“THE BLUE CORAL”

This animal, known scientifically as Heliopora coerulea (Pallas, 1766), is an extant octocoral with a massive skeleton (up to a meter in diameter that can be columnar, plate-like, or branched). The blue color of its skeleton is often obscured by the brownish to gray-brown color of its living tissues. The skeleton itself contains iron salts that produce its unique blue color. Furthermore, the skeleton consists of fibrocrystalline aragonite (a type of calcium carbonate). It is a hermatypic zooanthellaete species with polyps in the skeleton. Each polyp has eight tentacles.

Heliopora colonies are variable in their shape, ranging from branching forms (with blunt ends) to encrusting forms.

Heliopora coerulea: 8.5 inches [21 cm] wide and 6 inches [15.2 cm tall], from the Philippines. 

Close-up of a part of the specimen of H. coerulea shown above.

The surface of the skeleton is smooth and perforated by cylindrical pits of two sizes: 1) widely-spaced “tiny black holes” up to 0.25 mm diameter and encircled by a stellate margin and 2) much smaller tubular-shaped openings used by the autozooids (part of the internal canal system that contains the zooanthellae = the symbiotic algae).

An even more close-up of the same specimen of H. coerulea shown above. Notice the two different sizes of openings in the skeleton: the autozooids = black holes (each one up to 0.25 mm diameter and encircled by a stellate margin) and the much smaller (near microscopic size) tubular openings that, according to Wood (1983) enclosed extensions of the internal-canal system that contained the zooxanthellae.

Heliopora coerulea is a species that can tolerate thermal changes. It has a fossil record since the Cretaceous. The morphology of this octocoral has changed little since then.

The geographic occurrence today of H. coerulea is confined to the tropics: Indian Ocean and western Pacific Ocean, including the Japan and the Great Barrier Reef of Australia. It lives nearshore on reefs, in depths below two meters. It is a vulnerable species.

Its classification is: 

Phylum Cnidaria

Subphylum Anthozoa

*Class Octocorallia (lack true septa)

Order Helioporacea (includes only one genus: Heliopora)

Family Helioporidae

Genus Heliopora

Species H. coerulea

**Species H. hiberniana

In addition to the “Blue Coral,” octocorals include soft corals (such as Tubipora = the red-colored organ-pipe coral), sea pens, and gorgonians (sea fans and sea whips). There are about 3,000 known species of octocorals. They have colonial polyps with eight-fold symmetry (note: true corals have hexa-radial symmetry).

Zoe et al. (2018) reported a new species of living Heliopora, namely H. hiberniana, from offshore areas in north Western Australia. It differs from the “Blue Coral” by having a white skeleton, more slender branches, and some differences in skeletal morphology.

References Cited or Used:

en.wikipedia.org/wiki/Blue_coral

gbif.org/occurrence/1039258522

   [this online site is via the Invertebrate Zoology Division, Yale Peabody Museum]

Wood, E. M. 1983. Corals of the world. T.F.H. Publications. Neptune City, New Jersey, 256 pp.

Zoe, T.R., and seven others. 2018. Integrated evidence reveals a new species in the ancient blue coral genus Heliopora (Octocorallia). Scientific Reports 8, article no. 15875.