58 Causes of Emigrations 



line with terrestrial flies and beetles. The archer fish in Siam shoots 

 insects down into the water where it devours them, certain of the 

 gobies run out over the land where they catch ants and spiders 

 (Pearse, 1933) , ?nd climbing perch at twilight or during rains leave 

 ponds and streams to search for insects in grassy fields. Various 

 crabs have not left the ocean but continually run out over the land 

 to hunt for food, and some of these (fiddler crabs) probably never 

 seek food under water. 



Respiration 



There are great diiferences in the amount of oxygen consumed 

 by various marine animals. Baldwin (1924) gives the following 

 figures, stated as cubic centimeters per gram per hour: 



Mackerel 0.726 Nereis 0.291 



Scup 0.301 Amphitrite 0.131 



Sea bass 0.070 Cerebratula 0.172 



Flounder 0.063 Starfish 0.019 



Tautog 0.062 Sagartia 0.026 



Squid 0.601 Dactylometra 0.019 



Phascolosoma - 0.367 



At 12 ^C. the oxygen consumption of the scup was reduced 21%; 

 at 4°C., 40%. In light, a scup consumed 0.156 cc; in dark, 0.115 

 cc. Evidently, more complex and active animals consume more 

 oxygen than do simple sluggish animals. 



Respiration is more difficult in fresh water than in the ocean, but 

 currents may help compensate for lowered salinity (Prenant, 1929) . 

 The presence of monocarbonates in sea water makes the elimination 

 of carbon dioxide easier. This factor alone prevents certain animals 

 from invading brackish water. It is easier for marine animals to 

 invade brackish water in the tropics than in cooler parts of the 

 earth because monocarbonates are present (Schlieper, 1928). Ma- 

 rine fishes enter streams in the tropics more readily than elsewhere, 

 probably because of the richer lime content there (Breeder, 1933). 



