Bubble coral (Plerogyra sinuosa)
|Also known as:||bladder coral, green bubble coral|
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).
For further information on the conservation of coral reefs, see:
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- Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
- Asexual reproduction: reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by fission (or in plants ‘vegetative reproduction’); part of the organism breaks away and develops into a separate individual. Some animals, including vertebrates, can develop from unfertilised eggs; this process, known as parthenogenesis, gives rise to offspring that are genetically identical to the parent.
- Colonies: 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.
- Larvae: 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.
- Photosynthesis: 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.
- Polyp: 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.
- Septa: in a coral, radial elements that project inwards from the corallite wall (the skeletal wall of an individual coral polyp).
- Spawning: the production or depositing of large quantities of eggs in water.
- Symbiotic relationship: relationship in which two organisms form a close association. The term is now usually used only for associations that benefit both organisms (a mutualism).
- Zooplankton: tiny aquatic animals that drift with currents or swim weakly in water.
IUCN Red List (March, 2011)
CITES (March, 2011)
- Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townville, Australia.
World Association of Zoos and Aquariums - Green bubble coral (March, 2011)
- Veron, J.E.N. (1986) Corals of Australia and the Indo-Pacific. Angus and Robertson Publishers, UK.
- Dai, C.F. and Horng, S. (2009) Scleractinia Fauna of Taiwan. II. The Robust Group. National Taiwan University Press, Taipei, Taiwan.
- Wilkinson, C. (2004) Status of Coral Reefs of the World: 2004. Volume 3. Australian Institute of Marine Science, Townsville, Australia.
- 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.
- Miththapala, S. (2008) Coral Reefs. Coastal Ecosystems Series (Volume 1). Ecosystems and Livelihoods Group Asia, IUCN, Colombo, Sri Lanka.