PLANKTON OF THE GULF OF MAINE 211 



C. finmarchicus is likewise indifferent to changes in salinity within wide limits, 

 but I have been unable to learn that it is regularly abundant anywhere in water more 

 saline than about 35.3 per mille 10 (Farran, 1910). Thus high salinity is probably a 

 more effective barrier to its dispersal seaward abreast of the Gulf of Maine and thence 

 southward along the continental edge of North America than is high temperature, 

 though, to quote from an eminent student of this group (Willey, 1919, p. 176), 

 "the factor which determines the limit of southern dispersion of C. finmarchicus is 

 clearly neither a simple physical constant nor a single organic tropism," but "includes 

 the biological factors of food-supply and propagation." 



C. finmarchicus is regularly and abundantly present in considerably less saline 

 water (31 to 33 per mille) in the western side of the North Atlantic than Farran 

 (1910) set as the lower limit to its plentiful occurrence in the North Sea region (33.5 

 per mille), and apparently it was spawning actively in a salinity of only 29 to 30 

 per mille in Massachusetts Bay in May, 1920. Judging from its status in the en- 

 trances to the Baltic, however, and from its rarity within the latter, probably it can 

 not exist long in water much fresher than this, though it may reach brackish situations 

 as driftage. 



Economic importance. — The importance of C. finmarchicus in the general economy 

 of the Gulf of Maine and of all other seas where it abounds can hardly be over- 

 estimated. Certainly it is no exaggeration to call it the most important single 

 planktonic animal, probably more important in the gulf in its relation to both larger 

 and smaller organisms than all other copepods combined. It is the basic food for the 

 local mackerel, and is certainly a major article in the diet of the herring, alewife, and 

 shad while these are at sea. All the other fishes of the offshore waters of the gulf 

 that eat plankton at all may be expected to feed on Calanus more than on any other 

 single item. Through the medium of the herrings, which are nourished on it, Calanus 

 helps support the finback and humpback whales, Balsenoptera physalus and Megaptera 

 nodosa (the only whalebone whales now common in the gulf), though neither of these 

 feeds directly on copepods, their whalebone being too coarse (p. 97). On the other 

 hand, it is probable that Calanus makes greater inroads on the planktonic plants 

 on which it preys than do all other copepods combined, and conceivably it may 

 practically exterminate them locally and temporarily. 



Calanus gracilis Dana 



Dr. C. B. Wilson contributes the following note: 



This species has been reported from the western part of the Mediterranean and from the 

 Indian and Pacific Oceans as well as the Atlantic. Cleve (1900), in discussing the distribution of 

 Atlantic Copepoda, gave the northern and southern limits of this species as from the 44th parallel 

 north to the 35th parallel south. The Gulf of Maine, therefore, is about its northern limit, and it 

 would not be expected to appear in large numbers. Neither would it be widely distributed. It is 

 worthy of note that Pesta has reported it from a depth of 1,200 meters in the Adriatic, while Gies- 

 brecht gave 1,500 meters as the maximum depth limit. The few specimens found in the present 

 plankton were obtained in October from shallow water rather close to the shore [at two stations 

 off Marthas Vineyard and at one in Massachusetts Bay (see table, p. 298)]. 



i° Willey (1919. p. 176) records abundant Calanus at a salinity ol 35 per mille in the edge of the Oulf Stream between the Scotian 

 and Newfoundland Banks on June 1, 1915. 



