330 MARINE BIOTIC COMMUNITIES 



along the upper margin of the Balanus covered area forms a very ob- 

 vious line." He further states that the climax is reached only after 

 more than two and one-half years. The rapid replacement and over- 

 turn in the community constituents, however, stand out in contrast to 

 terrestrial phenomena, and marked changes in the arrangement and 

 abundance of certain constituents take place in short periods of time. 

 In the Bay of Fundy communities, the predation of the starfishes, 

 sea urchins, and whelks limits the downward extent of the biome, 

 while in the North Pacific physical factors appear to control the lower 

 boundary. In the Bay of Fundy and Gulf of St. Lawrence, the corre- 

 sponding biome or many of its constituents extend well below mean 

 low tide (Mossop, 1922; Huntsman, 1924; Newcombe, 1935 a; see 

 also Brandt, 1896). 



SuBTiDAL Barnacle-Gastropod Communities 



Marine communities have been so little studied from a quantita- 

 tive standpoint that only one examj^le of this community has been 

 even partially evaluated. It characterizes the sea floor about the San 

 Juan Islands in Puget Sound and on the west shore of Vancouver 

 Island. However, it is to be expected that it occurs over the con- 

 tinental shelf of the North Pacific in the clearer and more open 

 waters. The prevalence of large echinoderms and snails has led to its 

 designation as a sea urchin-gastropod community ( Shelf ord and Tow- 

 ler, 1925; Shelford et ah, 1935; Wisner and Swanson, 1935). Its suc- 

 cession, extent, etc., have been so little studied that it cannot be named 

 with certainty. 



Green Sea Urchin-Triton Community 



(Strongylocentrotus-Argobuccinum Biome) 



The most important sessile dominants are three species of Balanus, 

 of which B. nubilis Darw. is the largest and most conspicuous, along 

 with the sessile cucumber {Psoitis chitinoides H. L. Clark) , the rock 

 oyster {Pododesmus macroschisma Desh.), and scattered brachiopods. 

 These or similar species are always present; they do not, however, 

 cover large areas of bottom to the exclusion of other species, as the 

 tidal barnacles do. The control of the habitat and community is ef- 

 fected by slow-moving forms such as the sea urchins {Strong ylocen- 

 trotus drobachiensis Miill, and franciscanus A. Ag.) which commonly 

 are abundant, snails such as Argobuccinum oregonensis Red., Tricho- 

 tropis cancellata Hinds, Calliostoma costatum Mart., and numerous 

 crepidulas. Two or three sea cucumbers occur, the most noteworthy 



