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A fundamental characteristic in the classification of the hydromedusae is the presence or absence of a polyp stage. Anthomedusae, Leptomedusae, and Limnomedusae all have a fixed polyp stage, whereas the Trachymedusae and Narcomedusae exhibit direct development. Considerable diversity is evident in the polyp stages (Figure 3.6).

Anthomedusae are characterized as usually having a tall bell with gonads mainly on the stomach and manubrium or extending slightly along the radial canals. The hydranths (feeding polyps) of Anthomedusae are athecate, i.e. they lack a surrounding sheath (Figures 3.6b and 3.7), thus giving the group its secondary name of Athecata or Anthoathecata. Common genera include: Coryne, Bouganvillia, Podocoryne, Cladonema, Amphinema, and Leuckartiara.

Figure 3.6 Anatomy of hydrozoan polyps. (a) Composite diagram of a thecate hydroid colony showing different reproductive structures. (b) composite diagram of reproductive structures in an athecate hydroid colony; (c) cross section of a thecate gastrozooid (hydranth).

Sources: (a) Adapted from Bayer and Owre (1968), figure 55 (p. 38); (b) Bayer and Owre (1968), figure 54 (p. 38); (c) Bayer and Owre (1968), figure 53 (p. 38).

Leptomedusae are more dorsoventrally compressed with gonads only on radial canals. Common 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.