Solitary tunicates either live on their own, that is, apart from other individuals, or are gregarious, forming large, closely-packed clusters of the same species.  Unlike, other invertebrate aggregations, such as sea anemones Anthopleura elegantissima that are produced asexually, aggregations of sea squirts result mainly from the propensity of the larvae to seek out and to settle near adults of the same species.

Colonial sea squirts are either social or compound.  In the former, individuals of the colony have the body shape of solitary forms, but are inter-connected by basal stolons.  In compound forms each member of the colony, known as a zooid, has its own branchial siphon but shares a common atrial siphon with several other zooids usually arranged in a ring around the siphon (known as a "system"). In either case, the colony starts with a single settled larva, and subsequent growth is by asexual budding.  All zooids in a colony are genetically identical clones. 

Compound tunicates form smoothly textured, often colourful, growths on intertidal and subtidal rock surfaces.  The zooids in compound forms, such as Botryllus schlosseri, are embedded in a gelatinous matrix and are surrounded by blood vessels and ampullae. Drawings from Milkman 1967 Biol Bull 132: 229.

NOTE  lit. “animal” G. referring to an individual member of a tunicate colony

Close view of a "zooid system" of the colonial tunicate
Botryllus schlosseri. Photo courtesy Stanford University marine collection

Some colonial species, like Distaplia occidentalis, may share or partly share a common atrial siphon in compound fashion, but are connected by stolons in social fashion.  the different colours of Distaplia in the photo probably represent several different individual colonies. In the drawing on the Right, three zooids share a common atrial opening. Water flow, generated by beating cilia in the branchial basket, moves from outside the zooid, into the branchial siphon, through the branchial basket, and out the atrial siphon. Drawing from Mackie 1974 Can J Zool 52: 23.

Budding is effectively a cloning of new, smaller individual from an adult zooid, each of which has the same genetic makeup of the adult and, thus, of all the members of the colony.  Metandrocarpa taylori is a colonial tunicate of the social kind.  Its members are connected by stolons within which flow nutrients and energy material gained by feeding of any of the members. 

Note in the series below the pattern of budding that occurs after a number of larvae of Metandrocarpa are allowed to settle and metamorphose onto a piece of 15 x 15cm slate in the laboratory, and then placed in the intertidal zone off the California coast.  The colony start with 18 larvae, or 18 different genotypes.  Growth during each of 4mo in the ocean is indicated.  After 1mo, only 4 new buds have appeared (2 of these during a short stay in the laboratory). Following this, budding becomes geometric. Haven 1971 Biol Bull 140: 400.