As its common name suggests, Graham’s sheet coral (Agaricia grahamae) grows in large, thin plates, which are either flat or slightly curved, and often overlap (3)(4). Graham’s sheet coral is generally yellowish-brown to grey in colour, usually with pale margins (3)(4), and, like other corals, its colonies are made up of numerous tiny, anemone-like polyps, which secrete a hard skeleton (3). In Graham’s sheet coral, the polyps are widely spaced along the bottom of long, roughly concentric depressions (‘valleys’), which are V-shaped in cross-section and are separated by parallel ridges (3)(4)(5). The underside of the colony is smooth, lacking polyps(4). In this species, the individual coral skeletons, known as ‘corallites’, measure around 2 to 2.9 millimetres in diameter (4).
Graham’s sheet coral is similar in appearance to the related Agaricia lamarcki (Lamarck’s sheet coral), but has smaller corallites, more pointed tops to the colony ridges (5) and lacks the white polyp ‘mouths’ of the latter (3). It can be distinguished from Agaricia fragilis (fragile saucer coral) mainly by its longer valleys, larger corallites and the smoother appearance of its colonies(3)(5).
As in other corals, each polyp of Graham’s sheet coral possesses a ring of tentacles surrounding a central ‘mouth’, and uses stinging cells on the tentacles to catch tiny zooplankton. However, Graham’s sheet coral is also a ‘zooxanthellate’ coral, meaning that it receives most of its nutrition from single-celled algae, known as zooxanthellae, which live within its tissues. The zooxanthellae produce nutrients for the coral through photosynthesis, and in return receive a safe, stable environment in which to live. This relationship limits the coral to living in relatively clear, warm, shallow water where photosynthesis can take place, but enables it to grow faster and to create large reef structures (3)(6).
Coral colonies are able to reproduce asexually in a process known as budding, in which polyps divide into one or more new polyps. Corals are also able to reproduce sexually, usually by releasing large numbers of sperm and eggs into the water for external fertilisation(3). However, like some other Agaricia species (7), Graham’s sheet coral is reported to be a ‘brooding’ species (8), in which fertilisation occurs internally, the larvae (known as planulae) then developing inside the polyp before being released into the water (3).
Although relatively widespread and common throughout its range, Graham’s sheet coral faces a number of threats (1). Of major concern to corals worldwide is global climate change, which may lead to more severe, frequent storms and an increase in ocean temperatures, in turn leading to increased coral ‘bleaching’, in which the coral expels its zooxanthellae, often leading to death. Increased ocean acidity can also affect the ability of a coral to create its hard skeleton, and the combined effect of these stresses can make the coral more vulnerable to disease. Coral reefs also face many localised threats, including destructive fishing practices, human development, pollution and sedimentation (1)(6)(9)(10), and an estimated 20 percent of the world’s reefs have already been destroyed (9). High sedimentation, bleaching, and diseases such as ‘white plague’ have been identified as particular threats to Graham’s sheet coral (1).
Graham’s sheet coral is found in many Marine Protected Areas throughout its range, including the Florida Keys National Marine Sanctuary, Dry Tortugas National Park and Flower Garden Banks National Marine Sanctuary, USA (1). In US waters, it is illegal to harvest corals commercially (1), and international trade in Graham’s sheet coral should also be carefully controlled under its listing on Appendix II of the Convention on International Trade in Endangered Species (CITES) (2). There is a need for more research into the status of Graham’s sheet coral in deeper habitats (1), while more general measures to conserve corals include the expansion of protected areas, disease management, policies to tackle the threat of climate change, and further research and monitoring (1)(6)(9).
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Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
Type of asexual reproduction (reproduction that does not involve the formation of sex cells), in which new individuals develop from the parent organism, forming a swelling similar in appearance to a bud. The ‘bud’ slowly separates from the parent as it grows.
A group of organisms living together. Individuals in the group are not physiologically connected and may not be related, such as a colony of birds. Another meaning refers to organisms, such as bryozoans, which are composed of numerous genetically identical modules (also referred to as zooids or ‘individuals’), which are produced by budding and remain physiologically connected.
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.
Typically sedentary soft-bodied component of cnidaria, a group of simple aquatic animals including the sea anemones, corals and jellyfish. A polyp comprises a trunk that is fixed at the base, and a mouth that is placed at the opposite end of the trunk and is surrounded by tentacles.
Tiny aquatic animals that drift with currents or swim weakly in water.
Van Moorsel, G.W.N.M. (1983) Reproductive strategies in two closely related stony corals (Agaricia, Scleractinia). Marine Ecology Progress Series, 13: 273-283.
Bongaerts, P., Ridgway, T., Sampayo, E.M. and Hoegh-Guldberg, O. (2010) Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs. Coral Reefs, 29: 309-327.
Wilkinson, C. (2008) Status of Coral Reefs of the World: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Townsville, Australia. Available at: http://www.gcrmn.org/status2008.aspx
Carpenter, K.E. et al. (2008) One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science, 321: 560-563.
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