nonsessile

Nonsessile is a term used in biology to describe organisms or life stages that are not permanently attached to a substrate. They are capable of locomotion or dispersal and may move freely through their environment, at least for part of their life cycle. This contrasts with sessile organisms, which remain anchored to a surface for extended periods.

In practice, nonsessile encompasses a wide range of taxa and life stages. Free-swimming animals such as jellyfish, many fish, and planktonic organisms are nonsessile. Some organisms alternate between sessile adult stages and nonsessile larval or juvenile stages—for example, barnacles start life as free-swimming larvae but later attach permanently as adults. Many crustaceans, mollusks, and echinoderms exhibit mobility throughout most of their life, while others are sessile as adults but non-sessile earlier in development.

Mobility provides ecological advantages: dispersion to new habitats, escape from predators, and access to food resources or mates. Locomotion arises from various mechanisms, including muscular movement, ciliary or flagellar activity, and hydrodynamic or limb-based propulsion. The term is primarily used in zoology and marine biology and is less commonly applied to fungi and plants, where attachment to substrates is common; when used, nonsessile implies not permanently fixed.