Scyphozoa

Scyphozoa is a class of exclusively marine animals within the phylum Cnidaria, commonly known as true jellyfish or true jellies. Their name derives from the Greek *skyphos*, meaning a drinking cup, a reference to their bell-like shape. These creatures have been present from the early Cambrian period up to the modern day.

In terms of biology, most scyphozoans have two life stages. One is the planktonic medusa, which is most visible during warm summer months, and the other is a longer-lived, bottom-dwelling polyp that is less conspicuous and seasonally produces new medusae. The large, colorful jellyfish commonly seen in coastal waters worldwide belong to this class. Their size typically ranges from 2 to 40 cm in diameter, though the largest species, *Cyanea capillata*, can reach 2 meters across. Scyphomedusae inhabit all of the world’s oceans, from the surface to deep waters, but none are found in freshwater or on land. As medusae, they feed on crustaceans and fish, capturing prey with stinging cells called nematocysts located on their tentacles and oral arms. Some species are filter feeders, using their tentacles to strain plankton from the water.

Regarding reproduction, most scyphozoans are gonochoristic, with separate males and females. Their gonads are inside the stomach lining, and mature gametes are released through the mouth. After fertilization, some species brood their young in pouches on the oral arms, though planktonic development is more common.

Growth begins with a fertilized egg developing into a planular larva, which typically attaches to the seafloor. This larva becomes a tiny, sessile polyp called a scyphistoma. The scyphistoma reproduces asexually by budding, and then either transforms into a medusa or buds several medusae from its upper surface through a process called strobilation. The medusae start out microscopic and may take years to reach sexual maturity.

Anatomically, scyphozoans usually have four-part symmetry and a gelatinous internal material called mesoglea, which provides structural support. The mesoglea contains mobile amoeboid cells from the epidermis. They lack hard parts, a head, a skeleton, and specialized organs for respiration or excretion. Jellyfish can be up to 98% water, so they rarely fossilize. Unlike hydrozoan jellyfish, scyphomedusae lack a vellum, but they have a ring of muscle fibers in the mesoglea around the dome’s rim. By alternately contracting and relaxing these muscles, they propel themselves to escape predators or catch prey. The mouth leads to a central stomach with four radiating diverticula, and many species have a system of radial canals. Some genera, like *Cassiopea*, have additional smaller mouths on their oral arms. The digestive lining contains nematocysts and cells that secrete digestive enzymes. The nervous system is typically a distributed net of cells, though some species have organized nerve rings. In species without nerve rings, nerve cells concentrate in small structures called rhopalia, arranged around the umbrella’s rim. Each rhopalium is associated with sensory pits, a statocyst, and sometimes a pigment-cup ocellus.

Commercially, scyphozoa include the moon jelly *Aurelia aurita* and the enormous *Nemopilema nomurai*, which can disrupt fisheries between Japan and China. Jellyfish fished for food are scyphomedusae in the order Rhizostomeae, most of which live in warm water.

Taxonomically, the class Scyphozoa once included what are now separate classes Cubozoa and Staurozoa. Today, it comprises three extant orders, two of which fall under the subclass Discomedusae. About 200 extant species are recognized, though the true number is likely at least 400. The class is divided into subclasses Coronamedusae (order Coronatae) and Discomedusae (orders Rhizostomeae and Semaeostomeae), along with the extinct order †Byroniida.