A holdfast is like a root for seaweed

Close-up of kelp's anchoring holdfast threads
Tim Ridley/Dorling Kindersley/Getty Images

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 provide protection to many marine species from kelp crabs to tube worms, particularly their young. 

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Kennedy, Jennifer. "Holdfast." ThoughtCo, Aug. 23, 2017, thoughtco.com/holdfast-definition-2291716. Kennedy, Jennifer. (2017, August 23). Holdfast. Retrieved from https://www.thoughtco.com/holdfast-definition-2291716 Kennedy, Jennifer. "Holdfast." ThoughtCo. https://www.thoughtco.com/holdfast-definition-2291716 (accessed January 20, 2018).