The nature of the marine shell fauna of the Caribbean is extremely diverse, seeking shelter beneath the sand of beaches, under the rocks of coastal areas, in the soft deep mud of estuaries, in dark submarine caves, in the thick growths of marine prairies, and under branches of vast coral outcrops. It remains unobtrusive and often difficult to discover. Each species has its geographical distribution and populations vary in size, shape and colour according to environment; some animals migrate seasonally far afield. Tidal movement in the Caribbean is unspectacular other than during the spring and autumn equinoxes when shallow reefs and sand flats emerge briefly. The intertidal zone remains stable and undisturbed, with the exception of the passage of tropical storms and hurricanes, governing the lives of many gastropods and bi-valves.