CERATIUM SPECIES OF CARNEGIE COLLECTION 



25 



to the British Isles to a surface temperature of 12.°4 C. 

 The environmental conditions in situ varied as follows: 

 temperature, 10.°4 to 29.°4 C; salinity, 29.7 to 37.1 per 

 mille; pH, 8.0 to 8.39; phosphate, 2 to 99 mg P04/m3. 

 Nielsen (1934) classified C. extensum as a surface 

 species. The Carnegie data corroborate this classifica- 

 tion. The species was found with decreasing frequency 

 with Increase in depth (see table 17). If due correction 

 were made for contamination of the open nets, this fea- 

 ture would be more pronounced. 



Table 17. 



Records of occurrence of C. extensum 

 at three levels 



A=Number of records. B=Per cent of total number of 

 samples collected at that depth. 



22. Ceratium fusus (Ehrenberg) Dujardin 

 Figures llEE, 13A-D, chart 17, appendix table 21 



Ceratium fusus is a widespread cosmopolitan spe- 

 cies. It may be composed of subspecies but so far no 

 morphological differences warranting a division of the 

 species have been found. Jorgensen (1920) believed that 

 he could distinguish a tropical subspecies (seta [Ehren- 

 berg]) from the northern form of the species, but later 

 workers have not been able to verify this (Peters, 1934; 

 Nielsen, 1934). Peters found several different forms in 

 different parts of the South Atlantic, but theyintergraded 

 so much that he was not able to separate them satisfac- 

 torily. Nielsen considered the species to represent a 

 single genotype which is definitely modified as to size by 

 ecological conditions. His data for the South Pacific 

 show an increase in size under neritic conditions. 



In the Carnegie collection a statistical difference 

 was found between the forms growing in the different life 

 zones (see table 18). It is evident from these data thatt 

 the specimens from cold-water regions have a distinctly 

 greater diameter than those from the warm-water re- 

 gions as shown by both the means and extremes. The 

 length, however, does not show the same correlation. 

 The specimens from the warm Atlantic showed the great- 

 est average length, whereas those from the warm Pacific 

 had the smallest. It is to be hoped that a finer morpho- 

 logical analysis of this species will throw some light on 

 the taxonomic-ecologic problems involved. 



In the Carnegie collection C . fusus was found at ninety- 

 four stations — twenty-five in the Atlantic, and sixty-nine 

 in the Pacific. These stations were distributed from the 

 warmest to the coldest regions visited (see chart 17). 

 There are particular Unes of stations, however, where 

 C. fusus was not found. These distributional gaps cannot 

 be explained on the basis of general rarity of the species. 

 In the Carnegie collection there were 194 records of its 

 occurrence.. Of these, 109 were rare, 67 occasional, 11 

 common, and 7 abundant. Most of the records of higher 

 frequency, however, were from the cold Atlantic region. 



Table 18. Size variation in Ceratium fusus 



Region 



Total length 

 in microns 



Diameter 

 in microns 



and it should be noted that the lines of stations where 

 the species was not foimd are mostly in regions of low 

 phosphate content. Only 23 of the 194 records of occur- 

 rence were in water containing less than 10 mg P04/m3. 

 Thus, it is possible that C. fusus does not normally de- 

 velop to any great extent in oligotrophic water. 



Since the species is cosmopolitan, it was foimd in a 

 great range of conditions. The surface temperatures at 

 the stations where it occurred, varied from 7.°2 to 29.°5 

 C. The environmental conditions in situ were: temper- 

 ature, 2.°1 to 29.°5 C; salinity, 32.7 to 37.0 per mille; pH, 

 7.71 to 8.37; phosphate, 4 to 209 mg P04/m3. 



The very narrow body of C. fusus permits it to es- 

 cape from the larger meshed nets. Consequently there 

 are relatively more pump records for the species than 

 net records. It was found in only 18 per cent of the net 

 samples, but in 26 per cent of the pump samples. 



Nielsen (1934) classified C. fusus as a surface spe- 

 cies. This classification is probably correct. In the 

 Carnegie collection the number of records decreased 

 with increase in depth (see table 19). After correcting 

 for upper -level contamination in the open nets, it is ob- 

 vious that the species is much more frequent in the sur- 

 face layer. 



Table 19. 



Records of occurrence of C. fusus 

 at three levels 



A=Number of records. B=Per cent of total number of 

 samples collected at that depth. 



Subgenus EUCERATIUM 



23. Ceratium tripos (O. F. Muller) Nitzsch 



No species of the genus is so complex and variable 

 as C. tripos . For a time there was a tendency to sepa- 

 rate the components of this form into separate units with 

 specific rank. This movement was led by JOrgensen 



