1911] Michael: Chaetognatha of the San Diego Region. 143 



SIGNiriCANCE OF THE DATA CONCEENING THE DISTRIBUTION 

 OF S. BIPUNCTATA. 



In the foregoing pages it is demonstrated that S. hipunctata 

 is most abundant on the svirface during twilight when the tem- 

 perature of the water is between 15?9 and 17?5, and the salinity 

 between 33.605 and 33.648. Furthermore, we found the species 

 confined to the upper 150 fathoms of water, and mainly above 

 75 fathoms. Our data also indicate that during darkness and 

 intense light the species is most abundant and frequent in 15-20 

 fathoms, migrating to the surface only during the hours of twi- 

 light. In other words, the region of 15-20 fathoms is probably 

 the center from which the species migrates. Now what do these 

 facts taken together signify? 



Let us assume that the usual conditions of light, temperature, 

 and salinity occurring in 15-20 fathoms are those which determine 

 the migrations of this species. On this assumption, if the inten- 

 sity of the light in this depth during the day approximates twi- 

 light, the migration of the species toward the surface is readily 

 explained. Again, if the temperature in 15-20 fathoms is norm- 

 ally between 15?9 and 17?5, or less than 15?9, and the salinity 

 between 33.605 and 33.648, the tendency of the species to remain 

 on the surface in greater numbers when these conditions are 

 realized is also readily explained. 



What, then, are the usual conditions of light, temperature, 

 and salinity occurring in 15-20 fathoms? Concerning light, we 

 know its intensity below the surface rapidly decreases as the 

 depth increases, and it is generally agreed that it is very subdued 

 below 100 or 200 fathoms. Now, since the species begins to 

 accumulate on the surface at sundown and sunrise, it is certain 

 that the earlier or brighter evening twilight and the later or 

 brighter morning twilight initiates the upward migrations. This 

 fact, together with the time of day in which we found the maxi- 

 mum niunber of specimens in the various depths, indicates that 

 during the greater part of the day bright twilight conditions 

 normally occur in 15-20 fathoms. Again, with respect to tem- 

 perature, while observations were not taken in connection with 

 closing net hauls, our records show that the usual June and July 

 temperature in 15-20 fathoms is approximately 15°-16°. Finally 



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