320 BULLETIN OF THE BUREAU OF FISHERIES 



factor — -that is, the supply of available food — for this chastognath is both extremely 

 voracious and an active swimmer and hence would tend to gather at the levels, 

 and probably to some extent to congregate in the regions where the copepods on 

 which it chiefly preys are most abundant. Furthermore, it would naturally grow 

 fastest and breed most actively where food was most plentiful, tending to produce 

 and maintain an abundant local stock. 



It seems more probable that it is the dependence of S. eJegans on the calanoid 

 copepod plankton which, as remarked above (p. 30), is most plentiful in the mid- 

 levels, which accounts for the comparatively sparse sagitta population of the deepest 

 levels in the Gulf of Maine and not the comparatively high salinity at these depths, 

 for it thrives in still higher salinities in the North Sea region (Apstein, 1910). 



Temperature not only governs the distribution of S. elegans but also the size 

 to which it grows, a fact that has long been recognized. Indeed, three varieties or 

 subspecies of this species, one of them a large northern ("arctica"), another a smaller 

 boreal-temperate ("elegans"), have been recognized by von Ritter-Zahony (1911); 

 but Huntsman (1919) points out that these are not distinct, being connected by inter- 

 mediates. In fact, the Gulf of Maine collections suggest that the difference in size 

 between them probably is not hereditary at all, but the result of a direct physiological 

 influence of the environment on the individual, for the adults average decidedly 

 larger (up to 35 millimeters long) in March and April, when the temperature is near 

 its lowest for the year, than in summer. This is not the maximum size for the Gulf 

 of Maine, however, Huntsman (1919, p. 446) having recorded specimens of this 

 length with ovaries still immature, and he describes S. elegans up to 52 millimeters 

 long from the still colder waters of parts of the Gulf of St. Lawrence. He has also 

 pointed out that it matures sexually at a smaller size in high temperatures than in 

 low, as is the case with sundry other boreal planktonic animals — for example 

 Aglantha digitate. 11 



Sagitta serratodentata Krohn 



The fact that S. serratodentata is an annual immigrant to the Gulf of Maine and 

 not endemic there has been brought out in an earlier chapter (p. 58), and its tropical 

 origin and lines of dispersal have been discussed. It is safe to say there are no S. 

 serratodentata in the inner parts of the gulf in late winter or early spring, the visitors 

 of the previous summer all having perished, because our February and April 

 cruises of 1920 did not yield it anywhere within the continental edge except for a 

 single specimen in the southeastern part of the basin on March 11 (station 20064). 

 It is probably to be found in the warmer water along the slope abreast of the gulf, 

 however, throughout the year, for odd specimens were detected at our outer stations 

 off the southwest face of Georges Bank on February 22 (station 20044), and off Cape 

 Sable on March 19 (station 20077). 



In the year 1915 S. serratodentata had penetrated the eastern side of the gulf as 

 far as the neighborhood of Lurcher Shoal and the northeastern part of the basin by 

 May 10 (stations 10272 and 10273; Bigelow, 1917, p. 296), and by the last of that 

 month and first days of June the Canadian fisheries expedition found it at two 



71 For a discussion 01 other differences between the races of S. elegant living in high temperatures and in low see Huntsman 

 (1919). 



