COMPARISONS OF PLANKTON 



13 



light pcnctr.ilion. Altliou^li the copcpcicls taking part in 

 this deeper movement would not reach the surface, they 

 might he foLUid at tiie 50-meter or 100-meter level during 

 the night. 



If it be true that the coming of daylight furnishes the 

 incentive for the departure of the copepods from the 

 surface, it follows that the depth to which they retire 

 below the surface will vary with the intensity of the light. 

 : They will descend farther on a bright, sunny morning 

 than when it is cloudy and lowering. On a day that is 

 ushered in with clouds so dark and gloomy that the sun- 

 shine can scarcely penetrate them at all, the copepods 

 may well stay near the surface long after sunrise and 

 descend but a short distance. The varying intensity of the 

 light offers a good explanation of the fact that some 

 species are found close to the surface at one station and 

 considerably removed from it at another, perhaps the 

 very next one. Damas and Koefoed (1907) summed up 

 their work on the plankton of the Greenland Sea as 

 follows: "At any given place the organisms are distrib- 

 uted according to the degree of light for which they are 

 sensitive. They rise and fall according to the daily varia- 

 tions in light intensity. The level at which a form remains 

 is different from sea to sea. Species which live at the sur- 

 face in the polar sea are found in the depths under the 

 equator. Others that are seen to exist in the intermediate 

 layers in the north are only found in the abyss at the 

 south." Russell (1927/') confirmed these statements antl 

 added that copepods also show seasonal variation in their 

 vertical distribution, going deepest in mid-June when the 

 light intensity is greatest. 



Again, we cannot expect the same intensity of light to 

 affect different species equally, and here we have a reason 

 for the fact which is so evident in every one of the station 

 lists. Some species remain at the surface no matter how 

 intense the light becomes; the others migrate downward 

 but stop at different levels according to the amount of 

 influence exerted on them by the light. We are thus en- 

 abled to understand how it is that the copepods become 

 so distinctly stratified at every one of the stations in the 

 daytime and also why we find the same species concen- 

 trated in different strata on different days. Russell went 

 farther and declared: "In the sea each plankton animal 

 may have its own vertical zone in which it finds certain 

 conditions most favorable. This zone varies for different 

 species, for individuals of the same species at different 

 ages and stages of development, and even for the different 

 sexes." 



Although, therefore, this is not the first time that strati- 

 fication of the plankton under the influence of sunlight 

 has been proposed, it is here proved to exist among the 

 copepods over a much more extensive area of two oceans 

 than had hitherto been studied. The fact that every one 



of the station lists without exception gives definite evi- 

 dence of this stratification would .seem to warrant the 

 general statement that it exists everywhere in the open 

 ocean during the daytime. 



Michael (1913) discovered a similar stratification among 

 the chaetognaths of the San Diego region, California. He 

 concluded that each species had its own definite and 

 specific manner of vertical distribution just as truly as it 

 had its own specific morphological characteristics, and 

 that the two were more or less interrelated. Rose (1925) 

 made an extensive study of the biology of the plankton 

 at Roscoff on the coast of France. He included the entire 

 plankton in his study and, among the other forms, six of 

 the copepods most abundant at that locality. Later he 

 repeated his Roscoff experiments with other copepods at 

 Banyuls-sur-Mer on the Mediterranean. From these ex- 

 periments he concluded that below a certain inferior 

 light-intensity limit the copepods were positively photo- 

 tropic, above a certain superior light-intensity limit they 

 were negatively phototropic, and between the two limits 

 they were indifferent. He concluded that the daily 

 vertical migration is due to the following influences: (i) 

 Sunlight provokes the movement, directs it, and partly 

 regulates it. (2) Temperature modifies more or less the 

 action of the light energy, and when high enough may 

 even reverse the action of phototropism. These inter- 

 actions between temperature and phototropism have been 

 verified by other observers: Parker (K/J2) found that the 

 females of Liibidoceni aestiiHi hatl a strong positive photo- 

 tropism for light of low intensity, whereas the males had 

 a weak negative phototropism; these were unaltered in 

 both sexes by temperature changes between 10° and 35 

 C; in a strong light the female became negative, but the 

 male was not affected. Russell (1928) found that Calanus 

 fuimarchiciis was negative to light at medium tempera- 

 tures but became positive at 13° C and strongly so below 

 10° C; Metridia liicens was negative at ordinary tem- 

 peratures but became positive at 10° C; Acartia claiisti 

 was strongly positive at ordiitary temperatures but the 

 phototropism entirely disappeared at 28° C; Centropages 

 hamatus was positive up to 25° C, then became more and 

 more indifferent to light and was finally negative. (3) 

 Salinity, chemical composition, dissolved gases, etc. are 

 accessory factors but usually have little if any influence. 

 In exceptional conditions they may become of great im- 

 portance. These experiments were made near the shore 

 in comparatively shallow water, where physicochemical 

 conditions would have a somewhat increased influence. 

 In the open ocean the influence of the temperature and 

 accessory factors would be considerably reduced and that 

 of the sunlight correspondingly increased. Rose (1925) 

 suggested that the majority of the animals in the plank- 

 ton are adapted to an optimum intensity of light, and 



