A unique skeletal microstructure of the deep-sea micrabaciid scleractinian corals.

Micrabaciids are solitary, exclusively azooxanthellate deep-sea corals belonging to one of the deepest-living (up to 5,000 m) scleractinian representatives. All modern micrabaciid taxa (genera: Letepsammia, Rhombopsammia, Stephanophyllia, Leptopenus) have a porous and often very fragile skeleton consisting of two main microstructural components known also from other scleractinians: rapid accretion deposits and thickening deposits. However, at the microstructural level, the skeletal organization of the micrabaciids is distinctly different from that of other scleractinians. Rapid accretion deposits consist of alternations of superimposed "microcrystalline" (micrometer-sized aggregates of nodular nanodomains) and fibrous zones. In contrast to all shallow-water and sympatric deep-water corals so far described, the thickening deposits of micrabaciids are composed of irregular meshwork of short (1-2 μm) and extremely thin (ca. 100-300 nm) fibers organized into small, chip-like bundles (ca. 1-2 μm thick). Longer axes of fiber bundles are usually subparallel to the skeletal surfaces and oriented variably in this plane. The unique microstructural organization of the micrabaciid skeleton is consistent with their monophyletic status based on macromorphological and molecular data, and points to a diversity of organic matrix-mediated biomineralization strategies in Scleractinia.