A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic 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, while those of organisms that live in sandy substrates are bulb-like and very flexible, such as the holdfast of sea pens, allowing the organism(s) to pull the entire body into the substrate when the holdfast is contracted. The holdfasts of organisms that live on smooth surfaces (such as the surface of a boulder) have the base of the holdfast literally glued to the surface. The organism derives no nutrients from this intimate contact with the substrate, primarily because if the organism attempted to extract nutrients enzymatically from the substrate, the substrate would be eroded away, thereby increasing the risk of falling off the substrate.