VERTICAL MIGRATION RECONSIDERED 323 



cussed in a subsequent section. Perhaps a better way of stating the same hypothesis 

 would be to say that the vertically migrating animals, which live for part of the twenty- 

 four hours below the phytoplankton zone, come up into that zone for a period inversely 

 proportional to the density of the phytoplankton. 



An alternative hypothesis may be introduced under the influence of Harvey's 

 recent work. Considerations in a later section have led to the conception of a simple 

 system of currents, which could act on the vertically migrating animals in such a way 

 that their numbers became reduced in one area and increased in another without there 

 being any actual modification of their migrational range or rhythm. 1 Such a system 

 might explain the similar distribution of a large number of different animals in relation 

 to phytoplankton production, the latter being controlled by the grazing activities of the 

 former. These two hypotheses are not antagonistic. But it is thought that the second 

 method would only be very local and temporary in action, and would hardly account 

 for the correlations of zooplankton with phosphate values, which suggest the gradual 

 adjustment of distribution over a considerable period of time. In the present section 

 we will examine only the first proposition, that of the possible modification of vertical 

 distribution by the phytoplankton, and further consider the general significance of 

 vertical migration in terms of advantage to the organism. 



Russell (19286) found in the Plymouth area that during two seasons' observations 

 Calanus JinmarcMcus showed a change in its vertical distribution in the daytime during 

 summer months. From being low in the water in May and June it rose nearer the sur- 

 face in July, and in August and September, even in bright sunlight, was found close 

 against the surface. He had shown (in 19276) that Tomopteris helgolandica, and later that 

 post-larval young of teleostean fish (1930) and Sagitta (1933) have a similar change in 

 behaviour. He writes (1933) as follows: 



This appearance of Sagitta in the upper well-illuminated layers in July and August, and perhaps 

 September, is very striking. . . . 



That animals belonging to such widely different groups should all show this same type of change 

 in behaviour at this period of the year is very remarkable and it must surely indicate a phenomenon 

 of rather fundamental importance, and may have some significance in the relation of vitamin 

 content. 



Bigelow (1924) records Calanus finmarchicus swarming at the surface in the daytime 

 on two occasions in July and August in the Gulf of Maine. 



Nicholls (1933) in the Clyde area found male, female and stage V Calanus finmarchicus 

 more abundant in the upper layers in the daytime on two occasions in 1932 in May and 

 June, and more recently in describing the vertical distribution in 1933 Marshall, Nicholls 

 and Orr (1934) record that males, females and stage V were taken in larger numbers 

 in the top 10 m. in April and May than later in the year. 



Russell explains the difference in behaviour of his Calanus and Sagitta in late summer 

 from that of those in the early summer as being due to different broods reacting dif- 

 ferently to light. But now it is seen that the earlier broods of Calanus in the Clyde area 



1 Pp- 354-355- 



