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Phylum Cnidaria > Class Anthozoa > Subclass Alcyonaria/Octocorallia > Order Helioporacea
Blue coral
Heliopora coerulea
Family Helioporidae
updated Dec 2019
if you learn only 3 things about them ...
They produce a hard skeleton but considered soft corals because of their polyps.
Their skeleton is blue on the inside. But don't break the skeleton to see this!
They are considered living fossils.

Where seen? These living fossils are commonly seen on our Southern shores, sometimes forming many large colonies. Found among hard corals.

What are blue corals? Blue corals belong to Phylum Cnidaria. Although they produce a hard skeleton, they are NOT hard corals and are more closely related to soft corals. Blue corals are the only members of the Order Helioporacea, Family Helioporidae. They are also the only members of the soft corals (Subclass Alcyonaria) that contributes to reef-building, like hard corals do.

Living fossil: Blue corals are considered living relicts of fossil species known from more than 100 million years ago. Most other corals have an evolutionary age of only several hundred thousand years. Blue corals used to be dominant before the last Ice Age when the seas were warmer. They are now only found in warm tropical waters.

Pulau Jong, Jul 07

During mass coral bleaching, a small bleaching coral is seen
in the middle of large Blue coral colonies that appear normal.
Terumbu Pempang Tengah, Jul 16
Features: Blue corals are confusing. Firstly, they are often brown and don't appear blue at all. Secondly, although they have a hard skeleton, they are not grouped with other hard corals.

Colonies 15-30cm, polyps about 0.5cm. Their internal skeletons are blue, hence their common name. The blue colour is due to the iron salts that are incorporated into their skeletons. On the outside, they are usually brown because the thin layer of brownish living tissue that covers the outer surface of the skeleton. The skeletons are made of a different kind of calcium carbonate (fibro-crystalline argonite) that is more brittle than that of true hard corals that belong to Subclass Sclerectinia. Inside the skeleton are tubes, where the long, thin polyps are found, and a system of canals. Blue coral colonies are usually boulder shaped with knobs. They may also have thick leaf-like forms or columns, and may even be encrusting or plate-like.

Tiny polyps (about 0.5cm) have 8 tentacles with fine branches like other soft corals (True hard coral polyps have smooth tentacles in multiples of six). The polyps stick out of tiny holes (0.2cm) in the skeleton. They may be white or beige.

With polyps extended.
Pulau Semakau, Mar 05

Polyps with branched tentacles
more typical of soft than hard corals.

Beting Bemban Besar, May 11

A look inside a broken blue coral
showing the blue internal skeleton
and tiny channels when the polyps are.

Pulau Hantu, Apr 06
What do they eat? Blue coral polyps harbour microscopic, single-celled symbiotic algae (zooxanthellae) within their bodies. The algae undergo photosynthesis to produce food from sunlight. The food produced is shared with the host, which in return provides the algae with shelter and minerals. It is the zooxanthellae that give the animals their brownish colour. However, during mass coral bleaching, while other hard corals are bleaching in numbers, we usually don't see Blue corals undergoing the same kind of drastic colour change.

Blue food: This coral is preyed upon by a small nudibranch Doridomorpha gardineri that very closely resembles the surface of the coral.

Doridomorpha gardineri
Terumbu Bemban, May 21
Photo shared by Toh Chay Hoon on facebook.
Status and threats: Blue corals are not listed among the threatened animals of Singapore. However, like other creatures of the sea, they are threatened by human activities that degrade or destroy the habitat.

Blue corals on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

Pulau Biola, Dec 09

Terumbu Salu, Jan 10

Terumbu Berkas, Jan 10

Pulau Senang, Jun 10

Pulau Salu, Aug 10

Pulau Pawai, Dec 09

Pulau Berkas, May 10

Terumbu Raya, Mar 16
Photo shared by Marcus Ng on facebook.

Family Helioporidae recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.

  Family Helioporidae
  Heliopora coerulea

Grateful thanks to Toh Chay Hoon for finding and identifying nudibranchs that eat these corals.

  • Toh Chay Hoon & Rene Ong. 9 October 2015. New record of the nudibranch Doridomorpha gardineri in Singapore. Singapore Biodiversity Records 2015: 151-153
    Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004. Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
  • Fabricius, Katharina and Philip Alderslade, 2001. Soft Corals and Sea Fans. Australian Institute of Marine Science and the Museum and Art Gallery of the Northern Territoriy. 264 pp
  • Erhardt, Harry and Daniel Knop. 2005. Corals: Indo-Pacific Field Guide IKAN-Unterwasserachiv, Frankfurt. 305 pp.
  • Borneman, Eric H. 2001. Aquarium Corals: Selection, Husbandry and Natural History T.F. H Publications. 464 pp
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Ng, P. K. L. & Y. C. Wee, 1994. The Singapore Red Data Book: Threatened Plants and Animals of Singapore. The Nature Society (Singapore), Singapore. 343 pp.
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