No. 635] SHORTER ARTICLES AND DISCUSSION 



r,55 



fish return less quickly and seem to become readily acclimated 

 to the still waters of permanent or even temporary pools to 

 which they are directed as the tides recede. 



In August there is a period of over two weeks when actively 

 feeding killifish are almost completely absent from the marshes. 

 That temperature plays a most important part in this behavior 

 is indubitable. Shelf ord and Powers have shown (1915) that 

 the herring is sensitive to temperature differences as small as 0.2° 

 C. They have demonstrated that alkalinity and acidity are 

 more important than salinity. The herring and salmon experi- 

 mented with reacted to small fractions of a cubic centimeter per 

 liter of H„S and became negative to sea water which was slightly 

 more acid than the fresh. It is possible that increased tempera- 

 ture may bring into solution organic substances which alter the 

 alkalinity of the sea water or even render it acid near sources 

 of pollution. Johnstone has shown (1908) that the migration 

 of herring in Europe is closely associated with the salinity and 

 temperature of the sea. 



We may safely assume that Fundulus heteroclitus has an op- 

 timum temperature for its metabolism which will be higher when 

 the animal is weak and poorly nourished, but lower when it is 

 well fed. Thus a gradually increasing temperature while the 

 animal is feeding will finally result in such warmth that normal 

 metabolism is no longer possible and there will be no return to 

 the fresher waters until they become cooler. Another factor of 

 great importance in the inland movements of Fundulus is tin 1 

 fact that after spawning, the animals are shi<r<iish and hence in 

 no condition for a battle with the tides. This factor is probably 

 the one that causes almost complete disappearance of the larger 

 and the medium-sized Fundulus during August in the area 

 studied. 



Fall Migration 



Early in September large numbers of Fundulus heteroclitus 

 of small and medium size return to the marshes with the tides, 

 and they continue to run in and out until the water becomes 

 extremely cold. There are fewer individuals remaining in the 

 pools between tides, but many are still found variously dispersed 

 among temporary pools far inland. 



Their food is somewhat reduced so far as mosquito larvae are 

 concerned, but there are many other insects available, besides 

 small eggs and shrimps. 



