PLANKTON OF THE GULF OF MAINE 339 



temperatures between and 1° at several stations along the coasts of Maine and 

 Massachusetts in March (e. g., stations 20056, 20059, and 20062), frequently in water 

 of 2 to 4°, and the specimens taken by the Canadian fisheries expedition over the 

 Newfoundland Banks and in the Laurentian Channel in May, 1915, were probably 

 in water colder than 0° (see Huntsman, 1921, p. 86, for these records) and very 

 likely fractionally colder than — 1°. Thus this worm finds its optimum in compara- 

 tively low temperatures. Perhaps 15° might be stated as its absolute upper limit in 

 the Gulf of Maine, and, in fact, it is doubtful whether it could long survive water 

 warmer than 10 to 12°. 



On three occasions we have taken T. catharina in salinities as low as 31 to 32 

 per mille, 80 but the great majority of captures have been in water of 32 to 33 per 

 mille. The highest salinity in which our records positively establish its occurrence 

 in the Gulf of Maine (closing-net hauls at stations 20052 and 20055) is 33.7 to 33.8 

 per mille, and although we took Tomopteris at one station (10225) where the net 

 worked for a time in water as saline as 35 per mille, it is more likely that the odd 

 specimens that it brought back were picked up on its journey up or down through 

 the lower salinities of the superficial strata of water. 



Thus, these additional records obtained during 1920 and 1921 corroborate Hunts- 

 man's (1921, p. 90) conclusion that a salinity of approximately 33 to 34 per mille is 

 the upper limit for T. catharina off North America. On the other hand, it seems 

 well established that it never occurs in water much less saline than 32 per mille 

 except when it makes brief excursions to the surface, and our records thus support 

 Huntsman's (1921, p. 90) suggestion that low salinity is the factor that prevents it 

 from colonizing estuarine situations. In north European waters, as he remarks, the 

 relationship which T. catharina bears to salinity is quite different, for around Ireland 

 Southern (1911) found it only in water more saline than 34 per mille. Of course, it is 

 possible that this is a physiological difference between the American and European 

 races of this species; but the question also naturally occurs whether high salinity, 

 per se, acts anywhere as a bar to its dispersal, and whether it is not high temperature, 

 quite independent of salinity, which lays down a definite offshore bound for T. 

 catharina just outside the American continental edge as far northward as the Nova 

 Scotian banks. The rarity of T. catharina over the continental shelf along Nova 

 Scotia (Huntsman, 1921) is especially interesting in this connection, because the 

 temperatures and salinities there both fall within the limits in which it exists either 

 to the south or to the north. 



The most reasonable explanation for its peculiar distribution is that T. catharina 

 occurs chiefly in the immediate neighborhood of its centers of production, of which 

 there are but two in the seas under discussion — a major on the Grand Banks and a 

 minor in the Gulf of Maine. It does not breed in the intervening stretch of waters 

 because it requires a closer balance in its physical environment for successful breeding 

 than for vegetative existence. This, we may assume, it does not find in the Gulf of 

 St. Lawrence. 



It is therefore likely that when the geographic limits within which T. catharina 

 breeds successfully are better known, specimens taken elsewhere will prove to be 



» Station 20092, surface, 31.01 per mille; station 20093, surface, 31.92 per mille; station 20119, surface, 31.43 per mille. 



