8 



CERATIUM IN THE PACIFIC AND NORTH ATLANTIC OCEANS 



North Pacific, eastern North Pacific, and eastern South 

 Pacific. A comparison of the Ceratia distributions in 

 these regions Is very Interesting, and gives us a clue to 

 the factors controlling distribution, at least in the case 

 of some species. For these studies it is important to 

 consider first the most common tropical species so that 

 negative records in the transition zones can be given 

 some weight. Ceratium contortum is a good example of 

 a tropical distribution (chart 32). The species drops out 

 at surface temperatures of about 20° in the North Atlan- 

 tic, at above 20° off Japan, 15° off California, and 15° in 

 the southeastern Pacific. Many other species have a 

 similar distribution, some more closely confined to cer- 

 tain temperatures and some less. Ceratium massillense 

 (chart 38) is a common tropical form whose limits of 

 distribution parallel rather closely the 15° C isotherms 

 in the four transitions. Some species show a definite 

 restriction to tropical water except for a particular dis- 

 placement by a current. A good example of this is Cera- 

 tium hexacanthum which is within the 20° isotherm at 

 the three transitions in the Pacific, but In the Atlantic it 

 ts apparently carried by the currents to Iceland where 

 the surface temperature is less than 10°. Conversely, 

 there are species which are restricted to warm water 

 throughout their ranges. Examples of these are C. breve 

 (chart 21) and C. lunula (chart 31). 



A few forms are found in the cold water but not in 

 the tropics, or only in insignificant numbers there. 

 Ceratimn arcticum (chart 47) is an example. Still a 

 third general type of distribution Is found in such forms 

 as C. fusus (chiairt 17) which occur In cold water as well 

 as in warm, and are truly cosmopolitan. 



On the basis of the distributions of the Ceratium 

 species at the Carnegie stations, a classification con- 

 sisting of three main categories was devised: tropical, 

 subpolar, and cosmopolitan. The tropical, in turn, was 

 divided into three regions. Those species which were 

 rather closely restricted to surface temperatures of 19° 

 or above were grouped together as intolerant tropical 

 species. Those which transgressed a little into the 

 transition zones were called slightly tolerant tropical 

 (orms and, finally, those which were carried far beyond 

 the tropical regions were designated very tolerant tropi- 

 cal forms. This classification has no category for tem- 

 perate species such as most previous classifications 

 had. The authors believe that there are no truly temper- 

 ate oceanic species of Ceratium. The temperate lati- 

 tudes are populated by tropical and cosmopolitan forms 

 with occasional appearances of subpolar forms. Follow- 

 ing is the geographic classification of the Carnegie 

 Ceratia based on the above scheme. 



Slightly Tolerant Tropical 



Very Tolerant Tropical 



C. azoricum C. massiliense 



C. arletinum C. extensum 



C. hexacanthum 



Cosmopolitan 



C. horrldum C. fusus 



C. furca C. petersii (?) 



C. tripos subsp. atlantlcum 



Subpolar 



C. llneatum C. macroceros subsp. 

 C. pentagonum subsp. macroceros 



pacificum C. articum 

 C. compressum (?) 



Currents 



Currents are certainly important in the distribution 

 of Ceratia. They cause displacements in the normal 

 distribution by carrying populations into regions where 

 they have not developed, and they create displacemtBts 

 of the normal range of environmental conditions so that 

 species can develop in regions which otherwise would 

 not be favorable for them. The current factor in distri- 

 bution, however, is the most difficult one to demonstrate 

 by a gross world survey such as that of the Carnegie . 

 The value of Ceratia as current indicators is undoubted- 

 ly much greater than the Carnegie data would indicate. 

 The importance of such indicators is evidenced most in 

 temporary current anomalies which can be observed 

 satisfactorily only l>y a continued study of a particular 

 area. 



It shoxild be emphasized that a current which dis- 

 places a flora from its normal 'distribution also displaces 

 the environmental conditions. When the displaced water 

 mixes with the adjacent water, it may no longer be suit- 

 able for the organisms displaced and they disappear. 

 That the amoimt of mixing in such currents is great is 

 indicated by the rather sharp delimitation of many of the 

 Ceratium species in the Carnegie collection. This is 

 particularly striking in the Kuroshio and Gulf Stream, 

 two currents well known for their velocity and general 

 Influence. In these currents the tropical floras change 

 their fades rather rapidly when the surface tempera- 

 tures drop below 19° C. 



Peters (1934) considered that ocean currents played 

 a great role in the distribution of Ceratia in the South 

 Atlantic. He was able to give comparatively few in- 

 stances of displacements of species, however, in the 

 areas he studied. 



