Some, including the sessile stauromedusae and the polyp stages of metagenetic free-swimming medusae, occur as part of the benthos. Representatives of the group are found in all oceans and range bathymetrically from sea surface to abyss. All are marine. Of about 175 species recognized worldwide, six occurring in the Woods Hole area are considered here. The medusae of Scyphozoa are unique in being typically much larger and anatomically more developed than the polyp.
Jellyfishes undergo a complex life cycle.
The ephyra grows rapidly into an adult medusa, completing the life cycle. Meanwhile, the basal part of the strobila reverts to a scyphistoma. After a period of growth this scyphistoma may reproduce asexually, or strobilate again, or both. While the medusa stages of scyphozoans are usually seasonal in occurrence, especially in temperate regions, their scyphistomae may be active all year. Under adverse conditions, the benthic stages may survive as dormant podocysts.