58 
BULLETIN OF THE BUREAU' OF FISHERIES 
copepods (p. 56), never more than two tropical animal species among the hosts of 
boreal animals. 
This scarcity of planktonic visitors of the tropical category within the Gulf of 
Maine and even over its shallow southern rim, when so rich a tropical surface fauna 
inhabits the inner edge of the Gulf Stream along the outer edge of the continental 
slope only a few miles without the 100-fathom contour, is fundamentally due to their 
inability to survive or to reproduce in the low temperatures of the coast water. 
Their sporadic and solitary occurrence there, contrasted with the considerable 
numbers and even communities of tropical planktonic animals that often drift close 
inshore west of Cape Cod, is explicable only on the assumption that the surface 
waters of the Gulf Stream very seldom overflow the barrier formed by Georges Bank 
an assumption corroborated by the physical character of the water. Nevertheless, 
the Gulf of Maine does owe to the tropical water indirectly, if not directly, one 
common and very characteristic summer visitor, the large chsetognath Sagitta serrato- 
dentata. This species, which is the dominant member of its systematic group in the 
coastal waters south of New York, occupies a rather peculiar faunal niche in the 
Gulf of Maine, for while it breeds only in the high temperatures of the Gulf Stream 
(so far as the area under discussion is concerned), great numbers drift into the cooler 
mixture zone along the edge of the continental shelf, where they thrive and grow 
to a much larger size than they do in the warmer waters farther offshore, either 
because lower salinities and temperatures especially favor their growth (though not 
their reproduction), or perhaps because of a richer food supply (p. 323, and Hunts- 
man, 1919). As a denizen of this mixed water, S. serratodentata is swept in abundance 
into the Gulf of Maine, where, because of its size and abundance, it is the most 
prominent of all the exotic immigrants, though it never attains a more permanent 
status there. 
Owing to its peculiar relationship to oceanic temperatures, all the Gulf of Maine 
records so far obtained for S. serratodentata have been for large specimens, the locali- 
ties of capture indicating considerable longevity for it within the gulf. It is strictly 
seasonal in its presence there, however, being so rare in winter and early spring that 
we have taken it only twice between December 1 and May 1, viz, in Massachusetts 
Bay on December 4, 1912 (station 10048), and again on January 16, 1913 (station 
10050). It appears in the eastern side of the gulf as early as the first week in May 
(p. 320, and Bigelow, 1917, p. 296), and by June it has spread generally over the 
eastern basin and into the Bay of Fundy as well as over the outer edge of the shelf 
off Cape Sable, and probably also all along the southern and eastern parts of Georges 
Bank, where we found it in July, 1914. This species penetrates the inner parts 
of the gulf so slowly during the early summer that in five years we have found it 
only once in the western and southwestern parts prior to August 1. Thereafter, 
however, it spreads so rapidly westward and southward along the coast of Maine 
that our August and September records for it cover the whole northern half of the 
gulf from Cape Ann right across to Cape Sable, including Massachusetts Bay, where 
it occurs regularly in late summer and autumn. 
The locations of the stations of capture and the fact that S. serratodentata is 
usually more numerous in the eastern than in the western side of the gulf (p. 322) are 
