66 Causes of Emigrations 



not stay long. At Tortugas a common predaceous fish, the gray 

 snapper, devours the ghost crab whenever it can. 



Johnstone (1908) suggests that salmon were originally marine 

 fishes and that they have developed the habit of spawning in fresh 

 water in order to protect their eggs from marine enemies. On the 

 California coast there is a remarkable smelt, the grunion, Leuresthes 

 tenuis (Ayers) . This fish comes out on sandy beaches on moon- 

 light nights during March, April, and May to lay its eggs. Just 

 after the highest tides little fishes of this species flop out on the 

 beach above the water, wiggle their posterior ends into the wet sand, 

 and deposit pods which contain about two thousand eggs each. 

 About two weeks or a month later when tides, and perhaps favor- 

 able winds, bring high water again, the young fishes hatch out and 

 are washed out of the sand into the ocean. "Other smelts lay their 

 eggs very differently, attached to the rocks or the bottom of the 

 ocean by slender stalks or filaments. Many species migrate into 

 brackish or even fresh water to spawn, while other species are en- 

 tirely confined to fresh water" (Thompson, 1919). The grunion 

 leaves the ocean to find a safe and suitable place for the develop- 

 ment of its eggs. 



Gobies are fishes which as a group range through a variety of 

 habitats from littoral waters to torrential mountain streams, and a 

 few are pelagic and bathybial. Certain of them are beach skippers. 

 These fishes live near water but show considerable reluctance to 

 enter it. If pursued they skip on top of the water, climb the roots 

 of mangroves, hide among thick vegetation, but seldom plunge be- 

 neath the surface or swim (Pearse, 1928a, 1933) . The eyes of beach- 

 skipping gobies are adjusted for far vision in the atmosphere, and 

 thus differ from those of most fishes. 



Among the enemies that threaten animals in any habitat are 

 parasites. The geographic distribution of parasites and their hosts 

 may be used to indicate the past history and relationships of ani- 

 mals (Metcalf, 1929). Ward (1910) made a careful study of the 



