PLANKTON OP THE GULF OF MAINE 
123 
fact, it may not have occurred at all, for the few specimens brought in by the deep 
hards 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 
vdth 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, 64 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. 
Repopulation 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. 
61 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. 
