The medusa-phase of some gelatinous species of the subphylum Medusozoa, a significant portion of the phylum Cnidaria, is referred to as jellyfish or sea jellies. Jellyfish are mostly free-swimming marine creatures with umbrella-shaped bells and trailing tentacles, however a few are stalk-anchored to the bottom. The bell has the ability to pulse in order to produce propulsion for extremely efficient movement.

The stinging cells on the tentacles can be utilized to catch prey and protect against predators. Jellyfish have a complicated life cycle; the medusa is the sexual phase, which produces planula larvae that spread far and undergo a sedentary polyp phase before sexual maturity.

Jellyfish may be found wherever on the planet, from the surface to the deep oceans. Scyphozoans (sometimes known as “real jellyfish”) are only found in the sea, however certain hydrozoans with similar appearances can be found in freshwater. Jellyfish of various sizes and colors may be found in coastal areas all over the world.

Most species’ medusae develop quickly, mature in a few months, and perish soon after mating, while the polyp stage, which is connected to the seafloor, may survive much longer. Jellyfish are the oldest multi-organ animal group, having existed for at least 500 million years, and perhaps 700 million years or more.

In certain civilizations, jellyfish are consumed by humans. In certain Asian nations, species of the Rhizostomae order are pressed and salted to eliminate excess water and are regarded a delicacy. They’ve been called a “ideal meal” by Australian experts, as they’re long-lasting, high in protein, yet low in calories.

They’re also utilized in science, where the green fluorescent protein that causes bioluminescence in certain species has been modified to serve as a luminous marker for genes introduced into other cells or creatures.

Jellyfish stinging cells, which are employed to subdue their food, can harm people. Every year, hundreds of swimmers are stung, with symptoms ranging from moderate pain to major damage or death; tiny box jellyfish are responsible for many of these deaths.

Jellyfish may create massive swarms when conditions are favorable, causing damage to fishing gear by overflowing fishing nets and clogging the cooling systems of power and desalination facilities that take their water from the sea.

Since 1796, the term “jellyfish” has been used to describe medusae and other related creatures, including comb jellies (ctenophores, another phylum). Although clam, cuttlefish, and starfish are not vertebrates, the name “jellies” or “sea jellies” is more recent, having been coined by public aquariums to avoid using the word “fish” with its current meaning of an animal with a backbone.

The terms “jelly” and “jellyfish” have been interchanged in scientific literature. Scyphozoans are the only “real jellyfish” according to several authorities.

Jellyfish are not a clade because they contain the majority of the Medusozoa, with the exception of a few Hydrozoa. The existence of citations on the following phylogenetic tree indicates the medusozoan groupings included by authorities. Wherever feasible, the names of included jellyfish are displayed in boldface; the inclusion of a named and referenced example implies that at least that species within its group has been referred to as a jellyfish.

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