PLANKTON OF THE GULF OF MAINE 51 



A similar community (notably Euchseta and the deep-water choetognaths) also 

 occupied the deeper water layers in the western basin in February and March, 1920 

 (p. 40), and deep hauls made there and in the southeastern part of the basin that 

 April gave much the same yield. Judging from hauls made in 1915, however, the 

 deep-water chaetognaths Eukrohnia Tiamata. and Sagitta maxima disappear altogether 

 from both the western and the northeastern deep troughs in May, not to. reappear 

 there until August, 20 a phenomenon interesting for its bearing on the lines of 

 immigration of these two species, neither of which breeds in the gulf, and as evidence 

 of the seasonal fluctuation of the bottom current. But it is possible that they 

 persist in the southeastern deep and in the eastern channel. 



It is probable that the Euchseta community of the western basin is at its lowest 

 ebb in May or June, for if the euphausiid shrimp Meganyctiphanes norvegica was 

 not wholly wanting there during those months in 1915, it was at least so rare that 

 the nets did not chance to pick up any specimens, although it was plentiful in the 

 eastern trough at the time. Meganyctiphanes repopulates the deep waters of the 

 western side of the gulf by midsummer, however, for we have found it there at all 

 our stations for July and August (p. 151), and the mammoth copepod Euchseta 

 norvegica is as constant, though not as abundant, an inhabitant of the deepest 

 waters of the gulf, season in and season out, as Calanus is of the upper strata. 



IMMIGRANT PLANKTONIC COMMUNITIES 



Besides the endemic boreal animals so far discussed (chiefly the Calanus com- 

 munity), which are the most important members of the animal plankton of the Gulf 

 of Maine, various immigrants enter it from time to time, as might be expected in 

 any maritime area where waters of diverse origin meet and mix, the details of such 

 immigrations varying with the ocean currents that give them birth and in which 

 their participants normally pass their existence. 



According to their adaptability to the temperatures and salinities which they 

 meet in the gulf, these involuntary visitors exhibit every degree of success as col- 

 onists, from inability even to survive for more than a few days or weeks to perfect 

 success in existing, growing, and breeding. The majority, however, occupy a middle 

 ground — able to live and grow to large size in the gulf but not to reproduce them- 

 selves there because of unfavorable temperatures or salinities, or at most breeding so 

 seldom that their continued presence in the gulf depends absolutely upon successive 

 waves of immigration from outside. Associated with their essentially exotic origin, 

 most of these immigrants are decidedly seasonal in their appearance within our 

 limits. 



To place clearly before the reader the faunal status of such wanderers, I must 

 emphasize here (what is perhaps the most essential factor in the biology of all pelagic 

 animals below the rank of fishes, and a truism to the oceanographer) their utter 

 inability to cany out voluntary migrations of more than a few miles at most from 

 place to place by swimming, for want of a continuous directive stimulus, though 

 they often perform extensive vertical movements. The horizontal migrations of 



:« Possibly in July, a month for which we have but one deep station. 



