Stomolophus meleagris L. Agassiz, 1862
Portuguese Man o' War, with trailing tentacle
Physalia physalis Linnaeus, 1758
Obviously jellyfish are not fish at all, but rather floating invertebrates (having no backbone or spinal cord). Most jellyfish have an umbrella-shaped structure, and long, thin, tentacles. The nematocysts on the tentacles of some species of jellyfish can inflict painful stings. (Certain species — for example Chironex fleckeri, a box jellyfish of northern Australian seas and southeastern Asian seas — can inflict lethal venom.) Most jellyfish are translucent or transparent. A pulsing “opening and closing” movement of their “umbrella” propels them through the water.
Most species are carnivorous. They use their tentacles to catch food, such as shrimp, fish, worms, and other jellyfish. Some jellyfish, however, do not have tentacles. They are a type of filter feeder. Pulsations of the jellyfish's body push water over the feeding apparatus where the planktonic animals in the water get trapped in mucus and passed through thousands of tiny mouths in the “stomach,” or gut. Another species of jellyfish lives off the commensal zooanthellae in its body. All that jellyfish has to do is stay near the sunny surface of the sea so that the zoonathellae can perform photosynthesis, creating nutrients for itself and its host.
The Portuguese Man o' War is a dangerous jellyfish relative with tentacles up to 60 feet long. When Man o' Wars wash up on the beach, they look a lot like pear-shaped, sky-blue balloons. Winds move colonies of jellies across the waves, so a lot of Portuguese Man o' Wars invade at the same time.
Jellyfish are composed of over 93% water and a fluid called mesoglea, which makes them very buoyant. When they wash up on the sand they die quickly, but the tentacles sting long after the animal dies. Do not mess with them. If you encounter a jellyfish in the ocean and get stung, get out of the water fast and wash the burning welts with soap and water. Apply rubbing alcohol or powdered meat tenderizer, and Vaseline petroleum jelly to the wound. (Some people advise applying vinegar.) Then apply ice. If a serious allergic reaction to the toxin occurs (generalized body swelling, breathing problems), get to a doctor as fast as possible.
The Cannonball Jellyfish, Stomolophus meleagris, frequently washes ashore on Carolina beaches. Cannonball Jellyfish lack true tentacles, so theoretically they do not sting. However, some individuals have reported mild stings from contact with Cannonballs.
Jellyfish and Man o' Wars are eaten by Sea Turtles, giant Ocean Sunfish, squids, other jellies, and Frigate birds, none of which are apparently affected by the stinging nematocysts.
People in some parts of the world eat jellyfish. Salted and dried jellyfish have long been considered a delicacy by the Chinese; some Orientals pickle and stew jellyfish and serve them appetizers, and in some areas jellyfish are eaten in the form of a paste or dried flour.
Jellyfish are members of the Cnidaria phylum. Their subphylum, Medusozoa, is divided into three classes, one of which is Scyphozoa. This class contains all the jellyfish. (The Portuguese Man o' War, Physalia physalis, is a siphonophore of a different class.)