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The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish (or "true jellies").
The class name Scyphozoa comes from the Greek language word skyphos (), denoting a kind of drinking cup and alluding to the cup shape of the organism. (1989). 9780030139192, Holt, Rinehart and Winston. ISBN 9780030139192
Scyphozoans have existed from the earliest Cambrian to the present.
As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells called nematocysts. The nematocysts are located throughout the tentacles that radiate downward from the edge of the umbrella dome, and also cover the four or eight oral arms that hang down from the central mouth. Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water.
Scyphozoans have no durable hard parts, including no head, no skeleton, and no specialized organs for respiration or excretion.Cartwright, P., Halgedahl, S.L., Hendriks, J.R., Jarrad, R.D., Marques, A.C., Collins, A.G., and Lieberman, B.S., 2007, Exceptionally preserved jellyfishes from the Middle Cambrian. PLOSONE Issue 10: e1121, p.1-7.Richards, H.G., 1947, Preservation of fossil jellyfish: Bulletin of the Geological Society of America, v. 58, p. 1221. Marine jellyfish can consist of as much as 98% water, so are rarely found in fossil form.
Unlike the hydrozoan jellyfish, Hydromedusae, Scyphomedusae lack a vellum, which is a circular membrane beneath the umbrella that helps propel the (usually smaller) Hydromedusae through the water. However, a ring of muscle fibres is present within the mesoglea around the rim of the dome, and the jellyfish swims by alternately contracting and relaxing these muscles.Morris, M., and Fautin, D., 2001, Animal Diversity Web: University of Michigan Museum of Zoology, " Scyphozoa., Accessed: September 28, 2008. The periodic contracting and relaxing propels the jellyfish through the water, allowing it to escape predation or catch its prey.
The mouth opens into a central stomach, from which four interconnected diverticulum radiate outwards. In many species, this is further elaborated by a system of radial canals, with or without an additional ring canal towards the edge of the dome. Some genera, such as Cassiopea, even have additional, smaller mouths in the oral arms. The lining of the digestive system includes further stinging nematocysts, along with cells that secrete .
The nervous system usually consists of a distributed net of cells, although some species possess more organised nerve rings. In species lacking nerve rings, the nerve cells are instead concentrated into small structures called rhopalia. There are between four and sixteen of these small lobes arranged around the rim of the umbrella, where they coordinate the muscular action allowing the animal to move. Each rhopalium is typically associated with a pair of sensory pits, a statocyst, and sometimes a pigment-cup ocellus.
The jellyfish fished commercially for food are Scyphomedusae in the order Rhizostomeae. Most rhizostome jellyfish live in warm water.
Class Scyphozoa
Reproduction
Most species appear to be gonochorists, with separate male and female individuals. The are located in the stomach lining, and the mature are expelled through the mouth. After fertilization, some species brood their young in pouches on the oral arms, but they are more commonly planktonic.
Growth and development
The fertilized egg produces a larva which, in most species, quickly attaches itself to the sea bottom. The larva develops into the hydroid stage of the lifecycle, a tiny sessile polyp called a scyphistoma. The scyphistoma reproduces asexually, producing similar polyps by budding, and then either transforming into a medusa, or budding several medusae off from its upper surface via a process called strobilation. The medusae are initially microscopic and may take years to reach sexual maturity.
Commercial importance
Scyphozoa include the moon jelly Aurelia aurita, in the order Semaeostomeae, and the enormous Nemopilema nomurai, in the order Rhizostomeae, found between Japan and China and which in some years causes major fisheries disruptions.
Taxonomy
Although the Scyphozoa were formerly considered to include the animals now referred to as the classes Cubozoa and Staurozoa, they now include just three Extant taxon orders (two of which are in Discomedusae, a subclass of Scyphozoa).Daly, Brugler, Cartwright, Collins, Dawson, Fautin, France, McFadden, Opresko, Rodriguez, Romano & Stake (2007). The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668: 127–182 About 200 extant species are recognized at present, but the true diversity is likely to be at least 400 species.
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