We saw hundreds if not thousands of these in a road rut serving as a high intertidal mud puddle. Light's Manual says, "Populations in the San Francisco Bay are endangered; they now live in high intertidal marshes, having been displaced by Ilyanassa obsoleta." Similarly Cohen and Carlton (1995) write, "The introduced Atlantic snail Ilyanassa obsoleta now occupies the Bay mudflat areas formerly occupied by the native snail Cerithidea californica. Each spring the two populations of these snails collide, and by mid-summer the exotic Ilyanassa restricts the native Cerithidea to high-marsh salt pannes (an environment too high in salinity for Ilyanassa and thus providing a habitat refuge for Cerithidea) through egg-string predation and direct competitive interference." This population seemed to be booming, but as they described, it was right a the high tide line, and the pool was very warm and probably very salty.