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Hydroids fool many people by looking like feathery plants. They are actually colonies of animals living interconnected with specialized functions. They attach not only to rocks, but to surfaces such as kelp and crabs. Hydroids are related to jellyfish and have two major life stages—one sedentary (small anemones ) and one drifting (tiny jellies). The tufts you find on the beach are composed of small colonies of numerous, tiny, anemone-like creatures.
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Source: Flotsam, Jetsam and Wrack.