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A holdfast is a root
Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
-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.Natural environments and the animals that live in them can be categorized as aquatic or terrestrial ecoregion ....
 sessile
Sessility (botany)

In botany, sessility is a characteristic of plants whose flowers or leaves grow directly from the Plant stem or peduncle ....
 organisms, such as seaweed
Seaweed

Seaweed is a loose colloquial term encompassing macroscopic, multicellular, benthos ocean algae. The term includes some members of the rhodophyta, phycophyta 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. The largest and most complex marine forms are called seaweeds....
, stalked crinoid
Crinoid

Crinoids, also known as sea lilies or feather-stars, are marine animals that make up the class Crinoidea of the echinoderms . They live both in shallow water and in depths as great as 6,000 meters....
s, benthic cnidarians, and sponges, to the substrate. 

Holdfasts vary in shape and form depending on both the species and the substrate type.






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Seaweed Holdfast
A holdfast is a root
Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
-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.Natural environments and the animals that live in them can be categorized as aquatic or terrestrial ecoregion ....
 sessile
Sessility (botany)

In botany, sessility is a characteristic of plants whose flowers or leaves grow directly from the Plant stem or peduncle ....
 organisms, such as seaweed
Seaweed

Seaweed is a loose colloquial term encompassing macroscopic, multicellular, benthos ocean algae. The term includes some members of the rhodophyta, phycophyta 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. The largest and most complex marine forms are called seaweeds....
, stalked crinoid
Crinoid

Crinoids, also known as sea lilies or feather-stars, are marine animals that make up the class Crinoidea of the echinoderms . They live both in shallow water and in depths as great as 6,000 meters....
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 of the substrate.