Tunicata (Sea Squirts and Salps)

Tunicates are suspension-feeding, solitary or colonial, marine invertebrates with sessile (e.g., sea squirts) or planktonic habit (e.g., salps). The body is compact like a barrel and covered by a gelatinous tunic (Ruppert, Fox, and Barnes 2004a). Although individuals have no evident segmental traits, a few structures in the adult pharynx, in the larval notochord, and in the circular musculature of some planktonic forms show a segmental arrangement.

In adults, the pharynx is basket-shaped with perforated walls whose function is central for nutrition, gas exchange, and excretion. These openings are often arranged in transverse rows along the main axis of the pharynx (Garstang 1892) and are intercalated by transverse circulatory vessels (Shimazaki, Sakai, and Ogasawara 2006), as well as transverse nerves (Burighel et al. 2001; Zaniolo et al. 2002). Adult zooids of some colonial species can also reproduce asexually by forming segmental epidermal constrictions in the abdomen (Brown and Swalla 2012).

In larval stages we find the notochord, a highly ordered stack of cylindrical cells that support the tail (Miyamoto and Crowther 1985) where each cell shows repeated morphology and anteroposterior polarity (Jiang, Munro, and Smith 2005; Hannibal and Patel 2013). Ascidian larvae also have a series of dorsal and ventral epidermal sensory neurons in the tail, but their arrangement is barely regular (Crowther and Whittaker 1994; Pasini et al. 2006). In general there are no segmental traits in the central or peripheral nervous system of ascidian larvae (Imai and Meinertzhagen 2007a. 2007b).

During development, the notochord is not formed progressively by terminal growth but by the rearrangement of forty progenitor cells that interdigitate and self- organize into an ordered single column (Miyamoto and Crowther 1985; Jeffery and Swalla 1997). This segmental arrangement, however, is transient; it disappears once the intercellular spaces between notochord cells fuse and form a hollow tube (Jeffery and Swalla 1997).

The formation of the first gill slits during development also involves a series of cellular rearrangements at the site of contact between ectodermal and endodermal epithelial sheets (Willey 1893; Manni et al. 2002). These primary gills elongate and subdivide increasing the number of openings per row, and entire rows can also subdivide generating the multiple transverse rows present in adult animals (Willey 1893; Berrill 1947; Shimazaki, Sakai, and Ogasawara 2006).

Additionally, in planktonic tunicates such as doliolids and salps (Thaliacea) the body is marked by a series of evenly spaced bands of circular muscles distributed along the anteroposterior axis. This hooplike musculature pumps water through the pharynx creating a jet thrust used for locomotion and filter-feeding (Ruppert, Fox, and Barnes 2004a). The regular arrangement of these muscle bands is gradually established during asexual budding, a common reproduction mode in thaliaceans. Bud primordia exhibit a dorsolateral mass of muscle tissue that—in concert with bud growth—extend ventrally, separate into initially misaligned individual bands, and finally reach the regular arrangement found in mature zooids (Berrill 1950).

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