sessile
Sessile is a term in biology with two distinct meanings:
In botany and medicine
In botany, sessile means "without a stalk", as in flowers (pedicel) or leaves (petiole) that grow directly from the stem or peduncle; however, in limnology, sessile vegetation are any organisms anchored to the benthic environment. The term is also used in medicine to describe tumors and polyps that lack a stalk, as opposed to those that are pedunculated.
In zoology
In zoology sessile animals are those which are not able to move about. They are usually permanently attached to a solid 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.
Sponges
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.
See also
 Sessile oak
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