Holdfasts are sturdy, but they can be easily ripped from the substrate by strong waves. During storm seasons, such as El Nino events, thousands of holdfasts are torn apart. The entire thallus—the complete seaweed—is set adrift in the ocean.

Photograph courtesy Claire Fackler, NOAA National Marine Sanctuaries

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Kelp is a giant seaweed that grows in two forms along the west coast of North America—giant kelp and bull kelp.