During the past two decades enormous progress has been achieved in the fields of development, evolution, immunology, natural products and ecology of ascidians. Their small genome, small cell number and (usually) short life-cycle make them an attractive model system for developmental biologists (Dehal et al., 2001; Nishida and Sawada 2001). Investigating the phylogenetic position of the subphylum Tunicata in relation to the other subphyla in the phylum Chordata is crucial to the understanding of possible mechanisms of chordate evolution (Swalla et al., 2000; Zeng and Swalla, 2005). The study of self/non-self recognition in ascidians provides important information regarding the evolutionary origin of the vertebrate immune system (Khalturin and Bosch, 2007). In addition, ascidians provide a fertile ground for studies in the field of natural products (Wang et al., 2007 for review), and play an important role in marine bioinvasions across the globe (Locke and Carman, 2009).
Kowalevsky discovered the chordate nature of the ascidian tadpole larva in 1866; the Ascidiacea were then reclassified as chordates rather than as molluscs (Raff and Love, 2004). Following Lahille (1886), the class Ascidiacea is now divided into three orders based on the structure of the adult branchial sac: Aplousobranchia (colonial), Phlebobranchia and Stolidobranchia. This is the current designation used by most ascidian taxonomists. All ascidians are hermaphrodites, having both male and female gonads, though many are not self-fertile. There are both solitary and colonial species; numerous excellent anatomical illustrations can be found in Monniot et al. (1991). Ascidian systematics is the domain of specialists, but a keen observer can differentiate the orders and most families, and recognize the well-described common species. Although you “cannot judge an ascidian by its cover”, underwater photographs can provide much information and assist in distinguishing one species from another (Monniot et al., 1991) especially if the fauna of the given region has already been studied. The invaluable monographs published by Van Name (1945), C. and F. Monniot (Monniot et al., 1991; Monniot and Monniot, 1996, 2001 and many others) and Kott (1985, 1990, 1992, 2001 and others) greatly assist in identifying the ascidian fauna worldwide to species level. Tabular keys to the families and genera of the world can be found in Monniot et al. (1991). A comprehensive listing of nearly all the publications on ascidians since 1995 can be found by clicking on the various newsletters listed at http://depts.washington.edu/ascidian