258 DISCOVERY REPORTS 



their way north into warmer waters at the end of the season, it may fairly be concluded, 

 in the present state of our knowledge, that the survival of the diatoms depends on the 

 small residue of the stock of whales that remains in Antarctic waters throughout the 

 winter (Harmer, 1931, pp. 153, 154)- How, then, does the film spread so rapidly upon 

 the "clean" arrivals from warmer waters in spring? The development of vast quantities 

 of microspores, which may be shed into the sea, aided by the schooling habit both of the 

 whales themselves and of the food they seek, together with the additional tendency to 

 local concentration caused by the position of South Georgia in relation to the ice-edge 

 and prevailing currents, furnishes the most probable explanation, as we have seen that 

 microspore formation is at a maximum at the beginning and end of the season. The 

 abundance of microspores at the end of the season will tend to ensure that a large 

 proportion of the whales that remain south throughout the winter become infected, 

 and the whole argument is greatly strengthened when the observations on Sei whales 

 during the latter part of the 1930-1 season are considered. As explained elsewhere 

 (p. 261) Sei whales, unlike the larger southern rorquals, make a brief journey southwards 

 towards the end of the whaling season. It is true that they are not captured when 

 a sufficiency of the larger and more profitable species are available, but it so happened 

 that during the second half of the 1 930-1 season whales were very scarce, so that it is 

 virtually certain that Sei whales were captured as soon as they appeared on the South 

 Georgia grounds. When they made their first appearance in February no diatom film 

 could be discerned upon them, but skin scrapings from the head region and mandible 

 showed the presence of vast numbers of the characteristic spores of C. ceticola, with, in 

 some cases, a few adult frustules. Towards the end of the month one Sei whale was 

 captured with film visible to the naked eye upon the snout. The records from the two 

 previous seasons also suggest that a period of about one month elapses between the 

 entry of the Sei whales into Antarctic waters and the appearance of visible diatom film 

 upon them, for as shown by Deacon (1933, p. 190, fig. 11) South Georgia lies near the 

 northern boundary of the Antarctic Zone. 



The colour of the diatoms forming the skin film was found to vary considerably be- 

 tween light yellowish green and dark yellowish brown. A brownish shade has been ob- 

 served in most living neritic diatoms, many of which are probably saprophytic in their 

 mode of nutrition, while true pelagic forms tend to develop a greener shade. However, 

 pelagic diatoms as seen eti masse generally appear brownish, it is when the samples are 

 preserved in salt-water formalin that the difference in the quality of the endochrome 

 appears. Under these conditions gatherings of pelagic diatoms always appear green, 

 while gatherings of neritic and skin-film species retain a yellowish tint. The colour of 

 the skin film in situ would not appear to give any reliable guide to the mode of nutrition, 

 as Stanbury (i 931, p. 651) has observed that diatom cultures show chromatic adaptation 

 to the quality of the light in which they are grown. The quality of the light falling upon 

 the skin film will obviously vary with its position on the whale's body while the latter 

 lies in the water (usually for some hours) before being flensed. This appears to be the 

 obvious explanation of the variation in colour of the skin film observed in situ. 



