Sessility (zoology) in the context of "Meroplankton"

The Ediacaran (/ˌiːdiˈækərən/ EE-dee-ACK-ər-ən; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (c. 635–538.8 Mya). These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

The Ediacaran biota may have undergone evolutionary radiation in a proposed event called the Avalon explosion, 575 million years ago. This was after the Earth had thawed from the Cryogenian period's extensive glaciation. This biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have almost completely replaced the organisms that dominated the Ediacaran fossil record, although relationships are still a matter of debate.

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

Ascidiacea, commonly known as the ascidians or sea squirts, is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer test or "tunic" made of the polysaccharide cellulose.

Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea (salps, doliolids and pyrosomes) and Appendicularia (larvaceans) swim freely like plankton, sea squirts are sessile animals after their larval phase: they then remain firmly attached to their substratum, such as rocks and shells.

Archaeocyatha (/ˈɑːrkioʊsaɪəθə/, "ancient cups") is a taxon of extinct, sessile, reef-building marine sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is thought that the centre of the Archaeocyatha origin is now located in East Siberia, where they are first known from the beginning of the Tommotian Age of the Cambrian, 525 million years ago (mya). In other regions of the world, they appeared much later, during the Atdabanian, and quickly diversified into over a hundred families. They became the planet's first reef-building animals and are an index fossil for the Lower Cambrian worldwide.

Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialized microtubules to ensnare and manipulate their prey. They live in both freshwater and marine environments, including some that live on the surface of aquatic animals, and typically feed on other ciliates. Instead of a single cytostome, each cell feeds by means of several specialized tentacles. These are supported by microtubules and phyllae, and have toxic extrusomes called haptocysts at the tip, which they attach to prey. They then suck the prey's cytoplasm directly into a food vacuole inside the cell, where they digest and absorb its contents. Most suctoria are around 15–30 μm in size, with a non-contractile stalk and often a lorica or shell.

Suctoria reproduce primarily by budding, producing swarmers that lack both tentacles and stalks but have cilia. They may also reproduce through conjugation, which is peculiar in involving cells of different size and often involves total fusion. The way buds form is the primary distinction between different orders of suctoria. Among the Exogenida, including common genera like Podophrya and Sphaerophrya, they appear directly on the cell surface. Among the Endogenida, for instance Tokophrya and Acineta, they form in an internal pouch and escape through an opening—and among the Evaginogenida, they form in a pouch that inverts before they are released.

A tubeworm is any worm-like sessile invertebrate that anchors its tail to an underwater surface and secretes around its body a mineral tube, into which it can withdraw its entire body.

Tubeworms are found among the following taxa: