20 



CERATIUM IN THE PACIFIC AND NORTH ATLANTIC OCEANS 



9. Ceratium pentagonum Gourret 

 Figures IOC, D, H-N, chart 9, appendix table 9 



This is a cosmopolitan species composed of three 

 subspecies: a widespread tropical form, subsp. tenerum 

 (Jorgensen); a subantarctic form, subsp. robustimi 

 (Cleve); and a northern Pacific form, subsp. pacificum 

 n. subsp. The authors agree with Nielsen (1934) that 

 Jorgensen' s var. subrobustum is specifically distinct 

 (see below). 



Subspecies tenerum is a widespread tropical form 

 somewhat more common than C. subrobustum . but with 

 a similar distribution except that it seems to be some- 

 what more tolerant of colder water. It was found at 

 eighty-one stations- -fifteen in the Atlantic and sixty- 

 eight in the Pacific. There were 189 records of occur- 

 rence: 152 rare, 34 occasional, and 1 common. Of 

 these records, 73 were from net samples, 114 from 

 pump samples. Expressed in percentages these are: 

 13.6 per cent of net samples, and 31.8 per cent of pump 

 samples. It is thus evident that the tropical form of C^ 

 ■ jentagonum is too small to be properly collected in the 

 tow nets. 



The subspecies was found in the Atlantic mostly in 

 the central line of stations; it was absent from the 

 warmer part of the Gulf Stream and Caribbean Sea. In 

 the Pacific its distribution is somewhat irregular, but 

 the records are probably incomplete there, owing to the 

 fact that the distinctness of C. subrobustum was not re- 

 alized at the time of the routine census of the samples, 

 so some of its records probably belong to C. pentagonum. 

 Ceratium pentagonum . however, extended beyond the 

 range of C. subrobustum in the southeastern Pacific and 

 off California. 



The surface temperatures at the stations at which 

 subsp. tenerum occurred, varied from 15.°0 to 29.°3 C. 

 The ranges of environmental conditions in situ were: 

 temperature, 13.°1 to 29.''3 C; salinity, 33.4 to 37.0 per 

 mille; pH, 7.76 to 8.47; phosphate, 3 to 233 mgP04/m3. 



Nielsen (1934) listed C. pentagonimi as a surface 

 species. In the Carnegie collection it was found less 

 frequently at 100 meters than at the surface and 50- 

 meter depths, and slightly more frequently at 50 meters 

 than at the surface. Thus it cannot be classed as a 

 shade species. Ceratium subrobustum . on the other 

 hand, is definitely a shade species, with its greatest fre- 

 quency at 100 meters (cf. tables 10 and 11). 



Table 10. Records of occurrence of C. pentagonum 

 subsp. tenerum at three levels 



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

 samples collected at that depth. 



Ceratium pentagonum is a remarkable species, in 

 that it has never been collected from the cold northern 

 Atlantic although it is represented in the subantarctic 

 by a cold-water subspecies. In the Carnegie collection 

 the distribution of the species in the North Atlantic 



terminated under 45° north latitude (station 13a), which 

 is somewhat farther north than previous authors had 

 found it. The surface temperature there, however, was 

 above 20° C. It apparently cannot endure transfer into 

 temperatures much below this value. 



The conspicuous absence of C. pentagonum in the 

 northern Atlantic was particularly noted by Peters (1934) 

 who mapped the distribution of subsp. robust\im in the 

 subantarctic waters of the South Atlantic. Peters em- 

 phasized the unequal distribution in which the species is 

 represented in the southern cold water by the most south- 

 erly member of the genus, but is entirely absent in the 

 cold waters of the North Atlantic. Subspecies robustum 

 doubtless populates the entire subantarctic region as 

 there are no barriers in this region. Thus, it probably 

 occurs in the southern latitudes of the Pacific. Peters 

 did notfind it in the Atlantic north of about 48°, however, 

 except where it was displaced along the South American 

 coast by the Falkland Current to about 32° south. This 

 subspecies was not found in the Carnegie collection prob- 

 ably because the stations were not far enough south. 

 The most southern station (station 60) was at about 40° 

 south. It is probable that the subspecies is displaced 

 northward in the Humboldt Current to 30° or so, but in 

 these latitudes the Carnegie stations were too far from 

 the coast to detect it. 



In view of this knowledge it was of particular inter- 

 est to search for this species in the cold waters of the 

 North Pacific. The investigations here revealed the 

 presence of a cold-water subspecies peculiar to this re- 

 gion, subsp. pacificum n. subsp. 



A series of forms was found south of the Aleutian 

 Islands and they have been grouped together imder this 

 subspecies. The extreme form (fig. 101) is remarkably 

 different from the other subspecies of C. pentagonum . 

 It is large (transdiameter 78 microns), with heavy walls 

 covered with thick ridges and lists, and the antapical 

 horns are long (equal to the transdiameter or longer). 

 All the forms in this cold-water region, however, do not 

 conform to this description, and in the west grade into 

 the southern form (see figs. IOC and J). Another varia- 

 tion is shown in figure lOD. These intergrades indicate 

 that the extreme form is of subspecific rank only. 



With the finding of this subspecies it was evident that 

 in the case of C. pentagonum there is a bipolarity in the 

 Pacific with subsp. robustum in the antarctic and subsp. 

 pacificum in the north, whereas in the Atlantic only the 

 southern form is represented. This distribution consti- 

 tutes the most convincing evidence of the planktological 

 isolation of the North Pacific waters (see p. 11). 



Subspecies pacificum was found at seven stations 

 and in seventeen samples (chart 9). The surface tem- 

 peratures of the stations where it occurred, varied from 

 7.°2 to 10.°5 C. The ranges of environmental conditions 

 in situ were: temperature, 4.°2 to 10.°5 C; salinity, 32.7 

 to 32.9 per mille; pH, 7.98 to 8.03; phosphate, 103 to 175 

 mg P04/m3. 



10. Ceratiimi subrobustum (JSrgensen) Nielsen 

 Figures lOA, B, E-G, chart 10, appendix table 10 



This is a rare, intolerant tropical species. Peters 

 (1934) found this form in the South Atlantic only in the 

 Falkland Current and for that reason considered it a 

 subspecies, raising it from the varietal rank given by 

 Jorgensen (1920). Nielsen (1934) found it in the Pacific 



