COMMUNITIES OF THE SEA BOTTOM 



349 



able. The variation of non-mobile species in the same community is 

 also illustrated in Fig. 39, page 181. 



The Nature of Dominance in Bivalve-Annelid Communities 



All the communities of this type that have been investigated in- 

 clude bottom-feeding, rapid-moving and often migratory fishes, which 



Fig. 84. — Communities of the North Atlantic interpreted in accord with the 



principles found among the North American land communities Fig. 54, page 255, 



and the marine communities of the shore waters of the North Pacific. (Fig. 83 



and pages 346-347, modified from Petersen, 1914.) 



range over two or three major communities as do the birds and large 

 mammals of the land. A second group of slow-moving and relatively 

 stationary, but usually important, influents, are ophiurids in the North 

 Atlantic and both asteriods and ophiurids in the North Pacific. They 

 are in part predatory and play an important role. Gastropods and 

 large crustaceans such as crabs and shrimps are present, but ordinar- 

 ily of lesser influence owing to the small numbers and, in the case of 



