All jellyfish are Cnidaria, an animal phylum that contains jellyfish, sea anemones, and corals, among others. They are all united in possessing stinging cells known as cnidocytes.
There are more than 10,000 species of Cnidaria, and roughly 4,000 of these make up Medusozoa—a group that contains all animals we think of as jellyfish. Jellyfish is the name for the swimming adult medusa stage of in the life cycle of these animals. The 4,000 or so medusozoans are divided into four different groups.
The polyp stage of Cassiopea xamachana (Medusozoa, Scyphozoa, Rhizostomeae) by Allen CollinsSmithsonian National Museum of Natural History
Scyphozoa are the most familiar jellyfishes, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason.
The jellyfish Cassiopea xamachana (Medusozoa, Scyphozoa, Rhizostomeae) by Allen CollinsSmithsonian National Museum of Natural History
Scyphozoans are most often noticed in their medusa body form, and there are at least 200 species.
The box jellyfish Carybdea (Medusozoa, Cubozoa, Carybdeida) by Allen CollinsSmithsonian National Museum of Natural History
Cubozoa are the box jellyfishes, named for their box-like bells. Some cubozoans, such as the sea wasps (species in the genus Chironex), produce some of the most potent venoms known. Cubozoan jellyfish also have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some even engage in elaborate (for a jellyfish) courtship behavior! There are at least 48 species known, living mostly in warmer waters. In 2011, Allen Collins, a jellyfish expert at the Smithsonian, and colleagues discovered a new species, which was named Tamoya ohboya in a public naming contest.
The stalked jellyfish Haliclystus californiensis (Medusozoa, Staurozoa, Haliclystidae) by Allen CollinsSmithsonian National Museum of Natural History
Staurozoa are the stalked jellyfishes, which don't float or swim through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.
The jellyfish Zancleopsis tentaculata (Medusozoa, Hydrozoa, Capitata) by Allen CollinsSmithsonian National Museum of Natural History
Hydrozoa is the largest group of jellyfishes, with some 3,800 species. The swimming medusa stages of this group are often small and inconspicuous, whereas the bottom-dwelling polyps, or hydroids, usually take the form of large colonies. A large number of hydrozoan species do not produce a jellyfish stage, and instead produce gametes in the polyp or hydroid stage.
In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota, 12,140 feet (3,700 meters) deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3,700 species of Hydrozoa.
Siphonophore (2018-04-28) by NOAAOriginal Source: NOAA Office of Ocean Exploration and Research, Gulf of Mexico 2018.
In the water column, one group of hydrozoans known as siphonophores can be quite spectacular. This group includes the notorious, surface dwelling Portuguese Man-o-Wars as well as many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets.
Siphonophore (2017-03-14) by NOAAOriginal Source: NOAA Office of Ocean Exploration and Research, Discovering the Deep: Exploring Remote Pacific MPAs
Siphonophores are colonial, composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. Different zooids are specialized for swimming, eating, prey capture or gamete production.
The freshwater jellyfish Craspedacusta sowerbii (Medusozoa, Hydrozoa, Limnomedusae) by Allen CollinsSmithsonian National Museum of Natural History
Jellyfishes are found in oceans worldwide, from the shallows to the deepest depths, and a few can even be found living in freshwater.
The hydroid of Pennaria disticha (Medusozoa, Hydrozoa, Capitata), with juvenile jellyfish in the process of being budded by Allen CollinsSmithsonian National Museum of Natural History