416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



coves and estuaries of Manila Bay. Later experiments were con- 

 ducted on the coast of Massachusetts, and these were supplemented 

 by observations in the estuaries and mangrove swamps near Santa 

 Marta, Colombia. Altogether 13 species have been watched with 

 considerable care, and their behavior has been similar in all essential 

 respects. 



GENERAL HABITS. 



Fiddler crabs are diurnal. Other crustaceans {Thalassina^ Cardio- 

 soma) which inhabit the ocean beaches with them may be found at 

 night if their haunts are visited, and they often stupidly sit in gi*eat 

 numbers, dazed by the glare of a light, but fiddlers retire when the 

 sun goes down and remain in the bottoms of their burrows until 

 morning. Fiddlers are said to be sometimes active on moonlight 

 nights, but, at any rate, a certain amount of light is necessar}^ to 

 bring them from their burrows. Prof. Holmes (1908) foimd that 

 Uca pugnax was strongly positively phototropic when tested under 

 laboratory conditions. 



Different species of fiddlers often select the exact habitat, in which 

 they are accustomed to dwell with great nicety. On the coast of 

 Massachusetts two sj)ecies live on the same beaches, but Uca puf/naco 

 usually digs its holes in mud, while U. pugilator prefers sand. In 

 the Philippines this specificity of habitat gives rise to fiddler zones 

 along the populous margins of the esteros : ^ (1) High along the edge 

 of the shore Uca forcipata is found; (2) this zone grades into one of 

 Z7. rathbunae just below, and is followed by (3) another in the softer 

 mud of the deeper parts of the estero, peopled by U. marionis and 

 U. marionis nitida. On the shores of Colombia U. mordax is found 

 in the clay near the mouths of rivers, and is also abundant with U. 

 minax in the soft mud among the roots in mangrove swamps. 



In addition to their diurnal habits and discrimination in the selec- 

 tion of sites for their burrows, fiddlers exhibit a third striking pe- 

 culiarity in their reactions to tidal changes. Countless individuals 

 are seen on the mud flats at low tide, and active feeding is carried on 

 then. The same is true when the tide is rising or falling. Wlien 

 the ocean threatens to cover the mouth of a burrow, however, a plug 

 of mud is carried to the hole and drawn down after the owner in such 

 a way as to shut him inside. During a period of high tides burrows 

 in low situations often remain closed for several days; during low 

 tides those on higher ground may be left open' day after day, though 

 the flats dry out to such an extent that crabs can not feed easily and 

 remain at the bottoms of their burrows. 



1 Estuaries. 



