Life in the Open Ocean. Joseph J. Torres

Medusae of the World). Both authors built on the work of luminaries preceding them, most notably A. G. Mayer’s three‐volume Medusae of the World. Recently, the benchmark works of Kramp and Russell have been supplemented with a new, multiauthor, treatment on the South Atlantic Zooplankton (Boltovskoy 1999) that gives a very nice summary of the systematics of the Cnidaria, though as in all things systematic it is not one that is universally accepted. Classification within the Cnidaria is very well treated by Mills et al. (2007) in the new Light and Smith’s Manual (J. T. Carleton, ed.).

      The classification scheme above includes recent consensus views on major groups within the Cnidaria and how they are related. Two groups stand out as unusual within the hydromedusae: the siphonophores and the “by‐the‐wind‐sailors” (sometimes known as the chondrophores). We will treat them separately because their biology is different from the remainder of the hydromedusae, even though they are now considered to be an order (Siphonophorae) and a family (Porpitidae) within the hydromedusae.

      If all the classification schemes are considered together, including those of Kramp and Russell, five basic divisions within the medusae of the hydromedusae are apparent: the Anthomedusae, the Leptomedusae, the Limnomedusae, the Trachymedusae, and the Narcomedusae. Species in the first three groups have a polyp stage in most instances and thus show alternation of generations. The second two groups show direct development. We will consider the medusae within the Hydromedusae to have the five basic divisions noted above, which most closely approximate the classical literature. Whether those divisions are orders or suborders is less important than the fact that the hydromedusae can be readily segregated morphologically using those divisions, and they have been for the better portion of 100 years.

The Hydromedusae

      Morphology Basics

The Cnidarians are all radially symmetrical (Figure 3.2); their parts are arranged around a central axis much like the spokes of a bicycle wheel are arranged around the hub. Each of the radii has names that are used in classification. The central axis or hub is the oral–aboral axis or gut. Between the “hub” and the “rim,” body parts are arranged concentrically (Figures 3.3 and 3.4). In perfect radial symmetry, a plane bisecting the animal in the axis of the hub will result in two perfectly equal halves, no matter where the plane is placed. Most often, and even in the hydromedusae, the morphology is modified to biradial (two equal halves) or tetramerous (four equal quarters) radial symmetry. The radial symmetry of medusae is important to their foraging strategy. Prey are equally accessible from all points of the compass, which, for an ambush predator, is a distinct advantage.

Figure 3.3 shows the basic body form of a hydromedusa. The outer, or exumbrellar, surface and the inner, subumbrellar, surface are separated by a deformable gelatinous acellular mesoglea. The mesoglea acts as a primitive skeleton, giving a medusa its characteristic shape and providing a surface against which muscle tissue can act to propel the medusa through the water.

Figure 3.2 Diagram to define the radii of a medusa. P. are the per‐radii on which the four primary radial canals lie. I. are inter‐radii and A. are ad‐radii.

      Source: Adapted from Russell (1954), figure 1 (p. 2).

Figure 3.3 Diagram of a medusa with one quadrant removed.

      Source: Adapted from Russell (1954), figure 2 (p. 3).

Swimming bells range in shape from tall to spherical to highly flattened in appearance (Figure 3.5a, Russell 1954). The stomach is a simple sack that projects downward from the subumbrellar surface and can vary considerably in length and morphology. The opening of the stomach or “lips” may vary from a simple circular opening to a highly crenulated and folded appearance. In some cases, the mouth is surrounded by small oral tentacles (Figure 3.5b).

Figure 3.4 Cnidarian structure. (a) Diagram of a hydroid polyp; (b) cross section of a hydroid polyp; (c) diagram of a hydrozoan medusa; (d) cross section of a hydrozoan medusa.

      Source: Adapted from Hyman (1940), figure 106 (p. 368)

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The radial canals originate at the four corners of the stomach and extend along the subumbrellar surface to join the ring canal that runs along the umbrellar margin. The umbrellar margin contains the marginal tentacles that are primarily used for subduing prey. The stomach, radial canal, and ring canal system provide for the distribution of nutrients to the medusa as a whole.

Figure 3.5 Medusae structure. (a) Diagrams of umbrella shapes of different medusae; (b) diagrams of mouths of different medusae.

      Sources: (a) Russell (1954), figure 3 (p. 4); (b) Russell (1954), figure 55 (p. 5).

      Morphological Detail and Life Histories

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