Hydrozoans vary in their feeding habits. Many trap small zooplankton with their tentacles. Some filter suspended particles (such as fish eggs and fecal pellets) from the water column. Some consume phytoplankton. A few groups contain symbiotic algae, and may get most of their nutritional needs from their symbiotes.

Pelagic hydrozoans, including siphonophore colonies and medusae, are known to show some selectivity in prey types, some taking mainly fish larvae, others taking soft-bodied invertebrates, others micro-crustaceans. They are also sensitive to chemicals produced by prey, and will move towards higher concentrations of these chemicals.

Large populations of hydromedusae may be significant ecological factors in pelagic marine ecosystems.

Foraging Behavior: filter-feeding

Primary Diet: carnivore (Eats eggs, Eats non-insect arthropods); planktivore ; detritivore