Corals in the cold waters along the coast of California don't build reefs like their tropical kin do. This coral does make its own outer skeleton: that cuplike limestone base underneath. A cup coral larva crawls on the rocky seafloor before settling. After cementing its limestone skeleton to a rock, the coral is set for life.
Rocky reefs are important homes for many kinds of fishes and invertebrates. But "rockhopper" trawls, used in commercial fishing, can leave reefs a tumbled wasteland, unable to recover for decades.
Reef-building corals form huge colonies, but cup corals live solitary lives, taking refuge in their individual "cups."