Acroporidae Family of Corals

Staghorn Coral, Acropora cervicornis.  A representative of the Acroporidae Family of Corals.

Phylogeny: The Acroporid Corals are members of the Acroporidae Family of Small-polyped Stony Corals. The Acroporidae Family has two hundred seven-seven global species, with only two of these species found in the Gulf of Mexico. Like Sea Anemones, Jellyfish, and Hydroids, Acroporid Corals belong to the Phylum Cnidaria. They are in the Class Hexacorallia and the Order Scleractinia.

Distribution:  The Acroporid Coral is found in the tropical waters of the Indian and Pacific Oceans, as well as the Western Atlantic Ocean. The Acroporid Corals require access to sunlight and are found in shallow water, with only a few species exceeding depths of 50 m (164 feet). They grow on hard substrate such as buoy chains, pilings, rocks, and wreckage. They are prone to damage by sediment or disturbance by snorkelers, divers, or other beach goers.

Morphology: The Acroporidae Corals have radial symmetry, a hollow digestive cavity, and specialized stinging structures contained within the tentacles surrounding the mouth. Their polyps have a flower-like appearance, representative of the Class Anthozoa. In addition, their polyps produce a hard, calcareous skeleton, representative of the Order Scleractina. They live in tightly packed colonies where new generations build upon the skeletons of previous generations, forming coral reefs. Acroporid Corals fall under the category of hard, or stony corals, as compared to the soft corals, or octocorals. Acroporid Corals are characterized as having small polyps and an axial corallite at the end of each branch. Growth occurs by secondary corallites budding from the terminal corallite. They are the most prolific of the reef-building corals. They are also the fastest growing and the most species-rich family. Corals in this family may be encrusting, branching, or tabletop. The Branching Corals are often known as staghorn corals and may have branches reaching 2 m (6 feet 6 inches) in length. The Tabletop Corals reach 3 m (9 feet 9 inches) in diameter. The Acroporid Corals are blue, brown, cream, green or pink in color.

Ecosystem Roles:  Acroporid Corals have a symbiotic relationship with zooxanthellae, single-cell dinoflagellates. Zooxanthellae live within certain coral polyps, sea anemones, jellyfish, and nudibranchs. They produce energy by means of photosynthesis and the energy is passed along to their hosts, sometimes providing the majority of the host’s total energy needs. In return, the host provides nutrients, carbon dioxide, and a secure, sunlit, platform for the zooxanthellae. During periods of chemical or thermal stress the corals can eject the zooxanthellae to reduce metabolic stress. This causes the coral to appear white. This process is known as coral bleaching. Coral bleaching can be an indicator of an unhealthy environment, or a natural response to a short-term condition such as El Niño. The coral will replace the zooxanthellae after the stress is resolved, or it will acquire a different species of zooxanthellae that is better accommodated to the new environment. In either case, the polyp risks starvation if it is unable to replace the zooxanthellae quickly. Acroporid Corals supplement the energy provided by zooxanthellae by using their tentacles to capture amphipods and other plankton from the surrounding water. The tentacles then pass the food to the mouth. Because these polyps cannot change location, the Acroporid Corals are found in areas where moving water, caused by wave action or current, brings them more food. They are preyed upon by crabs, fish, gastropods, polychaete worms and starfish.

Reproduction:  The Acroporid Corals are hermaphrodites, that reproduce via broadcast spawning. This event occurs once a year, when the coral colony produces mass amounts of sperm and egg. The resulting gametes undergo three stages of development until it grows into a settled coral. Mortality rates are high during these stages and early settlement. Sediment density can impact both larvae survival and larvae settlement. In addition, reproduction can occur asexually through fragmentation.