Figure 3.34 Dimorphism of Physalia. (a) Right handed (left sailing); (b) left handed (right sailing).

The calycophorans are the Olympic swimmers of the siphonophores. They lack a pneumophore and are often much smaller than the cystonects and physonects, but are, with a few exceptions, very capable swimmers. In the diphyids particularly, the nectophores have powerful swimming muscles and face directly backward, resembling a miniature spacecraft (Figure 3.27d). Table 3.8 gives the swimming speeds of several siphonophores. The diphyids may cruise using only the smaller posterior nectophore (Bone and Trueman 1982), keeping the larger anterior nectophore in reserve for escape. Keep in mind that for small species such as Chelophyes, the escape swimming velocities are many body lengths per second.

      Buoyancy

As Mackie (1974) observed, “Cnidarians are fishermen, not hunters.” As a group, the siphonophores are the most committed to a fisherman’s lifestyle, the best examples being the very large species such as Apolemia, which are several meters in length. When observed from a submersible with their feeding tentacles extended, they even resemble a drift net, ambushing their prey in the dimly lit waters of the mesopelagic zone. Flotation is therefore particularly important to the siphonophores’ lifestyle. Neutral or positive buoyancy is achieved in two ways: by the use of floats, as in the cystonects and physonects, and by exclusion of heavy ions, as in the calycophorans.

Table 3.8 Swimming speeds (cm s⁻¹) of siphonophores as estimated in situ by divers.

      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).

The cystonects are entirely dependent on their float for buoyancy (Figure 3.35a). For genera such as Rhysophysa that remain in the midwater, the ability to adjust their position in the water column may come either from secretion of gas into the float or release of gas from it. An apical pore in the float allows for gas release. In Physalia, the large pneumophore is kept inflated as it fishes from the surface. In both species the gas within the float is carbon monoxide, secreted by gas glands intimately associated with the float.

      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

As truly open‐ocean species, siphonophores are found throughout the water column from the pleustonic surface layers to very great depth (8000 m, Vinogradov 1970). Though vertical distributions for individual species can be extensive, most siphonophores exhibit depth ranges over which they are typically most abundant. Those ranges are the same as seen with most other open‐ocean taxa: epipelagic (0–250 m), mesopelagic (250–1000 m), and bathypelagic (>1000 m) (Pugh 1999).