326 DISCOVERY REPORTS 



midday, and that towards dusk there is a fairly rapid rise into the upper 250 m. of water, followed by 

 a descent again in the early morning. 



It seems probable, though by no means certain, that those animals which in daytime are between 

 500 and 750 m., do not migrate into the upper 100 m. during the night but remain within the 250- 

 100 m. level, whereas those animals that spend the day between 500 and 250 m. probably enter the 

 top 100 m. 



There is no evidence from these data of the midnight sinking displayed by many other diurnal 

 migrants. It may well occur, however, for it is unlikely to be detected with time intervals as widely 

 spaced as these. 



Various authors have shown that the extent of diurnal migration varies in some animals with their 

 age, size and sex. Russell (1931) found that the smaller specimens of Sagitta elegans Verrill, 1873, 

 rose to the surface before the larger ones at dusk and were later in leaving the surface at dawn ; Southern 



o 



5 o 

 100 



^ aso 



s 00 



»9) 



oa 



BO) 



ADOLESCENTS AND ADULTS 

 ALL STAGES 



— 1 



30 



I T 1 I 1 



O S 10 15 20 25 



MEAN LENGTH (MM) 

 Fig. 10. Variation of the mean length with depth of all stages and of adolescents and adults only taken in the N70V 

 between 2000 hrs. and midnight. The numbers of specimens upon which the means are based are shown in brackets. 



and Gardiner (1932) showed that the reverse applied to Daphnia longispina O. F. Muller (1785) in 

 Lough Derg. Motoda (1953), working at Lake Shikotsu in Japan, found that at midday the copepodites 

 of Acanthodiaptomus yumanacensis (Brehm) were nearer to the surface than the adults. He also 

 observed that the males of this species showed a greater sensitivity to light than the females. Gushing 

 (195 1 ) suggests that 'some animals become more sensitive to light as they grow older and others 

 become less so '. 



In Fig. 10 the mean lengths of adolescents and adults taken by the N70V between 2000 hrs. and 

 midnight on the Greenwich meridian, have been plotted against depth. We are only concerned here 

 with the relative lengths at different depths, the actual lengths are dependent upon the time of year 

 the hauls were made. There is little doubt that there is a definite tendency for the smaller individuals 

 to inhabit the upper 100 m. during this period whereas the larger ones are found below this level. 

 It does not follow that the larger animals are never found in the upper layers, for specimens of over 

 35 mm. have frequently been taken in the loo-o m. NiooB at night; nor does it mean that the 



