Despite their protective stinging cells, hydrozoans are prey for many types of predators. A variety of snails and worms graze on polyps and stolons, as do some fish and crustaceans. Fish also consume medusae and pelagic colonial hydrozoans, as do some sea turtles (especially leatherbacks), ctenophores, and other cnidarians, including larger hydrozoans.
A variety of predators have the ability to consume the stinging cells of hydrozoans without triggering them. These predators then sequester the stinging cells in their body to defend them against their own predators. Nudibranchs are particularly well known for this ability, but some species of ctenophores, turbellarian flatworms, and priapulids can store cnidocysts as well.
Nearly all hydrozoans protect themselves with their cnidocysts. Some colonial species have specialized polyps that grow large tentacles armed with dense batteries of these stinging cells or grow large rigid spines. Many colonial polyps secrete a rigid protective layer over stolons and polyp tubes. This layer is often made of chitin, some groups produce a mineral skeleton. Free-swimming medusae cannot use rigid protection, but do defend themselves with stinging cells. There is evidence that some also contain toxic compounds that discourage predators from eating them. Most hydrozoan medusae also follow the diel migration pattern common to many planktonic organisms -- sinking below the limit of light penetration to avoid visual predators during the day, and then rising towards the surface at night in pursuit of prey.