IOS THURLOW C. NELSON. 



VII. 



Tin-: DISTRIBUTION OF CTENOPHORES AS EFFECTED BY SALINITY 



AND TEMPERATURE. 



Mayer ('14) has emphasized the superior temperature adjust- 

 ment of marine animals of the arctic and temperate zones over 

 individuals of the same or of related species in the tropics. 

 Mnemiopsis leidyi in Barnegat Bay illustrates this superiority of 

 adjustment in a striking manner. Owing to the typography and 

 small tidal flow of Barnegat Bay, summer temperatures there 

 may rise well above 26 C. During most of the summer of 1923 

 the water temperature varied between 24 and 25 C., with 

 specific gravity over the region within 4 miles south of Seaside 

 Park ranging from about i.oiio to 1.0180. 



Bigelow ('15) found Mnemiopsis leidyi in July in greatest 

 abundance over the inner half of the continental shelf off the 

 New Jersey coast, in water of a specific gravity between 1.0246 

 and 1.0252. It was not found in the very salt Gulf stream on 

 the one hand nor where the specific gravity fell below 1.0244 at 

 the mouth of Chesapeake Bay, on the other. The upper limit of 

 temperature at which this ctenophore was found was 24.5 C., the 

 lower about 15.5 C. 



He gives the range of Pleurobrachia pileus as unbroken from 

 Labrador at least to Pamlico Sound. He found it more generally 

 distributed in the coast waters than any other ccelenterate, with 

 local swarms south as well as north of Cape Cod. The warmest 

 water in which it was found was 20.5 C., the coldest about 6 C. 

 The range in specific gravity was from about 1.0241 to approxi- 

 mately 1.0267. Nearly all the specimens taken were from the 

 deeper waters, whereas the majority of Mnemiopsis were found 

 in the first fathom. The swarms of Mnemiopsis and Plcuro- 

 brachia were found to be mutually exclusive; never were these 

 two forms taken side by side. 



The distribution of Pleurobrachia in the deeper waters, together 

 with the lower temperatures observed, indicate that Pleurobrachia 

 is adjusted to colder water off the New Jersey coast than is 

 Mnemiopsis. The high summer temperatures of the estuaries 

 such as Barnegat Bay form a barrier which it, unlike Mnemiopsis, 

 is unable to pass. In the extreme southern part of its range 



