THE OLDER STAGES 209 



be of considerable significance that it is in the neighbourhood of the Princess Martha-Coats Land 

 coast (p. 213) that particularly strong movements in the cold deep stratum have been reported. 



Although our nets so far have not revealed it happening, the migration of the gravid females 

 to great depths would appear to be well within the powers of this large and vigorous euphausian. 

 It seems in fact it could happen quite quickly, Hardy and Bainbridge (1954) having shown experi- 

 mentally that its lesser counterpart in northern waters, Meganyctiphanes norvegica, is able to swim 

 vertically upwards at a speed of 92-8 m./hr.^ for i hr. and vertically downwards at a speed of 128-8 m./hr. 

 for the same period. In so far as the much larger krill are concerned, if the downward speed recorded 

 for M. norvegica could be sustained in nature, sustained that is over long periods, it would mean that 

 the gravid females could swim down to say 2000 m., spawn, and return spent to the surface in a 

 matter of about 32 hr. Indeed, if the sustained horizontal speed of the krill against the tidal stream 

 directly observed by Gunther (p. 155) is any indication of their capabilities in the vertical plane, it 

 would appear that they could accomplish the same journey inside 8 hr. In other words they would go 

 down and up in a night. If a deep spawning migration did in fact take place so rapidly, and above 

 all if it were undertaken by single individuals and not by the massed formations of females to which 

 our evidence seems to point (pp. 233-4), it could conceivably have escaped our notice simply 

 because the area in which we have operated is so vast and our observations in it, comprehensive though 

 they be, are yet so widely scattered. 



We are always, however, confronted with the fact that such a rapid descent as we are postulating 

 would only be possible if the gravid females were utterly indifferent to the enormous changes in pressure 

 involved. For if they do go at high speed to great depths to spawn, down to say 3000 or 4000 m., they 

 would encounter quite suddenly pressures of from 300 to 400 atmospheres, and although Regnard 

 (1891)2 has shown experimentally that Cyclops and Daphnia have recovered after 5 min. at 600 atmo- 

 spheres ( = a depth of about 6000 m.), we do not know if predominantly surface living animals like 

 the Antarctic krill could equally endure and survive such enormous environmental change. That they 

 must both endure and survive it, if the deep spawning we are postulating is a fact, is clear from the 

 pronounced occurrence at the surface (Table 39) of those that have shed their eggs.^ They have in 

 fact even been reported (Bargmann, 1945) actively feeding there.* In the present state of our know- 

 ledge, therefore, the most we can say is that deep spawning, if it takes place, and above all if it takes 

 place rapidly, postulates in the gravid females an astonishing measure of accommodation to gross 

 environmental change, and that whether or not they can in fact adjust themselves to it perhaps only 

 future experiment, backed by the extensive use of large, very deep, closing nets, will eventually show. 

 In the meantime it might be noted, in the face of the Regnard experiments, as Russell (19276) 

 remarks, pressure does not seem to be a factor of very great importance in the vertical distribution 

 of plankton animals. 



Finally, it might be observed, although the large haul of eggs at Station 2594 is suggestive of deep 

 oceanic spawning, their developmental condition, the majority of them containing advanced or very 

 advanced Nauplii (p. 184, Fig. 22), points rather to their having been laid much nearer the 



^ An even higher experimental rate of cUmb is claimed for this species by Bainbridge (1953) who states that it is known 

 to be able to swim vertically upwards at a speed of about 128 m./hr. for several hours on end and to be capable of bursts of 

 climb as high as 271 m./hr. We must always bear in mind, however, that although these phenomena have been observed in 

 the laboratory it does not necessarily follow that the same would occur in nature (Moore and Bauer, i960). 



^ Cited by Russell (19276). 



^ The swarms of spent females recently reported by Nemoto (1959) from whales' stomachs would also postulate that if they 

 do go deep to spawn they must return alive to the surface. 



* In his field notes Mr Fry records that a gravid female taken on 19. i. 38 was placed that day alive in a jar on the deck of 

 'Discovery 11'. On 22. i. 38 it shed its eggs. It was alive the night of 25-26. i. 38 when it moulted. It was killed and pre- 

 served with its moult on 26. i. 38. 



23 DM 



