REPORT Sclerite calcification and reef-building in the fleshy octocoral genus Sinularia (Octocorallia: Alcyonacea) M.-S. Jeng • H.-D. Huang • C.-F. Dai • Y.-C. Hsiao • Y. Benayahu Received: 29 October 2010 / Accepted: 26 April 2011 / Published online: 15 May 2011 Ó Springer-Verlag 2011 Abstract Alcyonacean octocorals in tropical reefs are usually not considered as reef builders. Some Sinularia species, however, are capable of consolidating sclerites at the colony base to form spiculite. Nanwan Bay, southern Taiwan, features both fossilized and recently formed boulders composed of spiculite, thus demonstrating the role of Sinularia in contributing to the reef structure. Section radiography of an 18.5 kg spiculite boulder demonstrated a regular density banding of 3–6-mm intervals. Core survey indicated spiculite coverage of 25–30% on the live reef and of 30–40% on the uplifted boulders. Cores taken from living Sinularia revealed a distinct transition from discrete sclerites to compact spiculite and amorphous calcium carbonate cementing the sclerites. In the widespread S. gibberosa, sclerite formation appeared to start intracel- lularly, followed by a prolonged extracellular calcification process. At the calcification site, multiple sclerocytes formed expanded pseudopod-like membranes that inter- connected, forming multicellular vesicles (MCVs) around the sclerites. The MCVs and the pseudopods disappeared at sclerite maturation, followed by degradation of the scle- rocytes around the mature sclerites. At the colony base, granular vesicles were distributed among the sclerites, indicating a cementing process in progress. These findings suggest that colonies of Sinularia are able to cement sclerites and consolidate them at their base into spiculite, thus making them reef builders. Keywords Reef-building Á Octocorals Á Spiculite Á Sinularia Á Calcification Á Sclerite cementation Á Taiwan Introduction It is well accepted that the carbonate skeletons of stony corals form the foundations of coral reefs. New skeleton, in the form of aragonite crystals, is deposited externally by the calicoblastic epithelium of these corals onto previously formed ones (Le Tissier 1991; Muscatine et al. 1997). Octocorallia produce calcite sclerites within fleshy tissue as internal support (Fabricius and Alderslade 2001) and are generally not considered to contribute to the reef structure. The discovery of rocks composed of octocoral sclerites, termed spiculite (Konishi 1981; Accordi et al. 1989), led to the suggestion that fleshy octocorals may contribute to reef construction and growth (Schuhmacher 1997). Most spic- ulites are made of consolidated sclerites from octocorals of the family Alcyoniidae (order Alcyonacea). Cary (1931) first focused attention on sclerite masses left by colonies of Alcyonium (= Sinularia) confertum in American Samoa. Later, below flourishing living colonies of the fleshy octocoral, Sinularia, cemented layers of sclerites were found on modern coastal reefs in the Ryukyu Islands (southern Japan) and Australia that had been accreting for Communicated by Geology Editor Prof. Bernhard Riegl M.-S. Jeng (&) Á Y.-C. Hsiao Biodiversity Research Center, Academia Sinica, Taipei 11529, Taiwan e-mail: jengms@gate.sinica.edu.tw H.-D. Huang National Museum of Natural Science, Taichung 40453, Taiwan C.-F. Dai Institute of Oceanography, National Taiwan University, Taipei 10617, Taiwan Y. Benayahu Department of Zoology, Tel Aviv University, 69978 Ramat Aviv, Tel Aviv, Israel 123 Coral Reefs (2011) 30:925–933 DOI 10.1007/s00338-011-0765-z