PLANKTON OF THE GULF OF MAINE 153 



this apparent preference for considerable depths is the fact that the small surface 

 net captured no fewer than 111 large specimens in the center of the gulf on April 17, 

 1920, at 2 p. m. (station 20113), while the haul from 120 meters took only three, 

 though there were many of these shrimps at 1 10 meters, but none on the surface only 

 35 miles distant to the westward (station 20114), that same day. S. I. Smith (1879 

 p. 89) likewise found it in shoals on the surface "on the mackerel ground" off Casco 

 Bay, both day and evening during the warm months 40 years ago. It swarms on 

 the surface in the Eastport-St. Andrews region in midsummer and earl}- autumn, 

 as just remarked (p. 147), and although recent records for it in Massachusetts Bay 

 have all been from depths of 40 meters or deeper, quantities of Meganyctiphanes 

 were taken at the surface at the mouth of the bay on July 7, 1894, in dip nets from 

 the rail of the Grampus; and they were so abundant there at a depth of less than 2 

 fathoms two days later that a large number found their way into the fish well of the 

 vessel (Hansen, 1915). Thus, while the normal habitat of Meganyctiphanes is in the 

 low temperatures and darkness of the deeper strata in the trough of the gulf, it may 

 rise to the surface anywhere at any time. In the Eastport region it may be brought 

 up involuntarily by the active stirring of the water which takes place there, and the 

 constancy of this type of vertical circulation may account for the regularity of its 

 presence at the top of the water there, expecially in view of the low surface tem- 

 perature that characterizes that locality (10 to 12° in summer and early autumn). 

 The Massachusetts Bay region, with surface readings of 16 to 18°, is nearly the 

 warmest part of the gulf in midsummer, so Meganyctiphanes is not prevented from 

 making occasional excursions upward to the top of the water even by temperatures so 

 high that a prolonged stay would probably prove fatal. Furthermore, such excur- 

 sions in this part of the gulf during the warm months involve voluntary upward 

 swimming, the vertical currents being weak and the water highly stable, with its 

 density much the lowest at the surface. Neither do they correspond to the diurnal 

 vertical migrations shared in by many copepods (p. 25), because the appearances of 

 Meganyctiphanes at the surface appear to be independent of the time of day. There- 

 fore, the actual captures so far recorded do not indicate any definite phototropism 

 on its part, positive or negative, although it is doubtful whether it could long survive 

 the full illumination of bright sunlight. 



Experience in most parts of the Gulf of Maine is therefore in line with Paulsen's 

 (1909) conclusion that when Meganyctiphanes visits the surface in Icelandic waters 

 it is not as a direct response to temperature (to which I may add salinity) or to the 

 degree of illumination, but in pursuit of food. It is also brought up by vertical 

 currents, where these are active. 



The depth at which Meganyctiphanes is most plentiful is more definitely limited, 

 and the relationship between its vertical occurrence and temperature is closer in 

 North European waters than in the Gulf of Maine. Off Ireland, for instance, and 

 in such parts of the North Sea as it visits, this euphausiid lives chiefly in the deeper 

 layers of water, reaching its maximum, according to Tattersall (1911), at about 200 

 meters. In the Skager-Rak (Kramp, 1913, p. 542) it carries out a more or less 

 definite vertical seasonal migration, always seeking the coldest level, which leads 

 it to the surface in winter and down to lower levels in summer. 



