Holdfast

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For the woodworking tool used to hold down wood on a workbench, see hold fast

A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, colonial cnidarians, and sponges, to the substrate.

Holdfasts vary in shape and form depending on both the species and the substrate type. The holdfasts of organisms that live in muddy substrates often have complex tangles of root-like growths. The holdfasts of organisms that live on smooth surfaces (such as the surface of a boulder) have the ends literally glued to the surface, in some cases, the holdfast simply being the actual base of the organism's stalk glued to the surface.