Joseph J. Torres

Life in the Open Ocean


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genera include Laodicea, Aequorea, Obelia, Eirene, Mitrocoma, Eutima, Phialidium. Polyps are thecate (have a surrounding sheath, Figure 3.6a and c). The life cycle is shown in Figure 3.8a.

Schematic illustration of (a) The hydrozoan polyp Coryne eximia with newly liberated medusa Coryne eximia; (b) a single hydrozoan medusa Coryne eximia produced by its polyp stage.

      Source: Redrawn from Mayer (1910), Vol. 1, (p. 56).

Schematic illustration of life cycles of hydrozoans.

      Sources: (a) Adapted from Bayer and Owre (1968), figure 153 (p. 101); (b) Bayer and Owre (1968), figure 155 (p. 102); (c) Bayer and Owre (1968), figure 154 (p. 102).

Schematic illustration of hydrozoan medusae.

      Sources: (a) Kramp (1959); (b) Mayer (1904), plate IV.

      Basic Characteristics

      The Scyphozoa are exclusively marine. They occur from the surface to bathypelagic depths and from polar to tropical oceans. The medusoid stage dominates the life history. When present, the polypoid stage, termed the scyphistoma, is small and sessile. Scyphomedusae range in size from bell diameters of 2 cm to 2 m. About 223 species have been described.

      All three orders of Scyphomedusae are pelagic: the Coronatae, Semaeostomae, and Rhizostomae. The coronate medusae are all found in deep water and, because of that, tend to have very wide‐ranging distributions. Semaeostome and rhizostome medusae are found primarily in coastal waters. Even so, some species, e.g. the semaeostome Pelagia noctiluca and the cannonball jelly Stomolophus meleagris, exhibit considerable latitudinal range (Mianzan and Cornelius 1999).

Schematic illustration of scyphomedusae.

      Sources: (a) Bayer and Owre (1968), figure 100 (p. 65); (b) Kaestner (1967), figure 5‐10 (p. 98); (c) Redrawn from Mayer (1910), figure 368.

      A further difference between the scyphozoan and hydrozoan medusae is their average size. Though large hydromedusae and young scyphomedusae do overlap in size, scyphomedusae are typically quite a bit larger. Large representatives can reach a meter in diameter (e.g. Cyanea, Desmonema, Stygiomedusa) and the largest recorded, Cyanea arctica, reaches 2 m (Hyman 1940). Adult hydromedusae range in size from 1–2 mm to 20 cm in diameter.

      Morphological Detail and Life Histories

      General

      Most scyphomedusae are dioecious, though incidences of hermaphroditism have been found. Chrysaora hysocella initially produces male gametes and then female (Arai 1997). In most cases, the fertilized egg develops into a planula larva, which settles to the bottom and grows into the scyphozoan polyp stage, known as a scyphistoma. Scyphistomae may reproduce asexually by budding additional scyphistomae or may produce medusae by strobilation. A strobilating polyp develops transverse fissures, which separate from the stalk to form free‐swimming ephyrae (larvae), which then rapidly grow into adult medusae.