Named for its resemblance to elk antlers, elkhorn coral (Acropora palmata) is structurally complex with many large, thick branches. As with staghorn coral (Acropora cervicornis), the dominant reproduction mode of elkhorn coral is asexual fragmentation. New colonies form when branches break off of a colony and reattach themselves to the substrate. In addition, sexual reproduction through spawning each year in August or September also occurs. Individual colonies are concurrently male and female — a condition called simultaneous hermaphrodites. Although millions of eggs and sperm, called gametes, are released, few coral larvae survive to settle and grow into new colonies. Because so little sexual reproduction takes place, genetic diversity may be very low in remnant populations. Low genetic diversity makes coral populations more susceptible to being decimated by disease. Like staghorn coral, elkhorn coral has been listed as federally threatened under the Endangered Species Act.
Colonies of elkhorn coral are relatively fast growing, with branch length increasing about 2 to 4 inches per year. Colonies reach maximum size in about 10 to 12 years. Coral age can be determined by counting coral growth rings the same way that tree rings are counted to determine tree age. Elkhorn colonies typically grow in water depths of less than 20 feet but may be present down to a depth of about 65 feet. The species ranges geographically from its northern limit in Biscayne National Park to its southern limit in Venezuela. Large, continuous elkhorn thickets once extended along the front side of most coral reefs and supported a diverse assemblage of other invertebrates and fish. Once the most common coral in the Caribbean and known as the “giant redwoods of the reef,” today it is more common to see rubble fields made up of dead elkhorn coral and only a few isolated living colonies.