The class Medusa has approximately 200 species of jellyfish, with a wide distribution. “True jellyfish” have a diverse range of habitats; they can be found in salty estuaries, bays; the pelagic zone of the ocean and the abyssal depths of the ocean. Pelagic Cnidarians are found in the pelagic zone of the ocean. The term “Pelagic” literally defines the open ocean. Scyphozoan jellyfish are keystone species, being a primary and important predator in their marine ecosystems. They have been a keystone species in their ecosystems throughout the Phanerozoic period and also since the mid-Cambrian period, when fossils of the major animal groups first appeared. (Hagadorn et al. 2002; Cartwright et al. 2007; Hagadorn and Belt 2008 cited by Dawson) An ancient origin of the aggregate phenomenon of scyphomedusae, extracted from marine fossil deposits dating back to the end of the Cambrian, Hagadorn et al. 2002 cited by Dawson). Scyphozoans are increasingly criticized as a "boring" species, causing countless problems around the world. Aggregations and swarms of scyphozoan jellyfish can harm important aquacultures and injure swimmers. (Reword******They are also known as an invasive species clogging the water intakes of power plants, due to the high concentration of nitrate located in the waters surrounding the plant, thus having a negative impact on the mode of human life (reword) In addition to this, jellyfish also trigger poor oxygen conditions in certain waters where, synchronously, the same areas containing high numbers of jellyfish carcasses are deposited***** Rewording. ) (Arai 1997; Mills 2001; Hay 2006; Graham and Bayha 2007; Purcell et al. 2007; Pitt et al. 2009; Richardson et al. 2...... middle of article...... d by Dawson), Lychnorhizidae, alternative to recent taxonomic behavior, which includes three distinct families (Kramp 1961 cited by Dawson). The last and fifth way of conflict concerns the family Nausithoidae. This family is paraphyletic and/or polyphyletic (Fig. 3-5). Even with the oppositions and questionable relationships within the crowns, the phylogenetic locations of shallow tropical waters, Nausithoe sp. and the genetically distant deep-sea species Nausithoe atlantica and Nausithoe rubra collectively refer to Nausithoidae as non-monophyletic. Additionally, the identification of Atorella as nausithoid (Mills et al. 1987 cited by Dawson) isolates Nausithoidae as a non-monophyletic species, which allowed them to interpret their trees as confirming recognition of the family Atorellidae (Eggers et Jarms 2007 cited by Dawson).).