Aggregations of hydromedusae depend on establishment and survival of populations, as well as on passive and behavioral factors leading to uneven distributions of individuals within those populations. Recent years have seen an increase in knowledge about hydrozoan budding, feeding, growth and respiration. However there is still little work on the effects of environmental factors, such as temperature or salinity, or on predation. The physical factors causing aggregations such as thermoclines and Langmuir circulation are better documented in the vertical than the horizontal plane. Pertinent behavioral patterns include responses to light, pressure, discontinuities of temperature or salinity levels, and prey.