Fig. 15- 



An auxospore of Dactyliosolen antarcticus. 

 X500. 



SPORE FORMATION OF PLANKTON DIATOMS 327 



the auxospore was already visible, though still adhering to the two halves of the original 

 narrow cell that gave rise to it by rupture of the connective zone at one side. 



Auxospore formation in some Chaetoceros spp. has been seen quite frequently but 

 not recorded systematically, for most of the stages are too early to enable one to deter- 

 mine their numbers with certainty in pre- 

 served samples. One good example of the 

 process in Thalassiosira antarctica, after 

 mid-season when that species is rapidly de- 

 creasing in numbers, has also been observed. 



The formation of resting spores has been 

 noted with certainty in a few species, most 

 frequently in Rhizosolenia alaia, Rh. simplex 

 and Rh. tnincata. In the autumn of 1938 

 in the Scotia Sea and at South Georgia, 

 most of the population of Biddulphia striata, 

 which predominated in the scanty phyto- 

 plankton present, was in process of forming 

 resting spores as described in the separate 

 notes on that species. 



It will be noted that so far as these scanty observations go resting-spore formation 

 would seem to follow marked decreases in the numbers of the population as conditions 

 become unfavourable, precisely as one would expect. It is even probable that the so- 

 called type phase, or winter phase, of Eiicampia balaustiim, are the resting spores of the 

 species, which is abundant only in the summer moelleria phase. 



THE FEEDING OF PLANKTON ORGANISMS 

 Some progress has been made with the examination of the stomach contents of 

 Euphausia stiperba and other important plankton animals. The observations were 

 aimed at the determination of ' competitors ' and ' enemies ' of that most important of 

 Antarctic plankton animals, but have only reached a preliminary stage. 



All the Euphausia superba examined have contained recognizable diatom remains, 

 and Foraminifera have been the only animals identified with certainty in their stomachs. 

 Euphausia frigida, the Copepods Rhincalaniis gigas, Calaniis acutus and C. propinquus, 

 and the Pteropods Limacina helicina and Cleodora sulcata, were all found to have been 

 feeding on plankton diatoms. The great difficulty in the proper interpretation of these 

 findings lies in the different degree of silicification of the cell walls of different diatom 

 species. Those identifiable with certainty in the stomachs of plankton organisms are 

 those most strongly silicified— the same that remain recognizable in bottom deposits 

 and in bird guano, such as: Fragilariopsis, Thalassiosira, other Discoidae, fragments of 

 Thalassiothrix, of spines of Chaetoceros criophilum, terminal spines of Rhizosolenia spp., 

 etc. As I have already pointed out (Hart, 1934, pp. 11, 186) the more typically oceanic, 

 less strongly silicified forms are probably quite as important as food for the planktonic 



9-2 



