Architectural and Mechanical Properties of the Black Coral Skeleton (Coelenterata: Antipatharia): A Comparison of Two Species.

Black coral skeletons are laminated composites, composed primarily of chitin fibrils and non-fibrillar protein. This paper examines mechanical properties of the composite and the architecture of the chitin component. Two species are shown to differ significantly in their tensile strength and fibril structure. The skeleton of Antipathes salix, a Caribbean species of commercial value, is stiffer, harder, darker, more dense, and more hydrophobic than Antipathes fiordensis from New Zealand. The chitin fibrils constitute a greater proportion of the skeleton in A. salix, where they are helically wound in an anticlockwise pattern within layer. Adjacent layers of skeleton are arranged with relatively small layer-to-layer fibril biases. There is no evidence of "helicoidal" structure in this skeleton. The fibrils in A. fiordensis are also wound anticlockwise within layer, but with rather large fibril biases between layers, giving the appearance of a meshwork. Large-scale helicoidal patterns with apparent rotations of 180{deg} characterize this material. Skeletal architecture is compared with the cuticle of insects and other arthropods. The skeletons of both species exhibit spines characteristic of the Antipatharia. We suggest that these have a significant reinforcing effect on the strength of the skeleton, contributing to an overdesign for the habitat in which these organisms presently occur.