The study presents the microanatomy of the polyps of the reef-dwelling octocoral Sarcophyton auritum. We demonstrate the presence of its unique collagen fibers in the colony by means of Masson Trichrome histological staining. Based on peptide profiling, mass spectroscopy analysis confirmed that the fiber proteins were homologous with those of mammalian collagen. Histological and electron microscopy results showed that six of the eight mesenterial filaments of the polyps possess an internal, coiled, spring-like collagen fiber. High-resolution electron microscopy revealed for the first time in cnidarian collagen the interwoven, three-dimensional arrangement of the fibrils that comprise the fibers. Some fibrils feature free ends, while others are bifurcated, the latter being attributed to collagen undergoing fibrogenesis. Along with the mass spectroscopy finding, the coiled nature of the fibers and the fibril microanatomy show a resemblance to those of vertebrates, demonstrating the conserved nature of collagen fibers at both the biochemical and ultrastructural levels. The location, arrangement, and small diameter of the fibers and fibrils of S. auritum may provide a highly protective factor against occasional rupture and injury during the bending of the octocoral's extended polyps under strong current conditions; that is, providing the octocoral with a hydromechanical support. The findings from the microanatomical features of these unique fibers in S. auritum, as well as their suggested function, raise the potential for translation to biomedical applications.
© 2016 Marine Biological Laboratory.