A species of ascidians, commonly called sea squirts. As adults they are hermaphrodites that live as filter feeders in shallow seas. They are sometimes called tunicates because each is enclosed in a tough tunic made up of tunicin, a cellulose-like fiber. Fertilized eggs develop into free-swimming larvae, and each tadpole has a prominent notochord and dorsal nerve tube. For this reason, they are classified as Urochordates, a primitive branch of the Acraniata (q.v.). Ciona has 14 chromosomes and a genome size of about 156 mbp. Its 15,900 ORFs are more closely packed than those of most vertebrates. About 1/6 of the Ciona genes have vertebrate homologs. The Ciona homologs are generally present in only a few copies compared to the multiple copies found in vertebrate gene families (i.e., actin genes or myosin genes). See Classification, Eumetazoa, Deuterostomia, Chordata, Acraniata, Urochordata; Chronology, 2002, Dehal et al.; isoforms, tunicin.
Subjects: Genetics and Genomics.