Coral reefs form rings. Not occasionally, not in special conditions — the annular shape recurs across scales from meters to kilometers, a geometric signature that appears wherever reefs grow.

The classical explanation is geological: the coral sits on a subsiding island, growing upward to stay in the light while the island sinks. Darwin’s atoll theory. It works for large atolls but doesn’t explain ring shapes at smaller scales where there’s no island to subside around.

A numerical model coupling coral growth with water flow produces the rings from scratch, without geology. Corals grow where resources arrive via currents; growing corals modify the currents; modified currents change where resources arrive. The feedback loop between organism and fluid spontaneously generates annular formations. Colonization and mortality depend on resource supply and hydrodynamic stress, and the interplay between these two forces creates a ring as naturally as convection creates hexagonal cells.

The model reproduces observed scaling laws and fractal dimensions across multiple reef datasets. The ring isn’t stamped onto the landscape by geology or biology alone — it’s the stable pattern that emerges when growth and flow negotiate. The coral shapes the current that shapes the coral.

Darwin got the mechanism right for the wrong scale. At the scale where his geology applies, the rings are geological. At every other scale, they’re hydrodynamic. The shape is the same; the cause is not.