Hydrozoans are both predators and prey for many marine organisms, and large seasonal blooms of medusae may strongly affect local fish and zooplankton populations. Some species of polyps are hosts for symbiotic algae, and many large pelagic forms have symbiotic hyperiid amphipods living on or in them. There is even a small species of fish, Nomeus gronovii, that lives in association with Portuguese man-o-wars. Some polyp colonies grow on the shells of hermit crabs, providing them protection.
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