Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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in parentheses following Gulf of Mexico seasons and North Atlantic zones are the number of SCUBA dives.

Schematic illustration of dimorphism of Physalia.

      Buoyancy

      Source: Field Studies of Fishing, Feeding, and Digestion in Siphonophores, D. C. Biggs, Marine & Freshwater Behaviour & Physiology, 1977, table 1 (p. 263). Reprinted by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).

Species Number measured Undisturbed (“normal” speed) Escape speed
Agalma okeni 27 2–5 10–13
Nanomia bijuga 2 25
Forskalia spp. 10 1–3 2–5
Stephanophyes superba 3 10–15
Rosacea cymbiformis 5 1–3 3
Sulculeolaria monoica 5 2–5 12–16
Chelophyes appendiculata 6 7–16 23
Diphyes dispar 3 1–3 5–10

      The physonects depend to a varying degree on the float for buoyancy. Streamlined genera such as Nanomia that have much of their mass in the nectophores and little gelatinous tissue below them (Figure 3.35b) are apparently quite dependent on the float to maintain their position in the midwater, being negatively buoyant without it. In contrast, genera such as Agalma with considerable gelatinous tissue below the nectophores (Figure 3.35c) depend to a much greater degree on the lift provided by the gelatinous tissue.

      Calycophorans, having no float, must swim constantly, sink, or rely on their own tissues for lift. Jacobs (1937) first demonstrated that the gelatinous tissues of siphonophores provided lift, and later studies (Robertson 1949; Bidigare and Biggs 1980) provided the mechanism: selective replacement of heavier ions within the tissue, notably the replacement of sulfate with chloride. The lift provided by sulfate exclusion is sufficient in siphonophores and other nearly neutral gelatinous species to enable station‐keeping in the water column. It is worth noting that other important pelagic groups, such as the Crustacea (e.g. Sanders and Childress 1988), also exploit sulfate exclusion as a buoyancy mechanism.

      Vertical Distribution

Schematic illustration of degrees of dependence on gelatinous parts for flotation.

      Source:


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