Boulder star coral  (Montastraea annularis)

Classified as Endangered (EN) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

Prior to 1994, the wide variability exhibited in the appearance of Montastraea annularis was attributed to the different environmental conditions in which it occurs (3). However, scientists have since discovered that it actually comprises a species complex that can be divided into three distinct species: the type specimen, M. annularis, together with two newly described species, M. faveolata and M. franksi (1) (3) (4).

Like other colony-forming corals, colonies of M. annularis are composed of numerous small polyps, which are soft-bodied animals, related to anemones. Each polyp bears numerous tentacles that direct food into a central mouth, where it is digested in a sac-like body cavity. One of the most remarkable and ecologically important features of corals is that the polyps secrete a hard skeleton, called a ‘corallite’, which over successive generations contributes to the formation of a coral reef. The coral skeleton forms the bulk of the colony, with the living polyp tissue comprising only a thin veneer (4). In M. annularis, the colonies are formed by long, thick columns, with only the top parts supporting living tissue. The colour of the living colonies is usually golden brown to tan, but sometimes appears grey or green (3).

This common species occurs in the Caribbean, the Gulf of Mexico, Florida, the Bahamas, and Bermuda (1). 

Montastraea annularis is found at shallow and intermediate depths, from 1 to 20 metres (3).

Like many coral species, M. annularis is zooxanthellate, which means that its tissues contain large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship, in which the algae gain a stable environment within the coral's tissues, while the coral receives nutrients produced by the algae through photosynthesis. By harnessing the sun's energy in this way, corals are able to grow rapidly and form vast reef structures, but are constrained to live near the water surface (4). While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (5). 

Around one third of the world’s reef-building corals are threatened with extinction (6). The principal threat to corals is the rise in sea temperature associated with global climate change. This leads to coral bleaching, where the symbiotic algae are expelled, leaving the corals weak and vulnerable to an increasing variety of harmful diseases. Climate change is also expected to increase ocean acidification and result in a greater frequency of extreme weather events such as destructive storms. This is not to mention the localised threats to coral reefs from pollution, destructive fishing practices, invasive species, human development, and other activities (1) (6).

In addition to being listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which makes it an offence to trade M. annularis without a permit (2), this coral falls within several Marine Protected Areas across its range. To specifically conserve M. annularis, recommendations have been made for a raft of studies into various aspects of its taxonomy, biology and ecology, including an assessment of threats and potential recovery techniques (1).

For further information on the conservation of coral reefs see:

• Reef Check:
  http://www.reefcheck.org/
• The Coral Reef Alliance:
  http://www.coralreefalliance.org/