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Hydrozoan and scyphozoan medusae can have large impacts on local zooplankton prey fields, particularly when prey and predator densities, and therefore encounter probabilities, are high (Table 3.2). Most of the studies on diet and feeding in medusae have taken place in coastal systems with an eye toward describing interactions of medusae with the larvae of commercially important fish species. More general studies on field‐caught medusae (Table 3.3) reveal a varied diet that fluctuates with available prey (Mills 1995). It includes copepods, chaetognaths, fish eggs, fish larvae, larvaceans, other medusae, euphausiids, mysids, decapods, and ctenophores.

Impacts of medusae vary considerably and depend largely on predator density. Purcell and Arai (2001) demonstrated that prey‐removal rate by the hydromedusa Aequorea victoria ranged from 0.1 to 73% of available herring larvae per day from coastal waters off Vancouver Island, British Columbia, depending upon predator concentrations. Clearly, medusan predation can have a profound influence on larval survivorship, particularly when wind and wave or reproductive activity act to concentrate weakly swimming prey and gelatinous predators in one location.

The radial symmetry, stinging tentacles, and gelatinous character of medusae make them highly effective as predators, particularly as ambush predators. However, they also may find themselves as prey in the diets of other medusae. In particular, the semaeostome scyphomedusae often have hydromedusae in their diet when the smaller medusae are available in quantity, e.g. during early spring (Purcell 1991). At this time, no scyphozoan medusa is known to prey exclusively on other medusae, but it may be that the narcomedusae, the slow swimming hydromedusae important in the mesopelagic zone, specialize on other jellies (Purcell and Mills 1988).

Table 3.2 Predation rates, clearance rates, and predation effects from field observations of gelatinous predators feeding on fish eggs and larvae. Prey consumed percentages are estimated consumed daily in situ.

Source: Adapted from Purcell and Arai (2001).

Species	Size	Prey type (density)	Prey eaten (no. • pred−1 • d−1)	Clearance ratesa (no. • pred−1 • d−1)	Prey consumed (% • d−1)	References
Siphonophore
Physalia physalis	na	Larvaea (~0.2 m−3)	120	600 000	60	Purcell (1984)
Rhizophysa eysenhardti	na	Larvaea (28 m−3)	9	311	28.3	Purcell (1981a)
Medusae
Aequorea victoria	33–68 mm	Larvaec (<10 m3)	13 ± 13	5650 ± 6114	18 ± 29	Purcell (1989, 1990), Purcell and Grover (1990)
Aequorea victoria	33–68 mm	Larvaec (10–100 m−3)	55 ± 48	1357 ± 908	49 ± 35	Purcell (1989, 1990), Purcell and Grover (1990)
Aequorea victoria	33–68 mm	Larvaec (<100 m−3)	91 ± 47	288 ± 210	33 ± 32	Purcell (1989, 1990), Purcell and Grover (1990)
Aurelia aurita	6–50 mm	Larvaed (na)	1.6	na	2.6–4.4	Möller (1980)
Chrysaora quinquecirrha	40–70 mm	Eggse (avg. 164 m−3)	343 ± 419	2213 ± 1625	14 ± 4	Purcell et al. (1994)
Chrysaora quinquecirrha	40–70 mm	Larvaee (avg. 43 m−3)	86 ± 136	1818 ± 1861	29 ± 4	Purcell et al. (1994a)
Stomolophus meleagris	55 mm	Eggsb (na)	Na	3120	na	Larson (1991)
Ctenophore
Mnemiopsis Leidyi	40 ml (live vol.)	Eggse (224 ± 178 m−3)	42 ± 33	128 ± 58	9 ± 14	Purcell et al. (1994a)

na = not available

^a Calculated from data in source.

^b Mixed species or unidentified.

^c Pacific herring.

^d Atlantic herring, Clupea harengus Linnaeus.

^e Bay anchovy.

Table 3.3 Summary of data on stomach contents of field‐caught Scyphomedusae.

Source: Adapted from Arai (1997), table 3.2 (pp. 69–71).

Species (size range if available)	Common prey items (in order of predominance)	Source/references
Aurelia aurita (2.5–150 mm)	Copepods, herring, hydromedusae, crustacea, tintinnids, cladocera, rotifers (in small Aurelia)	Matsakis and Conover (1991), Mironov (1967), Möller (1980), Olesen et al. (1994)
Aurelia aurita (80–300 mm)	Copepods, veligers	Hamner et al. (1982), Kerstan (1977)
Chysaora quinquecirrha (<6 mm)	Protozoa and rotifers	Purcell (1992)
Chysaora quinquecirrha (18 to >31 mm)	Copepods, fish eggs	Purcell (1992), Purcell et al. (1994b)
Cyanea capillata (40–700 mm)	Fish larvae, ctenophores, hydromedusae	Plotnikova (1961)
Cyanea capillata	Larvacea, cladocera, fish eggs, fish larvae, copepods	Fancett (1988)
Drymonema dalmatinum	Medusae	Larson (1987b)
Pelagia noctiluca	Fish eggs, copepods, cumacea, chaetognaths	Larson (1987b)
Pelagia noctiluca	Copepods, decapods, cladocerans, chaetognaths	Giorgi et al. (1991)
Periphylla periphylla	Copepods	Fosså (1992)
Phacellophora camtschatica	Fish larvae, larvacea, gelatinous zooplankton, copepods	Purcell (1990)
Pseudorhiza haeckeli	Fish eggs, fish larvae, copepods, larvacea, decapod larvae	Fancett (1988)
Stomolophus meleagris (21–83 mm)	Veligers, copepods, tintinnids	Larson (1991)