Symmetrical brain coral (Diploria strigosa)

KingdomAnimalia
PhylumCnidaria
ClassAnthozoa
OrderScleractinia
FamilyFaviidae
GenusDiploria (1)

Classified as Least Concern (LC) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

A reef-building species that forms massive spherical colonies, the symmetrical brain coral often develops the very foundation of coral reefs in the tropical western Atlantic (3). Like other brain corals, it is named for the system of meandering grooves and ridges on its outer surface, which resembles the brain of higher animals. Each groove contains a single, long polyp (soft-bodied animal, related to the anemone), surrounded by tentacles that direct food into the mouth, where it is digested in a sac-like body cavity (3) (4). One of the most remarkable and ecologically important features of Scleractinia 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. Living colonies of the symmetrical brain coral range in colour from purple-brown to grey or green, often with the groove floors being a contrasting paler colour (4).

Found in the Caribbean, the Gulf of Mexico, the Bahamas, Bermuda and off the Florida coast (1).

The symmetrical brain coral lives in shallow reef environments down to depths of 47 metres, although it is most common above 10 metres. It is perhaps the most widely distributed of the Diploria species and is found in a variety of reef habitats, including those in exposed areas and in bays with high sediment-loads (1) (3) (4).  

Like many coral species, the tissue of the symmetrical brain coral contains large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship in which the algae gain a safe, 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. While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (4).

Like many other species of hermaphroditic coral, the symmetrical brain coral can reproduce both sexually and asexually. When reproducing asexually, the eggs are fertilised by sperm inside the polyp walls, but when reproducing sexually, the sperm and eggs combine in the water column, with a peak in egg and sperm release occurring in mid-August. Larvae subsequently develop, and when released, float passively in the currents as part of the zooplankton community, before settling on the ocean floor. Here the larvae undergo several stages of metamorphosis to form sessile young polyps. These polyps immediately commence asexual reproduction, known as ‘budding’, to produce additional polyps (3).  

With an estimated 20 percent of the world’s coral reefs already destroyed, the symmetrical brain coral faces many of the threats affecting coral reefs globally (5) (6). Worldwide there is increasing pressure on coastal resources resulting from human population growth and development. There has been a significant increase in domestic and agricultural waste in the oceans, poor land-use practices that result in an increase in sediment running on to the reefs, and over-fishing, which can have ‘knock-on’ effects on the reef (5). However, the major threat to corals is global climate change, with the expected rise in ocean temperatures increasing the risk of coral ‘bleaching’, in which the stressed coral expels its zooxanthellae, often resulting in the death of the coral. Climate change may also lead to more frequent, severe storms, which can damage reefs, and rising carbon dioxide levels may make the ocean increasingly acidic. Such stresses can also make corals more susceptible to disease, parasites and predators, such as the crown of thorns sea star (Acanthaster planci) (5) (6) (7). 

Despite the wealth of threats that the symmetrical brain coral faces, it is still relatively common throughout its range, and is thought to be the most abundant of all the Diploria corals. However, of all the Diploria species, it is also thought to be the most susceptible to black band disease and white plague, both of which are caused by infectious bacteria and can cause partial or total mortality of colonies, and as a consequence of this, some local declines in the species’ population have been observed (1). The number of outbreaks of these diseases has increased dramatically in recent years, and is most frequent in reefs already affected by pollution and degradation (1) (8).

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 this species without a permit, the symmetrical brain coral also forms part of the reef community in numerous marine protected areas, including the Florida Keys National Marine Sanctuary (1) (2). It is also protected by law in Bermuda under the Coral Reef Preserve Act and the Fisheries Protected Species Order, both of which prohibits the removal of any coral species from designated protected areas (3). To specifically conserve the grooved brain coral, recommendations have been made for a raft of studies into various aspects of the species’ biology, population status, habitat and threats to its survival (1).

For further information on the conservation of coral reefs, see:

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  1. IUCN Red List (September, 2010)
    http://www.iucnredlist.org/
  2. CITES (September, 2010)
    http://www.cites.org/
  3. Evans, J. (2005) Symmetrical Brain Coral (Diploria strigosa). In: Wood, J.B. (Ed.) Marine Invertebrates of Bermuda. Bermuda Institute of Ocean Sciences. Available at:
    http://www.thecephalopodpage.org/MarineInvertebrateZoology/Diploriastrigosa.html
  4. Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townville, Australia.
  5. Wilkinson, C. (2004) Status of Coral Reefs of the World: 2004. Volume 3. Australian Institute of Marine Science, Townsville, Australia.
  6. 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.
  7. Miththapala, S. (2008) Coral Reefs. Coastal Ecosystems Series (Volume 1). Ecosystems and Livelihoods Group Asia, IUCN, Colombo, Sri Lanka.
  8. Green, E.P. and Bruckner, A.W. (2000) The significance of coral disease epizootiology for coral reef conservation. Biological Conservation, 96: 347-361.