THE LARVAL STAGES 121 



to Other plankton animals and their responses to pressure Hardy (1956) asks, 'Have they some small 

 pressure gauge — perhaps a tiny gas-filled vesicle expanding and contracting against a nerve ending? Or 

 is it just the varying effect of pressure upon some vital chemical reaction within the body ? ' As Bruun 

 (1957^) has said, ' It is definitely known that the hydrostatic pressure affects the physiological process, 

 but this is known so far only from experiments with surface animals or deep-sea bacteria. It seems, 

 however, safe to conclude that more knowledge about this pressure factor in relation to the bathyal and 

 abyssal species may clear up many controversies. Some species have a greater tolerance towards varia- 

 tions in hydrostatic pressure than others.' The tolerance of the climbing larvae, it seems, must indeed 

 be enormous, and perhaps it is specific to each individual stage during this early developmental phase. 



A parallel phenomenon operating in the reverse direction is provided by the migration, or more 

 correctly perhaps sinking, of full grown Sagitta gazelle (David, 1955) to enormous depths in order 

 to shed their eggs, and both movements perhaps have their roots far back in phylogenetic history. 

 As Young (1954) has recently said, 'We can say that the organism is instructed by the information it 

 receives from heredity, from its environment and from whatever memory store it has built up during 

 its Ufetime '. Huxley (1912) also remarks on the many structures and habits of animals 'that can only 

 be made fully intelligible through their history '. 



In Eraser's original experiments (p. 55) gravid females taken on the surface, or from near the surface, 

 were kept on deck in small aquaria in the Antarctic summer air. There, at atmospheric pressure, they 

 shed their eggs. 'Attempts to induce segmentation ', however, were unsuccessful. In the same condi- 

 tions Fry also got gravid females from the surface to lay their eggs but he too found they would not 

 divide. Other laboratory workers have had more success. Lebour, for instance (p. 119), working with 

 Nyctiphanes couchii, was able to rear this species from the egg right through to the Third Calyptopis 

 stage. Again this was done at atmospheric pressure, in this instance, however, with a neritic species 

 in which eggs, larvae and adults naturally occur near the surface or at their deepest never very far 

 away from it. These findings distinctly suggest that in the Fraser and Fry experiments it was the vital 

 element of pressure that was missing and that in Eiiphmisia superba the liberated eggs might be expected 

 to divide only if gradually subjected to the increasing pressures, and ultimately to the enormous pressure 

 maxima, they would naturally encounter if, as seems possible (p. 184), they sink in the sea. The 

 resultant larvae might then be induced to develop by reversing the pressure trend. As ZoBell (1954) 

 and ZoBell and Morita (1956) have recently shown with bacteria brought up from the deep-sea floor, 

 reproduction is more rapid at high than at low pressures, luminescent forms only beginning to show 

 their light when re-subjected to the immense pressures of their natural environment. 



The vital role of pressure, we may well indeed say enormous pressure, in the early developmental 

 history of this species seems to be strongly emphasised by our widespread failure to find any evidence 

 of successful hatching other than at great depths (p. 201) in oceanic water. Hatching has not so far 

 been found to be a successful event in shallow shelf seas. 



Finally, it may be noted, it must happen occasionally that local upwelling of the bottom water, as 

 for instance (p. 102) at Stations 823 and 2594, is assisting both eggs and larvae to rise. 



In so far as the laboratory will be concerned, perhaps alone concerned, in explaining the pheno- 

 menon of the climbing krill, and other equally little understood phenomena of the open sea, Rae (1958) 

 has recently called attention to our pressing need for establishing 'a reasonable variety of healthy 

 plankton species in captivity ', adding that if we succeed in so doing 'we open up entirely new facilities 

 for a wide range of studies that have not yet been applied to oceanic animals '. 



It seems likely then, that in their ascent towards the surface the larvae are carried in a current with 

 a southerly component^ for a period which at its shortest might be put at 10 days and at its longest at 



^ Except perhaps (p. 123) in part of the Atlantic sector. 



