COMPARISON OF ATLANTIC AND PACIFIC WATERS 



11 



It would be supposed that any species originating in the 

 Atlantic would, in time, be swept to the Indian and Pa- 

 cific oceans since the currents in the far southern 

 waters are predominantly from west to east. Whether 

 the Pacific tropical species could find their way to the 

 Atlantic by the southern connection is less certain. 



The water connection of the two oceans to the north 

 by way of Bering Strait and the Arctic Ocean certainly 

 would be expected to constitute a barrier to Ceratium 

 species, at least to the tropical forms. Bering Strait 

 is shallow and narrow, with water temperatures around 

 8° C. It is not likely, however, than an organism could 

 pass from here to the Atlantic without passing through 

 water of 0° C or lower. 



In the light of this knowledge it is interesting to 

 compare the Ceratium floras of the Pacific and Atlantic 

 oceans. An examination of the Carnegie lists of species 

 for the two oceans shows that most of the species are 

 common to both oceans. Nevertheless, there are some 

 significant differences. Nine of the species occurred in 

 only one ocean and this, significantly was the Pacific. 

 One of these, C. axlale . has been foimd in the Atlantic by 

 others, so it will be disregarded. The others are as fol- 

 lows: C. aultii. C. bigelowli . C. carnegiei . C. deflexum. 

 C. fllicorne . C. geniculatum . C. peter sii. C. bohmii. 



None of the Atlantic species were absent from the 

 Pacific. In Nielsen's comparison of the two oceans, he 

 cites two examples of Atlantic forms which are absent 

 from the Pacific, namely, C. longlnum Karsten (=C. 

 arcuatum longlnum In Peters, 1934), and C. mlnutvun 

 Jorgensen. These forms, however, have now been found 

 abundantly in the Pacific in the Carnegie collections (see 

 pp. 35 and 22). Otherwise Nielsen's comparison of the 

 two oceans agrees well with our own, except that he lists 

 C. himiile as absent from the Atlantic. It was found in 

 that ocean in the Carnegie collection. 



K is true, of course, that the species peculiar to the 

 Pacific are rare forms and may yet be discovered in the 

 Atlantic. In the case of at least three of the species, 

 (C. bigelowli. C. deflexum . and C. fllicorne) . however, 

 this is not likely, as many records of their occurrence 

 are now being acctmiulated. For Instance, in the Carne- 

 gie collection alone there are thirty-seven sample rec- 

 ords for C. bigelowli . one hundred five for C. deflexum. 

 and twenty -seven for C. fllicorne. 



R shoxild be noted that the eight species peculiar to 

 the Pacific are strictly, or only slightly tolerant, tropi- 

 cal species, not occurring at stations where the surface 

 temperature was less than 20° C. Thus, these species 

 might well find it impossible to pass around Cape Horn 

 in the cold southern water. 



Therefore, it seems quite possible that the above- 

 mentioned seven species have originated in the Pacific 

 Ocean and must forever remain in the warm regions of 

 that great ocean, being barred from the Atlantic by the 

 great southern extension of South America, whereas the 

 forms that have originated in the Atlantic have found an 

 easy migration to the Pacific by way of the Indian Ocean. 



We must also bear in mind that the other species of 

 the two oceans may not be as similar as present work- 

 ers believe. After all, the morphology of Ceratium is 

 known only grossly; very few species have been anal3^ed 

 in detail, and none of them completely. Perhaps real 

 specific characters have been overlooked. In this con- 

 nection it is important to examine some differences in 

 the distribution of some of the species which, according 

 to present taxonomic methods, are specifically Identical 



in the two oceans. Five of the tropical species were 

 decidedly more tolerant to cold water in the Atlantic 

 than in the Pacific; namely, C. furca. C. extensum . C. 

 horrldum . C. hexacanthum . and C. tripos anan^ j niir , 

 whereas one cold water species, C. arcticum was appar- 

 ently more tolerant to warm water in the Atlantic than 

 in the Pacific (see charts 6, 16, 44, 48, 18, and 47). 

 Ceratium horridtmi . in addition, seemed to show a dif- 

 ference in its relation to ollgotrophlc water in the two 

 oceans, being found practically only in eutrophlc water 

 in the Pacific but in ollgotrophlc as well as eutrophlc 

 water in the Atlantic. These differences in distribution 

 in the two oceans strongly suggest that the genetic com- 

 plex of the forms in the two oceans is not identical. The 

 representatives of the two oceans may be different spe- 

 cies or subspecies or maybe "physiological subspecies." 

 The solutions to these problems must await a more mi- 

 nute morphological examination of the forms in question. 



About 20 per cent of the tropical species of Ceratium 

 in the Carnegie collection show either morphological or 

 distributional features peculiar to one ocean, indicating 

 an isolation of the tropical waters of the Atlantic and Pa- 

 cific. The evidence for the isolation of the North Pacific 

 cold-water region, however, is even more convincing. 



Although there are certain similarities between the 

 Ceratium floras of the cold North Pacific and cold North 

 Atlantic regions, there are, on the other hand, some very 

 striking differences which can be accounted for only on 

 the assumption that these two oceans are biologically iso- 

 lated from each other. The similarities in the two floras 

 are expressed by the occurrence in both regions of the 

 two subpolar species C. lineatum and C. arcticum. 



The differences in the floras of the two regions are 

 more striking than the similarities. In contrast with the 

 tropical floras these differences are not one-sided, that 

 is, the "mono-oceanic" forms are not all in one ocean 

 so that, in contrast with the southern oceans, a mutual 

 isolation is indicated. 



The significant forms in this case are not species, 

 but subspecies. The first of these to consider is C. furca . 

 This species, as broadly considered, is cosmopolitan. It 

 was noticed, however, that in the Atlantic there was a 

 hiatus between the tropical records and the cold-water 

 records (chart 6). Although no morphological difference 

 could be discerned between the southern and northern 

 forms, it was suggested that they represented at least 

 physiological subspecies (see p. 18). When we turn to 

 the Pacific we find that the species is represented only 

 in the warm-water regions except for one station in re- 

 g;ion m off Japan, which must be considered a displace- 

 ment by the Kuroshlo. The species is absent along all the 

 rest of the cold-water area traversed by the Carnegie . 



The case of another species, C. macroceros . is 

 somewhat more convincing. Inasmuch as the subspecies 

 are morphologically easily distinguishable. Subspecies 

 g alUcum is characteristic of all warm-water regions. It 

 is widespread over both the Atlantic and Pacific. The 

 species is represented in the cold North Atlantic by sub- 

 sp. macroceros. In the cold North Pacific, however, the 

 species is conspicuously absent (see chart 37). An as- 

 sumption of the isolation of the North Pacific based on 

 the above two examples alone would not be very conclu- 

 sive since It would be based on negative evidence In a 

 region not well Investigated, namely, the North Pacific. 

 The following example, however, doesnot carry this weak- 

 ness and, taken with the above two, forms a convincing 

 proof of the isolation of that ocean. 



