Scyphozoa (Jellyfish)

(invertebrate zoology) A class of the phylum Cnidaria; all members are marine and are characterized by large, well-developed medusae and by small, fairly well-organized polyps.

(Jellyfish)

Phylum: Cnidaria

Class: Scyphozoa

Number of families: 20

Thumbnail description
Large, soft-bodied, gelatinous marine invertebrates that swim by contracting their umbrella-shaped swimming bell and catch small prey by means of stinging tentacles

Evolution and systematics

The class Scyphozoa includes four orders, 20 families, 66 genera, and about 200 species. The four orders are Stauromedusae, the stalked jellyfish; Coronatae, the crown or grooved jellyfish; Semaeostomeae; and Rhizostomeae.

Animals in the phylum Cnidaria may have one or both of two body forms, the benthic polyp and the pelagic medusa. The four orders within the class Scyphozoa emphasize these two forms to different degrees. Specifically, in the order Stauromedusae, there is only a benthic stage, which is considered the medusa. In the orders Coronatae, Semaeostomeae, and Rhizostomeae both stages occur in most species, with the medusa stage being the largest and most conspicuous.

The phylum Cnidaria is considered to be of early evolutionary origin, but the position of the Scyphozoa relative to other cnidarian classes (Anthozoa [corals and anemones], Cubozoa [box jellyfish], and Hydrozoa [hydroids, hydromedusae, fire corals, and siphonophores]) is uncertain. It is debated whether the polyp or the medusa form is most primitive. The scyphozoans are related most closely to cubozoans, which were placed in the same class until recently. They have similar body forms and life cycles. The scyphozoans are related least to the Anthozoa. Molecular evidence suggests that Anthozoa represents the most primitive class in the phylum Cnidaria.

The fossil record of Scyphozoa is poor. Radially symmetrical impressions have been interpreted to be casts of primitive scyphomedusae. Recently, a large number of scyphomedusae that apparently were stranded and buried on a beach was discovered in central Wisconsin in the United States. Other fossil groups that may be ancient scyphozoan polyps are the conulariids, which were similar to modern coronate polyps and were present from the Ordovician to the late Triassic, and Bryonia from the Upper Cambrian and Ordovician, which is from the extinct order Bryoniida of the Scyphozoa.

Physical characteristics

Most species in the class Scyphozoa, excluding the order Stauromedusae, have two life stages, the predominant medusa stage (up to 80 in, or 2 m, in diameter) and the small, inconspicuous polyp stage (less than 0.13 in, or 4 mm, long). The medusa, or jellyfish, stage has a saucer- to umbrella-shaped body with two epithelial layers (epidermis and gastrodermis) separated by a thick layer of mesenchyme, a gelatinous connective tissue containing cells. Near the edge of the bell in the orders Coronatae and Semaeostomeae are tentacles used in feeding. The tentacles have millions of microscopic intracellular organelles called nematocysts that evert a hollow thread from a capsule and may inject toxin into or entangle their small prey (zooplankton, fish eggs and larvae, or other gelatinous species). In the Semaeostomeae and Rhizostomeae, there are four oral or mouth arms on the underside (concave) of the bell, which also have stinging nematocysts for feeding. The polyps, called scyphistomae, can form colonies of individuals by budding or, in the case of coronate polyps, true colonies that have a chitinous sheath. Polyps are cup-shaped, attached to the substrate by a "foot," and with the central mouth surrounded by a single ring of tentacles with nematocysts.

Stalked jellyfish in the order Stauromedusae attach to seaweed or sea grasses by an aboral stalk. The main body (calyx) is funnel- or goblet-shaped and grows to 1.2 in (3 cm) wide, with eight arms, each bearing a cluster of as many as 100 short, clubbed tentacles. In the common genus Haliclystus, between each arm there is an adhesive disk, by which the animal can attach to move about. The gonads extend down the arms. The mouth is located at the inside center of the funnel-shaped body. Coloration varies; it may be shades of green, brown, yellow, or maroon and often matches the color of the substrate, making these jellyfish difficult to see. This form is considered the medusa stage, and there is no polyp or swimming stage.

Jellyfish in the order Coronatae have a deep groove around the aboral surface that separates the swimming bell into a central disk and a peripheral zone, which has lappets. One thick tentacle emerges between lappets on the upper surface of the bell; depending on the species, there are between eight and 36 tentacles. The mouth opens into a large, pouchlike stomach on the underside of the bell. Most of the species are deep living, and thus the central disk is colored dark red to maroon, making it invisible at depths and presumably concealing the bioluminescence emanating from consumed prey. Most of the medusae are small, less than 2 in (5 cm) in diameter or bell height, but some species may attain 6 in (15 cm) in diameter. The known polyps are colonial and covered with a chitinous sheath.

Adult jellyfish in the order Semaeostomeae generally are large, up to 80 in (2 m) in diameter, but usually less than 12 in (30 cm). The swimming bell is flat to hemispherical in shape. The bell edge may have lappets, or it may be smooth. From eight to hundreds of tentacles are present at the bell margin or beneath the bell. In the center of the concave side of the bell are four diaphanous or frilled oral arms that lead to the central mouth. The bell ranges from translucent to opaque and from white to dark orange in color, and it may have radiating stripes in some species. The polyp stage is small and may form groups of individuals by budding.

The rhizostome medusae also are large, up to 80 in (2 m) in diameter. The swimming bell is hemispherical and very firm in texture and lacks tentacles at the edge. The bell margin has eight or 16 lappets. The four oral arms of rhizostome medusae are fused and usually very elaborate, with many tiny tentacles and small mouths for feeding. There may be clublike projections from the oral arms. The medusae are translucent to opaque, with colors ranging from white to dark red; some have patterns that include stripes and spots. The polyp stage is small and may form clusters of individuals by budding.

Distribution

Scyphozoans are found in all marine waters. Most Stauromedusae are found in cool waters along temperate to sub-polar shorelines spring through autumn. Coronate medusae generally occur at great depths, where temperatures are a cool 40–46°F (5–8°C), but a few species occur in subtropical to tropical waters. Semaeostome species are the predominant large medusae in polar to temperate oceans, and they also inhabit subtropical and tropical waters. The scyphistomae of those species are active only in warm months; however, they can become dormant and survive through winter months. Rhizostome species live mostly in tropical waters, with only a few species found in subtropical or temperate regions. The medusae of shallow-living scyphozoans are seen during late spring to early autumn in surface waters of temperate to polar seas. In tropical waters and among deep-dwelling species, medusae may be present all year.

Habitat

Scyphozoan medusae are found from surface waters to abyssal depths, and the polyps are attached to hard surfaces, such as pilings, shells, and rocks, at various depths, depending on the species. Most Stauromedusae are seen at intertidal down to shallow subtidal depths, usually attached to benthic plants (algae or sea grasses). One species is known from deep hydrothermal vent communities. Coronate medusae typically are found at mesopelagic depths (1,625–4,875 ft, or 500–1,500m), but a few species occur near the surface. Deep-living species may have polyps at abyssal depths, but the polyps of shallow-living species are on shallow substrates. Semaeostome and rhizostome medusae occur most abundantly near shore in surface waters above 165 ft (150 m), where food supplies are greatest. Their polyps also are found at shallow depths, often on the underside of structures away from direct light. Some semaeostome species are deep living, and their polyps generally are not known. There are no known deep-dwelling rhizostome species.

Behavior

Jellyfish behavior generally is simple, owing to their simple nervous system. Stauromedusae move around on the substrate by somersaulting, which they accomplish by alternating adhesion of the basal disc with that of tentacles or adhesive pads located between the tentacles of some genera. The most noticeable behavior of jellyfish is the rhythmic pulsation of the swimming bell, which moves them through the water for feeding and respiration. The swimming pulsations are coordinated by nerve centers around the edge of the bell. At the bell margin there also are sensory clubs (rhopalia), each consisting of a light-sensing organ (ocellus) and a gravity-sensing organ (statocyst); thus, medusae can sense light and dark and can determine their orientation in the water column. Semaeostome and rhizostome jellyfish swim continuously. This is important for oxygen exchange, which occurs over the entire body surface, and for feeding. The swimming of several species is known to be against flow in the water column; the result is that they all swim in the same direction and may become concentrated in convergences, like bales of hay stacked up to dry. Some species move up in the water column at night and down in the day ("diel vertical migration"). The scyphistomae (polyps) are able to move by the so-called foot and its extensions. They feed when prey makes contact with their tentacles, which have nematocysts; the jellyfish contract the tentacles and bring the prey to their mouths.

Feeding ecology and diet

All scyphozoans feed with tentacles or tentacle-like projections that have millions of microscopic intracellular organelles called "nematocysts." Some nematocysts act to paralyze or kill the prey, whereas others entangle them. Stauromedusae catch prey by the tentacles and fold the arm inward to bring the prey to the mouth. Many coronate medusae do not swim actively while feeding but instead remain nearly motionless with their tentacles extended above the bell. For semaeostome and rhizostome medusae, the pulsations of the swimming bell force water through the tentacles and create vortices that may bring prey into contact with the tentacles and oral arms. For semaeostome medusae, when a prey item is immobilized on a tentacle, the tentacle contracts and transfers the prey to an oral arm. The prey is moved by cilia up the inside of the folded oral arm to the mouth and into one of the four gastric (stomach) pouches, where short, fingerlike projections wrap around the prey and secrete digestive enzymes. For rhizostome medusae, prey capture is by the small tentacles on the oral arms, which transfer the prey to one of the many small mouths nearby.

Most species feed on small crustaceans that predominate in most habitats. Stauromedusae consume epibenthic crustaceans, including gammarid amphipods and harpacticoid copepods. Medusae in the other orders primarily eat abundant calanoid copepods but also eat other small zooplankton, such as cladocerans, larvaceans (= appendicularians), and chaetognaths. Many semaeostome species also feed on other gelatinous species, including scyphomedusae, hydromedusae, siphonophores, and ctenophores. It is of particular interest that several species are known to consume the eggs and larvae of fish. Thus, scyphomedusae may be detrimental to fish populations, both by consuming the zooplankton foods needed by fish larvae and zooplanktivorous fish species, like herring, and by feeding on the young fish directly.

Reproductive biology

Scyphozoans generally reproduce both asexually and sexually. The benthic forms, Stauromedusae and scyphistomae (polyps) of the other orders, reproduce asexually by budding new polyps or cysts from the body or foot. Scyphistomae, which are present in most species of all orders except Stauromedusae, also produce the medusa stage by an asexual budding process called strobilation. Strobilation typically takes place at a certain time of year and is triggered by environmental factors, which differ by species; these factors include rising (spring) or falling (autumn) temperatures or changes in light levels. During strobilation, the polyp undergoes transverse segmentation, forming one to several small medusae, called "ephyrae." The process requires days to weeks, depending on temperature.

The fully formed ephyrae break free by swimming pulsations. The ephyrae grow into sexually mature medusae over the course of a month or longer. The medusae of most species have separate sexes, but a few species are sequential hermaphrodites. The males and females are indistinguishable except by examination of the gonads. No mating occurs. Sperm strands are released into the water by males and are taken up by the females during feeding. The gonads surround the gastrovascular cavity, and eggs may be fertilized in the ovary or after they are released into the gastrovascular cavity. In most species the fertilized eggs develop into small ciliated larvae (planulae) that swim to a suitable substrate, attach, and develop into polyps. In some species, the planulae are retained (brooded) by the female before settlement. Some species lack a polyp stage.

Conservation status

No species of Scyphozoa is listed by the IUCN.

Significance to humans

Scyphozoan jellyfish have direct and indirect effects on humans, many of which are negative. Swimmers fear them for their painful stings. All jellyfish sting, but the stings of small specimens and those with short tentacles often are not painful to humans. The genera Chrysaora and Cyanea are known for painful stings. Scyphozoan stings are painful but not deadly. More painful and dangerous stingers are in the class Cubozoa (box jellyfish) and the class Hydrozoa (specifically, the Portuguese man of war, Physalia physalis).

Fish populations and commercial fisheries may be affected detrimentally by jellyfish. Jellyfish may occur in great abundance, and, if they are caught in fishing nets, their great weight may cause the nets to rip or the fish catch to be damaged. Jellyfish eat the pelagic eggs and larvae of fish as well as the small zooplankton prey of fish larvae and zooplanktivorous fish species. Therefore, jellyfish both eat fish and compete with them for food. Jellyfish also appear to be intermediate hosts for some parasites of fish. Jellyfish have been a nuisance to fish farms, where they break up on the fish impoundments and sting and kill the fish, and to power plants, where they may clog the cooling water intake screens, sometimes causing the plants to suspend operations. On the positive side for fish and fisheries, the juveniles of at least 80 species of fish, many of which are commercially important, associate with large jellyfish. While the relative advantages of such associations are not known, they are thought to benefit the fish partners most.

Jellyfish also have a place of value in human enterprise. In Japan and China jellyfish are an important food and have been exploited for more than 1,700 years. In China they are considered a culinary delicacy and are thought to have medicinal value. A multimillion-dollar commercial fishery exists for at least 10 species of rhizostome medusae throughout Southeast Asia, and a fishery for Stomolophus meleagris has been started in the Gulf of Mexico. The swimming bell of the jellyfish is processed in a mixture of salt and alum and packaged for distribution. The semidried jellyfish is rehydrated, desalted, blanched, and served in a variety of dishes. The prepared jellyfish has a special crunchy texture.

Owing to their great beauty and the relaxing effect of their swimming pulsations, jellyfish have been a great success as specimens in public aquariums and even as household pets. Over the past decade, considerable advances have been made in jellyfish husbandry, and several species are on display at aquariums worldwide. In Japan jellyfish are kept as pets in special aquariums.

Species accounts

Resources

Arai, Mary N. A Functional Biology of Scyphozoa. London:Chapman and Hall, 1997.

Cornelius, Paul F. S. "Keys to the Genera of Cubomedusae and Scyphomedusae (Cnidaria)." In Proceedings of the Sixth International Conference on Coelenterate Biology, edited. by J. C. Den Hartog. Leiden, The Netherlands: National Natuurhistorich Museum, 1997.

Franc, André. "Classe des Scyphozoaires." In Traité de Zoologie: Anatomie, Systématique, Biologie. Vol. 3, Cnidaires, Cténaires, edited by Pierre-P. Grassé. Paris: Masson, 1993.

Mayer, A. G. Medusae of the World. Vol. 3, The Scyphomedusae. Washington, DC: Carnegie Institution, 1910.

Mianzan, Hermes W., and Paul F. S. Cornelius. "Cubomedusae and Scyphomedusae." In South Atlantic Zooplankton, edited by Demetrio Boltovskoy. Leiden, The Netherlands: Backhuys, 1999.

Purcell, Jennifer E., W. Monty Graham, and Henri J. Dumont. Jellyfish Blooms: Ecological and Societal Importance. Developments in Hydrobiology, no. 155. Dordrecht, The Netherlands: Kluwer Academic, 2001.

Purcell, Jennifer E., Alenka Malej, and Adam Benovic. "Potential Links of Jellyfish to Eutrophication and Fisheries." In Ecosystems at the Land-Sea Margin: Drainage Basin to Coastal Sea, edited by Thomas C. Malone, Alenka Malej, Larry W. Harding Jr., Nenad Smodlaka, and R. Eugene Turner. Washington, DC: American Geophysical Union, 1999.

Wrobel, David, and Claudia Mills. Pacific Coast Pelagic Invertebrates: A Guide to the Common Gelatinous Animals. Monterey, CA: Monterey Bay Aquarium, 1998.

Brodeur, Richard D., Claudia E. Mills, James E. Overland, Gary E. Walters, and James D. Schumacher. "Evidence for a Substantial Increase in Gelatinous Zooplankton in the Bering Sea, with Possible Links to Climate Change." Fisheries Oceanography 8, no. 4 (1999): 296–306.

Kramp, P. L. "A Synopsis of the Medusae of the World." Journal of the Marine Biological Association of the United Kingdom 40 (1961): 1–469.

Larson, Ronald J. "Feeding in Coronate Medusae (Class Scyphozoa, Order Coronatae)." Marine Behavior and Physiology 6 (1979): 123–129. ——. "Scyphomedusae and Cubomedusae from the Eastern Pacific." Bulletin of Marine Science 47 (1990): 546–556. ——. "Diet, Prey Selection and Daily Ration of Stomolophus meleagris, a Filter-Feeding Scyphomedusa from the NE Gulf of Mexico." Estuarine Coastal and Shelf Science 32 (1991): 511–525. ——. "Riding Langmuir Circulations and Swimming in Circles: A Novel Clustering Behavior by the Scyphomedusa Linuche unguiculata." Marine Biology 112 (1992): 229–235.

Mills, Claudia E. "Jellyfish Blooms: Are Populations Increasing Globally in Response to Changing Ocean Conditions?" Hydrobiologia 451 (2001): 55–68.

Purcell, Jennifer E. "Predation on Zooplankton by Large Jellyfish (Aurelia labiata, Cyanea capillata, Aequorea aequorea) in Prince William Sound, Alaska, USA." Marine Ecology Progress Series 246 (2003): 137–152.

Purcell, Jennifer E., and Mary N. Arai. "Interactions of Pelagic Cnidarians and Ctenophores with Fishes: A Review." Hydrobiologia 451 (2001): 27–44.

Gershwin, Lisa. "Medusozoa Home Page." [June 13, 2003].

Jellies and Other Ocean Drifters. Video. Monterey Bay Aquarium, 1996. "The Jellies Zone." [June 13, 2003]. .

Ocean Drifters. Video. National Geographic Explorer, 1993.

Mills, Claudia E. Home Page. June 10, 2003 [June 13, 2003]. .

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The Shape of Life: Life on the Move: Cnidarians. Video. Sea Studios, Monterey, CA, 2001.

[Article by: Jennifer E. Purcell, PhD]

A class of the phylum Coelenterata, containing five living orders—Stauromedusae, Cubomedusae, Coronatae, Semaeostomeae, and Rhizostomeae—and a fossil order, Stromatoporoidea. They are all marine and usually take two forms, the polyp, or scyphopolyp, and the medusa, or scyphomedusa. However, some are polyplike and sessile throughout their lives, while others are always pelagic and lack the sessile polyp stage. Among the Coelenterata, the Scyphozoa are characterized by having well-developed medusae of large size and fairly well-organized polyps of small size.

The scyphomedusae are generally found near the coast. Exceptions are certain pelagic forms and the Coronatae, which are abyssal. Most of the Stauromedusae have a circumpolar distribution, while the Cubomedusae and Rhizostomeae are found mostly in warm and tropical seas. Scyphomedusae are carnivorous, except the Rhizostomeae which are plankton eaters. Some Semaeostomeae and Cubomedusae are injurious to humans because of their nematocysts. On the other hand, some Rhizostomeae are used as food in the Orient. See also Coelenterata.