A holdfast is a root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

-like structure that anchors aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 sessile

Sessility (botany)

In botany, sessility is a characteristic of plants whose flowers or leaves are borne directly from the stem or peduncle, and thus lack a petiole or pedicel...

 organisms, such as seaweed

Seaweed

Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

, other sessile algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, stalked crinoid

Crinoid

Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...

s, 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 pen

Sea pen

Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...

s, 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.