UNIVERSITY OF CALIFORNIA PUBLICATIONS 



IN 



ZOOLOGY 



Vol. 8, No. 1, pp. 1-7 May 8, 1911 



THE VERTICAL DISTRIBUTION OF 



EUCALANUS ELONGATUS 



IN THE SAN DIEGO REGION DURING 1909 



BY 



CALVIN O. ESTERLY 

 (Contribution from the Laboratory of the Marine Biological Association of San Diego) 



This species is a very common one in the San Diego region, 

 ranking next to Calanus fi}imarcJiicus in that respect. Euca- 

 lanus is characterized by the remarkable transparency of the 

 body; the living animals are practically invisible in a dish of 

 water. The body is also almost entirely colorless ; the median eye 

 may reveal a slight reddish tint and some individuals have one or 

 two oil drops in the hinder part of the cephalothorax that are of a 

 faint orange or reddish color. In some cases the bristles on some 

 of the appendages are of a more or less evident orange. How- 

 ever, these copepods are the most difficult to see of any with which 

 I am familiar. 



Because of its transparency and lack of noticeable color, 

 Eucalanus is highly adapted for its life as a plankton animal 

 (Steuer, 1910, p. 275), and on this account, also, it has seemed 

 to me that its vertical distribution during day and night would 

 be interesting. It does not seem likely that if vertical migrations 

 take place they could add much to the protection of the animals 

 by leading to their removal from levels at which they would be 

 easily visible during the day and so apt to be preyed upon. The 

 animals would be practically invisible at any level. 



The collections on which this paper is based were made with 



