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Ascidians (Phylum: Chordata, Class: Ascidiacea), or sea squirts, are the largest and most diverse class of the sub-phylum Tunicata (also known as Urochordata). They comprise approximately 3000 described species found in all marine habitats from shallow water to the deep sea. There are no freshwater species, and most cannot tolerate salinities below about 20‰. Recent phylogenomic studies suggest that they are actually the sister group to the vertebrates (Bourlat et al., 2006), although this conflicts with rRNA and mitochondrial data (Swalla and Smith, 2008). Adult ascidians bear little resemblance to typical chordates, though their short-lived non-feeding tadpole larvae clearly exhibit the four fundamental characteristics of the phylum: a dorsal tubular nerve cord, notochord, rudimentary pharyngeal gill slits and a post-anal tail. Another important character is the presence of the endostyle in the pharynx that will evolve as the thyroid gland in vertebrates. Following settlement, the lecithotrophic larvae undergo metamorphosis during which they lose all of these characteristics except for the endostyle and the gill slit rudiments in the pharynx (Millar, 1971), which become functional and multiply to form the branchial sac. The name "tunicate" (sub-phylum Tunicata) comes from the polysaccharide-containing tunic that envelops the animal and forms a somewhat flexible skeleton (Swalla and Cameron, 2010). Various proteins and blood cells occur in the tunic, and spicules in a few species (Monniot et al., 1991; Hirose 2009). Ascidians filter their food from the water-column via an oral siphon that brings water into the branchial sac where food items such as microalgae are filtered onto a mucus net; water, feces and gametes are expelled through an excurrent siphon. Particles suspended in the current are trapped along the wall of the branchial sac in a mucus net produced by the endostyle. The net pores range from 0.1 to 1 µm, allowing ascidians to filter even very small particulate matter, primarily in the range of 0.5 to 10 µm diameter (Bak et al., 1998; Bone et al., 2003). Several solitary species are cultured for food in Japan, Korea, France and Chile (Lambert, 2005) or extracted straight from rocky shores for human consumption or bait (Branch et al. 2010).
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