PLANKTON OF THE GULF OF MAINE 121 



and summer (Massy, 1909), and that it is as erratic in its occurrence in the North 

 Sea as it is in the Gulf of Maine. 



Limacina has been taken at about 50 per cent of our stations over the conti- 

 nental slope between the longitudes of New York and Cape Sable in late winter, 

 spring, summer, and early autumn, though never in great numbers. Only one 

 specimen was taken at our most oceanic station (1021S, July, 1914), where the plank- 

 ton as a whole was tropical, nor did we find it associated with the warm-water 

 pteropods at our outermost stations south of New York in 1913. 



Being typically boreal in its affinity to temperature, it is not to be expected in 

 the warm waters of the so-called Gulf Stream off the American littoral except as an 

 accidental and probably short-lived straggler from the cooler coastal zone, but in 

 more northern seas Limacina occurs chiefly in what is generally known to European 

 oceanographers as the "Atlantic" water. This, for example, is the case south of 

 Iceland, where it appears in great shoals, and it is with the general drift of this water 

 (which is warm in contrast to the polar currents) that Limacina penetrates the 

 Norwegian sea (Paulsen, 1910), for it is not at home in the icy cold Arctic water of 

 comparatively low salinity. 



Most of the records of Limacina in the gulf have been from subsurface hauls, 

 for which the precise depths can not be stated because made with open nets; but 

 most of them have apparently come from comparatively shoal levels, for when two 

 hauls have been made at different depths below the surface the shallower has 

 usually taken the most Limacina. On the whole, the most prolific depth zone 

 may be stated as from 20 to 25 meters down to about 80, which corroborates 

 Paulsen's (1910) generalization that Limacina lives chiefly shoaler than 50 meters 

 in north European seas, though it has occasionally been taken much deeper. 



In summer we have never detected Limacina on the surface during the hours 

 of bright sunlight. In August, 1913, for example, "it was only once taken on the 

 surface (station 10103), although a surface haul was made at every station, usually 

 with a net of the same mesh as the one in which Limacina was taken in the depths" 

 (Bigelow, 1915, p. 303), that one occasion being at 7 p. m. On several occasions 

 during August, 1914, however, and the summer and autumn of 1915 (stations 10247, 

 10264, 10294, 10295, 10308, 10329, and 10333), surface tows between sunset and 

 sunrise have yielded it in some numbers. This suggests that Limacina, like many 

 other planktonic animals, performs a more or less regular diurnal migration in summer, 

 rising toward the surface during the dark hours, to sink again at sunrise. The fact 

 that the surface captures of Limacina (10 stations) 62 on our March and April cruises 

 of 1920 were made invariably either in the dark or during the twilight hours between 

 sunset and sunrise shows that this also takes place in spring, but perhaps not in 

 autumn and early winter, when the sun is at its lowest. 63 This habit certainly is 

 not so characteristic of Limacina in the more northern seas, where the sunlight is 



« Limacina retroversa was taken at the following stations during the spring of 1920: 20044, 20045, 20046, 20O4S, 20063, 20057, 

 20060, 20001, 20064, 20065, 20067, 20068, 20070, 20071, 20088, 20091, 20094, 20105, 20107, 20110, 20114, 20116, 20119, 20120, 20126, 20129; and 

 at the following in the winter and early spring of 1920-21: 10488, 10490, 10491, 10493, 10495, 10496, 10497, 10501, 10502, 10505, 10509, 

 10510, 10511. For earlier Gulf of Maine records of this pteropod see Bigelow, 1914, 1915, 1917, and 1922. 



•i We lack direct information on this point, our surface hauls for that season having been made with small, flne-meshed nets, 

 through which so little water filters that the apparent absence of Limacina may not be significant. 



