Data Sheet 1_Scleroblasts, the sclerite forming cells in octocorals, form a continuous network throughout the mesoglea.docx

Octocorals build characteristic sclerites within the mesoglea, traditionally described as being produced by isolated ectoderm-derived scleroblasts adjacent to the endoderm-derived canals. We revisited scleroblast organization using thick sections, cellular labeling, and confocal imaging with 3D reconstructions in two model species: the soft coral Sarcophyton sp. (Malacalcyonacea) and the precious red coral Corallium rubrum (Scleralcyonacea). In both taxa, scleroblasts were not disseminated as cell clusters but instead formed a continuous scleroblastic network intertwined with the canal system. In C. rubrum, a single sleeve-like network of scleroblasts formed an extracellular compartment enclosing growing sclerites. In Sarcophyton, two cellular networks co-occurred: a primary tubular network lining canals and a secondary dendritic meshwork, where sclerites developed within pocket-like compartments. We also observed cytoplasmic bridges spanning the mesoglea, physically linking canal cells and scleroblasts. These findings overturn the longstanding view of isolated scleroblasts and instead support a tissue-level, networked model of octocoral biomineralization.