Most organisms are motile, but some important ones, like corals and sponges, are sessile. Sessile organisms use passive feeding methods and have developed ways to protect themselves from predators. Sessile marine organisms have been common since the dawn of multicellular life.
Sessile organisms stay in one place, while motile organisms are mobile. Most organisms are motile, but many important organisms, including corals, sponges, barnacles, tunicates, bryozoans, polychaete worms, some bivalves, and most brachiopods are sessile. Naturally, all land plants remain in one place. Some animals have motile larval stages and sessile adult stages, or vice versa.
Animals that remain in place should use passive feeding methods, especially filter feeding. Sessile plants use photosynthesis for energy in all but rare cases. Animals have developed a variety of interesting means of extracting nutrient fragments from the water, where they almost always live: tentacles, filters and pumps. Mobile animals, which are by far the most common and complex, have at their disposal a much greater number of means of obtaining food, but at the same time their nutrient requirements are greater.
Sessile marine organisms have been extremely common since the dawn of multicellular life. Most of the early animals, which made up a set called the Ediacaran fauna, were sessile. During the Ordovician period about 480 million years ago, there was an increase in the number of filter-feeding organisms, suggesting that concentrations of small marine animals (plankton) became more abundant during the period. Immediately prior Cambrian animals were mostly scavengers or bottom predators.
Organisms that stay in one place need an effective way to protect themselves from predators: they can’t swim away. Usually these are structural elements made up of chemicals such as calcium carbonate or silica, or for plants, lignin (bark). The tools of choice are the shell and the nematocyst (stinging cell), the latter popular with cnidarians (like coral) and the former among others. Some sessile animals, such as the Pompeii worm found around volcanic vents on the sea floor, build a long tube around themselves as they grow and are able to retreat into it. Sponges are one of the few sessile organisms with no obvious defense mechanisms, except for their low nutritional value and stomach-irritating spicules (ears) made of calcium carbonate or silica.
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