PLANKTON OF THE GULF OF MAINE 
371 
hand, its habit of rising to the surface on warm summer days brings it into water of 
16° and upward, while, on the other, it has been taken in the gulf in water as cold 
as 2.5°, and there is no reason to doubt that it can survive the minimum to which the 
temperature of any part of the gulf ever chills. Nor is it surprising to find it in 
extremes as wide apart as this, for the species is practically eurythermal in its geo- 
graphic distribution (p. 369). As I have previously pointed out (Bigelow, 1915, 
p. 323), its optimum salinity in North American waters is from about 32 per mille to 
about 34 per mille, but since it lives in decidedly more saline water in the North 
Sea region its absence from the saltest water of the Gulf of Maine does not mean 
that high salinities are unfavorable to it but is due to its neritic habit and to its 
preference for the uppermost stratum of water. 
It is not unlikely that the vertical movements of Pleurobrachia are influenced 
by the density of the water in which it lives . 10 Although there does not seem to 
be any connection between the occurrence of Pleurobrachia and density within a 
range of 1.022 to 1.026, we have never found it (probably it can not float or swim) 
in water lighter than 1.022, seldom, indeed, in specific gravity lower than 1.023. 
On the other hand, the presence of Pleurobrachia has never been established in water 
heavier than 1.027 in the Gulf of Maine or anywhere off the coast of North America, 
which may explain its failure to sink into the heavier bottom water of the deep basin 
of the gulf. 
Mertensia ovum (Fabricius) 
This cold-water ctenophore, so abundant in Arctic seas (Mortensen, 1912) and 
especially along the eastern coasts of Labrador and Newfoundland (Bigelow, 1909a), 
reaches the Gulf of Maine only as an immigrant from the north and is short ved 
there. Its faunal status being discussed elsewhere (p. 59), I need only add that 
recent records of it in the gulf are confined to spring and early summer at the follow- 
ing localities and dates: 
Eastern basin, May 6, 1915 (station 10270), in surface, 50-0-meter, and 150-0- 
meter hauls, a total of about 20 specimens; near Lurcher Shoal, Maj^ 10, 1915 
(station 10272); off the mouth of Penobscot Bay, June 14, 1915 (station 10287). 
It is present through a longer season off southern Nova Scotia, for we have taken 
it along the Shelburne profile both in March, 1920 (stations 20075, 20076, and 20077), 
and in June, 1915 (stations 10291 and 10294); and off Halifax in August (Bigelow, 
1917, p. 249). During some years it appears in the Gulf of Maine in autumn, for 
Alexander Agassiz (1865, p. 29) records it as “exceedingly common in Eastport 
Harbor during the month of September,” a record indisputable because of his excel- 
lent figures and description. Fewkes (1888, p. 212) similarly speaks of it as “the 
common tentaculated ctenophore” at Eastport and at Grand Manan during the 
summers of 1885 and 1886, but his failure to mention Pleurobrachia, which is actually 
so abundant there, suggests the possibility that he confused the two genera. 
Large Mertensia are unknown south of Massachusetts Bay, and indeed only 
one adult has been taken even there (A. Agassiz, 1865), but its young may travel 
as far west and south as New Jersey during the cold season (Mayer, 1912). 
ls Rose (1913) has experimented on the flotation of this ctenophore in waters of varying densities 
