PLANKTON OF THE GULF OF MAINE 



181 



Time 



March 

 April . 

 May.. 



Surface 

 hauls 



Per cent 

 14.5 

 11 

 3 



Vertical 

 hauls 



Per cent 



In several instances the greater percentage on the surface was the result of a 

 definite concentration there, proved by the capture of hundreds of specimens in the 

 surface net at several stations where A. longiremis was so scarce deeper down that 

 the vertical net missed it altogether — for instance, off the Isles of Shoals on March 

 5 (station 20061); off the northern edge of Georges Bank, March 11 (station 20063); 

 on its eastern edge and southern slope, April 16 (stations 20108 and 20109); and, 

 notably, on March 23, off Yarmouth, Nova Scotia (station 20083), where the richest 

 surface catch of all was made (711 specimens) . At a rather larger number of localities 

 the yield of the vertical nets was considerable, where few or none were taken on the 

 surface, as shown in the following table: 



The most that can be said from this is that at times A. longiremis tends to gather 

 at the surface, both in spring and in midsummer, but that on other occasions it keeps 

 at least a few fathoms down. The hauls here listed give no evidence of diurnal 

 migrations, for the richer surface catches were more often between 9 a. m. and 5 p. m. 

 than at night, and, on the other hand, several of the hauls in which it most predomi- 

 nated in deeper levels were between sunset and sunrise. 



A. longiremis has been found over a very wide range of salinity, being common 

 in water as brackish as 6.72 per mille in the Baltic and as salt as 35.32 per mille in 

 the English Channel. In the Gulf of Maine it occurs well within these limits. It is 

 likewise eurythermal over a wide range of temperature, being present in the guff 

 indifferently in water as warm as 16° and as cold as 0.3° to 2°. The physical limits 

 within which it reproduces locally have not been determined, but the presence of 

 juveniles in August (p. 177) proves that reproduction takes place successfully in summer 

 temperatures, probably upwards of 10 to 12°. 



Acartia tonsa Dana 



This species was originally described from Port Jackson, Australia, and was 

 reported by Giesbrecht (1892) from the west coast of South America, and from the 

 Malayan Archipelago by Cleve (1901). On the one side of the North American 



