PLANKTON OF THE GULF OF MAINE 143 



tropical and warm-temperate form which undoubtedly reaches the gulf from the 

 warmer waters offshore and not from the cooler seas to the east and north. Its 

 local presence is sure evidence of an influx of such water into the gulf. 



As I have noted elsewhere (Bigelow, 1917, p. 284), Th. gregaria is much less 

 common in the gulf than Th. inermis, or, I may add, than Th. longicaudata; but 

 the records for 1912 (Bigelow, 1914a, p. 412), 1914, and 1915 (Bigelow, 1917, p. 285), 

 show that in summer it is to be expected anywhere on Browns and Georges Banks, 

 along the continental slope south of Nova Scotia, in the Eastern Channel, and in 

 the inner parts of the gulf as well (fig. 51). We have never found Th. gregaria in 

 any abundance anywhere in the gulf north of the offshore banks, but we took it in 

 numbers on the western part of Georges Bank on July 20, 1914 (station 10216), 

 and Hansen (1915) detected it in the gatherings from two deep stations south of 

 Marthas Vineyard. Curiously enough, however, in spite of its well-established 

 warm-water origin, we did not find it at our saltest and warmest station east of Cape 

 Cod, where the plankton was distinctly tropical in aspect (station 10218, July 21, 

 1914), nor did it appear in the tow nettings along the slope from Georges Bank to 

 the latitude of Chesapeake Bay during July, 1916. Our records for this species 75 prove 

 that it is more seasonal in its occurrence in the Gulf of Maine than are its northern 

 relatives, nearly all being for August; and its history in 1915 in particular, when 

 it was not detected until August, although we made frequent tows in various parts 

 of the gulf during the spring and early summer, shows that it increases in numbers 

 and penetrates farther and farther into the gulf with the advance of summer. Its 

 presence there seems short lived, however, for we did not find it at all during October, 

 1915, or November, 1916; and although the tow yielded an odd specimen off Glouces- 

 ter on December 23, 1912, we sought it in vain in December, 1920, and January, 

 1921, and during the late winter and spring of 1920. Probably the correct explana- 

 tion for its absence from the Gulf of Maine during the cold half of the year is that 

 the species vanishes thence when the stock that has entered the gidf during the 

 summer perishes at the onset of autumnal cooling. It does not reappear until the 

 surface waters are once more sufficiently warm for its existence, which means mid- 

 summer. Thus it closely parallels Sagitta serratodentata (p. 58) in its status in the 

 gulf, and there is no reason to suppose that Th. gregaria ever breeds successfully 



there. 



Thysanoessa rascliii, M. Sars 



This species (fig. 52) resembles Th. longicaudata in its Arctic-boreal nature 

 (Kramp, 1913; Zimmer, 1909), and ranges southward along the European coast 

 to the northern part of the North Sea, to the longitude of Nantucket and probably 

 still farther, off North America; but, as I have noted in an earlier report (Bigelow, 

 1917, p. 284), it is much less common in the Gulf of Maine in summer than is either 

 Th. inermis or Th. longicaudata. It was not detected there at all in the hauls of 

 July and August, 1912, and appeared at only three stations within the limits of the 

 gulf during the summer of 1914 — two of them in its northeastern part and the third 

 off Marthas Vineyard (Bigelow, 1917, p. 282). It was not detected at all during the 



" For lists of the Gulf of Maine records of Th. gregaria, 1912 to 1915, see Bigelow, 1914a, p. 411, and Bigelow, 1917, p. 282. 

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