Siphonophorae
Geographical and vertical distribution

There are very few, if any, wide ranging studies of the distribution of siphonophores in the South Atlantic Ocean. The data presented in IdentifyIt (file "Siphonophorae") are, thus, based on the sparse records in the literature. Only the maximal latitudinal limits of distribution are given, although this may mask some of the finer details, such as the presence of certain species in certain water masses. The data on the main vertical range for each species is largely based on data in the Southampton Oceanography Center database.

Most siphonophore species are truly oceanic and rarely appear inshore A few are mainly neritic, such as Muggiaea atlantica and Muggiaea kochi, and there appears to be a mutual exclusion between them, such that they almost never occur in the same area at the same time (Mackie et al., 1987). The oceanic species can be roughly divided into boreal, tropical and equatorial species (Margulis, 1972), although such a scheme does not take account of the depth distribution of some. Thus, some species are clearly restricted to warm, superficial waters, while the deeper-living, bathypelagic species can occur at almost any latitude as the temperature at these depths does not vary greatly. The zoogeographic distribution of siphonophores in the North Atlantic Ocean has been addressed by Pugh (1977) and Mackie et al. (1987). These studies indicate that the species composition of a siphonophore population is influenced by the various water masses present. For instance, there was a clearly defined and very abundant population of siphonophores above the permanent thermocline present at lower latitudes. In addition, different populations were found in North and South Atlantic Central Waters. It is most likely that the same situation will pertain in the various water masses of the South Atlantic, but there are too few data to demonstrate this. Although there are clear correlations between hydrographical features and siphonophore populations, it is probable that this is an indication of other factors that have a more immediate impact on their distribution, such as dietary preferences (Mackie et al., 1987).

In the North Atlantic maximal species diversity was reached at ca. 20°N (Mackie et al., 1987) with a slow decrease in numbers toward the equator, and a more rapid one toward higher latitudes. The latter was mainly the result of the disappearance of epipelagic species living above the permanent thermocline. Nonetheless, there appear to be more species in Antarctic waters than there are in Arctic ones; and several of those species are indigenous to that region, for example Pyrostephos vanhoeffeni and Diphyes antarctica. There are also one or two bipolar species that have never been recorded at lower latitudes in the Atlantic, these being Muggiaea bargmannae and possibly Gilia reticulata.

There is also a change in species diversity in the vertical plane, with fewer species being found in the colder, deeper waters. Individual species can occur over quite wide depth ranges but, in general, they can be classified as epipelagic (0-250 m), mesopelagic (250-1000 m) or bathypelagic (>1000 m). However, several species have a relatively restricted distribution within the mesopelagic zone. In the North Atlantic there was a marked discontinuity, at ca. 42°N, between the more southerly and more northerly siphonophore populations in the top 1000 m of the water column. Although the number of species present on either side of this discontinuity was very similar, there was a change over in the specific population and a dramatic increase in biovolume to the north, with just a few species predominating. To the south the siphonophore population consisted mainly of small epipelagic species (chiefly in the families Diphyidae and Abylidae). To the north the deeper living but larger species of the families Prayidae, Hippopodiidae and Clausophyidae became predominant. It is probable that similar changes will occur across the various major fronts, for instance the Subtropical Convergence and the Polar Front (=Antarctic Convergence), in the South Atlantic; but again there are no detailed studies.

Several epipelagic siphonophore species undergo diel vertical migrations over a depth range of up to 200-250 m. However, such migrations may not always be apparent because, in some species the rate of depth change is relatively slow so that there is a sinusoidal wave of depth change throughout the day and night (Mackie et al., 1987). In addition, it may be that the depth range of sampling is too coarse to establish any small-scale diel vertical migrations (Pugh, 1984). The factors that might initiate these migrations, and the processes involved in them are discussed by Pugh (1977; Pugh, 1984). It is presumed that siphonophores undertake such migrations in order to remain in the vicinity of their prey, which often migrate to superficial waters at night. Physonect species, with a gas-filled float, are known to be important contributors to deep-scattering layers, and some have a pore to allow release of gas from their float that may facilitate a diel vertical migration.