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Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Phylum: Cnidaria
Classis: Hydrozoa
Subclassis: Trachylinae
Ordo: Trachymedusae
Familiae: Geryoniidae - Halicreatidae - Petasidae - Ptychogastriidae - Rhopalonematidae

Vernacular names
日本語: 硬水母目

Trachymedusae belong to the phylum Cnidaria and the class Hydrozoa, among the 30 genera are 5 families containing around 50 species in all, the family Rhopalonematidae has the greatest diversity.[1]

Trachymedusae are identifiable by their umbrella edge which lacks any lobes. The tentacles at the edge of the umbrellas are solid or solid and hollow, there is a thickened tissue ring that has a large number of nematocysts, the radial canals number from 4 to 6 to 8 and more than 8, though 8 is the most common amount found. The sensory clubs can be open or closed with the endodermal axis. The gonads are generally located at the radial canal or where the radial canal and the manubrium connect. The cnidome may have stenoteles. Trachymedusae reproduce sexually during the medusae stage lacking a polyp stage.[1][2]

Primarily found in the deep ocean, where they are recorded at depths of seventy to two thousand metres.[3][4]
Interactions with humans

Interactions with humans are few as this species primarily occurs at great depth. Although it has been found that hydrozoans have increased in numbers in recent years, the increase in Liriope tetraphylla, a species of Trachymedusae, does not appear to be affecting the total zoo-plankton populations they prey upon within the Sea of Marmara.[5]

"World Hydrozoa Database - Species". Retrieved 2016-03-13.
Kramp, P. L. (November 1961). "Synopsis of the Medusae of the World" (PDF). Journal of the Marine Biological Association of the United Kingdom. 40: 237–265. doi:10.1017/S0025315400007347. Archived from the original (PDF) on 4 March 2016. Retrieved 14 March 2016.
Lindsay, Dhugal; Hunt, James; Hashimoto, Jun; Fujiwara, Yoshihiro; Fujikura, Katsunori; Miyake, Hiroshi; Tsuchida, Shinji (2000). "Submersible observations on the deep-sea fauna of the south-west Indian Ocean: preliminary results for the mesopelagic and near-bottom communities" (PDF). JAMSTEC Journal of Deep Sea Research, 16. Retrieved 2016-03-27.[dead link]
Stübing, Dorothea; Piepenburg, Dieter (1998). "Occurrence of the benthic trachymedusa Ptychogastria polaris Allman, 1878 (Cnidaria: Hydrozoa) off Northeast Greenland and in the northern Barents Sea". Polar Biology. 19 (3): 193–197. doi:10.1007/s003000050234. Retrieved 2016-03-27.
Noyan Yilmaz, Izzet (8 February 2014). "Collapse of zooplankton stocks during Liriope tetraphylla (Hydromedusa) blooms and dense mucilaginous aggregations in a thermohaline stratified basin". Marine Ecology. Retrieved 2016-03-13.

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