CERATIUM SPECIES OF CARNEGIE COLLECTION 



29 



and probably has been confused by some with the latter 

 species, although the original figures of Pavillard (1905) 

 and Jorgensen (1911, 1920) were not C. filicorne. In the 

 Carnegie collection this species was identified without 

 any difficulty, and no intergrades were foimd between it 

 and either C. euarcuatxun or C. svmmetricum . 



Ceratium filicorne has a distribution of particular 

 interest. In the Carnegie collection it occurred only in 

 the South Pacific. This distribution is unique for the 

 Ceratium species. Since the only other records for the 

 species are those of Nielsen (which are also from the 

 South Pacific), its known distribution is limited to that 

 region. Nielsen's records run from the Tuamotu Islands 

 to Fiji. In the Carnegie collection there are three sta- 

 tions for the species between the Tuamotus and Samoa, 

 i.e., in the same general region of those of Nielsen; the 

 other stations, fifteen in number, are in the southeast- 

 ern Pacific. These, however, are limited to the warm 

 Pacific region, although they Include all the stations (4) 

 In the eastern extension of the region. 



This is one of the most extraordinary distributions 

 found in the genus. If it were an extremely rare species 

 this distribution would not be remarkable as it might be 

 explained on the basis of incomplete sampling. This 

 probably is not the explanation in this case, however, 

 since the species was not found In particularly small 

 numbers within its range. Of the twenty-seven records 

 of occurrence only slxwere rare, whereas fourteen were 

 occasional, and seven were common. Fifteen of the rec- 

 ords were from net samples and twelve were from pump 

 samples. Why the species is not found in the equatorial 

 regions is an enigma. Is there something peculiar to the 

 South Pacific necessary for the development of C. fili- 

 corne ? 



The surface temperatures at the stations where the 

 species occurred, varied from 19.°0 to 28.°7 C. The en- 

 vironmental conditions in situ were: temperature, 14.°3 

 to 27.°6 C; salinity, 34.4 to 36.3 per mille; pH, 7.17 to 

 8.27; phosphate, 8 to 32 mg P04/m3. 



There was only one record from water with less than 

 10 mg P04/m3. Nielsen (1934) designated C. filicorne 

 as a genvilne shade species, and this is borne out by the 

 Carnegie data, as can be seen in table 25. It was found 

 predominantly in the 100-meter samples, and only a sin- 

 gle specimen was fovuid in the surface samples. 



Table 25. 



Records of occurrence of C. filicorne 

 at three levels 



Rare 



Occasional 



Common 



Total 



0.3 



0.3 



1.9 



21 



7.9 



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

 samples collected at that depth 



30. Ceratium symmetricum Pavillard 

 Figures 15H-L, chart 24, appendix table 28 



Paulsen (1931) suggested that the name C. gracile be 

 supplanted because of the confusion of synonyms and the 



differences of opinion of Jorgensen (1911, 1920) and 

 Pavillard (1923). The present authors agree that the 

 name should be dropped, not because of the confusion of 

 these authors' opinions, however, but because the origi- 

 nal drawing of Gourret (1883, pi. I, fig. 1) is not identi- 

 fiable. 



There is a considerable difference of opinion as to 

 what segregation of forms should be made within this 

 group. Peters (1934) listed only two forms; C. gracile 

 gracile and C. gracile svmmetricum. He included var. 

 orthoceros Jorgensen in his subspecies gracile. It is 

 doubtful, however, if he found coarctatum . since his draw- 

 ings do not show it. Nielsen (1934), on the basis of some 

 statistical work on size and horn spread, decided that the 

 various forms he found could be lumped into a single uni- 

 fied species. It is doubtful, however, if his measurements 

 of horn spread properly express the difference in body 

 shape and origins of horns--which are the distinguishing 

 features of the segregates of this group. The present au- 

 thors believe, on the basis of the study of the Carnegie 

 material, that coarctatum and orthoceros are sufficiently 

 distinct to justify the retention of their names and the 

 recording of their distributions. There is sufficient in- 

 tergradation between them, on the other hand, that they 

 cannot be given more than varietal value. Variety coarc- 

 tatum is characterized by long, more or less parallel, 

 horns; var. svmmetricum by widely spreading horns; 

 var. orthoceros by short close horns, and the right ant- 

 apical in particular, with an abrupt anterior turn. 



The three forms of the species are not subspecies 

 in the geographic sense; they are all found within the 

 same regions. For this reason their records have been 

 combined on the chart (chart 24) although the individual 

 records are noted on the distributional list (appendix table 

 28). 



The species as a whole is a fairly common warm- 

 water form. In the Carnegie collection it was found at 

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

 in the Pacific. In the Atlantic these were all in the warm 

 Atlantic region. The surface temperatures at these sta- 

 tions were all above 24.°0 C. In the Pacific the species 

 occurred in the warm Pacific region and throughout the 

 southeast Pacific region. At the northern limits of the 

 warm Pacific region the species occurred where the sur- 

 face temperatures were as low as 19.°1 C. In the south- 

 east Pacific region It occurred at surface temperatures 

 as low as 15.°0 C. Thus, it was classified as a slightly 

 tolerant tropical species. 



Table 26. 



Records of occurrence of C . symmetricum 

 at three levels 



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

 samples collected at that depth. 



Its relative abundance is also shown by the number 

 of sample records. There were 156 recordsof occurrence, 

 of which 86 were rare, 59 occasional, and 11 common. It 



