Ascidians (class Ascidiacea), commonly known as sea-squirts, are a class of sessile, filter-feeding chordates who live solitarily or colonially inside an extracellular “tunic.” Although the adults have no overt chordate features, their free-swimming tadpole-like larvae have a tail supported by a notochord, homologous to the spinal cord in vertebrates, and a hollow nerve cord like that of other chordates (Sasakura et al. 2012). The following pictures detail the process of larval settlement and metamorphosis in a colonial ascidian Distaplia occidentalis, a fouling organism on floating docks in NE Pacific marinas.
Cloney, R. A. (1978). Ascidian metamorphosis: review and analysis. In: Settlement and metamorphosis of marine invertebrate larvae. Chia, F.-S. and Rice, M.E. (eds). Elsevier. New York. pp. 255-282.
Flores, A. R. and Faulkes, Z. (2008). Texture preferences of ascidian tadpole larvae during settlement. Marine and Freshwater Behaviour and Physiology. 41(3): 155-159.
Sasakura, Y., Mita, K., Ogura, Y., and Horie, T. (2012). Ascidians as excellent chordate models for studying the development of the nervous system during embryogenesis and metamorphosis. Development, Growth, and Differentiation. 54(3): 420-437.