MICROPLANKTON 219 



the negative temperature anomaly was greatest. This can scarcely be due to the effect of temperature, 

 however, for the species is often abundant in water as cold as this (in the absolute sense, not in terms 

 of iso-anomaly) far to the south on the Atlantic side of Cape Peninsula. Further, it may be equally 

 abundant in warm waters of the Agulhas current on the east coast, as far north as Durban and probably 

 beyond. This implies temperature tolerance throughout the range io° to 23 ° C. at the least. The dis- 

 continuity in distribution off the south-west coast could, however, be due to a markedly stenohaline 

 reaction on the part of D. tripos. The area from which we found it almost absent coincides with the 

 low salinities observed northwards from the Orange river mouth throughout the area of maximum 

 upwelling activity to Luderitz Bay and beyond. 1 



The point has been mentioned here because Schiller (1933) who has done so much to clear up the 

 taxonomy of the Caudata group, added the trenchant note 'Ob immer stenohaline?' concerning the 

 distribution of D. tripos, which suggests that this trait had already been noticed in other regions. 



Diversity of the microplankton 

 The rich variety of the microplankton in the region surveyed may be judged from the list of all 

 208 categories recorded in the routine counts (Table 12). This table also shows their frequency of 

 occurrence, and the frequency with which they occurred as dominants in each of the two separate 

 •series of thirty-nine stations, repeated at approximately the same positions, and in the smaller sub- 

 sidiary series as shown by the column headings. 



Table 12. List of all microplankton categories recorded among routine counts, with data on their 

 frequency of occurrence, and dominance, as shozvn by column headings 



Species or category 



DlATOMACEA 



Melosira sphaerica Karsten 



Skeletonema costatitm (Grev.) Cleve 



Stephanopyxis palmeriana (Grev.) Griinow 



S. turris (Grev. & Arnott) Ralfs 



Thalassiosira condensata Cleve 



T. excentrica Karsten 



T. hyalinum (Griin.) Gran 



T. rotula Meunier 



T. subtilis (Ostenfeld) Gran 



Thalassiosira spp. non det. 



Bacterosira fragilis Gran 



Coscinodiscus gigas Ehrenberg 



C. janischii A. Schmidt 



C. parvulus Karsten 



C. radiatus Ehrenberg 



Coscinodiscus spp. non det. 



Actinocyclus spp. non det. 



Planktoniella sol Wallich 



Hemidiscus cuneiformis Wallich 



Actinnptychus senarius Ehrenberg 



Asterolampra spp. non det. 



Asteromphalus heptactis (Brebisson) Ralfs 



Biddulphia longicruris Greville 



B. mobiliensis Bailey 



B. regia (Schultze) Ostenfeld 



Cerataulina pelagica (Cleve) Hendey 



Triceratium favus Ehrenberg 



Triceratium spp. non det. 



1 Here Dietrich (1950) has shown iso-lines of negative anomaly of 0-5-1-5 % salinity, with normal values north and south, 

 and but a little distance seaward. 



'3 



