1911] Michael: Chaetognatha of the San Diego Region. 149 



TABLE 57 



Two Series of Hauls containing S. serratodentata made with the Nansen 

 Closing Net on March 17, 1910, at 32° 50:0 N and 117°30:0W. 



Hauls made between 4:20 and 7:34 P.M. 



Number of specim'ns 

 Haul Per 50 



number 

 2104 

 2100 

 2099 

 2095 

 2094 

 2090 

 2089 

 2081 



This table shows that the species was most abundant above 75 

 fathoms during late afternoon and early evening, while below 

 this depth it was most abundant during early afternoon. It is 

 significant that this result is very similar to that derived from 

 consideration of the average number of specimens obtained by 

 both horizontal and vertical nets. It is quite certain, therefore, 

 that the species migrates toward the surface when light is sub- 

 dued. Owing to the small number of animals obtained it is im- 

 possible to determine accurately its center of migration, but the 

 data suggest that this center is between 100 and 200 fathoms. 



All our data show that the species was rarely found above 10 

 fathoms, which suggests the question: Why did it occur on the 

 surface? Of the fourteen hauls in which it appeared, six were 

 made between H and 10 p.m., one between 10 a.m. and noon, 

 three between 8 and 10 a.m., and four between 6 and 8 a.m. It 

 seems, therefore, that their occurrence on the surface was not 

 correlated with intensity of light, or time of day. Again, fifty- 

 four specimens were obtained when the temperature of the water 

 was between 15?5 and 17?8, eight when it was between 18?6 

 and 19?3, and fourteen when it was between 20?5 and 22?2. 

 This possibly means that low temperatures caused them to 

 remain in greater numbers on the surface, but it can not explain 

 what brought them there. Finally, the salinities corresponding 

 to the fourteen hauls varied from 33.563 to 34.069 so that it 

 seems impossible that this was a factor. Their occurrence on the 

 surface was undoubtedly due to some unusual surfaca conditions. 

 Low temperature was possibly one, but what the others were is 

 not apparent in our data. 



