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Appendicularians are exclusively marine holoplanktonic urochordates. They are distinguished from other tunicates by the persistence of a notochord in the adult stage. They have a body formed by a trunk and a characteristic tail, which is generally several times longer than the trunk (Esnal 1981, 1996a, 1996b, Fenaux 1998a). Most of them are small plankters, not exceeding 5 mm in length, although some species can be as long as 25 mm. Appendicularians show a peculiar feeding mode based on the function of a complex mucous structure, commonly known as “house,” which covers the anterior part of the trunk. Water is pumped through a filter that is part of the house, which retainsavariableamountofparticleswhosesizeandnature varies from colloids to nanoplankton (Bedo et al. 1993, Esnal 1996b). Appendicularians are important elements in thedynamicsoftheplanktontrophicwebsastheyandtheir discarded houses are consumed by several other pelagic organisms, including fish larvae and adults (Tokioka and Suárez-Caabro1956,Alldredge1976,AlldredgeandMadin 1982). Their role as secondary producers has been reevaluated in recent surveys, and they are far more important than previously noticed (Hopcroft and Roff 1998). This group is among the most frequent constituents of the marine oceanic zooplankton; in estuarine and coastal areas they are often among the most abundant zooplanktonic taxa (Castellanos, Gasca, and Esnal 1994). They are very widely distributed in the world oceans; most species are regarded as warm-water forms, but several species are related to cold and temperate waters (see Fenaux, Bone, and Diebel 1998). Appendicularians are mainly epipelagic forms; their vertical migration range is relatively narrow, restricted to a depth range of 0–200 m; they are presumed to be most abundant within the 100–200 m layer (Fenaux 1966). It has been suggested that they are more diverse in deeper waters and that many meso- and bathypelagic appendicularian taxa are yet to be discovered in these environments (Fenaux 1992, 1993b). New methodological developments such as tripulated submersibles and ROVs (remotely operated submersibles) have allowed the observation and study of the distributional patterns of deep-living appendicularians (Gorsky and Fenaux 1998) and the collection of undescribed forms (Hopcroft and Robison 1999). Aside from the taxonomic complexity of this group and their small size, the fact that they can be easily damaged during plankton hauls constitutes an additional difficulty to the identification process; this is particularly 1217 74 Appendicularia (Urochordata) of the Gulf of Mexico Iván A. Castellanos and Eduardo Suárez-Morales  Appendicularia. After a drawing at UMMZ Internet website (http://animaldiversity.ummz.umich.edu/) modified by F. Moretzsohn. 1218 ~ Appendicularia (Urochordata) which turned out to be a synonym of T. fertilis (see Fenaux 1993a).Afteragapofseveraldecadeswithoutasinglepublication on this group in Gulf waters, Escamilla-Sánchez (1996) reported 3 species of Oikopleura off the northern coast of the Yucatán Peninsula. In the northern Gulf of Mexico, Dagg et al. (1996) recorded O. dioica in a work on the feeding ecology of the appendicularians. In the western Caribbean, with influence in the southwestern Gulf zooplankton fauna, the appendicularians were studied by Castellanos, Gasca, and Esnal (1994), Castellanos and Gasca (1998a), and Castellanos (2003). Castellanos and Gasca (1998b) provided a key for the identification of 14 species that include some of the species known to be distributed in the Gulf of Mexico. Recently, Sato and Dagg (2004) recorded 11 species of 5 genera in the Mississippi River plume and found 2 species of Oikopleura as being the most abundant in the area; these data have not been published yet and were not considered herein. Taxonomy and Number of Species Described The current taxonomy of the group was strongly influenced by the early classifications and criteria proposed by Fol (1872) and Lohmann (1933). The class Appendicularia is subdivided into 3 well-defined families: Kowalevskiidae , containing only one known genus and 2 species , Kowalevskia oceanica Lohmann, 1899; and K. tenuis Fol, 1872. The family Fritillariidae includes around 30 species arranged in 3 genera, and the Oikopleuriidae has 11 genera and 38 species (Fenaux 1998b). Overall, Fenaux (1993a) recognized up to 65 species and 14 genera within the Appendicularia. Several species have been described since then, including some in Mesoikopleura (Fenaux, 1993b) and the deep-living Mesochordaeus erythrocephalus Hopcroft and Robison, 1999, in Monterey Bay, California (Hopcroft and Robison 1999); additionally, other speciesremainunpublished,suchasanewspeciesof Oikopleura from a Norwegian fjord (by P. R. Flood) and new mesopelagic forms of Fritillaria by R. Hopcroft. Hence, the currently recognized number of appendicularian species rises to a figure close to 72 species, distributed in 15 genera. In the Atlantic Ocean the...

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