UNIVERSITY OF CALIFORNIA PUBLICATIONS 



IN 



ZOOLOGY 



Vol. 16, No. 21, pp. 393-400 March 3, 1917 



THE OCCURRENCE OF A RHYTHM IN THE GEO- 

 TROPISM OF TWO SPECIES OF PLANKTON 

 COPEPODS WHEN CERTAIN RECURRING 

 EXTERNAL CONDITIONS ARE ABSENT 



BY 



CALVIN O. ESTERLY 

 (Contribution from the Scripps Institution for Biological ResearchI 



It is known that rhythmic activity may occur in animals even 

 when periodic changes in external conditions, that may have caused 

 the rliythm in the first place, can no longer affect the organisms 

 directly. Examples of such behavior have been given bj' Holmes 

 (1911, pp. 16, 79, 80, 155-158) and other instances may be found in 

 the paper by Menke fl911). Activities of this character in strictly 

 pelagic animals of the plankton have not been observed, so far as 1 

 know, under experimental conditions. 



The marine eopepods Acartia tonsa Dana and A. clausi Giesbrecht 

 show a periodic change in behavior under uniform external conditions. 

 Temperature is a modifying factor, however, for tonsa. Both species 

 are common and easily obtained by towing from the pier of the 

 Scripps Institution. Tonsa has been taken all the time from August, 

 1916, to the present date, wliile clausi appeared about November 1. 



Acartia tonsa. in the ordinary diffuse light of the laboratory, is 

 for the most part negatively geotropic, providing the animals have 

 been obtained at the surface. As might be expected, the individuals 

 do not all show the same response, but most animals of most sets stay 

 at the surface or near it when in a vertical container. The animals 

 were released at the top of a flat-bottomed tube 50 cm. high and 

 30 mm. in diameter, and the tube was left standing about 75 cm. 

 from the north windows. Tlie column of water was considered to be 



