262 DISCOVERY REPORTS 



Dactyliosolen mediterraneus. This last provides the greatest enigma, being found in great numbers 

 inshore during the first (autumn) survey, but also among the dominants on the outer shelf and offshore 

 (especially in the northern part of our area) in spring. It is possible that more restricted temperature 

 requirements may account for this anomaly if the spring temperatures inshore are suboptimal for this 

 species. Conversely, Corethron criophilum, undoubtedly an oceanic form in most of its phases, here 

 showed a very definite maximum inshore in spring, which might be explained by 'preference' for 

 lower temperatures. 



The ecological relationships of the ' Pennatae ' (Table 27) at group level are not very clear, chiefly 

 because it is an arbitrary ' unnatural ' assemblage of the diverse ' reversionary plankton forms ' with 

 sundry tychopelagic species, introduced solely to assist in concentrating the data to within manageable 

 proportions. At the specific level, however, most of them can be seen to conform to definite distribu- 

 tional trends much more clearly than most of the solenoids. 



Fragilaria karsteni, Asterioriella japonica and the rarer tychopelagic species all had a definitely inshore, 

 neritic distribution. Fragilaria karsteni was most abundant in spring and Asterionella japonica in 

 autumn. Thalassiothrix longissima was an offshore form with a spring maximum, and so in the main 

 was Fragilaria granulata, which never overlapped its congener. 



The only important ' Pennatae ' not clearly assignable to one of the distributional trends that we now 

 feel to be recognizable (somewhat dimly, perhaps) are Nitzschia delicatissima and N. seriata. These 

 are among the most ubiquitous, cosmopolitan, panthalassic diatoms known, as all marine plankton 

 workers must agree. Here they were prominent at all distances from land especially in spring. 

 N. seriata was the more important inshore, while N. delicatissima showed maximum relative importance 

 in the scanty offshore catches in autumn, though not nearly so numerous there as it was on the 'outer 

 shelf during both surveys. 



Table 27. Relative importance of the group Pennatae, and percentage of the several species within the 

 group. Results arrayed according to distance from the land as shozvn, omitting those from the Orange 

 river line 



{a) Average total diatoms 

 {b) Average total 'Pennatae' 

 bja as percentage 



Average for each category as percentage 

 of the average total for the group {M 

 category jb as %) 



Fragilaria granulata 



F. karsteni 



Asterionella japonica 



Thalassiothrix longissima 



Thalassionema nitzsch hides 



Striatella sp. non det. 



Navicula membranacea 



Navicula spp. non det. 



Pleurosigma capense 



Pleurosigma sp. non det. 



Nitzschia closterium 



N. delicatissima 



N. lojigissima 



N. seriata 



12-77 



64- 1 5 

 0-06 



0-04 

 o-oi 



0-02 



o-8i 



0-31 



0-06 



2177 



o-iS 



48-95 



5-42 

 i-54 

 4-35 



0-03 

 0-17 

 o-oi 



8-24 



3!-24 



604 



3-36 



29-04 

 i-68 



118 



0-06 



3 1 " 



