li)19 J Esterly: Reactions of Various Plankton Animals 



LITERATURE 



The first investigators to study diurnal migration from the experi- 

 mental side were Groom and Loeb (1890). They worked with the 

 larvae of a barnacle, Balanus perforatus. Their general conclusion 

 (]). 173) is that the animals move down because they are negative 

 toward the intense light of day, while they ascend at night because 

 they are at least not negative to light of the intensity that prevails 

 then. The authors consider that their experiments show "... dass 

 die ganze Erscheinung der periodischen taglichen Tiefenwanderung 

 der Tiere eben nur dadurch moglich ist, dass dieselben erstens helio- 

 tropisch sind, das heiszt, dass sie durch den Lichtstrahle gerichtet 

 werden; und zweitens, dass der Heliotropismus. . . Abends (im 

 schwachen Licht) positiv, am Morgen (bei starkem Licht) negativ 

 ist." They state, however (p. 176) that the matter is different with 

 certain copepods ; they could not establish that there is a change in 

 heliotropism in these forms. 



A few years later Loeb (1893) studied several different kinds of 

 smaller pelagic animals. Polygordius laiwae are made positively helio- 

 tropic by cooling, and negative by warming (p. 90). Increase of 

 concentration (by adding NaCl to sea water) has the same effect as 

 cooling, while decreasing the concentration by adding fresh water to 

 sea water causes the negative response (p. 94). Copepods, "probably 

 Temora longicornis" are affected the same way as the worm larvae 

 (p. 96). Loeb states (p. 104) that he has worked with copepods, 

 larval Crustacea, worms and mollusks, and has not found a pelagic 

 animal of these sorts that would not become permanently or tem- 

 porarily positive to light under certain conditions. Geotropism, how- 

 ever, as well as light has some effect in determining the vertical dis- 

 tribution since positive heliotropism and negative geotropism and 

 negative heliotropism and positive geotropism work in the same direc- 

 tion (p. 105). A number of years later (Loeb, 1906) it was found 

 that copepods are made positively heliotropic by CO., or any dilute 

 acid (p. 565), as is Daphnia (p. 568). Fresh water Gammarus also is 

 made strongly positive, but this condition endures only a few seconds 

 (p. 570). The larvae of Balanus become positive in water to which 

 carbon dioxide is added (p. 575). 



A general statement of the bearing of such experimental results 

 on the migration was given by Loeb in 1908. He regards light as 



