alt="Schematic illustration of life cycles of hydrozoans."/>
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. Strobilation takes two forms, monodisk strobilation, 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 Coronatae are deep‐dwelling jellyfish mainly inhabiting mesopelagic to bathypelagic depths, though some genera (Linuche, Nausithoe) are found in surface waters. Each has a characteristic deep furrow, the coronal groove, which divides the exumbrella into a central disc and a peripheral zone and gives the order its name (Figure 3.11). Below the coronal groove, the peripheral zone is divided into two subregions. First, the thickened deeply grooved pedalia, and below the pedalia and centered on the inter‐pedaliar grooves, the marginal lappets. The pedalia most often have a single, solid tentacle. Coronate medusae are heavily pigmented, usually deep burgundy in color, and most species have stiff noncontractile tentacles that are often held above the bell.
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.
Figure 3.11 Coronatae. (a) The coronate scyphomedusa Periphylla (helmet jellyfish). (b) The coronate scyphomedusa Nausithoe (crown jellyfish).
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