changes. The branching morphology created homes and protection for many other reef organisms. Since its decline, no other coral species has been able to fulfill these ecosystem functions. As mentioned, the species faces many threats, which include but are not limited to disease, temperature-induced bleaching, and physical damage from hurricanes. The BRT determined that Elkhorn coral was not at risk for extinction, but could become so in the foreseeable future because its population sizes were low and the severity of threats was predicted to increase.
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