Sessility (zoology)

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.

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 tiny larval cochineals to a new host.

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.”

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