Jellyfish

Underwater Jellyfish

Jellyfish are most recognised because of their jelly like appearance and this is where they get their name. They are also recognised for their bell-like shape and tentacles. Jellyfish are free-swimming members of the phylum Cnidaria. They have several different basic morphologies that represent several different cnidarian classes including the Scyphozoa, Staurozoa, Cubozoa and Hydrozoa. The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; "medusa" is another word for jellyfish. Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water. Most of the information about jellyfish that follows in this article is about scyphozoan jellyfish, or scyphomedusae. These are the big, often colorful, jellyfish that are common in coastal zones worldwide.

Underwater Jellyfish

Some of the most common and important jellyfish predators are other species of jellyfish, some of which are specialists in eating jellies. Other predators of jellyfish include tuna, shark, swordfish, and at least one species of Pacific salmon, as well as sea turtles. Sea birds sometimes pick symbiotic crustaceans from the bells of jellyfish near the surface of the sea, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.

Jellyfish are commonly displayed in aquaria in many countries. Often the tank's background is blue and the animals are illuminated by side light to produce a high contrast effect. In natural conditions, many jellies are so transparent that they are almost impossible to see.

Holding jellyfish in captivity presents other problems. For one, they are not adapted to closed spaces. They depend on currents to transport them from place to place. To compensate for this, professional exhibits feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the kreisel for this purpose.