Box jellyfish
Although the notoriously dangerous species of box jellyfish are largely restricted to the tropical Indo-Pacific region, various species of box jellyfish can be found widely in tropical and subtropical oceans, including the Atlantic Ocean and the east Pacific Ocean, with species as far north as California ("Carybdea confusa"), the Mediterranean Sea ("Carybdea marsupialis") and Japan (such as "Chironex yamaguchii"), and as far south as South Africa (for example, "Carybdea branchi") and New Zealand (such as "Copula sivickisi").
Phacellophora camtschatica
This cool-water species can be found in many parts of the world's oceans. It feeds mostly on smaller jellyfish and other gelatinous zooplankton, which become ensnared in the tentacles. Because the sting of this jellyfish is so weak, many small crustaceans, including larval crabs ("Cancer gracilis") and Amphipoda, regularly ride on its bell and even steal food from its oral arms and tentacles. The lifecycle of this jellyfish is well known, because it is kept in culture at the Monterey Bay Aquarium. It alternates between a benthic stage attached to rocks and piers that reproduces asexually and the planktonic stage that reproduces sexually in the water column; both males and females occur in the plankton.