DISTRIBUTION OF THE CHAETOGNATHA 221 



originating and maintaining the species found in the Southern Ocean, and in this connexion it is not 

 therefore surprising to find that when overlapping does take place it is between species of two separate 

 genera, Eukrohnia and Sagitta, and not between species of the one genus. 



WINTER DISTRIBUTION 



Fig. 13 shows the vertical distribution of the four common species in 0° in August. At that time of the 

 year the ice edge was in about 57° S, so that only a small part of the antarctic zone could be sampled. 



S. gazellae. The main subantarctic concentration has moved from the surface layer down to the 

 250-100 m. horizon, but is still in the same latitudes. A small antarctic maximum appears in about 

 56° S which seems to be analogous with the March concentration (see Fig. 3) in the same latitude. 

 This concentration also has descended from 100-50 m. to 250-100 m. 



S. marri. The section does not go far enough south to sample the main part of the population of the 

 species, and is therefore based on very small numbers which are not very informative. There is, 

 however, a suggestion of a deeper winter distribution as found by Mackintosh (1937) for the 80° W 

 line. An interesting feature of this section is the extension of part of the population into the Sub- 

 antarctic in the 1 500-1000 m. layer. 



S. maxima. The main concentration is a little deeper in this line than it is in the summer one, and 

 extends rather further south. 



E. hamata. This species shows a clear movement to deeper water in the winter and possibly a slight 

 southward extension of the main concentration. 



Other species (not shown in the section). The August line in o^"" does not commence so far north as the 

 March one, so it is difficult to draw any conclusions about the subtropical species. S. serratodentata 

 was present in very small numbers at Station 2385 at 250-100 m. and S. decipiens at the same station 

 at 500-250 m. An August line (2867-2875) in the western Indian Ocean sector, which began just 

 north of the subtropical convergence showed S. serratodentata present in the 50-0 m. haul in quite 

 considerable numbers (465 per 250 m. haul) and S. decipiens present in the 500-250 m. horizon. 



THE DISTRIBUTION OF THE MATURE STAGES 

 An interesting feature which is apparently common to all the most abundant Southern Ocean chaeto- 

 gnaths is the habit of descending into deep water to breed. This has been clearly shown for S. gazellae 

 (David, 1955, p. 270). In E. hamata animals at states III and IV (Kramp, 1939) (see footnote, p. 204) 

 are only found below 750 m., though animals at stage V (' var. antarctica ') are sometimes found at the 

 surface as well as in deep water. Specimens of S. maxima at stages III and IV are also found below 

 750 m. Very few specimens of S. marri at stage IV have been taken, but specimens at stage III are 

 also only found below 750 m. These three latter species differ from S. gazellae in that the very young 

 stages are found throughout the top 1000 m. of water, whereas very young S. gazellae are found only 

 in the top 250 m. 



It is difficult to imagine anything advantageous to the animals in this performance of deep spawning, 

 which is also a habit of S. lyra and S. planctonis in the subtropics, but it is possibly a behavioural relic 

 of some deep-living ancestral form. Since there are relatively fewer barriers to the spread of animals 

 in deep water than there are to the spread of those at the surface, it seems quite likely that radiation 

 in the group may have been via deep-living forms. 



