Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although a few are anchored to the seabed by stalks rather than being mobile. The bell can pulsate to provide propulsion for highly efficient locomotion. The tentacles are armed with stinging cells and may be used to capture prey and defend against predators. Jellyfish have a complex life cycle; the medusa is normally the sexual phase, which produces planula larva that disperse widely and enter a sedentary polyp phase before reaching sexual maturity.
Hydrozoa are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
The chondrophores or porpitids are a small and very unusual group of hydrozoans classified as the family Porpitidae. Though it derives from an outdated name for this lineage, some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the two genera contained therein.
Turritopsis dohrnii, also known as the immortal jellyfish, is a species of small, biologically immortal jellyfish found worldwide in temperate to tropic waters. It is one of the few known cases of animals capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary individual. Others include the jellyfish Laodicea undulata and species of the genus Aurelia.
Solmissus, or dinner plate jellyfish, is a genus of hydrozoans. Its species are unique among cnidarians in that they actively hunt for prey as opposed to passively waiting for plankton to pass by. They are found in the deep waters of Monterey Bay, California. They are most likely to be found in the deep sea, mid water. They grow to be 20 cm (7.9 in) in diameter. These hydrozoans feed on gelatinous zooplankton, including salps and doliolids, ctenophores, jellyfish, and copepods. However, Solmissus may be limited to feeding on soft-bodied prey by the type of nematocysts on their tentacles (Mills).
Anthoathecata, or the athecate hydroids, are an order of hydrozoans belonging to the phylum Cnidaria. A profusion of alternate scientific names exists for this long-known, heavily discussed, and spectacular group. It has also been called Gymnoblastea and, Anthomedusa,Athecata, Hydromedusa, and Stylasterina. There are about 1,200 species worldwide.
Hydroidolina is a subclass of hydrozoans in the phylum Cnidaria. It contains the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the class Hydrozoa in older classifications and were placed at order rank. Hydroidolina also includes, however, the highly advanced colonial jellies of Siphonophora, which were not included in the "Hydroida".
Leptothecata, or thecate hydroids, are an order of hydrozoans in the phylum Cnidaria. Their closest living relatives are the athecate hydroids, which are similar enough to have always been considered closely related, and the very apomorphic Siphonophora, which were placed outside the "Hydroida". Given that there are no firm rules for synonymy for high-ranked taxa, alternative names like Leptomedusa, Thecaphora or Thecata, with or without the ending emended to "-ae", are also often used for Leptothecata.
Rhopalonematidae is a family of hydrozoans. The family comprises 15 genera and 36 species.
Turritopsis is a genus of hydrozoans in the family Oceaniidae.
Lovenellidae is a family of hydrozoans. Their hydroids live together in upright stolonal or sympodial colonies, and their gonophores are pedunculate free-roaming medusae. The relationships of this fairly small but distinctive radiation to other members of the order Leptothecata are not well understood at present.
Haleciidae is a family of hydrozoans. Their hydroid colonies emerge from a creeping hydrorhiza and usually form upright branching colonies, although some species' colonies are stolonal. Their gonophores are typically sporosacs, growing singly or bunched into a glomulus. They remain attached to the hydroids or break off to be passively drifted away; in a few, the gonophores are naked.
Narcomedusae is an order of hydrozoans in the subclass Trachylinae. Members of this order do not normally have a polyp stage. The medusa has a dome-shaped bell with thin sides. The tentacles are attached above the lobed margin of the bell with usually a gastric pouch above each. There are no bulbs on the tentacles and no radial canals. Narcomedusans are mostly inhabitants of the open sea and deep waters. They can be found in the Mediterranean in large numbers.
Vampyrocrossota is a genus of hydrozoans of the family Rhopalonematidae. The genus only contains one species, Vampyrocrossota childressi. Unlike many hydromedusae, these animals do not have a sessile stage. Rather, they spend their entire lives in the water column as plankton. It is the only known species with a medusa that is truly black.
Csiromedusa medeopolis is a species of hydrozoan described in 2010. It was discovered in the estuarine waters of the River Derwent near to the Commonwealth Scientific and Industrial Research Organisation's Marine and Atmospheric Research branch in Hobart, Tasmania, Australia. C. medeopolis has been described as presenting a new family and genus as well as species.
Benthocodon is a genus of hydrozoans of the family Rhopalonematidae. The genus contains two known species: Benthocodon hyalinus and Benthocodon pedunculatus, however due to the small size and red pigmentation, they can easily be confused with related genera. Unlike many hydromedusae, these jellyfish do not have a sessile stage. Rather, they spend their entire lives in the water column as plankton. The genus Benthocodon can be found near the sea floor in the Pacific Ocean from Antarctica to California to the Arctic Ocean.
Turritopsis nutricula is a small hydrozoan that once reaching adulthood, can transfer its cells back to childhood. This adaptive trait likely evolved in order to extend the life of the individual. Several different species of the genus Turritopsis were formerly classified as T. nutricula, including the "immortal jellyfish" which is now classified as T. dohrnii.
Maeotias is a genus of hydrozoans in the family Olindiidae. It is a monotypic genus with only a single species, Maeotias marginata, commonly known as the Black Sea jellyfish or brackish water hydromedusa and often referred to as Maeotias inexpectata in the literature. It was first described from the Don and Kuban estuaries of the Sea of Azov, and also occurs in the Black Sea, all of which are areas of low salinity. It has been recorded in several other estuarine locations around the world and is regarded as an invasive species.
Porpita prunella is a marine species of hydrozoan organisms within the family Porpitidae. It consists of colonies of zooids. Very little is known about this species, as there have been no confirmed sightings since its discovery in 1801 and naming by Haeckel in 1888. Being in the chondrophore group, it is likely that its behaviour is similar to the other species of the genera in the family. However there are also serious doubts as to its very existence as a separate species and may in fact be a synonym for Porpita porpita instead.
Aglantha digitale is a species of hydrozoan in the family Rhopalonematidae. It is found in the Arctic and sub-Arctic where it is one of the most common jellyfish. It is unusual in having both a slow swimming action, through pulsating its bell, and a rapid escape response.
