The four major temperate zones of the ocean surface. The Tropical Zone (TR) is delimited by the 20 °C isotherm for the coldest month. It is bordered by a Warm‐Temperate Zone (W‐T), a Cold‐Temperate Zone (C‐T), and a Cold or Polar Zone (C).
Source: Adapted from Briggs (1995), figure 55 (p. 209).
Epithelial Conduction vs. Neural Conduction
Neural pathways exist both in the zooids and in the stem of siphonophores. Communication takes place between them, affording a primitive centralization of coordination. In fact, the neural tissue in the stem of some physonects and calycophores has coalesced to form giant axons that run along the midline of the stem (Mackie et al. 1987). Figure 3.37 is a “wiring diagram” that nicely explains the neural organization of a physonect, including a visualization of the epithelial pathways.
Conductive pathways are more easily defined in the Siphonophora than in the hydromedusae and scyphomedusae. This is partially because there has been more neurophysiological research on that group but is also because the neural network in siphonophores is less diffuse, unlike the medusae with their multiple nerve nets.
Figure 3.37 Simplified wiring diagram of a physonectid siphonophore. Only ectodermal nerve pathways are included. ex. ect., exumbrellar ectoderm; mu. circ., circular muscle; mu. long., longitudinal muscle; mu. rad. vel., radial muscle of velum (“fibres of Claus”), sub. end., subumbrellar endoderm.
Source: Adapted from Mackie et al. (1987), figure 35 (p. 187).
Figure 3.38 Porpitidae. (a) Pelagic polyp colony and medusa of Porpita porpita. (b) pelagic polyp colony (“by‐the‐wind‐sailor”) and medusa of Velella velella.
Sources: (a) Bouillon (1978); (b) Bouillon (1984).
The Cnidaria Formerly Known as Chondrophora
Classification
The Chondrophora is a legacy taxonomic ranking for a unique group of hydrozoans commonly known as “By‐the‐Wind Sailors.” The name is still widely used to refer to them and will be used here. There are two genera delineating the two morphs within the group: with and without sails (Figure 3.38). Together they form the Porpitidae, a family now classified within the suborder Capitata in the order Athecata and subclass Hydroidolina.
Before any of the present schemes, the chondrophora were considered to be highly specialized members of the Siphonophora. A glance at Figure 3.38 will show you why. They are small colonies of zooids attached to the underside of a chitinous float and are most often encountered in flotillas of varying size. The flotillas form a mini‐ecosystem that is exploited by a number of molluscan species taking advantage of the “moving island” as a substrate and source of food. A chance encounter with a raft of Velella is one of the real treats of being at sea.
The reason for classification changes over the years is a shift in what was considered the dominant life stage. If the medusa is considered as the primary taxonomic determinant, the “by the wind‐sailors” group most closely align with the anthomedusae (Athecata). The best known life stage, the “by the wind‐sailors” themselves, is considered to be an aberrant or highly specialized polypoid stage.
Characteristics of the Chondrophoran Medusa
Chondrophoran medusae are very small, 3 mm tall by 2 mm wide, and therefore would be quite easy to miss. They also seem to be rare; they weren’t reported in the Atlantic until 1980 (Larson 1980). The chondrophorans are analogous to a floating, single hydranth of an athecate hydroid in structure and development as well as in some behavioral traits. For example, they exhibit whole‐organism contractile behavior known as concert behavior.
Velella medusae are brown in color due to high concentrations of zooxanthellae in the subumbrella. The presence of zooxanthellae and the fact that they were first collected in situ by blue‐water divers strongly suggest an epipelagic life habit for the medusa stage of Velella.
The medusa itself has sensory papillae on the exumbrella, four marginal bulbs, two with pairs of tentacles, one of each pair short, and a conical manubrium. The bell is cylindrical in shape with a flat apex (Larson 1980).
Evolution Within the Chondrophora
There were two schools‐of‐thought regarding the evolution of the siphonophores themselves. One supported the idea that siphonophores were highly modified medusoid organisms, giving rise by budding from the sub‐umbrella to secondary medusae and polyps (Haeckel 1866; Hatschek 1888). The other school‐of‐thought regarded siphonophores as floating colonies of hydroid polyps, showing division and specialization of labor, and budding off medusae (Leuckart 1848; Vogt 1854; Agassiz 1883).
Chondrophora were recognized as a special case by LeLoup (1929) and Garstang (1946), who believed them to be polypoid organisms showing affinities to Tubularia‐type hydroids, as they are classified today. Thus, the new scheme, which may seem to be off‐base from the morphological perspective, not only has more substance than was initially obvious but has a long history of argument behind it.
Like other Anthomedusae, Tubularia is an athecate hydroid. It does not release free medusae and its hydranth bears numerous gonophores whose eggs develop first into planulae then into actinulae before release (Figure 3.39). In contrast, both Vellela and Porpita have a free medusa stage, and these are presumably dioecious. Either the fertilized egg develops into planula‐conaria larva, or planula larvae are released from medusae to become conaria.
Feeding in the Chondrophora
Both Porpita and Velella feed on copepods, both cyclopoid and calanoid, with the size of prey increasing with the size of the predator (Table 3.10). In addition, both genera have zooxanthellae, but it is not clear what contribution the symbionts make to their overall nutrition.
Like their (however distant) relatives, the cystonect siphonophores, the chondrophores are ambush predators that move exclusively where the wind carries them, enacting no active pursuit of prey. It is assumed that prey items that blunder into their stinging tentacles are conveyed to the mouth in a way similar to feeding in the siphonophores and medusae.
Locomotion
The common name for the group, the by‐the‐wind‐sailors, is accurate and descriptive. Like the man‐o‐war Physalia, Velella and Porpita are part of the pleuston, those organisms floating at the air–water interface. Like Physalia, Velella occurs in right‐hand and left‐hand sailors, so named for the direction they will sail in when blown by a wind normal to the long axis of the sail. Porpita, which has no sail, is nonetheless still at the mercy of the wind, which drives all surface water movement.