Holdfast (biology)

For other uses, seeHoldfast (disambiguation).

Aholdfastis aroot-like structure that anchorsaquaticsessileorganisms, such asseaweed,other sessilealgae,stalkedcrinoids,benthiccnidarians,andsponges,to thesubstrate.

Holdfast torn from the sea floor by a storm

Eocrinoidholdfasts on anOrdovicianhardgroundinUtah

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. These projections are calledhapteraand similar structures of the same name are found onlichens.The holdfasts of organisms that live in sandy substrates are bulb-like and very flexible, such as those ofsea pens,thus permitting the organism 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 flattened bases which adhere to the surface. The organism derives no nutrition from this intimate contact with the substrate, as the process of liberating nutrients from the substrate requiresenzymaticallyeroding the substrate away, thereby increasing the risk of the organism falling off the substrate.

The claw-like holdfasts ofkelpsand other algae differ from the roots ofland plants,in that they have no absorbent function, instead serving only as an anchor.