Chaetognatha
Geographic distribution

Some chaetognath species are known as excellent indicators of water masses because of their close relationships with environmental variables. Relations between species and hydrology have been shown on both sides of the South Atlantic. [Surface currents in the South A ].

Along the African coast, Neto (1961) reported abundances of species off Angola during two cruises from about the same period (June-December and August-January). She found 18 species (however, Sagitta serratodentata and Sagitta tasmanica were then still considered subspecies, so the real abundance ranking is different from the one given in that paper). Sagitta friderici was dominant in both cruises, curiously with almost identical percentages (22.6% and 23.0%, respectively). She noted the surprizing discovery of Sagitta setosa, which she considered an indicator of the cold Benguela Current. This is probably correct since the warm-water species Sagitta enflata was absent from the stations where Sagitta setosa appeared. Furnestin (1962) mentioned 15 species between 0° and 15°S, with Sagitta friderici representing 45.7% of all chaetognaths. In a few samples between 0° and 7°40'S, off the mouth of the Congo River, Furnestin (1966) found only 8 species, with Sagitta enflata largely dominant (57%), and Sagitta friderici replaced by its closest relative Sagitta tenuis. Ducret (1968) recognized 24 species, including Sagitta megalophthalma (which was later separated from Sagitta bipunctata, Dallot and Ducret, 1969), from three cruises (February to June) off Congo and Angola. The dominant species were Sagitta enflata (49.8%) and Sagitta hexaptera (20.5%). She emphasized the scarcity of Sagitta friderici (5 specimens out of ~135,000!), attributing it to the position of the stations off the continental shelf and mostly over bottom depths in excess of 1000 m; the only 3 specimens recorded were caught at the three stations closest to the coast. She also emphasized the presence of Sagitta hispida at all stations, which was regarded earlier as a neritic form. She also identified Sagitta bedoti, an Indo-Pacific species, transported northward from South Africa to 15°20'S by the Benguela Current.

In a synthesis of the Atlantic African plankton, Furnestin (1970) characterized various planktonic assemblages with chaetognath ecological species groups. Moreover, three Eukrohnia species have been discovered in this area: Eukrohnia proboscidea (Furnestin and Ducret, 1965), Eukrohnia macroneura and Eukrohnia flaccicoeca (Casanova, 1986c). Duró and Gili (1996) listed 19 species off Namibia, describing fine-scale relationships with the Benguela hydrodynamic system. It appeared that Sagitta setosa, representing more than 70% of the specimens, was found only in the cold upwelled waters, along with a smaller population of Sagitta tasmanica. Another population of Sagitta tasmanica was associated with warmer Angolan waters, which were well characterized by the dominant species Sagitta enflata. They also reported two Indo-Pacific species, Sagitta pacifica at 26°S, and Sagitta robusta up to 18°S. These species are more numerous off the west coast of South Africa (Heydorn, 1959; Masson, 1972, fide Schleyer, 1985).

On the American side of the South Atlantic, particularly off Argentina, papers on chaetognaths are more numerous, with data on seasonal changes, unfortunately lacking on the African side. In a large area along the north Brazilian coast between 3° and 13°S to more than 30°W, Nogueira Paranaguá (1963-64) mentioned only 6 species in 66 samples from surficial tropical waters, the most abundant being Sagitta serratodentata (63.4%), Sagitta enflata (16.9%) and Pterosagitta draco (13.1%). To the south, off São Paulo State, Almeida Prado (1968) recognized 7 species at two sets of stations sampled during 2 and 3 years in shelf and coastal areas. Sagitta enflata, Sagitta friderici, Sagitta hispida and Krohnitta pacifica were dominant. Her results showed that the abundance of Sagitta enflata, a shelf species, decreased regularly towards the coast, while conversely that of Sagitta friderici, a typical coastal species, increased. Their distribution was linked to water masses, except for the two last-mentioned species, for which such a relationship was difficult to establish. Three other species appeared: Sagitta minima, associated with shelf and tropical waters, and Sagitta serratodentata and Pterosagitta draco, both carried by oceanic tropical waters into shelf waters.

Along the southern Brazilian shelf, Resgalla (1993) studied many samples collected during two cruises between 31°40'S and 33°45'S, in winter and summer, at various depths from 0 to 500 m. He noticed seasonal variation in abundance and species composition, representing the influence of coastal and subantarctic waters in winter, and tropical water in summer. Eighteen species were identified, 16 during the winter and 15 in summer, of which 13 were common to the two seasons. Maximum abundances were registered on the shelf between 0 and 25 m, with 529 specimens mö-3 in winter and 723 in summer. Highest densities of Sagitta tenuis, the only neritic species, occurred in winter, and those of Sagitta enflata in summer. Moreover, the morphometric and reproductive characteristics of the former depended on the two kinds of coastal waters present in the area, cold and warm. Some species were good indicators of water masses: Sagitta tasmanica (subantarctic), Sagitta serratodentata (tropical waters of the Brazil Current), and Krohnitta subtilis and Sagitta decipiens (subtropical). Sagitta gazellae, Sagitta maxima and Eukrohnia bathypelagica were found only in winter, and Krohnitta pacifica and Sagitta bipunctata only in summer.

Boltovskoy (Boltovskoy, 1973a; Boltovskoy, 1974a) pointed out polymorphism in Sagitta serratodentata and Sagitta tasmanica off Argentina; in this area the former is associated with the warm and warm-temperate waters of the Brazil Current, while the latter dwells in the cold waters of the Malvinas (=Falkland) Current. Mostajo (1973) established that in March-April Sagitta friderici was the main chaetognath in shelf waters south of 44°S, and that Eukrohnia hamata, Sagitta gazellae and Sagitta tasmanica indicated the influence of cold Antarctic and subantarctic waters. The same species, plus Sagitta maxima, were identified in February by Kapp (1980) south of ~ 38°S, but at this time Sagitta tasmanica was the most widespread, while Sagitta friderici was caught only south of the Península Valdés. South of 34°S, in August, Boltovskoy and Mostajo (1974) reported 11 species, those of the genus Eukrohnia in subantarctic waters, and some others, i.e. Pterosagitta draco, Sagitta enflata, Sagitta hexaptera and Sagitta minima, in tropical-subtropical waters. The limit between the two groups was around 36-37°S. Boltovskoy (1975a) listed 16 species from two consecutive cruises (July 1973 and January 1974), and suggested that temperature is not the sole factor in any really dynamic pelagic zonation. He proposed faunistic groupings involving Chaetognatha, Pteropoda, and Foraminifera, showing that associations of species belonging to various phyla characterize particular water masses. In 1978, he expanded this concept including many other taxa in addition to the chaetognaths. In a few samples off southern Brazil, Uruguay, and Argentina, Boltovskoy (1975b) found 14 species and suggested an interesting inter-specific competition between Sagitta enflata and Sagitta tenuis (considered as a synonym of Sagitta friderici by the author). This recalls Almeida Prado's (1968) report of a similar inverse relationship between Sagitta enflata and Sagitta friderici.

Boltovskoy (1981f) reviewed the Chaetognatha of the western South Atlantic furnishing data on systematics, biology, ecology and distribution. One year later, Dadon and Boltovskoy (1982) defined 9 biogeographic areas in the Southwestern Atlantic, using indicator-species groups including Chaetognatha, Pteropoda, and Euphausiacea. Mazzoni's thesis (1990) is a valuable contribution to the study of the chaetognaths of Argentine shelf waters during many cruises, from ~ 36°S to 55°S. Maximum and minimum abundances were observed in winter and spring, respectively. Only 5 species were caught: Eukrohnia hamata and Sagitta gazellae in southern subantarctic waters, Sagitta tasmanica transported north by the Malvinas (=Falkland) Current, Sagitta friderici in temperate coastal waters north of 48°S, and Sagitta minima in a single sample from the northern reaches of the area surveyed. Numerous data on abundance, biological cycles, parasitism, etc. were also included in this work.

Papers on the South Atlantic as a whole are scarce. Thiel (1938) studied the "Meteor" collections, but unfortunately did not treat this abundant material totally to advantage. He thought that many species were closely related or even identical, e. g. Sagitta bipunctata, Sagitta friderici, and Sagitta robusta (probably = Sagitta hispida), were treated as the Sagitta bipunctata group; Sagitta maxima, Sagitta lyra, and Sagitta gazellae as the Sagitta maxima group, etc. So he provided distribution maps for only 10 "species". More interesting is Alvariño's (1969) work on the whole Atlantic, based on many different cruises. For the South Atlantic she listed 29 species, providing not only distribution maps but also occurrences in other oceans. She also incorrectly raised the Atlantic populations of Krohnitta pacifica to specific rank, as Krohnitta mutabbii .

Thirty nine species have so far been reported from the South Atlantic between 0 and 60°S, but this number will still increase when, as it seems likely, most if not all the 17 heterokrohniid species living north and south of this area are sampled here with adequate near-bottom gear. Indeed, the bentho-planktonic habitat of this family explains why Heterokrohnia mirabilis, which is the least bottom-linked, remained for about 55 years the sole species of the genus. Dawson (1968) was the first to suspect this behavior when he found some specimens in bottom trawls but not in plankton hauls above. This was confirmed later when four new Heterokrohnia species and a new genus, Archeterokrohnia, were discovered at one station off Mauretania a few metres above the bottom at depths of 4000 m (Casanova, 1986a, 1986d). The biodiversity in this new habitat seems very high; indeed, new species are described in almost every new near-bottom sample deeper than 700 m. Today, 10 are known from the Bay of Biscay, off Arcachon; by comparison with about 15 planktonic species in the water column above. In this sector, bentho-planktonic species can live as high as 400 m above the sea bed (water depth 2400 m) in a layer corresponding to the so-called nepheloid bottom water mass (Casanova, 1993b, 1994). These waters are characterized by homogeneous temperatures and salinities, resulting from particles thrown into suspension by strong mixing (Vangrieshem, 1985). Also certain to be expected off Argentina are at least Heterokrohnia mirabilis, which is known in the Antarctic, and three other species of heterokrohniids recently described from around the Antarctic Peninsula (ca. 60°50'S; Kapp and Hagen, 1985; Hagen and Kapp, 1986).

Biodiversity is about one third higher on the African than on the American side. Indeed, 38 species are present on the former. The sole species missing is Sagitta helenae, an American coastal endemic (its record off Freetown by Bainbridge, 1960, is a misidentification). Off South America, only 25 species are known, perhaps 26 if Sagitta decipiens, not recognized by Alvariño (1969), has been overlooked and confused with its closest relative Sagitta sibogae (the latter was named Sagitta decipiens sensu Ritter-Záhony, 1911, until, and sometimes after, Pierrot-Bults', 1979, revision; while the former was then named Sagitta neodecipiens Tokioka, 1959). Among the 13 other species missing along South America, 5 may still be recorded in the future, i.e. Sagitta bierii, Sagitta megalophthalma, Eukrohnia proboscidea, Eukrohnia macroneura, and probably also Eukrohnia flaccicoeca, since all but the last one have been identified in other areas. The other 8 chaetognaths, however, most probably do not occur in the western South Atlantic: Sagitta setosa, which is an East Atlantic endemic, and 7 Indo-Pacific Sagitta which are advected around the Cape of Good Hope into South African waters by the Agulhas Current. [Surface currents in the South A ].

The warm Agulhas current (AgC) occupies a very reduced area of the South Atlantic, but it nevertheless makes a significant contribution to the biodiversity of the west coast of Africa. The southbound coastal warm Brazil Current (BrC) and the northbound cold Malvinas (=Falkland) Current (MC) along the American side, the northbound cold Benguela Current (BeC) along the African side, and the Subtropical Convergence (StC) and Antarctic Convergence (=Polar Front) (AC) are the main hydrological phenomena influencing the distribution of species. The influence of these currents accounts for the oblique (SW-NE) latitudinal limits of the species as seen on many distribution maps related to the next section.

The general horizontal distribution patterns of characteristic chaetognath species groups in the South Atlantic are as follows (see for abbrevations for currents Surface currents in the South A):

—Antarctic-subantarctic species
The northward limits of Sagitta gazellae and Sagitta marri are roughly defined by the StC (Antarctic- subantartic A-B).

—Temperate species
Sagitta tasmanica occurs between the AC and the StC, mainly in subantarctic waters throughout the South Atlantic, and Sagitta setosa, also cold water, is restricted to the African side (Temperate C). Both species disappear in the tropical region, to reappear in the temperate North Atlantic.

—Tropical species
Sagitta hispida and Krohnitta pacifica are limited southward by a line joining the terminations of BrC and BeC (Tropical D-E).

—Tropical-subtropical species
The three epipelagic species belonging to this group, Sagitta enflata, Sagitta bipunctata and Pterosagitta draco, range from the equator to about 30°S in the middle of the ocean, but can reach 40°S along the American and African coasts (Tropical-subtropical F). Six other species live slightly deeper: Sagitta decipiens, Sagitta sibogae, Sagitta lyra, Sagitta hexaptera, Sagitta serratodentata and Krohnitta subtilis (Tropical-subtropical G-K). The distributions of the first two are lumped (Tropical-subtropical G) because, as previously noted, they are not always distinguished. The southernmost records of most of these are in the western South Atlantic.

—American species
Only one species, Sagitta helenae is known to be restricted to the warm American neritic waters, ranging south to about 35°S (American and Indo-Pacific L).

—Indo-Pacific species
Seven Indo-Pacific chaetognaths are known to occur off the west coast of South Africa: Sagitta regularis, Sagitta neglecta, Sagitta pacifica, Sagitta pulchra, Sagitta ferox, Sagitta robusta, and Sagitta bedoti, the last two reaching southern Angola (American and Indo-Pacific L). They are advected from the Indian Ocean by the AgC to the area off Capetown, and can sometimes be carried north by the BeC (Heydorn, 1959).

—Bilateral species
Two of these are neritic: Sagitta friderici (Bilateral M) and Sagitta tenuis (Bilateral N) The former is common on the African side, but less abundant off the American continent. The opposite is true for the latter, which is found only between 0 and 10°S off Africa, and often north of 48°S off America. The two are sometimes incorrectly considered synonymous. A third species, Sagitta minima (Bilateral O), belongs to this group, although it can be present farther out in the open ocean.

—Widespread species
Many species are known throughout almost all the South Atlantic Ocean. Two of them, the very abundant Eukrohnia hamata (Widespread species 1 P) and Sagitta maxima (Widespread species 1 Q) are bipolar. They are more or less epiplanktonic from 60° to about 40-45°S, and then seek colder (and deeper) waters northward; they are often present on the continental shelf off southern Argentina. The others are deep species. Among them, Eukrohnia fowleri (Widespread species 1 R), Sagitta macrocephala (Widespread species 1 S), Sagitta planctonis, and Sagitta zetesios (Widespread species 2 T) are the most common, almost always present in deep samples. Because of the controversial taxonomic status of Sagitta planctonis and Sagitta zetesios, both have been lumped in the corresponding distributional map (Widespread species 2 T). Indeed, they are considered separately by some authors, such as Alvariño (1969), who found the former south of 30°S and the latter north of 35°S; on the other hand, Pierrot-Bults (1969) treated them as forms of the same species. Five other Eukrohnia species belong to this group, although their distribution is more or less scattered (Widespread species 2 U-W): Eukrohnia bathyantarctica, Eukrohnia bathypelagica, Eukrohnia proboscidea, Eukrohnia macroneura and probably Eukrohnia flaccicoeca. Eukrohnia bathyantarctica was presumed an Antarctic endemic (David, 1965), but subsequently it has been found in many other oceanic areas, including the South Atlantic off Argentina (Boltovskoy and Mostajo, 1974) and Angola (Casanova, 1986c). Off Angola , all five of these Eukrohnia occur, and the latter three were originally described from this area. However, they are not endemic: Eukrohnia proboscidea has been reported from the Caribbean Sea, and Eukrohnia macroneura from the Bay of Biscay and the Pacific coast of Mexico. As for Eukrohnia flaccicoeca, it might be found in the area of Eukrohnia bathypelagica, its closest relative, but in deeper layers.

—Unclassified species
These include Sagitta bierii, reported off Angola and south of Capetown, and Sagitta megalophthalma, also off Angola (Unclassified species X). The former, described from the East Pacific, is found along the African side to the Iberian coast, while the latter also lives in the Mediterranean and Caribbean Seas.