PLANKTON OF THE GULF OF MAINE 123 



fact, it may not have occurred at all, for the few specimens brought in by the deep 

 hauls may have been picked up by the nets close to the surface on their journey down 

 or up; and the scarcity, if not absence, of this species in the coldest water along 

 Nova Scotia is sufficient evidence that it is not an immigrant to the Gulf of Maine by 

 that route. The general thesis that it is not at home in water of Arctic temperatures 

 is further corroborated by Doctor Huntsman, who informs me that Limacina retro- 

 versa is scarce, if not wanting, in the Gulf of St. Lawrence, where, by contrast, its 

 larger Arctic relative (L. helicina) is very plentiful. 



I have pointed out elsewhere (Bigelow, 1917, p. 299) that L. retroversa occurs 

 in numbers in waters of widely varying salinity in the Gulf of Maine, which agrees 

 with experience in European seas; but in spite of its tolerance for variations in salinity 

 it is clearly characteristic of the Salter rather than of the fresher waters of the gulf. 

 Thus, it has been detected at only five stations out of 55, where the upper 10 meters 

 or so have been fresher than 31.5 per mille; never in any numbers except where the 

 underlying layers were much Salter (e. g., station 10294, surface 31.06, 80 meters, 

 32.79 per mille). While such evidence is perhaps not conclusive for an organism 

 so sporadic in its local appearances and disappearances, at least it justifies the working 

 hypothesis that L-. retroversa is seldom to be expected in water fresher than, say, 

 31.5 per mille, and not likely to persist in much lower salinities. About 31.06 per 

 mille is the lowest salinity in which it has certainly been taken within the limits of 

 the gulf, and Paulsen (1910) has already suggested the probability that when this 

 pteropod chances to stray into water much fresher than 30 to 31 per mille it perishes. 



The dependence of L. retroversa on comparatively high salinity may have as 

 much to do with making Massachusetts Bay and the coastal belt of the gulf generally 

 unfavorable for it in spring as has its avoidance of very low temperatures. 



Until the seasonal cycle of these two sets of phenomena — biologic and hydro- 

 graphic — has been followed more closely, the dependence of the former on the latter 

 can only be stated in the most general terms. However, it is important for an 

 understanding of the biology of this pteropod to emphasize the probability that 

 there is a causal relationship between the seasonal expansions and contractions in its 

 geographic range in the Gulf of Maine, on the one hand, and local and seasonal 

 differences in the salinity of the water, on the other. We find in this a resasonable 

 explanation for the fact that while winter chilling to 2° to 3° probably is the cause 

 which banishes L. retroversa from the coldest parts of the gulf in winter, 04 it does 

 not reappear near the coast in regions where the effect of the spring freshets 

 in lowering the salinity persists longest into spring and summer (Massachusetts 

 Bay, for example) until several months after the water has warmed to a point 

 favorable for its existence, and until a considerable increase has taken place in the 

 salinity of the upper 40 meters or so. In such locations, therefore, low salinity is 

 probably responsible for its protracted absence, which continues until the water is 

 once more salt enough for its liking. 



Kepopulation of the coastal zone by Limacina after its annual period of scarcity 

 might take place in one of two ways — either by local survival or by immigration. 



•< From parts of the Bay of Fundy and from the inner parts of Massachutests Bay and probably from all along the shore in 

 cold winters. 



