Darwin explained atolls by subsidence: volcanic islands sink, coral keeps growing upward, and eventually only the ring remains. The mechanism requires geology — millions of years of tectonic motion sculpting the reef from below.
But satellite surveys find the same ring-shaped geometry in tiny patch reefs sitting on flat seafloors. No volcano. No subsidence. No geological explanation at all. The annular form appears at scales from meters to hundreds of kilometers, with consistent geometric signatures including scaling laws and fractal dimensions that hold across five orders of magnitude.
Llabrés et al. (arXiv:2603.16933) model coral growth coupled to ocean currents and show that the interaction between reef growth and hydrodynamic stress produces ring structures without any geological precondition. As a reef grows, it diverts water flow around itself. The accelerated currents at the reef edges deliver more nutrients, promoting growth there. The stagnant interior receives less, growth slows, and eventually the center erodes or fails to keep pace. The ring emerges from the feedback between the growing structure and the flow it modifies.
The geometry is scale-free because the mechanism is scale-free. Whether the reef is ten meters or ten kilometers, the same coupling between growth and flow operates. The fractal dimensions and scaling exponents the model produces match those observed in global reef datasets.
Darwin was right about atolls but wrong about the mechanism being geological. The ring is hydrodynamic. The shape of the structure determines the flow; the flow determines where the structure can grow. The form builds itself through a conversation between organism and current — and subsidence, when it happens, merely reinforces a geometry that water was already sculpting.