16 



CERATIUM IN THE PACIFIC AND NORTH ATLANTIC OCEANS 



in the South Pacific. In the Carnegie collection it was 

 pretty well scattered throughout the tropical waters of 

 the Pacific, occurring at seventy-five stations and 

 sometimes in water of somewhat lower temperature 

 than that of the Atlantic. Off Japan it occurred to 40.°5 

 north, surface temperature 15.° 9 C (station 117). Off 

 Calif ornia it occurred to 34 ° north, surface tempera- 

 ture 19.°1 C (station 31). In the southeastern Pacific it 

 occurred south of Easter Island to 34 ° south, surface 

 temperature 19° C (station 57). In the extreme south- 

 eastern Pacific it was not common, occurring at only 

 four stations east of the line of stations in the longitude 

 of Easter Island. 



Although widespread, this species is never very 

 abundant. There were 182 records of occurrence, 144 

 of which were rare and 38 occasional. Of these records, 

 164 were from the net samples and 18 from the pump 

 samples. 



Ceratium gravidum was classified as a shade spe- 

 cies by Nielsen (1934). In oligotrophic water he found 

 that it had its main distribution in the levels from 200 to 

 100 meters, although he found it in the upper levels in 

 eutrophic water. The Carnegie data substantiate this 

 classification of Nielsen. The total number of records 

 (in percentage) at 50 meters is more than three times 

 that at the surface; the number of 100-meter records 

 shows an increase over that for 50 meters (see table 4). 

 This distribution is shown also by the rare and occasion- 

 al records separately. Thus, C. gravidum is a decided 

 shade species. 



Table 4. 



Records of occurrence of C. gravidimi 

 at three levels 



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

 samples collected at that depth. 



In the Carnegie collection as a whole, C. gravidum 

 seemed to be found in both oligotrophic and eutrophic 

 water. Sixty-one of the 182 records of occurrence were 

 in water containing less than 10 mg P04/m3. The sur- 

 face temperatures at the stations where it occurred, 

 varied from 15.°9 to 29.°5C. The ranges of environmen- 

 tal conditions in situ were: temperature, 10.°6 to 29.°3 C; 

 salinity, 33.4 to 37.0 per mille; pH, 7.76 to 8.44; phos- 

 phate, 2 to 153 mg P04/m3. 



Variation. -- Ceratium gravidmn is a variable spe- 

 cies. Jorgensen (1911) divided it into three varieties and 

 later (1920) described a fourth variety. These have been 

 more or less accepted by later workers (Nielsen, 1934 

 and Peters, 1934), although apparently no one hitherto 

 has had sufficient material to test the validity of these 

 varieties. In the Carnegie studies hundreds of specimens 

 were examined, and it was evident that no taxonomlc 

 grouping could be made of the wide variations in body 

 form found in this species. Representative specimens 

 are shown in figures 3 and 4. Each of these drawings is 

 representative of a group of individuals examined and 

 other variations between these were found. Thus, there 



is a complete intergradation between the wide rotund 

 form shown in figure 3A (which is even wider than JOr- 

 gensen's var. latum) and the narrow form shown in fig- 

 ure 4U (which, in turn, is narrower than JOrgensen's 

 var. angustum ). The characteristics of the antapical 

 horns such as length, thickness, curvature, and diver- 

 gence (which have also been used in differentiating the 

 varieties) also vary in such a manner that no varietal 

 segregations are indicated. An attempt was made to 

 correlate the variations of C. gravidum with geographic 

 location, or with different water masses. No correlation 

 was found; the different variations were indiscriminately 

 scattered throughout the areas investigated. Peters 

 (1934) found the different varieties in the same regions 

 of the South Atlantic. Nielsen (1934) found var. latum to 

 be restricted to the eutrophic water of the eastern part 

 of the South Equatorial Current, whereas var. elegans 

 and angustum were found there as well as in the oligo- 

 trophic western part and in the East Australian Current. 

 Nielsen's results are not corroborated by the Carnegie 

 collections. M the latter, specimens as wide or wider 

 than var. latum were, indeed, found in the eutrophic 

 water in the region of the Galapagos Islands, but they 

 also occurred in the oligotrophic water of the central 

 part of the North Equatorial Current. 



Subgenus BICERATIUM 



4. Ceratium digitatimi Schutt 

 Figure 5, chart 4, appendix table 4 



This is a rare intolerant tropical species, one of 

 the most rare of the Carnegie collection. B is apparent- 

 ly restricted to warm water as it never approached the 

 20° surface isotherm except in the region of Easter Is- 

 land. It is also remarkable that it was never found near 

 large land masses, although Jorgensen (1920) foimd it in 

 the Mediterranean. Peters (1934) found it at only four 

 stations in the South Atlantic in his "main distribution 

 region" of warm oligotrophic water. On the Carnegie 

 cruise it was found only at two stations in the Atlantic, 

 in the North Equatorial Current. 



Nielsen (1934) is the only previous author to report 

 C. digitatum from the Pacific. He foxind it at three wide- 

 ly separated stations in the South Equatorial Current. In 

 the Carnegie collection it was found at twenty-three 

 widely separated stations, from Easter Island to Guam 

 and to a point midway between Hawaii and California. 

 The most northern station is 33.°5 north; the most south- 

 ern, 31.° 5 south. On the German South Polar expedition 

 C. digitatum was found to 31 ° south. This seemed extra- 

 ordinary to Peters (1934) who did not find it farther south 

 than 20° south on the Meteor expedition. He explained the 

 fact by stating that it was probably carried south in the 

 Agulhas Current. The Carnegie southern records in the 

 southeastern Pacific may thus be explained by a drift 

 from the northeast, in the large vortex characteristic of 

 that region. The absence of this species off Japan, how- 

 ever, where it should be carried by the extension of the 

 North Equatorial and Japan currents, and in the North At- 

 lantic Drift, are noteworthy and can only be explained on 

 the assumption that the species is so rare that its detec- 

 tion anywhere in the ocean must be considered fortuitous. 

 Its limits of distribution, thus, will not be well defined 

 until much more survey work has been done. 



Ceratium digitatum is always found singly or in small 



