Staghorn Corals

Acropora is a genus of small polyp stony coral in the phylum Cnidaria.^[3] Some of its species are known as table coral, elkhorn coral, and staghorn coral. Over 149 species are described.^[4] Acropora species are some of the major reef corals responsible for building the immense calcium carbonate substructure that supports the thin living skin of a reef.

Anatomy and distribution

Depending on the species and location, Acropora species may grow as plates or slender or broad branches. Like other corals, Acropora corals are colonies of individual polyps, which are about 2 mm across and share tissue and a nerve net. The polyps can withdraw back into the coral in response to movement or disturbance by potential predators, but when undisturbed, they protrude slightly. The polyps typically extend further at night to help capture plankton and organic matter from the water.

The species are distributed in the Indo-Pacific (over 100 species) and Caribbean (3 species). However, the true number of species is unknown: firstly, the validity of many of these species is questioned as some have been shown to represent hybrids, for example Acropora prolifera;^[5] and secondly, some species have been shown to represent cryptic species complexes.^[6]

Threats

Symbiodinium, symbiotic algae, live in the corals' cells and produce energy for the animals through photosynthesis. Environmental destruction has led to a dwindling of populations of Acropora, along with other coral species. Acropora is especially susceptible to bleaching when stressed. Bleaching is due to the loss of the coral's zooxanthellae, which are a golden-brown color. Bleached corals are stark white and may die if new Symbiodinium cells cannot be assimilated. Common causes of bleaching and coral death include pollution, abnormally warm water temperatures, increased ocean acidification, sedimentation, and eutrophication.

In 2014 the U.S. Fish and Wildlife Service listed ten Acropora species as 'threatened'.^[7]

Reef-keeping

Close-up of a network of Acropora polyps

Most Acropora species are brown or green, but a few are brightly colored, and those rare corals are prized by aquarists. Captive propagation of Acropora is widespread in the reef-keeping community. Given the right conditions, many Acropora species grow quickly, and individual colonies can exceed a meter across in the wild. In a well-maintained reef aquarium, finger-sized fragments can grow into medicine ball-sized colonies in one to two years. Captive specimens are steadily undergoing changes due to selection which enable them to thrive in the home aquarium. In some cases, fragments of captive specimens are used to repopulate barren reefs in the wild.^[8]

Acropora species are challenging to keep in a home aquarium. They require bright light, stable temperatures, regular addition of calcium and alkalinity supplements, and clean, turbulent water.

Common parasites of colonies in reef aquariums are "Acropora-eating flatworms" Amakusaplana acroporae,^[9] and "red bugs" (Tegastes acroporanus).

Species

Acropora (Acroporidae) at French Frigate Shoals, northwestern Hawaiian Islands

A. tenuis cells of the IVB5 line and symbiosis with photosynthetic dino­flagellate Breviolum minutum (Suessiales) — in vitro: Symbiotic inter­actions of coral cells (b and c) and dino­flagellates (x and y). Coral cell b inter­acted with symbiont x, but did not in­cor­porate it, whereas coral cell c endo­cytosed both x and y.

The following species are recognised in the genus Acropora:^[10]

References