Sedentary Animal

any one of the mostly marine bottom-dwelling animals attached to a substrate as adults. Sedentary animals include many protozoans, for example, some foraminifers, heliozoans, and infusorians (including vorticellae and stentors). Also sedentary are scyphozoans, sponges, hydroids, and coral polyps—many of them living in the tubes of polychaetous worms. Other sedentary animals are barnacles, some bivalve mollusks (for example, oysters and tridacnas), all bryozoans, brachiopods, phoronids, pterygobranchiates, pogonoforans, stalked crinoids, and ascidians.

Many sedentary animals are colonial forms, but solitary species frequently form large masses. Adaptations to a sedentary mode of life characteristic of many of these animals include the presence of a solid calcareous skeleton, hermaphroditism, increased fecundity, and the presence of tentacles to trap prey or devices to filter food particles from the water (sediment feeders). The transition to the sedentary mode of life frequently led in the course of evolution to a simplification of organization and to the origin of radial symmetry. Attachment to other animals may be the initial stage of transition to parasitism.

Sometimes included among sedentary animals are certain almost motionless, animals that are not attached to a substrate, for example, Mya mollusks, which pass their entire lives in a single hole, and some burrowing animals.