Botryllus schlosseri, commonly known as the star ascidian or golden star tunicate, is an invasive, colonial ascidian tunicate that grows on slow-moving, submerged objects, plants, and animals in nearshore saltwater environments.
Individual zooids may grow to 3 millimetres (0.12 in) in size, with colonies reaching 50 millimetres (2.0 in) long .
Its range has spread over the last 100 years to a nearly worldwide extent. Ranging in the western Atlantic Ocean from the Bay of Fundy to North Carolina, this is "the most common colonial tunicate in North America."
This species can be distinguished from Botrylloides sp. by the pattern of zooid growth. B. schlosseri zooids emanate from a center in the manner of the arms of a star. Also, there usually are fewer zooids per cluster (5–8 in B. schlosseri and 10 or more in Botrylloides). There are many colors in which this species can be found, ranging from orange, blue and grey. A colony can be easily be separated from the main body to form an independent colony usually referred to as a subclone. Two colonies may also fuse together if they share common alleles for historecognition.
Clones of Botryllus schlosseri have been maintained in continuous laboratory culture for several decades, with new adults developing from buds that form from the body wall of existing adults. Under typical culture conditions, asexual reproduction occurs on an approximately two week cycle, during which a new bud will grow and begin to actively feed, while the adult it emerged from regresses and is eventually re-adsorbed.
When sexually productive, these Botryllus are known to produce,"yellowish-white or pale orange tadpole larva" exhibiting an oval outline.
In a study conducted by Heather Boyd, from Stanford University, it was concluded by comparing morphological and genetic experiments that "Botryllus from the Monterey and from Woods Hole, Massachusetts, may be designated contingently as Botryllus schlosseri."