Coral is far from being the only organism involved in the construction of a coral reef. Other calcareous organisms such as coralline algae, foraminifera and molluscs may also be significant. And, of course, there are those delicate artistes known as bryozoans. Many bryozoans tend to be underestimated as reef components because, as well as being relatively small, they often prefer to settle in more cryptic habitats such as around and under coral gravel (Kobluk et al. 1988).
The organism in the SEM photo at the top of this post is one reef-inhabiting bryozoan, Crepidacantha longiseta. This species belongs to the ascophoran bryozoans, i.e. the zooid is protected dorsally by a calcified frontal wall, and each feeding zooid is associated with two long whip-like avicularia (the exact function of bryozoan avicularia is debated, but they are generally believed to be related to defense and/or cleaning the surface of the colony). In the top image, the feeding zooids are represented by the keyhole- or cartoon-fish-shaped openings, while the avicularia are positioned to either side of the main opening. Other Crepidacantha species may have the avicularia in different positions; they also differ in the length of the avicularia and the shape of the primary orifice (Tilbrook et al. 2001).
Crepidacantha longiseta is found in cryptic habitats around coral gravel in shallower waters, but may be found in more exposed positions as the water gets deeper, below about 25 m (Martindale 1992). It has been found in Vanuatu, Brazil, the Caribbean and Mauritius, and is presumably pantropical in its distribution (Tilbrook et al. 2001).
Kobluk, D. R., R. J. Cuffey, S. S. Fonda & M. A. Lysenko. 1988. Cryptic Bryozoa, leeward fringing reef of Bonaire, Netherlands Antilles, and their paleoecological application. Journal of Paleontology 62 (3): 427-439.
Martindale, W. 1992. Calcified epibionts as palaeoecological tools: examples from the Recent and Pleistocene reefs of Barbados. Coral Reefs 11 (3): 167-177.
Tilbrook, K. J., P. J. Hayward & D. P. Gordon. 2001. Cheilostomatous Bryozoa from Vanuatu. Zoological Journal of the Linnean Society 131: 35-109.