Sessile is a term in biology with two distinct meanings:
In botany
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.
In zoology
In zoology sessile means "simpel animal", sessile animals are those 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.
Sessile animals
Sponges Most of the 7000 spices of sponges are marine animals; only about 50 spices live in the fresh water. Sponges are sessile animals that come in a wide variety of sizes and shapes that are adapted to differnt movement patterns of water. Sponges reproduce both sexually and asexually. In most species, a singel 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. Scientists at the Norwegian university U.I.O, have results from experiments that show that over 75% of the sponges reproduce asexually.
Clumping is a behavior in an animal, usually sessile, in which individuals of a particular species group close to one another for beneficial purposes. kate