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 110 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 early 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 fives 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. 
