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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.
Most of the 10,000 species of sponges are marine animals; only about 100 species live in fresh water. Sponges are sessile animals that come in a wide variety of sizes and shapes and are adapted to different movement patterns of water. Sponges reproduce both sexually and asexually. In most species, a single individual produces both egg and sperm, but individuals do not self-fertilize. Water currents carry sperm from one individual to another. Asexual reproduction is by budding and fragmentation.
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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