Appendicularia
Geographic and vertical distribution

Appendicularians are conspicuous members of marine zooplankton, occurring in all oceans, from pole to pole. They occur in both neritic and oceanic environments, being more abundant in the former. Highest diversities have been reported from the warmest waters, but many of the species are eurythermic, extending broadly into temperate and cold waters (e.g., Forneris, 1957; Fenaux, 1966; Bückmann and Kapp, 1975; Esnal, 1978, 1981b, 1986).

The general distribution pattern outlined by the early expeditions for the South Atlantic (Lohmann, 1896, 1928, 1931; Lohmann and Bückmann, 1926; Lohmann and Hentschel, 1939) and subsequent reviews (Forneris, 1957; Fenaux, 1966; Bückmann, 1974; Esnal, 1981b), is still valid without major modifications. Some new records permit extension of the southward distribution of several species, like Fritillaria fraudax to 58°S, Fritillaria pellucida to 65°S, Fritillaria tenella to 57°S, and Oikopleura albicans to 48°S; while the Antarctic species Oikopleura gaussica reaches as far north as 51°S (Esnal, unpublished data). All this information has been compiled in App-table that shows the latitudinal distribution of the species so far recorded in the South Atlantic.

Highest numbers of species were found in the equatorial zone, in the Gulf of Guinea to the south of South Africa (De Decker, 1973; Lazarus and Dowler, 1979), and along the Brazilian coast to Mar del Plata (Fenaux, 1966). The most abundant species of the family Oikopleuridae are, in decreasing order, Oikopleura longicauda and Oikopleura fusiformis, followed by Oikopleura rufescens and Oikopleura cophocerca. In coastal areas Oikopleura dioica predominates, being particularly abundant near the mouth of the Río de La Plata estuary (Fenaux, 1966; Esnal, 1972). In Fritillaridae, Fritillaria formica, Fritillaria pellucida, and the closely related species Fritillaria borealis and Fritillaria sargassi are most abundant in warm waters. Traditionally, the latter species has been considered a form of Fritillaria borealis, regarded as cosmopolitan, distributed from pole to pole, and with three formae: typica, intermedia (imprecisely characterized), and sargassi (Tokioka, 1940, 1960; Fenaux, 1966, 1967; Bückmann and Kapp, 1975).

Esnal et al. (1996) re-examined the problem of the infraspecific variability of Fritillaria borealis, establishing the existence of two clearly defined groups considered species, and reverting to the classification of Lohmann (1896), who regarded F. borealis as ranging from pole to pole, and Fritillaria sargassi as present only in warm and cool waters. It is probable that in F. borealis, with its vast geographic range, infraspecific forms exist, and that intermedia could be, as Tokioka (1960) and Fenaux (1967) stated, a variant of very difficult identification, occurring in warm waters or in the mixing areas between cold and warm waters. The characterization of infraspecific formae (Van der Spoel, 1971) is very useful in oceanographic studies, since their occurrence is probably governed by environmental conditions. However, unless these formae can be identified with reasonable ease, they are meaningless for ecological surveys or biogeographical analysis.

Many of the taxonomic and distributional problems posed in the past remain unsolved owing to the scarcity of materials for the species involved. For example, Oikopleura gaussica, Oikopleura drygalskii, Oikopleura valdiviae, and Oikopleura weddelli, considered endemic to the Antarctic Ocean by Lohmann (1928), have also been found in subantarctic waters of the Southwestern Atlantic (Lohmann and Hentschel, 1939, Udvardy, 1958; Esnal, unpublished data). Tokioka (1964) questioned the taxonomic validity of the latter three species, suggesting that they could be mere variants of Oikopleura gaussica. Lohmann and Hentschel (1939) cited 8 other species of this genus from Antarctic and subantarctic waters, but their material and drawings were lost during the Second World War, and were never published. In Fritillaria, the only species considered by Lohmann (1928) to be endemic to the Antarctic Ocean is Fritillaria antarctica, but Lohmann and Hentschel (1939) found it in subantarctic waters as well as in warm waters off the Brazilian coast. Also in this case, Tokioka (1964) questioned the validity of this species based on its marked affinity with Fritillaria fraudax, which is regarded as a warm water species. F. fraudax was considered to be very rare in the Southwestern Atlantic (Esnal, 1978, 1981b), but subsequent samplings revealed that it frequently occurs at high densities south of 41°S (Esnal, 1986), reaching as far south as 58°S.

Aside from the endemic forms, Lohmann (1928) reported findings of most of the warmer water species in Antarctic waters, stating that these records could be occasional and restricted to the deeper waters (below 100 m), where temperatures are slightly higher. While warmer-water plankters do indeed occur south of the Polar Front in association with the slightly warmer Warm Deep Water, at depths over 200-300 m (Abelmann and Gersonde, 1991; Abelmann, 1992; Boltovskoy and Alder, 1992), Tokioka (1964) rejected such a possibility on the basis of the scarce zonal variation in the physico-chemical characteristics from 400 m depth to the surface. These species, as well as some of the supposedly endemic ones, may be regional ecological forms of widely distributed species, which indeed seems to be the case of the pair F. antarctica - F. fraudax.

Lohmann and Hentschel (1939) pointed out that Oikopleura longicauda, the most common species in warm and temperate waters of all oceans, is restricted in the Atlantic to waters warmer than 15°C. Interestingly, it is also present in the Antarctic Ocean. On the other hand, Oikopleura fusiformis, which is normally found on the Argentine continental shelf up to 61°S, does not occur in Antarctic waters. However, Esnal (1986) noticed that both species are widely distributed southward: O. fusiformis ranges south of 64°S in the Pacific Ocean, whereas O. longicauda reaches the Antarctic Convergence in all oceans (thus, Lohmann’s, 1928, is the only record from the Antarctic Ocean). It is quite probable that these supposedly disjunct distributions are an artifact resulting from the scarcity of data and inadequate sampling.

In the Southwestern Atlantic, the Patagonian shelf usually hosts an impoverished warm water fauna, while Antarctic species transported northward by the Malvinas (=Falkland) Current occur regularly along the continental slope. Lohmann and Hentschel (1939) mentioned an area of abundant appendicularians, extending from Rio de Janeiro to the Gulf of San Matías, centering on a locale south of the mouth of the Río de La Plata. South of this area appendicularian abundances drop drastically, with the exception of isolated spots of high concentration around Cape of Good Hope and the Malvinas (=Falkland) Island. The reports by Esnal (1972, 1973) and Bückmann (1974) confirm this distribution pattern.

There is little quantitative information on appendicularian absolute abundances to allow comparisons among different oceanic areas. Dadon and Esnal (1995), reported values of 2400 and 3700 individuals per 104 mö3 for Oikopleura longicauda and Oikopleura dioica off southern Brazil, and Esnal et al. (1985), found 244000 per 104 mö3 for the latter species off northern Brazil. The last figure, although high, is fairly lower than the maximum densities recorded by Seki (1973) in Saanich Inlet: 25.6 million per mö3.

Generally, appendicularians are most abundant around 100-200 m (e.g., Lohmann, 1896; Lohmann and Hentschel, 1939; Fenaux, 1968). However, some species seem to be constant members of the zooplankton community in meso- and bathypelagic environments (Barham, 1979; Davoll and Youngbluth, 1990; Fenaux and Youngbluth, 1990, 1991; Gorsky et al. 1991; Hamner and Robinson, 1992; Fenaux, 1992, 1993). Until very recently, our knowledge of the diverse and often abundant macroplankton fauna of the deep layers was very limited, since traditional plankton sampling methods were inappropriate for capturing many of these fragile animals, which were almost invariably destroyed. Nowadays, thanks to the use of manned submersibles, the situation has changed, and new appendicularian species have been described recently from Mediterranean, Bahamian, and Bermudan deep waters (Fenaux and Youngbluth, 1990, 1991; Fenaux, 1992, 1993). So far, no equivalent surveys have been performed in South Atlantic waters.