Chemistry and structure of skeletal growth rings in the black coral Antipathes fiordensis (Cnidaria, Antipatharia)

The skeleton of the black coral Antipathes fiordensis is a laminar structure displaying major and minor growth rings. The skeleton is composed primarily of protein and chitin organized into successive microlayers. Each microlayer is tightly bound to the next by an organic cement that is periodically opaque in section, and occurs as clusters throughout the skeleton. In contrast, the skeletal microlayers themselves are never opaque. The apposition of multiple, opaque cement lines between skeletal layers is primarily responsible for the growth ring pattern.