Chaetognatha
Vertical distribution

Species live at different depths, but temperature, age of specimens, and light intensity have a more or less marked influence on their vertical distribution. This is obvious for temperature in the case of Eukrohnia bathyantarctica. In the Antarctic, it dwells between 1000 and 2400 m (mainly between 1500 and 2000 m, and is rare between 1000 and 1500 m) (David, 1965). In the Caribbean, Owre (1972) reported it as shallow as 477 m (at 9.34°C), but mostly below this level where the temperature was around 4-5°C. In the Bay of Biscay its upper limit was considerably deeper, at about 1200 m, since here the temperature was still around 9°C owing to the warm Mediterranean waters spreading around 1000 m (Casanova, 1993b). Off Argentina, from ~ 44 to 40°S, it was sampled between 400 and 0 m (Boltovskoy and Mostajo, 1974) along with Eukrohnia hamata and Eukrohnia bathypelagica. Off North Angola, on the other hand, it was the deepest Eukrohnia, living between 1500-1600 and 900 m, but not above 900 m where five other species of the genus occurred (Casanova, 1986c). This also explains why some species, such as Eukrohnia hamata or Sagitta maxima, are found only at depth in the tropical Atlantic while also at the surface in colder regions, a phenomenon called tropical submergence. The northern (off Newfoundland) and southern (off Angola) populations of Sagitta maxima are morphologically different (Ducret and Casanova, 1968), probably owing to reduced gene-flow resulting from the scarcity of specimens in the deep tropical Atlantic. Such a distribution could also be the case for Sagitta planctonis (Pierrot-Bults, 1975).

Juveniles of most species live shallower than their adults. Thus, Eukrohnia bathyantarctica caught above 400 m off Argentina had a maximum length of 16 mm, whereas those caught deeper off Angola reached 36 mm, according to the authors mentioned above. Off Angola, Ducret (1968) noticed that juveniles of Sagitta hexaptera live mainly between 200 and 0 m, but the adults are found below 200 m. In some cases, however, juveniles and adults occupy the same depth levels, such as in Sagitta macrocephala.

Diel migrations have been described repeatedly for chaetognaths. The direction of migrations depends on the species. Off São Paulo (Brazil), Almeida Prado (1968) observed that Sagitta enflata and Sagitta serratodentata sink to deeper waters at noon, Sagitta hispida and Krohnitta pacifica sink during daylight, and Sagitta minima does not migrate. In a detailed study of vertical migrations of different maturity stages in 10 species off Northern Namibia, related to the pycnocline, Duró et al. (1994) concluded that limited migration and staggered distribution of various species and stages were a space partitioning mechanism to avoid inter- and intraspecific competition.

In general, however, data on vertical patterns in the South Atlantic are scarce, and sometimes contradictory. For instance, Sagitta hexaptera was said to be deep mesoplanktonic off Africa (Ducret, 1968), but epiplanktonic off southern Brazil (Resgalla, 1993), depending on local hydrological conditions, season and time of sampling. In spite of these inconsistencies and paucity of information, four bathymetric species groups can be recognized.

—Epiplanktonic species (0-200 m)
Sagitta enflata, Sagitta hispida, Sagitta friderici, Sagitta tenuis, Sagitta helenae, Sagitta bipunctata, Sagitta setosa, the seven Indo-Pacific Sagitta (Sagitta regularis, Sagitta neglecta, Sagitta pacifica, Sagitta pulchra, Sagitta ferox, Sagitta robusta, and Sagitta bedoti), Sagitta gazellae, Sagitta minima, Sagitta megalophthalma, Krohnitta pacifica and Pterosagitta draco.

—Mesoplanktonic species (200-500 m)
Sagitta hexaptera, Sagitta serratodentata, Sagitta tasmanica, Sagitta bierii, Sagitta decipiens, Sagitta sibogae, Krohnitta subtilis and Sagitta lyra.

—Bathyplanktonic species (>500 m)
Eukrohnia fowleri, Eukrohnia bathypelagica, Eukrohnia proboscidea, Eukrohnia macroneura, Eukrohnia flaccicoeca, Eukrohnia bathyantarctica, Sagitta macrocephala and Sagitta marri.

—Eurybathial species
Eukrohnia hamata, Sagitta maxima, Sagitta planctonis, Sagitta zetesios.

Assignment to these groups must take into account the preceding reservations. Thus, some epiplanktonic species (e.g. Sagitta minima, Sagitta megalophthalma, Krohnitta pacifica and Pterosagitta draco) may be found below 200 m, while several of the mesoplanktonic ones (Sagitta hexaptera, Sagitta serratodentata, Sagitta tasmanica), sometimes considered shallow-mesoplanktonic (as opposed to deep-mesoplanktonic), also occur above 200 m. On the other hand, Sagitta macrocephala is always bathypelagic, and in a same area, Eukrohnia macroneura, Eukrohnia flaccicoeca and Eukrohnia bathyantarctica always live deeper than other Eukrohnia.