Life in the Open Ocean. Joseph J. Torres

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where the scyphistoma produces one ephyra at a time, and polydisk strobilation, where ephyrae are stacked up like dinner plates on the scyphistoma and are shed by repetitive transverse fission. Scyphistomae may live for several years. After a period of producing ephyrae, usually during the winter and spring, they resume life as a polyp until the following reproductive season.

      Coronatae

      The life histories of coronate scyphomedusae are poorly known, largely because most of them sport a mesopelagic lifestyle. The best‐known life cycle is that of Nausithoe, which shows a typical scyphozoan pattern similar to that of the semaeostome Aurelia. It has a colonial scyphistoma stage that produces ephyrae by polydisk strobilation. The medusae reproduce sexually, producing a planula larva that settles to form a scyphistoma to continue the cycle. It has been speculated that the deeper‐dwelling genera such as Periphylla and Atolla use a strategy of direct development like that of the semeaostome Pelagia. In a direct‐development strategy, the planula develops directly into an ephyra, bypassing the scyphistoma stage altogether. Evidence in support of this hypothesis is the large size of the eggs in Periphylla and Atolla, which would facilitate the direct developmental strategy (Larson 1986 in Arai 1997).

      Genera include Atolla, Linuche, Periphylla, Nausithoe, Stephanoscyphus, and Tetraplatia.

Schematic illustration of coronatae.

      Sources: (a) Adapted from Arai (1997), figure 1.5 (p. 7); (b) Redrawn from Mayer (1910), Vol. III, figure 352.

      Semaeostomae

      The semaeostomes include the medusae most typical of the class and most familiar to beachgoers. Medusae are large, typically 5–40 cm, with a bell shape ranging from saucer‐like (Aurelia) to bowl‐like (Chrysaora) and lack a coronal groove. Tentacles are found along or below the margin of the umbrella, which may be divided into eight or more lappets. Most typical of the semaeostomes are the long frilly oral arms that originate at and form the corners of the mouth (Figure 3.10).

Schematic illustration of life cycles of scyphozoans.

      Sources: (a) Bayer and Owre (1968), figure 161 (p. 105); (b) Bayer and Owre (1968), figure 162 (p. 106); (c) Adapted from Calder (1982), figure 4 (p. 156).

      Genera include Aurelia, Chrysaora, Cyanea, Stygiomedusa, and Pelagia. Pelagia, as the name implies, is common in offshore waters

      Rhizostomae

Schematic illustration of rhizostomeae.

      Sources: (a) Adapted from Uchida (1926); (b and c) Kaestner (1967), figure 5‐17 (p. 105); (d and e) Hyman (1940), figure 172 (p. 525); (f) Redrawn from Mayer (1910), plate 73.

      The life histories of rhizostome medusae are typical of the Scyphozoa (Figure 3.12c) in having a planula larva that settles to the bottom and forms a polypoid scyphistoma. Scyphistomae may strobilate to form ephyrae or may produce other scyphistomae by budding.

      The rhizostomes are chiefly a tropical–subtropical group inhabiting shallow waters, though two genera, Rhizostoma and Stomolophus, are found in temperate climes and may even form blooms. Stomolophus nomurai forms huge blooms in the Sea of Japan at intervals (Mills 2001). Rhizostomes are generally quite large, with sizes ranging from 4 to 200 cm across the bell.

      Genera include: Rhizostoma, Mastigias, Cassiopeia, Stomolophus, Cephea.

      The Cubomedusae, variously known as the box jellies, sea wasps, or fire medusae, comprise the Cnidarian class Cubozoa. The Cubomedusae were formerly considered to


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