The upper intertidal zone is the part of the shore that is covered by water only at high tide. The sublittoral zone is the shallow part of the ocean extending out to the edge of the continental shelf, that is, the part where sunlight penetrates all the way to the sea floor. Of the two, the upper intertidal zone features a greater diversity of species, perhaps because its sometimes harsh and changeable environment presents a greater variety of niches and requires a greater variety of adaptation in order to ensure survival.