1 8 Papers from the Marine Biological Laboratory at Tortngas. 



common fish around the shores of Loggerhead Key is the gray snapper, 

 Lutianus grisens, which patrols the waters close to the bank in search of 

 food. Besides this rather large fish there are also schools of a very much 

 smaller fish mentioned above, Atherina laticcps. These two species ordi- 

 narily occupy rather definite positions with reference to the shore line, the 

 atherinas close in and the gray snapper farther out. The latter, however, 

 prey upon the former and frequently make excursions shoreward, driving 

 the atherinas into the surf and finally out of the water upon the beach, 

 where they flop about helplessly. During the chase the gray snappers often 

 flounder in the surf, making considerable noise. When this occurs, almost 

 immediately ocypodas, if there are any out on the beach, run down and cap- 

 ture the little fish, returning with them to their burrows. They travel fast 

 and in a straight line to their prey, leaving no doubt in the mind of the ob- 

 server that the eyes are stimulated by the movements of the atherina. 



I have frequently brought about this same reaction by throwing small 

 fish from the window of the laboratory upon the sand below, in which case 

 the behavior would be the same as above, showing that the noise produced 

 by the gray snapper in the surf is not the only stimulus, if it is any at all, 

 that attracts the crabs. 



It might be claimed that the ocypodas reacted to the odor of the fish, 

 but this is probably not the case, since the olfactory sense is not very well 

 developed and since, when pieces of coral are substituted for fish, the be- 

 havior is the same up to a certain point. They will run directly to the coral, 

 often pick it up, but seldom take it to the burrow. 



In order to thoroughly test the influence of odor in the throwing ex- 

 periments the hands were thoroughly washed and ten clean chips of wood 

 were taken in the left hand and ten fish in the right hand. These were 

 then thrown alternately to the crab, with the result that the chips were run 

 after as if they were food. They were always examined, but not taken away. 



To prove that the ocypodas did not react through the tactile sense as a 

 result of the jarring of the sand when the objects were thrown upon it, 

 another experiment was tried. An atherina was tied to a long thread and 

 was thrown from the window in such a way that it would come within 

 about ID cm. of the sand, but would not touch it. Immediately an indi- 

 vidual would run directly toward the fish, but finding nothing on the ground 

 would attempt to capture the shadow cast by the fish. Failing in this the 

 ocypoda would usually remain still for some little time, apparently watch- 

 ing, and would then make another jump at the shadow. After several 

 trials, during which it would move away some distance and then return, 

 the crab would go back to its burrow or begin feeding along the drift line. 

 If then the bait was drawn up and after an interval of one or two minutes, 

 was thrown again, the same behavior would be repeated by the original 

 individual. Finally, if the experiment were repeated many times the crab 

 would no longer react by running toward the shadow. 



