THE VERTICAL DISTRIBUTION OF CERATIUM 



13 



Indicate that they are shade species. On the other hand, 

 Nielsen indicated C. hexacanthum as a shade species, 

 whereas the Carnegie data failed to substantiate this. 

 The agreement between the Carnegie and Dana shade 

 species and the "winter species" of Jorgensen (1920) 

 is also great. Only one of JOrgensen's winter species, 

 C. kofoidii . has not been classified as a shade species 

 by either Nielsen or the present authors. A comparison 

 of Jorgensen' s "winter species" with the shade species 

 of Nielsen and the shade species of the Carnegie is given 

 in table 1. 



Nielsen (1934) stated that Jorgensen's (1920) obser- 

 vations indicated that Ceratia are phototropic; the shade 

 species in the winter when the light intensity is low 

 come to the surface and thus are able to maintain a po- 

 sition in optimal light conditions. 



The authors agree that JOrgensen's observations in- 

 dicated a phototropic response on the part of the "winter 

 Ceratia," but do not believe that the vertical migration 

 of Ceratia is a simple light reaction phenomenon. In the 

 Carnegie collection the shade species were found most 

 abundantly at 100 meters. Surely the light intensity at 

 this depth even in summer could not be compared with 

 the light intensity at the surface in winter in the Medi- 

 terranean or anywhere else in the world for that matter. 

 It is scarcely logical to assume on the basis of the pres- 

 ent data that the shade species of Ceratia seek a zone of 

 a particular light intensity. 



Is it not possible that we have to deal here not only 

 with a phototropism but a trophotropism as well? In the 

 summer time the upper levels of the sea are depleted of 

 nutrient salts to a depth equal at least to that populated 

 by the phytoplankton. Obviously, the species of phyto- 

 plankton which are the most tolerant of shade would have 

 a decided advantage in the quest for nitrogen and phos- 

 phorus, which occur in the deeper levels in large quanti- 

 ties. Is it not possible that these forms have a positive 

 tropic reaction to nutrient salts or some associated con- 

 dition as well as a phototropic response? By such a 

 mechanism each species would maintain a position opti- 

 mal for photosynthesis. 



There is another, and simpler, explanation that may 

 account for the vertical migration of Ceratia. There 

 may be a simple reversal of phototropic response, de- 

 pending on the physiological condition of the organism; 

 in this case on the assimilation of inorganic nutrients or 

 some associated substance. The mechanism would be 

 such that with the assimilation of these substances the 

 organism is positively phototropic, thus remaining near 

 the surface in the winter; but with the diminution of this 

 assimilation the organism becomes negatively phototrop- 

 ic, and thus descends to lower levels of greater nutrient 

 content. 



It must always be borne in mind that the poverty of 

 nitrogen and phosphorus which land plants everywhere 

 are fighting, is accentuated to a high degree in the ocean. 

 V/hen a land plant dies, its nitrogen and phosphorus are 

 soon returned to the soil to be utilized by other plants. 

 On the other hand, when a planktonic plant dies, it sinks 

 below the growth zone and its nutrient elements are lost, 

 to be returned only after a long period of time except in 

 high latitudes and in certain peculiar regions. This is 

 particularly true of the tropics, where the thermal strat- 

 ification is extreme and continuous. In these regions it 

 is probable that the fertilization of the photic zone is 

 accomplished alone by the nocturnal visits of a sparse 



zooplankton. E is in such regions as this that the shade 

 species of Ceratium develop. Nielsen (1934) has already 

 observed that the shade species are all warm oceanic; 

 they do not occur in neritic conditions nor in the cold- 

 water southeast of New Zealand. The Carnegie obser- 

 vations show that none of the shade species are cold 

 water species, with the possible exceptions of C. arc- 

 ticum and C. horrldum . Of the twenty-nine species def- 

 initely or questionably shade species according to Car- 

 negie data, seventeen are intolerant tropical species, 

 eight are slightly tolerant tropical species, only two are 

 very tolerant tropical species, one is cosmopolitan (C. 

 horridum) . and one subpolar (C. arcticum) . 



The data concerning the depth at which C. arcticum 

 lives most abundantly are not conclusive, but they sug- 

 gest that it is a shade species. Ceratium arcticum . a 

 cold-water species, may be a shade species of another 

 type entirely. Shiviroff and Federoff (1938) have found 

 phytoplankton flourishing under the arctic ice cap. They 

 have not yet reported the species found, but probably C. 

 arcticiim is one of them as it is characteristic of arctic 

 currents. In the low illimiination occurring under the ice 

 it would be expected that only species with a high toler- 

 ance for shade would be found. The nutrients here are 

 rich. These forms, when living in exposed ice-free re- 

 gions, would then have the advantage of the ability to live 

 at greater depths than the species not tolerant of shade, 

 so they would be found at depths equal to that of the trop- 

 ical shade forms. 



Table 1. Shade species of Ceratium 



? indicates either insufficient data or that the results 

 were not conclusive. ^Favillardii. ''Molle andclaviger. 



