Fish and other vertebrates appear to have only three arrestins: no equivalent of arrestin-2, which is the most abundant non-visual subtype in mammals, was cloned so far. The proto-chordate C. intestinalis (sea squirt) has only one arrestin, which serves as visual in its mobile larva with highly developed eyes, and becomes generic non-visual in the blind sessile adult. Conserved positions of multiple introns in its gene and those of our arrestin subtypes suggest that they all evolved from this ancestral arrestin.[12] Lower invertebrates, such as roundworm C. elegans, also have only one arrestin.