S maxima 

 S.morri 



o 



214 DISCOVERY REPORTS 



salinity diagram (Fig. 6) shows. The very narrow tolerance range of the maximum concentrations of 

 the species, 34-67-34-7I %o and o-i to 1° C, suggests a restricted distribution, but within these limits 

 is a very large mass of water, and the section in 0° (Fig. 6) shows indirectly how extensive this water 

 mass is. The temperature/salinity diagram also shows clearly how parts of the population are ' lost ' by 

 being carried north to the antarctic intermediate water. These 'sterile expatriates' (Ekman, 1953) are 

 probably lost mainly by the movement of the antarctic surface water at the convergence where it sinks 

 to form the intermediate layer. 



The numbers in each line vary greatly, those in 0° 

 being much higher than in the other two lines. In 90° E 

 the 500-250 m. sample at Station 281 1 was spilt during 

 sorting aboard ship, and a large part of it was lost. It 

 would evidently have been a key sample, and the 

 contours from the other samples suggest that there 

 would have been quite a large concentration there. I have 

 indicated this by a dotted line in Fig. 5B. 



S. marri is perhaps the most characteristic antarctic 

 chaetognath of all, for although it lives in a water mass 

 whose origins are found in the subtropics of the southern 

 hemisphere, it only inhabits it when mixing and cooling 

 have reduced it to the very precise conditions of tempera- 

 ture and salinity found in the warm deep water of the 

 antarctic zone. 



Eiikrohnia sp. This species occurs very sparsely in 

 the vertical sections, always below 1000 m. Its 

 northern and southern limits are not yet known. It 

 occurs in the antarctic zone as the commonest 

 chaetognath below 1500 m. 



. o 

 ••■«•• 



9 340 I 2 



Fig. 6. The relative distribution of the two highest 

 orders of abundance of 5. marri and S. maxima in the 

 0° meridian in March, plotted against temperature and 

 salinity. For explanation of the lines on the figure see 

 legend to Fig. 4. 



S. maxima (Fig. 7). This species is the subantarctic counterpart of S. marri. Its northern limit 

 appears to be just within the subtropical zone at a depth of about 500 m., though it must extend 

 considerably further north in some places as it has been found in both the Peru and Benguela currents. 

 In the main, however, it is characteristically a subantarctic species. Its southern limit is usually not 

 far south of the antarctic convergence, but it occurs in small numbers further south than this ; in 80° W, 

 for example, it occurs to the ice edge. These southerly records must be regarded as losses of population 

 such as occur in S. marri to the north of its normal habitat, and in the case of S. maxima are probably 

 due to mixing of the antarctic intermediate water with the warm deep water in the vicinity of the 

 convergence. 



The species is most numerous between 150 and 500 m. but extends in smaller numbers to 1500 m. 

 It occurs very exceptionally in the subantarctic surface waters, but apparently never in the antarctic 

 surface waters. 



The main concentration of S. maxima is in the antarctic intermediate water, as reference to the 

 temperature/salinity diagram (Fig. 6) shows, but it is also found on either side of this water mass. 

 The salinity range for the larger concentrations is from 33-98 to 34-5 %o and the temperature range 

 is from 0-3 to 7-3° C, a much wider tolerance than that of S. marri, reflecting perhaps the more varied 

 conditions found in the moderate depths in the subantarctic zone. 



The sections (Fig. 7) show that in 80° W the species extends in considerable numbers south of 

 the antarctic convergence, while the other lines show the main concentrations ceasing well to the north 



