Life in the Open Ocean. Joseph J. Torres

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.

Limnomedusae have some of the characteristics of the Anthomedusae, Leptomedusae, and Trachymedusae. Gonads are either on the stomach wall with continuation along the radial canals or only on the radial canals. A sessile hydroid stage with or without tentacles is present but often reduced. The group was created by Kramp (1938), who named it the Limnomedusae because many of the species lived in fresh or brackish water (Russell 1954). Common genera include: Aglauropsis, Olindias, Cubaia, Vallentinia, Proboscidatyla, Craspedacusta. The life cycle is illustrated in Figure 3.8b.

Figure 3.7 (a) The hydrozoan polyp Coryne eximia with newly liberated medusa Coryne eximia; (b) a single hydrozoan medusa Coryne eximia produced by its polyp stage. Note the capitate (clublike) tentacles on the polyp.

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

Trachymedusae have a hemispherical or deep bell‐shaped umbrella with gonads usually confined to the radial canals. The life cycle lacks a polyp stage (Figure 3.8c). Tentacles are solid, and the subumbrellar surface exhibits a heavy musculature suggesting a strong swimming ability (Figure 3.9a). The velum is usually well developed. Common genera include: Liriope, Rhopalonema, Colobonema, Pantachogon, Crossota, Aglantha, Aglaura, and Halicreas. The trachymedusae are exclusively oceanic.

Narcomedusae are the most bizarre of the hydromedusae (Figure 3.9b). The sides of the umbrella are divided by peronial grooves so that the umbrellar margin may be lobed. They have a broad circular stomach that covers much of the subumbrellar surface, sometimes with peripheral pouches. Solid tentacles originate above the umbrellar margin. No radial canals are present. Narcomedusae are strictly oceanic. They have no true hydroid stage but may have parasitic larval development (Russell 1954). Common genera include Solmissus, Aegina, Aeginura, and Solmaris.

Figure 3.8 Life cycles of hydrozoans. (a) Obelia, a thecate hydroid with free medusae; (b) Limnocnida, a trachyline hydrozoan with a polypoid stage; (c) Aglaura, a trachyline hydrozoan without a polypoid stage.

      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).

Figure 3.9 Hydrozoan medusae. (a) The trachymedusa Aglaura hemistoma. (b) The narcomedusa Aegina citrea.

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

The Scyphomedusae

      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).

Scyphomedusae differ from Hydromedusae in that Scyphomedusae have no velum, the “skirt” that extends from the umbrellar margin into the subumbrellar space (see Figure 3.3). Additionally, they have gastric filaments in the digestive system, and the embryological origin of the gonads is endodermal (see Chapter 5 for a description of embryological germ layers). Symmetry in the Scyphomedusae is markedly tetramerous in both the polyps and medusae. This characteristic is most obviously manifested by the presence of four oral arms in many species and in the organization of the gastric system (Figure 3.10). The four oral arms lead to the lips of a central mouth that connects to the stomach via a manubrium or oral tube. Cnidoblasts on the oral arms aid in capturing prey and in defense. As in the Hydromedusae, nutrition is conveyed from the stomach to the periphery via radial gastrovascular canals. The position of the canals relative to the axis defined by the oral arms determines their name. Thus, the perradial canals lie in the same axis as the oral arms and the lips of the mouth and are considered the primary axes. In the middle of the quadrants defined by the perradial canals lie the interaradial canals. Finally, the adradial canals bisect the space between the perradial canals and the interradials. The body wall consists of an outer epidermis and inner gastrodermis separated by mesoglea. The mouth is the only opening to the digestive system; all food, waste, and gametes must move through the same opening.

Figure 3.10 Scyphomedusae. (a) General anatomy of a scyphomedusa. (b) Oral view of a scyphomedusa. (c) The scyphomedusan Chrysaora lactea.

      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.