Monday, January 21, 2013

Helioporacea Corals (Phylum Cnidaria: Order Helioporacea) of Singapore

Helioporacea corals (phylum Cnidaria, class Anthozoa, subclass Octocorallia, order Helioporacea or Coenothecalia) are radially symmetrical animals that live in a colony supported by a hard aragonite skeleton.

Blue Coral (Heliopora coerulea)
In Singapore and the Indo-West Pacific region as a whole, only one species from this order has been recorded - the Blue Coral (Heliopora coerulea). Like other cnidarians, blue corals possess explosive, harpoon-like cells called cnidocytes. Each cnidocyte contains a secretory organelle (cnidae), which can be a nematocyst that discharges a harpoon-like stinger carrying toxins, a ptychocyst that discharges sticky substances, or a spirocyst that discharges lasso-like threads. Hence while cnidocytes are often called "stinging cells", they do perform other functions apart from stinging.

Blue Coral (Heliopora coerulea)
The coral animal, or polyp, has a simple body comprising a stomach (coelenteron) and a mouth surrounded by tentacles (where most of the cnidocytes are located), appearing like a little flower. The tentacles occur in multiples of eight, and hence they are in the subclass Octocorallia. It does not have an anus, and thus the mouth performs both functions of ingesting food and removing waste. Corals of this order are the only octocorals that produce hard skeletons, and hence they are sometimes mistaken to be hard corals. Unlike the latter, however, they have eight feather-like tentacles per polyp instead of tentacles in multiples of six.

Blue Coral (Heliopora coerulea)
Despite the name, the colony usually appears brown. The common name comes from the blue skeleton, which can be seen sometimes when the colony has broken parts. The blue colour comes from iron salts in the calcium carbonate skeleton. A thin layer of soft tissue covering the skeleton gives the colony the brown colour, and this soft tissue is brown largely due to a brownish unicellular algae, zooxanthellae, living inside their tissues.

Blue Coral (Heliopora coerulea)
The zooxanthellae produce food through photosynthesis and share the food with the host corals, and in return get shelter and nutrients (waste products of the corals). The coral also actively feed on plankton with their tentacles. Sometimes, the corals may expel their zooxanthellae or the latter may leave the coral due to environmental stress, resulting in a phenomenon known as coral bleaching. Without the zooxanthellae giving the coral the brownish tone, the blue colour of the skeleton is revealed. The above features a partially bleached colony.

Blue Coral (Heliopora coerulea)
One of the main causes of coral bleaching is the rise in water temperatures. If the situation does not improve and the soft corals cannot recruit new zooxanthellae to replace the lost ones, they may die.

Blue Coral (Heliopora coerulea)
Taking a closer look at the blue coral colony exposed during low tide with the polyps retracted, the surface has many round depressions (corallites) which house the polyps. The space between the polyps has a furry appearance due to the numerous tiny tubes (coenencliymal coeca), which end blindly below and are closed above by the thin layer of soft tissue covering the exterior surface of the whole colony. These tiny tubes are set at right angles to the surface of the colony, and are linked to each other and the polyps by a network of canals just beneath the surface.

Blue corals can reproduce sexually or asexually. The colony can comprise all male or all female polyps. In sexual reproduction, the males broadcast their sperm into the sea to fertilise the females, which will brood the eggs internally. The fertilised egg develops into a larva, and will subsequently leave the colony (the development period varies). The swimming larva will then cement itself onto a suitable hard substrate, develop into a polyp and secretes a hard skeleton, and eventually clones (i.e. asexual reproduction) itself to form a colony.

Blue Coral (Heliopora coerulea)
Blue corals are a major component of Singapore's coral reefs, sometimes covering extensive areas, especially in the intertidal area. They are often called living fossils, as their fossil records can be traced to more than 100 million years ago. Unfortunately, due to the attractive blue colour of their skeletons, they are sometimes collected for decorative uses.

  • Babcock, R. 1990. Reproduction and development of the blue coral Heliopora coerulea (Alcyonaria: Coenothecalia). Marine Biology, 104: 475–481.
  • Bayer, F. M. 1979. The correct name of the helioporan coral Lithotelesto micropora Bayer & Muzik. Proceedings of the Biological Society of Washington 92: 873–875.
  • Bayer, F. M. & K. M. Muzik. 1977. An Atlantic Helioporan Coral (Coelenterata:Octocorallia). Proceedings of the Biological Society of Washington 90 (4): 975–984.
  • Chou, L. M. 1998. A Guide to the Coral Reef Life of Singapore. Singapore Science Centre. 128 pp.
  • Erhardt, H. and D. Knop. 2005. Corals: Indo-Pacific Field Guide. IKAN-Unterwasserachiv, Frankfurt. 305 pp. 
  • Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
  • World Register of Marine Species. 2012. Retrieved Jan 15, 2013, from

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