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Octocorals found to be reef builders
Written by Ming-Shiou Jeng, Ph.D.   

It is common knowledge that the aragonite skeletons of stony corals are the foundation of coral reefs. In contrast, octocorals deposit calcite sclerites as internal supporting structures, and are thought to be insignificant contributors to permanent reefs. Based on field surveys and microscopic examination of fossil boulders and live colonies, we conclude that components of permanent reef structures can be produced by sinularian octocorals.

Reef corals, also known as stony corals or scleractinian corals, represent hundreds of species that deposit calcium carbonate (mainly aragonite) frameworks. Soft corals, as the name indicates, possess fleshy colonies with embedded calcite (another form of calcium carbonate) sclerites. Their sclerites function as supportive structures, aid in defense against predation, and may help the colony to withstand drag forces caused by water flow. Since such sclerites do not form continuous structures, soft corals have long been considered as insignificant reef builders. Now, our study shows that at least one group of soft corals can consolidate discrete sclerites into solid reef structures.  

In 1997, a colony chunk of the soft coral Sinularia found in Kenting National Park, southern Taiwan, initiated this study. This block of coral flesh was dislodged by a storm and left on the beach. It contained desiccated tissue and a solid boulder of consolidated sclerites underneath. In subsequent surveys at the site, Sinularia sclerite boulders were both found on land and in the sea. Boulders weighing over one hundred kilograms (220 lbs) and numerous rolling stones of sclerite rocks were found throughout the beach. Core samples indicated a high abundance of sclerite rocks in the uplifted coral reefs; it was estimated that uplifted reef of the area contained approximately thirty to forty percent of sclerite boulders. Section radiographs of an 18.5 kilogram (41 lbs) boulder showed regular density banding similar to trees and massive reef corals, which indicated a distinct periodicity in its deposition.  

sclerite boulder

Close-up of the surface of a sclerite boulder on land (photograph: Dr. Ming-Shiou Jeng).

The coral reef of the area showed a soft coral cover of over fifty percent. Living colonies were core-sampled for examination of sclerite rocks. Approximately ninety percent of sampled Sinularia colonies, representing 22 species, contained significant sclerite rock layers underneath the colony. Situated between coral tissue and the solid sclerite rock was a thin transition zone of loosely cemented sclerites, demonstrating the sclerite consolidation process. Under a scanning electron microscope (SEM), sclerites at the colony base were found to be cemented by amorphous calcium carbonate, which covered the sclerite surface microstructure. When examining the fast-growing soft coral Sinularia gibberosa under a transmission electron microscope (TEM), inside the colony base where sclerites are consolidated, granular vesicles were found to be distributed among the sclerites, which may be secreting calcium carbonate adhesives.

live Sinularia core

Core sample of a living colony showing coral tissue with sclerite rock attached underneath (photograph: Dr. Ming-Shiou Jeng).

It is now clear that the ''sclerite rock'' phenomenon is actually common on coral reefs around the world. In addition to the previous records from the Red Sea and Okinawa Islands, we have found living Sinularia colonies together with sclerite rocks on reefs where alcyonacean soft corals are abundant. This study, however, revealed a continuous sclerite rock-forming zone in Sinularia colonies from the subtidal zone to uplifted reefs of Nanwan Bay, southern Taiwan.

Now, what is the relevance of octocorals being important reef builders? Fleshy octocorals are able to inhabit more turbid environments than scleractinian corals. Therefore, species from the genus Sinularia are capable of accreting material into reef structures in areas where scleractinian coral communities cannot develop. 

Sinularia stand

Large Sinularia colonies, here recorded by marine biologists, can contribute significantly to reef formation (photograph: Dr. Ming-Shiou Jeng).

References

Jeng M-S, Huang H-D, Dai C-F, Hsiao Y-C and Benayahu Y (2011). Sclerite calcification and reef-building in the fleshy octocoral genus Sinularia (Octocorallia: Alcyonacea). Coral Reefs DOI: 10.1007/s00338-011-0765-z.

Konishi K (1981). Alcyonarian spiculite: Limestone of soft corals. Proc. 4th Coral Reef Symp. 1: 643-649.

Schuhmacher H (1997). Soft corals as reef builders. Proc. 8th Int. Coral Reef Symp. 1: 499-502.