PLANKTON OF THE GULF OF MAINE 141 



and in summer. For instance, it was abundant on the edge of Georges Bank on 

 March 13, 1920 (station 20071), and on Browns Bank on July 24, 1914 (station 

 1022S) . This phenomenon and the fact that we have found it at most of our stations 

 along the continental slope abreast of Georges Bank and south of Nova Scotia, where 

 inermis has usually proved wanting, is no doubt correlated w T ith its oceanic nature, 

 and Hansen (1915) records Th. longicaudata from many localities over the slope 

 south of Marthas Vineyard, often in great abundance. 



Evidently this shrimp is a characteristic inhabitant of the cool band of water 

 of mixed origin which separates the tropical Atlantic (so-called "Gulf Stream") 

 water from the continental shelf. Probably it comes as a wanderer from the east 

 and north, and it may follow the outer part of the shelf at least as far south as the 

 latitude of Chesapeake Bay in cool summers, as in 1916 (Bigelow, 1922, p. 151) ; but 

 we have never found it at any station where the presence of a tropical planktonic 

 community has betrayed a large admixture of " Gulf Stream" water. Judging from 

 the boreal- Artie affinities of Th. longicaudata, it is probable that high temperatures 

 and salinities form an impenetrable offshore barrier to its dispersal off the coasts 

 of Nova Scotia and the United States. 



Bathymetric range. — We have yet to find Th. longicaudata on the surface in the 

 Gulf of Maine in summer, most of the records of it for the three months, July to 

 September, being in hauls from 80 meters or deeper, the shoalest from 50-0 meters 

 (two hauls). An interesting example of its preference for deep water is afforded 

 by its vertical distribution in the western basin on August 22, 1914 (station 10254), 

 when there were none on the surface, and, allowing for the use of different-sized 

 nets, many more at 235-0 meters depth than at 75-0 meters (Bigelow, 1917, p. 282). 

 Although it is not so closely confined to the deeper strata of water during the early 

 spring (for we found many on the surface over the eastern end of Georges Bank on 

 March 13, 1920 (station 20070), and a few on the surface in the western side of the 

 basin 10 days later (station 20087)) most of the spring records of the species in the 

 gulf have likewise been from depths greater than 75 meters. Thus, it finds its most 

 favorable habitat at a deper level than that of Th. inermis. 



Judging from the rather conflicting statements of European students (Holt 

 and Tattersall, 1905; Hansen, 1908; Tattersall, 1911; Kramp, 1913), Th. longi- 

 caudata is equally a deep-water form on the other side of the Atlantic, though it 

 comes right up to the surface of the water about Iceland (Paulsen, 1909). Probably 

 the warm layer that forms over the surface of most boreal seas in late spring and 

 summer acts as a barrier to its upward dispersal during the w r arm half of the year, 

 just as high temperature confines it offshore, abreast of the Gulf of Maine. At any 

 rate, its avoidance of the surface in summer and of the coastal zone at all seasons 

 makes it an inhabitant of low temperatures and comparatively high salinities in 

 the Gulf of Maine, where the water in which most of the stock lives ranges from 

 about 2° to about 10° in temperature and upward of 32.5 per mille in salinity. 



Whether Th. longicaudata breeds in the Gulf of Maine or appears there only as 

 an immigrant from the north is yet to be learned. Probably it is endemic there 

 in small numbers, like other planktonic animals with a similar affinity for low 

 temperature, but depends as much on more or less constant immigration from 



