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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)