COMMUNITIES OF THE SEA BOTTOM 327 



great within the biome. Its longitudinal extent is enormous, owing to 

 the sinuate nature of coastlines, and is expressible in thousands of 

 kilometers. 



Equivalent communities occur along most of the coasts of the 

 northern hemisphere, where not crushed off by shore ice. However, 

 in spite of the general circumpolar occurrence of Mytilus edulis as a 

 dominant, it is not possible to consider the North Atlantic and North 

 Pacific communities as associations of the same biome (Appellof, 

 1912; Pearse, 1913; Flattely and Walton, 1922; Beauchamp, 1923), 

 although Newcombe (1935, a, b) has applied the same name. The 

 other dominants are not the same, and the motile influents are all dif- 

 ferent. The two biomes, however, are of the same type and life form, 

 and belong to a closely related group similar to that of the coniferous 

 forest biomes of North America and Eurasia (see also Colton, 1916). 



Associations. The Balanus-Littorina biome of the North Pacific 

 is probably divisible into several associations, but only two appear to 

 have been fully identified in the North Pacific. These are the Bala- 

 nus-M. californianus association of the outer exposed shores and the 

 Balanus-M. edulis association, usually in the more protected places. 

 The former is the more definitely integrated and will be taken up 

 first. 



Balanus — M. californianus Association. This is best developed on 

 the open exposed shores and headlands of the Pacific coast of the 

 northern United States and southern Canada. Mytilus californi- 

 anus Conrad and Mitella polymerus (Sow.) are the most characteristic 

 species. There is a more vigorous growth of all species and commonly 

 a sharp separation into vertical groupings, termed faciations. This 

 is well shown in the results of a study on the west coast of Vancouver 

 Island (Table 13), where the sessile and motile species are separated 

 as two groups and arranged in the order of abundance. 



The belts shown are often less definite, and occasionally all the 

 species are mixed together. The variations are therefore properly 

 called faciations. 



Balanus-M. edulis Association. On the coast where extended 

 studies have been made, this association occupies the more sheltered 

 shores and waters of low salinity. Mytilus edulis and various species 

 of Fucus are most characteristic; Balanus glandula plays a more im- 

 portant role. Cribrina, the green sea anemone, IVIitella, the gooseneck 

 barnacle, and the ribbed mussel do not occur. There are also marked 

 differences in the less abundant species present. The arrangement of 

 the various dominants in this association has been studied by Rice (in 

 Shelford et al., 1935), who found in the case of barnacles that combi- 



