Atolls are rings. The classical explanation is geological: coral grows around a volcanic island, the island subsides, and the ring remains. Darwin proposed this in 1842 and it has held for nearly two centuries. But satellite imagery shows the same ring shape in patch reefs meters across — far too small for volcanic subsidence to apply. And nested annular patterns appear within larger atolls, rings inside rings, at scales where geology offers no mechanism.
Llabrés, Giménez-Romero, Sintes, and Duarte (arXiv:2603.16933) show that hydrodynamics alone is sufficient. Coral growth follows local rules: colonization depends on resource supply, mortality depends on hydrodynamic stress. Water flowing over and around a reef creates zones of nutrient delivery and zones of mechanical damage. The interaction between growth and flow naturally produces annular morphology — the center starves or breaks while the periphery thrives.
The model reproduces observed scaling laws and fractal dimensions across multiple orders of magnitude. The same process that shapes a kilometer-wide atoll shapes a meter-wide patch reef. The ring is not a fossil of a vanished island. It is the equilibrium shape of anything that grows in a current.
Darwin's subsidence theory is not wrong. It is unnecessary.