262 
PACIFIC SCIENCE, Vol. XI, July, 1957 
Fig. 12. Occurrence of Sagitta pacifica off Peru in 
1941. 
of the great variability of the species, it is the 
author’s opinion that S. setosa , living in water 
of reduced salinity, is also a form of the spe- 
cies and is interbreeding with the population 
in the eastern Atlantic. However, until this 
hypothesis can be definitely established or 
disproved, it seems best to retain the name 
S. tenuis for the species. The correlation of the 
morphologic variations of this species with 
its physical environment and the untangling 
of the involved synonymy will be treated 
more extensively in a paper on the general 
biology of the species now in preparation. 
In 1952 S. tenuis occurred at only three 
northern stations and was of the large friderici- 
like form. In 1941 this form was found to the 
south and the small tenuis - like form at the 
northern stations. Apparently in August of 
1952 the tenuis-like form (warm water) did not 
extend south of the Gulf of Guayaquil while 
in 1941 it reached south to the coast of Peru. 
CONCLUSION 
Although the material available is not as 
extensive as might be desired, there is no 
doubt that the May 1941 distribution of chae- 
tognaths off Peru was significantly different 
from the August 1952 distribution. The rela- 
tionship of the 1941 and 1952 samples to El 
Nino and El Aguaje is shown in Figure 14. 
The following hypothesis is advanced as 
an explanation of the data presented above in 
the hope that it will lead to more thorough 
and critical field and laboratory investigations 
to test its validity. 
Fig. 13. Distribution of Sagitta tenuis off Peru in 
1941 and 1952. Crosses with hatchure— present 1952, 
without hatchure — absent 1952. 
