12 



CERATIUM m THE PACIFIC AND NORTH ATLANTIC OCEANS 



This example concerns C. pentagomun . a widespread 

 tropical species. Peters (1934) found a cold-water sub- 

 species in the subantarctic waters of the South Atlantic, 

 subsp. robustum . the most southern representative of the 

 genus. He emphasized the remarkable absence of any 

 representative of the species in the cold northern waters 

 of the North Atlantic. The absence of the species in 

 these waters is well established as the region has been 

 thoroughly investigated. The Carnegie investigations in 

 the North Pacific, however, revealed a very divergent 

 subspecies, subsp. pacificum . which was found only in 

 the cold North Pacific region (see p. 20). It attained a 

 high degree of morphological distinctness in the coldest 

 water, although it intergraded with var. tenerum at its 

 southern limits. There can be no doubt that subsp. 



pacificum is not found in the Atlantic and, therefore, is 

 peculiar to the cold waters of the North Pacific. 



These three striking differences in the Ceratium 

 floras of the cold regions of the two oceans indicate that 

 a barrier to at least some of the subpolar species exists 

 between these two water masses. The nature of this 

 barrier, of course, is only a matter for speculation. 

 The hydrography of Bering Strait is very poorly known. 

 Any continuous current in either direction, however, 

 would not permit the development of the floras now ob- 

 taining in the two oceans, providing the species could 

 traverse the Arctic Sea. Whether they can cross this 

 ocean is also unknown. An investigation of the phyto- 

 plankton of the Arctic should throw some light on this 

 problem. 



THE VERTICAL DISTRIBUTION OF CERATIUM 



Karsten (1907) first described a special "shade 

 flora" of the ocean. According to him this consists 

 principally of Coscinodiscus, Planctoniella, and Goss- 

 leriella. He thought, however, that the genus Ceratium 

 was represented at different levels by different species 

 or varieties and designated C. gravidum Gourret; C^ 

 tripos azoricum CI. var. breve Ostf.. and C. tripos 

 glbberum Gourret as shade species. As Nielsen (1934) 

 states, an inspection of Karsten' s lists does not corrob- 

 orate this idea except In the case of C. gravidum . 



Schroder (1911) listed C. inflexum f. claviceps (=C^ 

 contrarium f . claviceps) . C. platvcorne . and C. limulus 

 as species which "avoid" the intense continued sunlight 

 of the "southern waters." 



Jorgensen (1920) made an Intensive study of the ver- 

 tical distribution of Ceratium in the Mediterranean. He 

 found that a great number of species which occurred at 

 the surface in the winter, inhabited the deeper levels in 

 the summer. He concluded that the surface summer flo- 

 ra is more or less indigenous to the Mediterranean, 

 whereas the winter species are dependent on a migration 

 from the Atlantic. 



Paulsen (1930) and Nielsen (1934) challenged this 

 migration theory of Jorgensen, but Bohm (1931) accepted 

 it. Peters (1934) could not establish any vertical distri- 

 bution from the material of the "Meteor" expedition. 



The Dana collected with closing nets at the following 

 steps: 200 to 100, 100 to 50, and 50 to meters. From 

 such material it was possible to test the Idea of the 

 "layering" of the species of Ceratium. Nielsen (1934), 

 who studied this collection, found that about one -third of 

 the Ceratium species occurring in the southern Pacific 

 Ocean must be designated "shade forms," whereas the 

 rest of the species Inhabit predominantly the upper lay- 

 ers. He found that the density of the plankton affects the 

 vertical distribution of the shade forms. They occur in 

 higher levels in the richer water, presumably because in 

 such regions there is not sufficient light for growth at 

 the lower levels. 



Nielsen (1934) compared the shade forms of Cera- 

 tium with the shade plants of the tropical rain forest, all 

 of which have their leaf surface increased in some way. 

 The leaves are thin and there is an Increase in assimi- 

 lating cells. So in Ceratium the cells are thin and crowd- 

 ed with chromatophores. The cell body (and apical horn) 

 may be expanded and crowded with chromatophores 



(chloraplasts) as in C. gravidum . or the antapicals may 

 be expanded and rich In chloroplast as in C. platvcorne . 

 C. claviger . C. ranipes. 



Long-horned forms are found among shade forms 

 as well as among surface forms, but the shade forms 

 always have the horns crowded with chromatophores. 



Of the surface forms there is not a single example 

 of surface expansion. Thus, the usual assumption, that 

 surface expansion in Ceratium is a flotation adaptation, 

 is erroneous. 



Since the Carnegie plankton collection contains sam- 

 ples collected at 50- and 100-meter depths as well as at 

 the surface, and, since the collection contains practically 

 all the marine species of the genus, it was possible to 

 test the theory of Nielsen in the case of each species. 

 Although the Carnegie collecting nets were open nets, 

 the duration of towing at the particular level was so much 

 greater than the time of hauling in, that the percentage 

 of "contaminants" would necessarily be small. These 

 contaminants, moreover, would be upper -level forms so 

 that they would not introduce any error into the calcula- 

 tions in the case of deep-water forms. 



For each species reported in the Carnegie collec- 

 tion, the number of records for each collecting level was 

 computed. Since there were more surface hauls than 

 deeper hauls, these numbers needed to be weighted. 

 Consequently they were computed as percentages of the 

 total number of samples collected at the particular depth. 

 Tables were compiled showing these values, as well as 

 the actual number of records, for the three levels. 

 Since an expression of the relative abxmdance of the spe- 

 cies at each level Is more significant than the mere pos- 

 itive record, the number of records of "rare," "occa- 

 sional," etc., and their percentages were computed as 

 well as the total number, and these were included in the 

 tables. 



Such an analysis of the Carnegie distributional rec- 

 ords showed that twenty of the species were definitely 

 more abundant in the deeper levels and nine were ques- 

 tionably so. All the twenty species showed an increase 

 In frequency from surface to 100 meters. The agree- 

 ment between these species and the species indicated as 

 "shade species" by Nielsen Is great. 



There are only three cases of definite disagreement. 

 Melsen classified C. subrobustum and C. trichoceros as 

 surface species, whereas the Carnegie data definitely 



