2 6o DISCOVERY REPORTS 



might be regarded as separate species, but the close anatomical similarity and an apparently common 

 life history suggests that the two may be regarded as racially, rather than specifically, distinct. It is 

 possible that the Antarctic Convergence is an isolating mechanism, and that the two races of S. gazellae 

 represent an early stage in the formation of a new species. As individuals of both races are found, and 

 can live, on the 'wrong ' side of the Convergence one may suppose that the irregularity and rapidity of 

 environmental changes in the region of the Convergence, more than the widely different, but stable, 

 conditions on each side of it represent the real barrier. 



Further, the distribution of the separate, though closely related, species S. lyra overlaps that of 

 S. gazellae slightly in the region of the subtropical convergence. It might be postulated that individuals 

 of the S. lyra "gazellae "-type crossed this convergence and successfully colonized the Subantarctic 

 zone, giving rise to the specifically distinct Small Northern S. gazellae form, and subsequently these 

 gazellae were equally successful in establishing themselves in the Antarctic zone, and produced the 

 Large Southern form. The suggestion that colonization by S. gazellae of the Antarctic from the Sub- 

 antarctic zone took place is supported by the evidence of its greater abundance in the Subantarctic 

 (see Fig. 17). Although the colonization of the Subantarctic was sufficiently long ago to enable the 

 Subantarctic population to become specifically distinct from the subtropical stock from which it 

 originated, the time since the colonization of the Antarctic zone has only been sufficient for the 

 formation of a new race. 



HORIZONTAL DISTRIBUTION 



S. gazellae is an oceanic species with a continuous circumpolar distribution in Antarctic and Sub- 

 antarctic water; Baker (1954) records it as the most consistently occurring of the Antarctic zooplankton 

 species which he has examined, being present in 96% of the hauls. 



Fig. 17, which shows the horizontal distribution of density of the species, has been compiled from 

 data from the 1 m. oblique nets (N 100 B) fished in the surface (100-0 m.) layer. Ommanney (1936) 

 has discussed the significance of the variations in fishing depth of these nets, and concluded that they 

 can be disregarded for most purposes. The shallow N100B is always fished for 20 minutes at a 

 standard speed of hauling, and thus the symbols on Fig. 17 represent numbers per 20-minute haul. 



It has not been necessary to take into account the time of day when the hauls were made, as Table 12 

 leaves little doubt that no diurnal migration takes place. 



Although this species has usually been regarded as an Antarctic form (Ritter-Zahony, 191 1), its 

 main centre of abundance, as Fig. 17 shows, is in the Subantarctic zone, and no catch in excess of 

 200 individuals per 20-minute haul has been obtained from Antarctic waters. Many of the negative 

 and very small hauls in Antarctic water shown on Fig. 17 were made in winter, and give the impression 

 that the Antarctic population is sparser than it is; regrettably there are not sufficient data to make 

 separate charts for winter and summer. 



There is no evidence that the species 'swarms' or forms 'shoals', and the consistency of its 

 occurrence suggests a fairly even distribution. 



The northern limit of the distribution of the species is the region of the subtropical convergence ; 

 in making Fig. 17 the mean position of this convergence has not been used, but for each line of stations 

 which has crossed it, its position has been found by reference to the continuous thermograph records ; 

 this convergence is a variable phenomenon, and the mean position given by Deacon (1937) is a 

 simplified representation of it. In areas where tropical currents such as the Aghulas or the S.E. 

 Australian current influence it, it is probably never stable, and the limits of the distribution of 

 S. gazellae fluctuate in a similar manner. Even when a contemporary position for the subtropical 

 convergence is used, some hauls of S. gazellae are occasionally taken north of it, for example at 



