Doliolida
Taxonomy

In the last years of the 19th century the classification of the doliolids was still quite confused (see Garstang, 1933). Herdman (1888) dealt with the doliolids collected by the Challenger Expedition and reported several species, some of them new to science. Unfortunately, his descriptions were incomplete and, in some cases, he did not even take into account high-quality previous information (see Borgert, 1894; Neumann, 1906; Garstang, 1933; Braconnot, 1970). The rich material obtained by the Plankton Expedition (Traustedt, 1893; Borgert, 1894) permitted significant advances in the knowledge of the group, which were subsequently expanded by the results of the German Tiefsee, Südpolar, and Meteor expeditions (Neumann, 1906, 1913; Krüger, 1939). However, it was Garstang (1933) who established the basis of the classification now used. At present two suborders and four families are recognized (Godeaux, 1996), with a total of 20 species (Garstang, 1933; Krüger, 1939; Tokioka and Berner, 1958a, b; Godeaux, 1996), 11 of which have been recorded from the South Atlantic Ocean (see Order Doliolida).

The life cycles of only a few species of the family Doliolidae are known, and each of these forms presents its own identification problems. Braconnot (1968, 1970, 1974) clarified existing confusion by characterizing the larval stage in the genus Doliolum and in Dolioletta gegenbauri and Doliolina muelleri. Species-level identification of the oozooid is particularly difficult because of the great similarity between the different species. Based on this fact, Godeaux (1961) proposed the concept of cryptic species to refer to this stage. Only the oozooid of Doliolina muelleri is easily distinguishable from other known oozooids. Although Braconnot (1970) recognized the difficulties in distinguishing these individuals, he proposed an identification key for young oozooids of the Mediterranean species, which we have incorporated in our diagnoses.

The growth of the oozooid involves tissue degeneration. Thus, in the "old nurse" stage, the organs which are normally used for classification have disappeared. Unlike the species of other genera, it is easy to identify the old nurses of Doliolum, in which the muscles are fused into a continuous sheath. Garstang (1933) proposed a morphometric method, based on the breadth of the muscular bands, to identify the nurses of Dolioletta gegenbauri and Doliolina muelleri, which has been followed by subsequent authors (e.g. Bary, 1960; Sewell, 1953; Tavares, 1967; Braconnot, 1970). Esnal et al. (1982) calculated a linear discriminant function, thus refining this method and reducing identification errors.

The phorozooid and gonozooid stages allow specific identifications more easily; therefore it is essentially on these individuals that the classification is based. The structural difference between these two stages depends exclusively on the presence or absence of gonads, and on the possession of a ventral peduncle with buds in the phorozooids.

The most commonly used diagnostic characteristics are the arrangement of the muscles of the body wall and the location of the different internal organs in relation to these muscles. The extension of the endostyle, the shape and extension of the gill-septum, the shape and location of the alimentary canal, and of the genital organs, have particular taxonomic value. The general morphology of doliolids is schematized in dol-larva, dol-young oozooid, dol-nurse, dol-gastrozooids, dol-phorozooid, dol-gonozooid, only the structures most important for identifying species have been emphasized.