Life in the Open Ocean. Joseph J. Torres

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Figure 3.25 Examples of the three suborders of siphonophores. Cystonectae; (a) Rhizophysa and (b) Physalia, respectively. Physonectae; (c) Physophora and (d) Agalma, respectively. Calycophorae; (e) Muggiaea and (f) Nectocarmen, respectively.

      Sources: (a) Pugh (1999), figure 3.2 (p. 495); (b) Pugh (1999), figure 3.1 (p. 495); (c) Pugh (1999), figure 3.16 (p. 497); (d) Kaestner (1967), figure 4‐35 (p.75); (f) Adapted from Alvarino (1983), figure 1 (p. 342).

Gastrozooids are the only members of a siphonophore colony that can ingest food and are sometimes called “siphons.” The name siphonophore means “siphon‐bearer,” a siphon in Greek and Latin being a tube or pipe. Gastrozooids have a tubular polyp‐like shape but no fringing tentacles at the mouth. Instead, a single long highly contractile tentacle emanates from the base with many side branches or tentilla (Figure 3.26a). The tentilla often terminate in distinctive structures thought to resemble the “prey of the prey” of the siphonophore, as well as in nematocyst batteries.

      Dactylozooids resemble gastrozooids without a mouth (Figure 3.26b). They usually have a simple basal tentacle instead of one bearing tentilla and are often called palpons. Dactylozooids may sometimes resemble nothing more than a large, particularly robust tentacle, particularly when they are associated with gonozooids. In those cases they are known as gonopalpons.

Table 3.5 Classification of the order Siphonophora.

      Source: Totton (1965), A Synopsis of the Siphonophora, British Museum of Natural History.

Gonozooids, which are polypoid in origin, are the structures that bear the reproductive gonophores, which are medusoid. They usually are branched stalks called gonodendra (Figure 3.26c)

Medusoid zooids comprise the swimming bells or nectophores (Figure 3.27), the bracts (Figure 3.28), the gonophores, and the gas‐filled float or pneumatophore (Figure 3.29).