26 



CERATIUM IN THE PACIFIC AND NORTH ATLANTIC OCEANS 



(1911, 1920). Later a tendency developed to lump these 

 units into a Formenkrelsor a few species. Peters (1934) 

 included under C. tripos all the varieties of pulchellum 

 (see Jorgensen, 1920). He divided the species into three 

 subspecies: atlanticum . pulchellum. and semipulchellum . 

 Nielsen (1934), on the other hand, recognized four sepa- 

 rate species in this group as follows: C. tripodoides . C. 

 semipulchellmn . C. pulchelltun . and C. tripos . He sepa- 

 rated the first three species from each other on the ba- 

 sis of statistical studies of individual plankton samples. 

 He found a grouping according to diameter and "antapi- 

 cal length." The last, C. tripos , which he found only in 

 the cold water in the region of Australia and New Zea- 

 land, he considered distinct because of its shorter apical 

 horn and stronger antapicals. These characters can 

 hardly be sufficient for the sepai'ation of a species in 

 this variable group. It is probable that the New Zealand- 

 Australian form is simply a southern subspecies or race. 

 Certainly Nielsen's drawings (figs. 32 and 33) do not 

 show sufficient distinctness for a separation from his C. 

 semipulchellum (fig. 30). 



The present authors lean toward the view of Peters 

 (1934) in regard to this group, with the exception that C^ 

 pulchellum (eupulchellimi of Jorgensen) has been treated 

 as a separate species. Considering the collection as a 

 whole, there was very little inter gradation between this 

 form and semipulchellum . Between semipulchellum and 

 tripos , on the contrary, there are many intergradations. 

 i.e., in various characters such as size, spread of horns, 

 and antapical curvature. It was considered advisable for 

 the present to treat these two units as subspecies. They, 

 in turn, are each composed of an endless number of va- 

 rieties or races, which, however, are not recorded here 

 as they are Impossible to separate with our present 

 knowledge of morphological characters in this genus. 



Ceratiimi tripos subsp. atlanticum (Ostenfeld) 

 Figures 13E-K, chart 18, appendix table 22 



This subspecies is more characteristic of colder 

 waters although it is sometimes found in the tropics; 

 The tropical forms are probably separate races; some 

 of them vary toward the semipulchellum form. 



In the Carnegie collection subsp. atlanticum was 

 found throughout the cold Atlantic region and at ten sta- 

 tions in the warm Atlantic region. In the Pacific it was 

 found in the cold North Pacific region to the west and 

 east but not in the most northern part. In the warm Pa- 

 cific region it was found at ten stations northof 27° north 

 latitude, and at two stations in the southeastern part. In 

 the southeast Pacific region it was found at thirteen sta- 

 tions. The total number of record stations for the sub- 

 species was sixty-one — twenty-six of which were in the 

 Atlantic and thirty-five in the Pacific. 



It was not found so often as subsp. semipulchellum 

 but a high percentage of the records were above "rare." 

 The total number of occurrences was 163, of which 49 

 were rare, 61 occasional, 45 common, and 8 abundant. 



Since the subspecies is cosmopolitan, the range of 

 environmental conditions was great. It was not, however, 

 foimd at the wannest stations of the Pacific. The range 

 of surface temperatures at the record stations in the At- 

 lantic was from 8.°9 to 28.°5C; in the Pacific from 6.°9 to 

 27.°1 C. The environmental conditions In situ were: tem- 

 perature, 6.°1 to 28.°2 C; salinity, 31.7 to 36.4 per miUe; 

 pH, 7.80 to 8.37; phosphate, 2 to 181 mg P04/m3. 



This subspecies, like the rest of the Formenkreis, 

 is a surface species, although it is also found in consid- 

 erable numbers In the deeper levels. As shown in table 

 20, subsp. atlanticum was found most frequently in the 

 surface samples. 



Table 20. Records of occurrence of C. tripos 

 subsp. atlantictun at three levels 



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

 samples collected at that depth. 



Ceratium tripos subsp. semipulchellum (Jorgensen) 

 Figures 13L-N, chart 19, appendix table 23 



This subspecies is a slightly tolerant tropical form 

 which occurs almost continuously in the warm regions 

 and the southeast Pacific region (chart 19). It is limited 

 to the tropical waters except in the southeastern Pacific 

 and northeast of Japan. In the Atlantic it was limited en- 

 tirely to the warm Atlantic region. In the Pacific it oc- 

 curred at one station northeast of Japan in the cold North 

 Pacific region, although not in water of low temperature; 

 the lowest surface temperature being 16.°1 C. In the 

 southeastern Pacific region it was found at all but three 

 stations. The surface temperatures at the record sta- 

 tions there were as low as 15° C. The total range of 

 surface temperatures was from 15° to 29.°5 C. The 

 ranges of environmental conditions In situ were: tem- 

 perature, ll.°4 to 29f4 C; salinity, 31.6 to 36.6 permUle; 

 pH, 7.76 to 8.47; phosphate, 2 to 198 mg P04/m3. 



The subspecies was found at a total of 127 stations-- 

 21 in the Atlantic and 106 in the Pacific. It is one of the 

 most common of the Ceratia. This is shown, not only by 

 the large number of stations at which it was found, but 

 also by the large number of single records of occurrence 

 and relative abundance. There were 391 records of oc- 

 currence, with 161 rare, 156 occasional, 72 common, and 

 2 abundant. Since it is a large form, it was found more 

 often in the net samples. There were 256 net records 

 and 135 pimip records. 



The subspecies definitely does not avoid oligotrophlc 

 water, as its record stations are unbroken through re- 

 gions of poor water, and 140 of the 391 records of occur- 

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



In regard to depth, Melsen (1934) states that seml- 

 pulchellum is a definite surface form. The Carnegie 

 data, on the contrary, indicate that it lives throughout the 

 upper 100-meter layer. The greatest percentage of 

 "rare" records was at 100 meters (see table 21); of 

 "occasional" records at the surface; of "common" rec- 

 ords at 50 meters; and the greatest total percentage was 

 at 50 meters. The large number of records for the 50- 

 and 100-meter levels cannot be explained entirely on the 

 basis of contamination of the open nets since there were 

 twenty-three stations scattered over the Atlantic and 



