The Scyphozoa are a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish". The class name Scyphozoa comes from the Greek word skyphos, denoting a kind of drinking cup and alluding to the cup shape of the organism. Most species of Scyphozoa have two life history phases, including the planktonic medusa or jellyfish form, which is most evident in the warm summer months, and an inconspicuous, but longer-lived, bottom-dwelling polyp, which seasonally gives rise to new medusae. Most of the large, often colorful, and conspicuous jellyfish found in coastal waters throughout the world are Scyphozoa. They typically range from 2 to 40 cm (0.79 to 15.75 in) in diameter, but the largest species, Cyanea capillata can reach 2 metres (6.6 ft) across. Scyphomedusae are found throughout the world's oceans, from the surface to great depths; no Scyphozoa occur in freshwater (or on land).
As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells called nematocysts. The nematocysts are located throughout the tentacles that radiate downward from the edge of the umbrella dome, and also cover the four or eight oral arms that hang down from the central mouth. Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water.
Aurelia aurita (also called the moon jelly, moon jellyfish, common jellyfish, or saucer jelly) is a widely studied species of the genus Aurelia. All species in the genus are closely related, and it is difficult to identify Aurelia medusae without genetic sampling; most of what follows applies equally to all species of the genus. The jellyfish is translucent, usually about 25–40 cm (10–16 in) in diameter, and can be recognized by its four horseshoe-shaped gonads, easily seen through the top of the bell. It feeds by collecting medusae, plankton, and mollusks with its tentacles, and bringing them into its body for digestion. It is capable of only limited motion, and drifts with the current, even when swimming.
The brown-banded moon jelly (Aurelia limbata) is much less common than the moon jelly and is distinguished by its dark brown margin and many-branched subumbrellar canals. Aurelia limbata, an epipelagic species which occurs in arctic waters. The umbrella diameter of this species may vary from 16 to 40 cm. Molecular analyses have demonstrated that all currently recognized morphospecies of Aurelia are polyphyletic, and that A. limbata includes at least two molecular species.
The northern sea nettle (Chrysaora melanaster), also called a brown jellyfish, is a species of jellyfish native to the northern Pacific Ocean and adjacent parts of the Arctic Ocean. (It is sometimes referred to as a Pacific sea nettle, but this name is also used for Chrysaora fuscescens; the name Japanese sea nettle was used for this species, but that name now exclusively means Chrysaora pacifica. This jelly's medusa can reach 60 centimeters in length with tentacles growing up to three meters. The number of tentacles is up to 24 (8 per octant). It dwells at depths of up to 100 meters, where it feeds on copepods, larvaceans, small fish, large zooplankton, and other jellies. The sting is mild, although can cause serious skin irritation and burning.
Phacellophora camtschatica, known as the fried egg jellyfish or egg-yolk jellyfish, is a very large jellyfish, with a bell up to 60 cm (2 ft) in diameter and sixteen clusters of up to a few dozen tentacles, each up to 6 metres (20 ft) long. It has traditionally been included in the family Ulmaridae, but is now considered the only member of the family Phacellophoridae. 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 (Strand & Hamner, 1988). 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 (Towanda & Thuesen, 2006). The life cycle of this jellyfish is well known (Widmer 2006), because it is kept in culture at the Monterey Bay Aquarium. It alternates between a benthic stage that is attached to rocks and piers that reproduces asexually and the planktonic stage that reproduces sexually in the water column; there are both males and females in the plankton.
The lion's mane jellyfish (Cyanea capillata), also known as hair jelly, is the largest known species of jellyfish. Its range is confined to cold, boreal waters of the Arctic, northern Atlantic, and northern Pacific Oceans. It is common in the English channel, Irish Sea, North Sea and in western Scandinavian waters down to Kattegat andØresund. It may also drift in to the south-western part of the Baltic Sea (where it cannot breed due to the low salinity). Similar jellyfish, which may be the same species, are known to inhabit seas near Australia and New Zealand. The largest recorded specimen found, washed up on the shore of Massachusetts Bay in 1870, had a bell (body) with a diameter of 2.3 metres (7 ft 6 in) and tentacles 37 m (120 ft) long. Lion's mane jellyfish have been observed below 42°N latitude for some time—specifically in the larger bays of the east coast of the United States.