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 (1S60) 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 Bolinopsis whole if they are not too 

 large for its widely distensible mouth to engulf, with digestive process so rapid that 



