FACTORS AFFECTING HORIZONTAL DISTRIBUTION 



9 



The authors tend to agree with Gran (1912) that the 

 nature of a Ceratlum flora is determined more by the 

 chemical-physical conditions of the water than by trans- 

 portation by currents. The fact that a warm current 

 carries a tropical flora into high latitudes is not opposed 

 to this argument, as present evidence indicates that such 

 a current will do this principally by carrying the tropi- 

 cal environment with it. 



In the Carnegie investigations there were four re- 

 gions where current influences would be expected to be 

 demonstrated. These concerned four great currents: 

 the Gulf Stream, Kuroshio, California, and Humboldt 

 currents. 



In the North Atlantic West Wind Drift or "Gulf 

 Stream" the intolerant tropical species dropped out at 

 about 45° north. The best examples of displacement, 

 however, were found here in the case of some of the tol- 

 erant tropical species. Thus, C. extensum was carried 

 to the British Isles to temperatures of 12.° 44 C (chart 

 16) and C. hexacanthum to Iceland in temperatures of 

 8.°92 C (chart 48). Unquestionably these are examples 

 of current displacements as C. extensum was not found 

 elsewhere in temperatures below 14.°97 nor C. hexacan- 

 thum below 18.°97. The occurrence of C. massiliense and 

 C.platycorne off Ireland should probably also be consid- 

 ered displacements (charts 38 and 35). Other tropical spe- 

 cies such as C. declinatum and C. ranipes were carried 

 over into region I only as far as station 3, where the 

 surface temperature was 15.°5 C (charts 28 and 36). 



In the corresponding current of the Pacific, the 

 Kuroshio, we find that the transition from tropical to 

 subpolar floras is more abrupt. Most of the tropical 

 species were not carried into water with temperatures 

 under 20 ° C . Only C. gravidum . C. candelabrum , and C. 

 tripos semipulchellum . of the tropical species, were 

 carried into water with surface temperatures as low as 

 15.°93 C (charts 3, 5, and 19). No evidence of any tropi- 

 cal species being carried into lower temperatures than 

 that in this region was found. 



Corroboration of the lack of displacement in the 

 Kuroshio was found in the distribution of the tropical 

 species at similar latitudes in the western Pacific off 

 California. If the currents were effective In displacing 

 the distributions significantly into different environ- 

 ments, then the tropical species should be found in cold- 

 er water off Japan than off California. The Kuroshio 

 should sweep populations into cold water and the Cali- 

 fornia Current should push the northern limit southward. 

 This was seldom found, however. The species which 

 disappeared at 19° C off Japan, reappeared at 19° C off 

 California. There were many variations between spe- 

 cies but, in the main, the tropical flora of the North Pa- 

 cific had its northern limits in the western and eastern 

 Pacific at the same isotherm, and this isotherm was at 

 approximately the same latitude, namely 40° north. 



There were two striking exceptions to this general 

 distribution, however; the distributions of C. breve 

 and C. lunula (charts 21 and 31). These species were 

 found rather consistently within tropical latitudes, that 

 Is, within 20° of either side of the equator. The ranges 

 of these species beyond these latitudes, however, does 

 not follow that of other common tropical species which 

 are usually found throughout the warm-water regions to 

 about 40° north and south. In the case of C. breve the 



records do not extend far north or east of Hawaii, 

 although in the western Pacific they extend to Japan. 

 They are absent from a large part of the southeast Pa- 

 cific region, although they extend to Easter Island. The 

 distribution of C. lunula is stUI more uneven in the 

 North Pacific. In the east it was found nowhere north of 

 20°, whereas in the west it was found continuously to 

 northern Japan, latitude 38° north. In the South Pacific, 

 however, it was everywhere within 20° south latitude. 

 Is it possible that these distributions are determined by 

 the current systems? Certainly such distributions can- 

 not be accounted for by simple current displacements as 

 a bodily displacement of the plankton would have dis- 

 placed other species as well. Could these curious dis- 

 tributions be the result of a specific susceptibility to 

 something? Is the absence of these species in the south- 

 east Pacific owing to some deleterious effect of the out- 

 wash of subantarctic and upwelled water from the south 

 and east, and is their absence from the region between 

 California and Hawaii owing to the combined effects of 

 the California Current and upwelling? The answers to 

 these questions cannot be given until further work Is 

 done. It is remarkable that only two species show such 

 distributions. The proof of the validity of these must 

 await further investigations. 



The equatorial currents apparently have little effect 

 on the distribution of Ceratia. This is probably because 

 they involve the movements of waters of very similar 

 nature. Although the temperatures of these waters may 

 vary from 20° to 29° C, the surface temperatures above 

 20° C apparently have no effect on the Ceratia. Nielsen 

 found that C. filicorne was found only In the warmest 

 parts of the South Equatorial Current, but this was not 

 corroborated by Carnegie data (see p. 28). No segrega- 

 tion of Ceratlum species within the equatorial regions 

 could be made on the basis of temperature. 



The last great current system to be considered is 

 the Himiboldt Current off South America. Unfortunately 

 the Carnegie stations did not run near the continent except 

 at Callao so the most highly developed part of the current 

 system was missed. The hydrography of this region is 

 complicated by a strong upwelling along the coast. The 

 changes in the environmental conditions brought aboutby 

 this upwelling are probably the same as those effected 

 by the Humboldt Current, which brings water from the 

 subantarctic. These changes consist In lowered temper- 

 atures and pH, increase in phosphate, etc., and a result- 

 ant Increase in plankton production. Thus it is impossible 

 to decide whether the biological conditions peculiar to 

 this region should be attributed to current phenomena or 

 to the other hydrographlc feature, namely, upwelling. 



Probably there is a displacement by the Humboldt 

 Current of antarctic forms northward along the southern 

 part of the South American coast southeast of the Carne- 

 gie track, but no evidence of this was observed at any 

 Carnegie station. Probably the only antarctic form of 

 Ceratlum is C. pentagonum robustum (Peters, 1934). 

 This did not occur in the Carnegie collection. 



The Humboldt Current is deflected westward, and 

 there is a more or less general movement of water away 

 from the South American coast in the southeastern Pa- 

 cific. How far westward the influences of this movement 

 are felt is an interesting problem. In the discussion of 

 the hydrographlc conditions, it was pointed out that the 



