Life in the Open Ocean. Joseph J. Torres
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Название: Life in the Open Ocean

Автор: Joseph J. Torres

Издательство: John Wiley & Sons Limited

Жанр: Биология

Серия:

isbn: 9781119840312

isbn:

СКАЧАТЬ href="#fb3_img_img_0f8632e4-783c-53b9-9078-53195334dfc5.gif" alt="Schematic illustration of the four major temperate zones of the ocean surface."/>

      Source: Adapted from Briggs (1995), figure 55 (p. 209).

      Epithelial Conduction vs. Neural Conduction

Schematic illustration of simplified wiring diagram of a physonectid siphonophore.

      Source: Adapted from Mackie et al. (1987), figure 35 (p. 187).

Schematic illustration of porpitidae.

      Sources: (a) Bouillon (1978); (b) Bouillon (1984).

      Classification

      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.

      Feeding in the Chondrophora

      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