Science, Tech, Math › Animals & Nature What Is a Holdfast in Marine Life? Share Flipboard Email Print kjohansen/Getty Images Animals & Nature Marine Life Key Terms Marine Life Profiles Marine Habitat Profiles Sharks Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Forestry Dinosaurs Evolution View More By Jennifer Kennedy Marine Science Expert M.S., Resource Administration and Management, University of New Hampshire B.S., Natural Resources, Cornell University Jennifer Kennedy, M.S., is an environmental educator specializing in marine life. She serves as the executive director of the Blue Ocean Society for Marine Conservation. our editorial process Jennifer Kennedy Updated March 07, 2019 A holdfast is a root-like structure at the base of an alga (seaweed) that fastens the alga to a hard substrate like a stone. Other aquatic organisms like sponges, crinoids, and cnidarians also use holdfasts to anchor themselves to their environmental substrates, which can range from muddy to sandy to hard. Types of Holdfasts and Substrates An organism's holdfast will differ in shape and structure depending on the substrate type and the organism itself. For example, organisms that live in sandy substrates will have holdfasts that are flexible and bulb-like whereas organisms in surrounded by muddy substrates may have holdfasts that resemble complex root systems. Organisms that anchor themselves to smooth, hard surfaces like stones or boulders, on the other hand, will likely have a holdfast with a flat base. The Difference Between Roots and Holdfasts Holdfasts are different from plant roots because they do not absorb moisture or nutrients; they serve only as an anchor. The alga doesn't get nutrition from the object that it is connected to, just a way to stay stationary. For example, the southern kelp has a claw-like holdfast that attaches it to mussels, rocks and other hard surfaces. Unlike plant roots, holdfasts can outlive the organism that relied on them. For example, while the sea kelp may only live for a month or two, kelp holdfasts can live and continue to grow for up to 10 years. Holdfasts can also provide shelter for other sea creatures. The tangled system of certain types of holdfasts can protect many marine species from kelp crabs to tube worms, particularly their young.