Structural investigations of octocoral sclerites

A common structural pattern (as observed under the SEM) in the main body of sectioned sclerites of the family Alcyoniidae, and octocorals in general, is the arrangement of acicular crystals in concentric layers. The crystals roughly follow the direction of the spicule axis, however, the sectioned tubercles of large Sinularia spicules have the acicular crystals oriented in the direction of the tubercles (i.e. perpendicular to the spicule axis), contrary to the sectioned tubercles of Sarcophyton and Lobophytum spicules. They reveal some rod-like structures, furcating the acute processes at the top of the tubercles. Sectioned small, club-like Sinularia sclerites show numerous tiny acicular crystals, oriented with their long axes at a fairly constant degree of inclination around a central axis. SEM studies of sectioned Cladiella sclerites show a granulate structure organized in concentric layers, but lacking acicular crystals. The Silurian Atractosella cataractaca show important characters in common with Recent alcyoniid species.