Scyphozoa
Chrysaora colorata
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Scyphozoa Götte, 1887
Orders
Coronatae Semaeostomeae Rhizostomeae

Scyphozoa is a class within the phylum Cnidaria^[1], sometimes referred to as the "true jellyfish".

The class name Scyphozoa comes from the Greek word skyphos (σκύφος), denoting a kind of drinking cup and alluding to the cup shape of the organism.

Scyphozoans range in geological time from the Ediacaran period^{[citation needed]} through the Recent.

Biology

Most species of Scyphozoa have two life history phases, including the planktonic medusa or jellyfish form, which is most evident in the warm summer months, and an inconspicuous, but longer-lived bottom-dwelling polyp, which seasonally gives rise to new medusae. Most of the large, often colorful, and conspicuous jellyfish found in coastal waters throughout the world are Scyphozoa.^[2] They typically range from 2 to 40 centimetres (0.79 to 16 in) in diameter, but the largest species, Cyanea capillata can reach 2 metres (6.6 ft) across. Scyphomedusae are found throughout the world's oceans, from the surface to great depths; no Scyphozoa occur in freshwater (or on land).

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 the tentacles to strain plankton from the water.^[3]

Anatomy

Scyphozoans usually display a four-part symmetry and have an internal gelatinous material called mesoglea, which provides the same structural integrity as a skeleton. The mesoglea includes mobile amoeboid cells originating from the epidermis.

Scyphozoans have no durable hard parts, including no head, no skeleton and no specialized organs for respiration or excretion.^[4][5] Marine jellyfish can consist of as much as 99% water and therefore are rarely found in fossil form.

Unlike the hydrozoan jellyfish, hydromedusae, scyphomedusae lack a velum, which is a circular membrane beneath the umbrella that helps propels 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.^[6] 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 diverticula 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 digestive enzymes.^[3]

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 rhopallia. 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 rhopallium is typically associated with a pair of sensory pits, a statocyst, and sometimes a pigment-cup ocellus.^[3]

Reproduction

Most species are dioecious, with separate male and female individuals. The gonads are located in the stomach lining, and the mature gametes are expelled through the mouth. After fertilisation, some species brood their young in pouches on the oral arms, but they are more commonally planktonic.^[3]

The fertilised egg produces a planula larva which, in most species, quickly attaches itself to the sea bottom. The larva develops into the hydroid stage of the life-cycle, 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. The medusae are initially microscopic, and may take years to reach sexual maturity.^[3]

Commercial importance

Scyphozoa includes the moon jelly Aurelia aurita^[7], in the order Semaeostomeae, and the enormous Nemopilema nomurai, in the Order Rhizosomeae, found between Japan and China and which in some years causes major fisheries disruptions.

Most of the jellyfish that are fished commercially for food are scyphomedusae in the order Rhizostomeae.^[8] Most rhizostome jellyfish live in warm water.^[9]

Taxonomy

Although the Scyphozoa was formerly thought to include the animals now referred to the classes Cubozoa and Staurozoa, it is now thought to include just three orders:

Class Scyphozoa

• Order Coronatae
  • Family Atollidae
  • Family Atorellidae
  • Family Nausithoidae
  • Family Paraphyllinidae
  • Family Periphyllidae
• Order Semaeostomeae
  • Family Cyaneidae
  • Family Pelagiidae
  • Family Ulmaridae
• Order Rhizostomeae
  • Family Archirhizidae
  • Family Cassiopeidae
  • Family Catostylidae
  • Family Cepheidae
  • Family Lobonematidae
  • Family Lychnorhizidae
  • Family Mastigiidae
  • Family Rhizostomatidae
  • Family Stomolophidae

References

External links

• The Classification and Distribution of the Class Scyphozoa

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