Sessility (motility)

Generally sessile Hydra attached to a substrate

In biology, sessility (in the sense of positional movement or motility) refers to organisms that do not possess a means of self-locomotion and are normally immobile. This is distinct from the second meaning of sessility which refers to an organism or biological structure attached directly by its base without a stalk.

Sessile organisms can move through outside sources (such as water currents) but are usually permanently attached to something. Organisms such as corals lay down their own substrate from which they grow. Other organisms grow from a solid such as a rock, dead tree trunk, or a manmade object such as a buoy or ship's hull.^[1]

Mobility

Sessile animals typically have a motile phase in their development. Sponges have a motile larval stage, which becomes sessile at maturity. In contrast, many jellyfish develop as sessile polyps early in their life cycle. In the case of the cochineal, it is in the nymph stage (also called the crawler stage) that the cochineal disperses. The juveniles move to a feeding spot and produce long wax filaments. Later they move to the edge of the cactus pad where the wind catches the wax filaments and carries the cochineals to a new host.

Reproduction

Many sessile animals, including sponges, corals and hydra, are capable of asexual reproduction in situ by the process of budding. Sessile organisms such as barnacles and tunicates need some mechanism to move their young into new territory. This is why the most widely accepted theory explaining the evolution of a larval stage is the need for long-distance dispersal ability. Wayne Sousa's 1979 study in intertidal disturbance added support for the theory of nonequilibrium community structure, “suggesting that open space is necessary for the maintenance of diversity in most communities of sessile organisms.”^[2]

Clumping

Blue mussels, Mytilus edulis, are sessile and exhibit clumping.

Clumping is a behavior in sessile organisms, in which individuals of a particular species group close to one another for beneficial purposes, and can be seen in coral reefs and cochineal populations. This allows for faster reproduction and better protection from predators.^[3]

Predominance in coastal environments

The circalittoral zone of coastal environments and biomes are dominated by sessile organisms such as oysters. Carbonate platforms grow due to the buildup of skeletal remains of sessile organisms, usually microorganisms, which induce carbonate precipitation through their metabolism.

Distinction from anatomical and botanical sessility

In anatomy and botany, sessility refes to an organism or biological structure that has no peduncle or stalk. See Peduncle (anatomy), Peduncle (botany) and Sessility (botany). A sessile structure has no stalk.

See also

• Anthozoa
• Ediacara biota

References

1. Pechenik, Jan (2016). Biology of the Invertebrates. ISBN 9781497006515.
2. WP. Ecology. 1979. Disturbance in Marine Intertidal Boulder Fields: The Nonequilibrium maintenance of species diversity. http://bio.classes.ucsc.edu/bio170/Readings/sousa%201979.pdf
3. James H. Thorp; Alan P. Covich (2001). Ecology and Classification of North American Freshwater Invertebrates. Academic Press. p. 213. ISBN 0-12-690647-5.