,2o DISCOVERY REPORTS 



from the rain of dead or dying phytoplankton sinking from above. Moreover, as Bargmann (1945) 

 points out, the external food supply is not of immediate concern to the larva when first it leaves the 

 egg, since the euphausian egg contains a large quantity of yolk, sufficient it seems to feed the young 

 larva for some little time after hatching. 



Groom and Loeb (1890) give the average speed of the vertically migrating nauplii of the acorn 

 barnacle, Balanus perforatiis, as about i mm. per sec, while Parker (1902) found that the copepod 

 Labidocera aestiva could climb 1-83 m. in 18 min. and descend the same distance in six. Assuming 

 these rates could be sustained upwards over long periods, in terms of time spent climbing up through 

 the warm deep current in our southern waters, they would work out at a minimum of 21 days for the 

 Balanus nauplii and at a minimum of 12 days for the Labidocera. Hardy (1953, 1956) finds that in 

 I hr. the nauplii of Balanus sp. can climb nearly 50 ft., approximately quadruple the rate of Groom 

 and Loeb's nauplii, but it seems doubtful if such a high speed could be sustained over the immense 

 distance involved in the developmental ascent. 



So far I have been assuming the newly hatched krill rise to the surface through their own efforts. 

 It may not be so, however, since for all we can tell they may be so constructed that they are able to 

 ' balloon ' upwards although more likely perhaps they are impelled by some tactic sense about which 

 we know nothing. However it comes about the long climb manifestly must demand of the rising 

 larvae a large, indeed astonishing, measure of adaptability to environmental change, change in fact 

 so gross, in so far for instance as pressure is concerned, ^ that one can scarcely imagine them surviving 

 it except there be an extremely slow and gradual upward trend. 



The experiments of Hardy and Bainbridge (1951a) and of Knight-Jones and Qasim (1955) show 

 that, starting from one atmosphere, certain plankton animals are responsive to small changes of from 

 one half to two atmospheres in pressure (= from 5 to 20 m. in depth), in general becoming more 

 active and swimming upwards in response to increases, and becoming less active, or totally inactive, 

 and allowing themselves to sink, when the pressure is lowered. This behaviour, or high ' barokinesis ' 

 as Knight-Jones and Qasim have called it, is usually they observe combined with negative geotaxis. 

 It might be supposed from these experiments that the animals, if originally collected on the surface, 

 were finding the slight pressure increases uncongenial and that they were rising in their experimental 

 tubes'- towards the levels of lower pressure they would naturally find in the sea. The developmental 

 ascent, therefore, could be described as an extreme instance of barokinesis, or an extreme instance of 

 negative geotaxis, or a combination of both. We can only speculate, however, the very living existence 

 of the larvae at the enormous pressures at which they hatch presenting a physiological phenomenon 

 that perhaps for long will remain unexplained. Much obviously has yet to be done, above all now in 

 the laboratory, before we can even begin to understand the * forces ' compeUing the larvae to rise or 

 the mechanism controlling the physiological adjustments necessary to bring their long climb to its 

 successful end. As Carthy (1957) has remarked of the smaller scale vertical movements involved in 

 diurnal migration, ' if pressure is to be the controlling stimulus, either there is a daily cycle of diflferent 

 preferences innate in the animal, or the effect of another changing stimulus is to alter the animal's 

 pressure preferendum'. In the young rising krill the pressure preferendum would manifestly be 

 subject to continuous, uni-directional, and in the long run, enormous, change. 



It will be interesting to discover what makes the newly hatched krill behave in this way, why they 

 should go up instead of down or why indeed they should not remain just where they are. Referring 



^ As Bruun (19576) has said, neither absence of sunhght nor low temperature is pecuHar to the deep-sea environment. 

 Both are the common heritage of many other pelagic animals living at intermediate depths or at the surface near the poles. 

 'The unique dominating factor is pressure'. 



'^ The Hardy and Bainbridge tube was 20 in. long. 



