Botryllus schlosseri

The "Golden Star Tunicate" Botryllus schlosseri is an invasive, colonial ascidian tunicate that grows on slow-moving, submerged objects, plants, and animals in nearshore saltwater environments.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 commmon colonial tunicate in North America." This species can be distinguished from Botrylloides sp. by the pattern of 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).
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A clonal hermaphrodite (Phylum Chordata, Subphylum Tunicata, Class Ascidian=sea squirts). It grows fast, reproduces weekly, and thus, a good model for genetic studies of protochordates. It also has a well-characterized allorecognition system called Fu/HC whose functions are prevention of fusion with non-kin and selective fertilization by sperm bearing Fu/HC alleles different from that of the egg. See a webpage about Botryllus.