PLANKTON OF THE GULF OF MAINE 
375 
and May have been small (15 to 20 mm. long) and immature — that is, were the 
product of the spawnings of the preceding summer and autumn — is evidence that 
no considerable production of this ctenophore takes place in the Gulf of Maine during 
the cold half of the year, and it is probable that the coming of spring sees the 
stock of this ctenophore at its lowest ebb for the year in all parts of the gulf. 
Beroes 30 millimeters long and upwards, such as we have taken in mid-April in 
Massachusetts Bay (station 20119), may be expected to grow so rapidly under the 
favorable conditions of food supply and temperature prevailing in May as to attain 
spawning size in June or early in July at latest. It is probable that the few that spawn 
in winter are the offspring of these early summer spawners, the development of those 
produced in late summer and autumn being arrested by the low temperature of 
winter, so that they do not mature until the following summer. Thus, particular 
groups of Beroe may produce either one or two broods per year, according to the 
rapidity with which they grow and the season at which they mature ; and while the 
chief production takes place from July to September, probably some spawn at all 
seasons except perhaps in early spring. 
It is worth emphasis here that A. Agassiz’s studies on the development of this 
ctenophore, corroborated by our own captures of its young in almost every month 
and at localities widely scattered, prove that Beroe is regularly endemic in the gulf, 
hence that the maintenance of the local stock depends chiefly on local production 
though it may be recruited more or less by immigration. 
Recent captures of Beroe support the suggestion made by Louis and Alexander 
Agassiz that it passes the winter at some little depth, for only 4 of our records for the 
cold half of the year (November to April) out of a total of 30 (and these for occasional 
specimens only) were from the surface, with one other from a 15-meter haul (Cape 
Cod Bay, station 20118, April 20, 1920). All our other winter-early spring captures 
of Beroe have been from depths of 40 meters and more. It may sink to a con- 
siderable depth in the Gulf during the cold season, for we took it with the closing net 
at 140-160 meters, and at 125-190 meters in the central part of the basin, March 2 
and 3, 1920 (stations 20052 and 20053). 
In summer Beroe frequently comes to the surface, most often during the midday 
hours, to sink again toward the end of the afternoon. This habit, long ago described 
by Louis Agassiz (1860) as well as by more recent authors, has repeatedly come under 
our own observation on the Grampus, notably during July and August of 1912, when 
we frequently saw large specimens of this ctenophore floating alongside the ship, 
usually in calm weather. On stormy days Beroe lies deeper, probably sinking below 
the limit of destructive wave action, and it is frequently taken at depths of 40 to 100 
meters, summer as well as winter. We have no evidence that this ctenophore ever 
descends into the deepest strata of the Gulf of Maine at any season (a single Beroe 
taken in a haul from 240 meters in the southeast part of the basin, July 23, 1914, 
station 10225, may have been picked up by the net on its journey down or up). 
The voracity of Beroe being commented on elsewhere (p. 108), I need only re- 
mark here that it has been described as preying greedily on other ctenophores in the 
Gulf of Maine, devouring Pleurobrachia and Bolin opsis whole if they are not too 
large for its widely distensible mouth to engulf, with digestive process so rapid that 
