Bubble coral  (Plerogyra sinuosa)

The bubble coral is classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (2). 

The bubble coral (Plerogyra sinuosa) is a rather strange looking coral, aptly named because of the grape-sized bubbles that enlarge its surface area and expose the symbiotic algae, or ‘zooxanthellae’, to more light. The bubbles protrude during the day, but shrink during the night when the tentacles expand to capture food (3) (4) (5). 

As in other colony-forming corals, colonies of the bubble coral are composed of numerous small soft-bodied anemone-like animals called polyps, (3). However, despite having numerous polyps, bubble coral colonies only have one large oral opening (4), where the food is digested in a sac-like body cavity (3). The colonies have irregularly-shaped septa, which produces a ragged appearance, and are initially shaped like an inverted cone, but become long and straight with age (6). Each polyp secretes 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 (3). Living bubble coral colonies tend to be green or blue (4).

The bubble coral is found in the Indian and Pacific Oceans, ranging from the Red Sea, Gulf of Aden and southwest Indian Ocean, across the northern Indian Ocean to Southeast Asia, Japan and the East China Sea, and into the West and Central Pacific Ocean (1).

The bubble coral is most frequently found on protected reefs in lagoons, where it grows on vertical faces or under overhangs. Large colonies are often found on flat surfaces in turbid waters. It is known to occur between depths of 3 and 35 metres (1) (3) (5).

Like many coral species, the bubble coral 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 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’s surface. While, on average, a zooxanthellate coral can obtain around 70 percent of its nutrient requirements from the photosynthesis of the zooxanthellae, the coral may also feed on zooplankton (3). 

Very little is known about the specific reproductive biology of the bubble coral, although it is likely to be able to reproduce both sexually and asexually. Asexual reproduction occurs via fragmentation, in which a branch breaks off a colony, reattaches to the substrate and grows. Sexual reproduction occurs via the release of eggs and sperm into the water. Some of the resulting larvae from these spawning events settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs that may be hundreds of kilometres away (3).

With an estimated 20 percent of the world’s coral reefs already destroyed, the bubble coral faces many of the threats that are affecting coral reefs globally (7) (8). 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 (7). 

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 (8). 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 starfish (Acanthaster planci) (7) (8) (9).

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 bubble coral also forms part of the reef community in numerous Marine Protected Areas (1) (2). To specifically conserve this coral, recommendations have been made for a raft of studies into aspects of its biology, population status, habitat and threats to its survival (1).

View information on this species at the UNEP World Conservation Monitoring Centre.

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

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