Scyphozoa

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There are approximately 200 species of Scyphozoans organized into four orders. Familiar scyphozoans include Aurelia (the moon jelly) and Cyanea (the lion's mane jelly). Scyphozoans live in all oceans, from the Arctic to tropical waters. Some inhabit the deep sea, but most live near the coastal waters. Most are motile animals, but members of the order Stauromedusae are sessile.

Scyphozoans exhibit the main characteristics of cnidarians. They have radial symmetry and are diploblastic, meaning that their body wall consists of the outer epidermis (ectoderm) and the inner gastrodermis (endoderm), which are separated by mesoglea. They have nematocysts, which are characteristic of the phylum. They undergo alternation of generations, with the medusa form being dominant. Scyphomedusae are the "jellyfish" with which most people are familiar.

The scyphozoan life cycle varies from order to order. The medusae are gonochoric. Fertilized eggs may be brooded for a time or may develop directly into a free-swimming, ciliated planula larva. A larva metamorphoses into a small polyp termed the scyphistoma. Scyphistomae commonly produce more scyphistomae asexually. Eventually, a scyphistoma becomes a strobila, in which the distal end develops into a medusa-like animal. The strobila may look like a stack of saucers, the saucers being the immature medusae called ephyrae. As formation of the ephyrae is completed, each breaks away and eventually grows into a sexually-reproducing adult medusa.

Scyphozoan polyps and medusae exhibit no cephalization and contain no brain, but in some species, light-sensitive eyespots are located along the bell margin of the medusa.

Scyphozoan medusae differ from those of hydrozoans in lacking a velum. A scyphomedusa locomotes by contracting and relaxing muscles of the bell. Contraction pushes water out, propelling the jelly in jet-like fashion. Surrounding the mouth of members of order Semaeostomeae are four oral arms that trail behind the bell and can reach a length of 40 meters. Nematocysts on the oral arms are used for defense and for capturing prey. Scyphozoans, like all Cnidarians, are all carnivores and some are filter-feeders. Many smaller jellies feed on food particles trapped from the water while larger ones prey on fishes or swimming invertebrates. Members of order Rhizostomeae lack central mouths; rather, each has structures much like the oral arms on which many reduced mouths open. An unusual member of the Rhizostomeae, the tropical jelly Cassiopeia, contains symbiotic dinoflagellates inside its body tissues, and lies upside down in sunny areas so its algae can photosynthesize; it receives most of its energy from the carbohydrates fixed by the algae.

Coloration in some scyphozoans comes from the gonads or other internal structures. The bell of some is deeply pigmented.

Scyphozoans can be a nuisance to humans when they wash up on beaches or if humans come in contact with them in the water. A sting from a jellyfish can be very unpleasant and may even cause death. Scyphozoans can be a nuisance to the fishing industry by clogging nets when they accumulate into shoals or groups, which can be many kilometers long. However, some people eat jellies, which are considered a delicacy. The fossil record for Scyphozoa is poor due to the fact that they are made mostly of water and lack hard parts.

References:

Barnes, R. D. Class Scyphozoa, pages 113-122 in Invertebrate Zoology. Philadelphia: Saunders College Publishing. 1987.

Kozloff, E. N. Class Scyphozoa, pages 120-127 in Invertebrates. Philadelphia: Saunders College Publishing. 1990.

Meglitsch, P. A. Class Scyphozoa, pages 123-127 in Invertebrate Zoology. New York: Oxford University Press. 1972.

Contributors

Michelle Morris (author), Daphne G. Fautin (author).

Glossary

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).