Life in the Open Ocean. Joseph J. Torres

digestion are taken up via phagocytosis by the endodermal cells of the gastrozooid or, in some cases, can be passed along to palpons via the contractions noted above and taken up there. Further processing takes place in the endodermal cells of both structures. Eventually the cells slough off from the endoderm and disintegrate, allowing the nutrients to be absorbed by the rest of the colony via the stem canal. Indigestible parts of prey are ejected from the gastrozooids the same way they entered, out of the mouth with the help of basal contractions. Palpons expel particulates through a pore in their tip.

      Diets and Selectivity

      Diets of siphonophores correlate roughly with the suborders. Calycophorans mainly consume small copepods, whereas physonects eat larger copepods as well as larger soft and hard‐bodied prey such as crustacean larvae, amphipods, ostracods, and pteropods. Cystonects appear to specialize on fish larvae. Even Physalia, though capable of taking fish greater than 4 cm in length, feeds primarily on fish larvae 2–20 mm in size (Purcell 1984). Overall, despite Physalia’s well‐deserved reputation as potent stingers, they appear to favor small weak swimmers as prey.

      Some siphonophores appear to capture particular prey species at a greater frequency than would be expected by the abundance of those prey items in the zooplankton community. To explain the apparent selectivity, we need to use the same principles of prey capture that were applied to medusae in the Madin (1988) model described above. Selectivity can be influenced by effectiveness of nematocysts in retaining prey as well as the probability of large vs. small prey encountering a tentacle.

Table 3.6 summarizes ingestion data from a variety of species in a variety of habitats. The take‐home lesson is the substantial impact of siphonophores as predators. In sufficiently high concentrations, siphonophores are capable of enormous local impacts on the zooplankton community. Those studies that have addressed their local influence as predators (Purcell 1982; Purcell and Kremer 1983) concluded that siphonophores were the most important gelatinous predators on copepods in two quite disparate locations: Friday Harbor in Washington and Catalina Island in the California Borderland.

Table 3.6 Ingestion rates of Siphonophores.

      Source: Adapted from Mackie et al. (1987), table 15 (p. 238), with the permission of Academic Press (Elsevier).

Siphonophore	Location	Prey type	Prey abundance	Feeding rate per individual • d−1 (carbon content; caloric content)	% of prey items in diet	% of prey population consumed • d−1	References
Physalia physalis	Gulf of Mexico	Fish larvae	0.2 m−3	Avg. 120 prey	94.1	60	Purcell (1984)
Rhizophysa eysenhardii avg. 8 gastrozooids	Gulf of California	Fish larvae	Avg. 28 m−3	Avg. 8.8 prey (7300 μg C; 107 cal)	100	28	Purcell (1981a)
Sphaeronectes gracilis 38.5 ± 9.6 gastrozooids	Southern California	Copepods	Avg. 250 m−3	8.1–15.5 prey (3.9–6.2 μg C; 0.06–0.09 cal)	100	2–4	Purcell and Kremer (1983)
Muggiaea atlantica avg. 22 gastrozooids	Friday Harbor, WA	Copepods	Avg. 9121 m−3	5.5–10.5 prey (2.6–4.2 μg C; 0.03–0.05 cal)	100	0.1–0.2	Purcell (1982)
Rosacea cymbiformis avg. 40 cormidia/colony	Gulf of California	Copepods	Avg. 1495–1695 m−3	89.4 prey (616–2068 μg C; 9.4–31.5 cal)	75.4	8	Purcell (1981b)

      Ecological Importance

      Even for gelatinous species, siphonophores are exceptionally difficult to enumerate owing to their delicate colonial structure. Nets tend to reduce them to fragments; those fragments are difficult to quantify as numbers of individuals. The most appropriate techniques for evaluating numbers of siphonophores are mainly visual counts, either from diver‐based observations or for deeper‐living species, using submersible‐based observations either manned or unmanned (Remotely Operated Vehicles – ROV’s, or AUV’s – Autonomous Underwater Vehicles), Techniques include using diver‐powered meter hoops and flowmeters, counting individuals as they passed through the hoops (Purcell 1981a, b) and, in a variation of the same theme, using larger (5 m × 5 m) grids towed behind a slowly moving boat while divers count ( Biggs et al. 1981, 1984). Options using submersibles mainly include evaluating nearest‐neighbor distances (Mackie and Mills 1983) and mounting a hoop in the front of the submersible.

In the open ocean, siphonophores are found at densities of less than 1/1000 m³. However, they can number 5–10 m⁻³ (Table 3.7) in more productive coastal regions (Mackie et al. 1987). Most often they are outnumbered in the open ocean by ctenophores, but their coastal numbers can exceed those of other gelatinous forms.

      Locomotion

Siphonophores differ considerably, by suborder, in their ability to move about. The cystonects, which have an apical float but no swimming nectophores to aid in propulsion, are limited to contracting and relaxing their stem for movement. Consequently, their swimming ability is weak at best. However, members of the Cystonectae, particularly Physalia, are drifters par‐excellence (Totton 1960; Woodcock 1971; Mackie 1974), using their float as a sail to cruise the open sea. Physalia has two basic morphs, a left‐hand sailor and right‐hand sailor, mirror images of one another that sail 45° to the left and right of the wind direction (Figure 3.34). The crest of the float provides an important part of the sail; its curvature and stiffness may be adjusted to form a characteristic “sailing posture” for most effective movement (Mackie 1974).

      Many of the physonects are capable swimmers, combining a float for buoyancy and a battery of nectophores for propulsion. The genus that has received the most attention is Nanomia, a capable swimmer often observed from submersibles. Mackie et al. (1987) described three swimming modes:

      1 Synchronous forward swimming, usually considered an escape response to stimulation of the siphosome, where all the nectophores contract together for one or two cycles and produce a velocity of 20–30 cm s−1, a respectable velocity for any small swimming species.

      2 Asynchronous