PLANKTON OF THE GULF OF MAINE 319 



art' in summer, though moderately plentiful at deeper levels in temperatures of 4 to 

 7°; but the small sizes were taken in all the surface hauls on that cruise, once in 

 some numbers (station 10399). With the continued cooling of the water the adults 

 must spread through the superficial stratum of water at some time during the late 

 autumn and winter to attain the distribution just described for March (p. 317), but 

 the horizontal hauls at our winter stations have not been adapted to show just when 

 this takes place. 



The data just outlined for the Gulf of Maine are directly in line with Hunts- 

 man's (1919, p. 465) observations based on the collections made by the Canadian 

 fisheries expedition, that off Nova Scotia the large S. elegans rise to the surface by 

 night during May and June while the surface temperature is still low, sinking again 

 during the hours of bright daylight, but are virtually absent from the surface during 

 July and August, night as well as day. 



The primary cause for this seasonal variation in the vertical distribution of 

 S. elegans is to be found in the temperature of the water, which, being uniformly 

 low during the early spring, then imposes no barrier to upward dispersal; but when 

 the vernal warming of the surface has proceeded to a certain degree, which may 

 tentatively be set at 10 to 12°, most of the Sagittas remain below the warm super- 

 ficial layer. The diurnal migration described by Huntsman (1919), together with 

 the fact that when S. elegans rises to the surface in the Gulf of Maine in July or 

 August this usually takes place at night, makes it probable that bright light as well 

 as high temperature to some extent limits its dispersal upwards. But, judging from 

 its vertical distribution in March and April, when it is at the surface day and night 

 indifferently, this is not the case until the sun attains a comparatively high declina- 

 tion, the inference being that while S. elegans is negatively tropic to light of more 

 than a certain intensity, its movements are little influenced by a paler illumination. 

 This warrants the following working hypothesis. In winter and early spring all levels 

 in the Gulf are sufficiently cool for S. elegans, and the illumination by the sun is not 

 so bright but what a certain number may regularly be found at the surface by day 

 as well as by night; but in late spring and early summer it is daily driven downward 

 for some meters by the sun, and by July and August the high temperature renders 

 the uppermost stratum of water unsuitable for its permanent presence, an unfavor- 

 able condition from which it can and does escape by sinking. Occasionally it rises 

 to the surface in summer, irrespective of temperature or of illumination. We found 

 an abundance of medium-sized specimens south of Nantucket Shoals, July 9, 

 1913 (station 10060), at 6 p. m., in a surface temperature of 16.1°, but it is not 

 likely that such upward incursions endure for more than a brief period, perhaps only 

 for a few hours. 



Huntsman and Reid (1921) have pointed out for the Bay of Fundy (and our 

 own observations corroborate them) that the young S. elegans tend to congregate 

 nearer to the surface than the adults. 



In the deeper strata of the gulf, below 20 meters or so, where the physical state 

 of the water is apparently favorable for the existence of S. elegans, the local varia- 

 tions in its abundance at different depths may be governed by quite a different 

 75898—26 21 



