Two-Dimensional Growth Patterns of Coral Nubbins

Coral body plans are constructed through repeated modular units, with polyps serving as the fundamental structural and functional units, yet the rules underlying tissue and polyp pattern formation remain poorly understood. This study investigated lateral, two-dimensional (2D) tissue and polyp expansion in the coral Stylophora pistillata under controlled laboratory conditions. Using the nubbin assay, we investigated the effects of colony, fragment origin (branch tips versus sub-apical fragments), and nubbin density on this 2D expansion assay. Nubbins from ten colonies (SC1–SC10) were grown on glass slides, and tissue expansion was quantified from digital images over six months. For three fast-growing colonies (SC1, SC2, and SC5), single-, double-, and triple-nubbin configurations were used to evaluate proximity effects. Across all colonies, lateral tissue area strongly correlated with polyp number (R² = 0.68), indicating a close relationship between surface expansion and polyp proliferation. Pronounced colony differences emerged: SC9 and SC1 exhibited the largest tissue areas, while SC5 developed compact, polyp-dense morphologies. Fragment origin did not influence 2D growth, suggesting the absence of apical dominance. Nubbin density influenced growth in a colony-specific manner; SC1 exhibited strong inhibition under crowded conditions, whereas SC2 and SC5 were largely unaffected. Collectively, these results suggest that intrinsic genetic factors and local spatial interactions, rather than a fragment’s position along a branch, are the primary drivers of 2D growth, highlighting the self-organizing nature of coral tissues and illustrating how controlled 2D systems can clarify the interplay between genetic regulation and local interactions in coral morphogenesis.