The American evolutionary biologist Paulyn Cartwright gives the following general definition: Typically, medusozoan cnidarians have a pelagic, predatory jellyfish stage in their life cycle; staurozoans are the exceptions [as they are stalked].

[14]The Merriam-Webster dictionary defines jellyfish as follows: A free-swimming marine coelenterate that is the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells.

[25] Stalked jellyfish are attached to a solid surface by a basal disk, and resemble a polyp, the oral end of which has partially developed into a medusa with tentacle-bearing lobes and a central manubrium with four-sided mouth.

Each individual has 24 eyes, two of which are capable of seeing colour, and four parallel information processing areas that act in competition,[38] supposedly making them one of the few kinds of animal to have a 360-degree view of its environment.

Experimental evidence for photosensitivity and photoreception in cnidarians antecedes the mid 1900s, and a rich body of research has since covered evolution of visual systems in jellyfish.

Although they lack a true brain, cnidarian jellyfish have a "ring" nervous system that plays a significant role in motor and sensory activity.

[41][42] Light-guided behaviors are observed in numerous scyphozoans including the common moon jelly, Aurelia aurita, which migrates in response to changes in ambient light and solar position even though they lack proper eyes.

[43] Box jellyfish possess "proper eyes" (similar to vertebrates) that allow them to inhabit environments that lesser derived medusae cannot.

[44][43] The lack of the ability to focus is due to the focal length exceeding the distance to the retina, thus generating unfocused images and limiting spatial resolution.

Eyes tend to differ in size and shape, along with number of receptors (including opsins), and physiology across species of box jellyfish.

[49] This is likely due to differences in lower molecular weight proteins and the subsequent lack of immunological reactions with antisera that other organisms' lenses exhibit.

[49] All four of the visual systems of box jellyfish species investigated with detail (Carybdea marsupialis, Chiropsalmus quadrumanus, Tamoya haplonema and Tripedalia cystophora) have invaginated synapses, but only in the upper and lower lensed eyes.

[50] Pupillary contraction appears to have evolved in response to variation in the light environment across ecological niches across three species of box jellyfish (Chironex fleckeri, Chiropsella bronzie, and Carukia barnesi).

bronzie inhabit shallow beach fronts that have low visibility and very few obstacles, thus, faster pupil contraction in response to objects in their environment is not important.

[50] Behavioral studies support the idea that faster pupillary contraction rates assist with obstacle avoidance as well as depth adjustments in response to differing light intensities.

In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing planula larvae.

The jellyfish polyp may be sessile, living on the bottom of boat hulls or other substrates, or it may be free-floating or attached to tiny bits of free-living plankton[68] or rarely, fish[69][70] or other invertebrates.

Meanwhile, the second vortex ring starts to spin faster, sucking water into the bell and pushing against the centre of the body, giving a secondary and "free" boost forward.

The mechanism, called passive energy recapture, only works in relatively small jellyfish moving at low speeds, allowing the animal to travel 30 percent farther on each swimming cycle.

Jellyfish achieved a 48 percent lower cost of transport (food and oxygen intake versus energy spent in movement) than other animals in similar studies.

[89] Jellyfish form large masses or blooms in certain environmental conditions of ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentration.

[113] Some jellyfish have a symbiotic relationship with single-celled dinoflagellates, allowing them to assimilate inorganic carbon, phosphorus, and nitrogen creating competition for phytoplankton.

[113] Their large biomass makes them an important source of dissolved and particulate organic matter for microbial communities through excretion, mucus production, and decomposition.

The upside-down jellyfish in the genus Cassiopea typically lie on the bottom of shallow lagoons where they sometimes pulsate gently with their umbrella top facing down.

[3] Fisheries have begun harvesting the American cannonball jellyfish, Stomolophus meleagris, along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asia.

[131] Rhizostomes, especially Rhopilema esculentum in China (海蜇 hǎizhé, 'sea stingers') and Stomolophus meleagris (cannonball jellyfish) in the United States, are favored because of their larger and more rigid bodies and because their toxins are harmless to humans.

[126] Traditional processing methods, carried out by a jellyfish master, involve a 20- to 40-day multi-phase procedure in which, after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed.

[126] Pliny the Elder reported in his Natural History that the slime of the jellyfish "Pulmo marinus" produced light when rubbed on a walking stick.

Professional exhibits as in the Monterey Bay Aquarium feature precise water flows, typically in circular tanks to avoid trapping specimens in corners.

[153][154] In Elba Island and Corsica dittrichia viscosa is now used by residents and tourists to heal stings from jellyfish, bees and wasps by pressing fresh leaves on the skin with quick results.