Appendicularia
Introduction

Appendicularians are planktonic tunicates characterized by the persistence of the notochord in the adult and the lack of a peribranchial cavity and cloaca (e.g., Brien, 1948; Berrill, 1950; Fenaux, 1967). The body is divided into two regions: a trunk, exceptionally exceeding 5 mm (as long as 25 mm in Bathochordaeus charon), and a tail, which is generally several times longer than the trunk (Oikopleurinae, Bathochordaeinae). Although appendicularians differ very much in appearance from the rest of the tunicates, the basic structural pattern of the trunk preserves the three characteristic tunicate parts: pharyngo-branchial, digestive, and genital. The anatomic features of these parts vary in the three families of the class (see Taxonomy below).

The first record of an appendicularian in the literature is by Chamisso and Eysenhart (1821), who identified it as a coelenterate (fide Fenaux, 1967). During the subsequent 30 years, in spite of the recognition of a relationship between these animals and the tunicates, they were thought to be larval stages of unknown adults. Huxley (1851) was the first to recognize them as adult animals; however, his opinion was not accepted by the majority of his contemporaries (fide Fenaux, 1967). Gegenbaur (1855), used the name of Copelata for these organisms, whereas Balfour (1881) used the term Perennichordata to differentiate them from the rest of the tunicates, which he called Caducichordata. Lahille (1890), called them Appendicularia (cf. Brien, 1948), and Herdman (1891) Larvacea. The terms Copelata, Appendicularia, and Larvacea are used interchangeably to refer to the class under the current classification scheme.

Appendicularians feed by means of a complex mucous structure, the "house", which is secreted by the epidermal cells covering the anterior part of the trunk (Lohmann, 1899a, b). House sizes vary from 6 mm to approximately 2 m, as in the giant appendicularians of the genus Bathochordaeus (Alldredge, 1977; Barham, 1979). The structure of the house, which entirely encloses the animal's body, (except in Fritillaridae), varies in the three families (Oikopleuridae-house, Fritillaridae-house, Kowalevskiidae-house). The most complex, and the best known in their morpho-functional aspects, are the houses of Oikopleuridae (e.g., Lohmann 1899a, b; Alldredge, 1977; Flood, 1978, 1991; Deibel et al., 1985; Deibel, 1986; Fenaux, 1986; Deibel and Powell, 1987; Flood et al., 1990). The animal uses its muscular tail to pump water through a complex food-concentrating filter that occupies a large portion of the total volume of the house (e.g., Deibel, 1986). This filter is capable of aggregating and concentrating particles suspended in sea water up to 1000 times, before they are ingested by the animal. The particles are removed from the suspension by an internal pharyngeal filter which is homologous to the branchial basket of other tunicates (e.g., Flood et al., 1992; Morris and Deibel, 1993). Appendicularians can consume, with varying degrees of retention efficiency, material ranging in size from fine colloidal (<0.2 mm) to pico- and nanoplankton (e.g., Deibel and Powell, 1987; Flood et al., 1992; Deibel and Lee, 1992; Urban et al., 1993; Bedo et al., 1993). This ability, in combination with high grazing rates (Paffenhöfer 1973, 1976; Alldredge 1976b, 1981; Deibel, 1988), indicates that appendicularians may exert significant grazing pressures on the pelagic environment.

A portion of the particles that appendicularians remove from the water is not ingested and remains associated with the house (e.g., Alldredge, 1976a; Gorsky et al., 1984; Bedo et al., 1993). In Oikopleuridae, houses are discarded and renewed up to 16 times a day during the animal’s life (Fenaux, 1985). Discarded houses constitute a particular type of marine snow aggregate, a substratum on which a rich community of associated organisms develops (e.g., Alldredge, 1972, 1976a; Silver et al., 1978; Alldredge and Youngbluth, 1985; Alldredge et al., 1986; Davoll and Silver, 1986; Caron et al. 1986; Davoll and Youngbluth, 1990; Steinberg et al., 1994). Sinking rates of discarded houses and free fecal pellets of Oikopleura dioica are similar to those of copepod fecal pellets. Thus, both the houses and the pellets have relatively long residence times in the water column, where they can be used as food by larger organisms (Fortier et al., 1994). It is well-known that appendicularians are sometimes important in the diet of several pelagic animals, including larval and adult fish (e.g., Tokioka and Suárez Caabro, 1956; Alldredge and Madin, 1982; Cailliet and Ebeling, 1990; Young and Davis, 1990; Castro, 1993). In the Southwestern Atlantic, Oikopleura dioica is consumed in great quantities by the anchovy Engraulis anchoita (Capitanio et al., 1997).

Discarded houses and fecal pellets from appendicularian populations living at mesopelagic depths may constitute a significant contribution to carbon sequestration in deep waters (e.g., Davoll and Youngbluth, 1990; Hamner and Robinson, 1992; Fortier et al., 1994; Steinberg et al., 1994).

The houses of some species of Oikopleuridae produce endogenous bioluminescent flashes and may contribute significantly to marine surface luminescence (e.g., Galt, 1978; Galt and Sykes, 1983; Galt et al., 1985). The buccal gland cells (Oikopleurinae-trunk) are considered to be responsible of delivering bioluminescent material into the house primordium (Fredriksson and Olsson, 1981, 1991).

With the single exception of Oikopleura dioica, appendicularians are hermaphroditic, and release their gametes directly to the sea. In Oikopleuridae, after the emission of spermatozoa through the spermiduct, the eggs are released by the rupture of the walls of the ovary and the body, thus resulting in the animal's death (Fenaux, 1963, 1967; Galt and Fenaux, 1990). Fecundity is high, and a single individual is able to produce hundreds of eggs (Last, 1972; Paffenhöfer, 1973, 1976; Fenaux, 1976; Fenaux and Gorsky, 1981, 1983; Galt and Fenaux, 1990). Generation times are very short and temperature-dependent, from 1 to 15 days in the case of O. dioica (Paffenhöfer, 1973, 1976; Fenaux 1976; Esnal et al. 1985; Galt and Fenaux, 1990; Hopcroft and Roff, 1995); and growth rates are very high (Paffenhöfer, 1976; King et al., 1980; Hopcroft and Roff, 1995). These characteristics determine an extraordinary production potential and may explain the known blooms, which are probably related to phytoplankton blooms and the absence of predators (e.g., Seki, 1973; Fortier et al. 1994; Hopcroft and Roff, 1995).