Physiological Differences between Marine Animals 

 from Different Depths 



Victor E. Shelford 

 University of Illinois, Urhana. 



INTRODUCTION. 



I 



It has been the custom of students of distribution to assume that 

 the physiological characters of individuals of a given 'species from 

 different environmental conditions are essentiallj'^ the same and that the 

 character of conditions may be determined by a study of the presence 

 or absence of different species. A most common type of work along 

 this line is that of students of distribution on land, who decide that 

 this or that factor controls distribution because a certain species is co- 

 incident with a uniform condition of the factor chosen. Further, of re- 

 cent years the students of sanitary science have from time to time tried to 

 use the occurrence of certain organisms as an index of contamination 

 of water. 



In view of these frequent apparent tacit assumptions that the 

 physiological constitution of a species remains essentially the same through- 

 out its range, it is a matter of utmost importance to know how far and 

 to what extent it actually varies with difference in surrounding condi- 

 tions. It is further of much importance from an ecological viewpoint 

 that these subjects be thoroughly investigated. The physiological foun- 

 dations of ecological work must be laid broad and deep and the question 

 of correlation of the physiological characters of animals with surrounding 

 environmental conditions, their variations in individuals of a given spe- 

 cies, is a matter of utmost importance. 



Physiological differences between animals of the same species from 

 different environmental conditions has been investigated by Alice (1) 

 who worked on isopods witli striking results. Pond individuals show 

 low percentage of positive rheotactic response while stream individuals 

 show high positive response to the same current. Tlie physiological 

 agreement between animals of the same or similar communities and the 

 physiological differences between different communities have been inves- 

 tigated by the writer (25). 



(157) 



