In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate.

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. Many sessile animals, including sponges, corals, and hydra, are capable of asexual reproduction in situ by a process of budding.

Clumping is a behavior in an animal, usually sessile, in which individuals of a particular species group close to one another for beneficial purposes.

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