DISTRIBUTION OF THE CHAETOGNATHA 



223 



THE PATTERN OF DISTRIBUTION 



The chaetognath fauna of the Southern Ocean is remarkable in the very small numbers of species 

 present, seven only, living and maintaining themselves beneath a surface area of about 21 million 

 square miles of sea, in comparison, for example, with the subtropical zone of the southern hemisphere 

 which has not nearly so great a surface area yet contains at least sixteen species if not more.^ This is, 

 however, in keeping with the well-known principle that cold waters support fewer species than warm, 

 though usually in larger numbers. 



Among other points which emerge from this study is the fairly high proportion of endemic species 

 — three out of seven. The antarctic benthic fauna is characterized by its high proportion of endemic 

 species, which would be expected by virtue of its isolation, geographic and physical (covering 

 temperature, salinity and other features), from the benthic fauna of the southern continents. The 

 planktonic fauna would not be expected to be so subject to isolation, yet evidently from the data 

 presented here it must be. The boundaries between water masses in the open ocean however are not 

 usually clear cut and distinct, and large-scale swirls or eddies are probably produced where water masses 

 meet. Salients of one water mass may from time to time be pinched off by another in the boundary 

 region, and gradually absorbed. Thus the fauna carried in such a salient may be found outside its 

 normal habitat ; but such stray records should not be regarded as typical of the range of a species. 



It is evident that barriers exist which limit the habitat of each species, and it seems likely that the 

 convergence areas can be regarded as effective in this respect ; nevertheless, there must also be other 

 factors involved, and although temperature or salinity considered separately indicates that there is con- 

 siderable overlap of tolerance in the various populations, the two factors in combination seem to have a 

 considerable effect (see Figs. 4, 6 and g). It is probable that if other physical factors were also considered 

 in combination, the actual tolerance of each species would be circumscribed within precise and narrow 

 boundaries ; but it must be remembered that this would probably only mean that a particular species 

 lived in a particular water mass, and that while the combination of physical factors identified the water 

 mass, they might not necessarily be the factors governing the distribution of the species. Biological 

 differences, as yet imperfectly understood, between various water masses, such as those which are 

 being investigated by Wilson (1953, 1954) and Wilson & Armstrong (1954), may well play an important 

 part in limiting the distribution of the plankton. 



SUMMARY 



The Chaetognatha of the antarctic and subantarctic zones can be classified broadly into three groups : 

 (a) 'endemic' species, (b) species common to other regions, and (c) exotic species which do not 

 maintain themselves in the area. There are three species in group (a), four in group (b) and eight in 

 group (c). The horizontal distribution of these is discussed, and the vertical distribution of the seven 

 species in groups {a) and (b) and two species of group (c) is shown by sections based on data from 

 vertical closing nets worked in the Atlantic, Indian and Pacific Ocean sectors of the Southern Ocean. 

 The results show that the distribution pattern is distinctive for each species, that the areas of maximum 

 abundance of species of the genus Sagitta are mutually exclusive, and that despite the relative 

 uniformity of the environment, these species are influenced by quite effective isolating factors. 



^ It is interesting to note that Tchindonova (1955, p. 307), commenting on the same phenomenon in the north Pacific, 

 says: 'While there are 23 diflFerent species known for the Yellow Sea and the Sea of Japan out of which 16 are warm, and only 

 two are cold water species, in the region of the Kurile-Kamchatka trench only seven species altogether have been found by us.' 



