42 U.S. NATIONAL MUSEUM BULLETIN 29 2 



least two marine crabs to spend much of the time above the high-tide 

 line. 



Terrestrial habitats. — Many of the beaches are studded with 

 water-worn rocks and entrapped debris (pi. 3a and b). Such areas 

 are the haunts of other crabs. Yet another crab is restricted largely 

 to the sandy beaches, where it digs shallow burrows in the exposed 

 sand. In the lower floodplains of rivers (pi. 4b) and on low, poorly 

 drained land, including a few mud flats (pi. 4a) and swamps, a dif- 

 ferent assemblage of crabs excavate their burrows. 



Freshwater habitats. — Probably nowhere else in the Caribbean 

 area does there exist an island with so many comparatively unpolluted 

 and little man-altered streams as occur on Dominica. The rugged 

 terrain has discouraged the agricultural development that has almost 

 denuded some of the West Indian islands, and the high annual rain- 

 fall keeps most of the stream beds flushed of garbage and other debris. 

 Only in the leeward coastal regions, where villages and the two major 

 ports, Roseau and Portsmouth, are densely populated, and along the 

 north coast are some of the streams obviously polluted. The lower 

 reaches of many of the sluggish streams on these coasts are cloudy 

 and frequently carry a heavy load of detergents. Elsewhere, with few 

 exceptions, the waters are clear, except after heavy rains, flowing with 

 a swift to moderate current even in the "dry season." (With an annual 

 rainfall as great as occurs on Dominica, perhaps one should refer to 

 it as the "comparatively dry season.") The largest river on the island, 

 the Layou, is usually clear from its headwaters to its mouth, and 

 what pollution it receives along its course is scarcely evident at the 

 coastal village of Layou. Probably responsible for the many clear 

 streams is the combination of the steep gradient of the beds (pi. 1; 

 see also Mitchell, 1966, pp. 90-91), the large volumes of water carried 

 by them, and, of considerable importance, the cascades and riffles 

 that maintain a near-saturation of the water with oxygen.^ 



Although most of the Dominican streams are permanent, a few of 

 them are temporary, ceasing to flow or becoming dry, particularly 

 during the winter months. Even though the decapods disappear when 

 the stream beds do become dry, they reappear quickly when the water 

 begins to flow again. Probably those areas that become dry actuaUy 

 have headwaters where water exists in reduced quantities all year, 

 but when there is comparatively little rain, the water percolates 

 through the soil before reaching the mouth. With increased volume 

 of water, the bed is refilled and drains to the sea, thus enabling 

 decapods in the headwaters, and perhaps some from the sea, to re- 

 invade the newly filled beds. 



* C. W. and D. G. Hart are preparing a report on the physical and chemical 

 characteristics of representative lakes and streams on Dominica. 



