Fig 1. Rocky intertidal pool.

Purple sea urchins occur in high densities in the intertidal and shallow subtidal where many are nestled in cavities (“pits”) carved out of the rock substratum. The “hand-in-glove” fit of the urchins to the pits is apparent upon close examination (inset). The sedimentary sandstone at this site (Bean Hollow, California, USA) is typical of many sites along the west coast of North America. The exposed rocks at this site are of the Upper Cretaceous Pigeon Point Formation, which is part of the Franciscan Complex that makes up much of the central coast of California.