338 MARINE BIOTIC COMMUNITIES 



accordingly climax communities, probably best designated as frag- 

 mented faciations. With reduction of numbers by fishing, the beds 

 are occupied by cockles and mussels. 



The habitats and communities of Ostrea virginica Gmel. on the 

 North American coasts are similar to those of the European species. 

 This oyster community, as well as the others referred to, is climax, 

 being capable of building its own substratum and maintaining itself 

 indefinitely. It is, however, always local or fragmented. In waters 

 of high salinity the oyster community is commonly exposed at low 

 tides. The size of the fragments and the area covered are much 

 greater in waters of low salinity. This is probably due to a variety 

 of physical and biological causes. Among these. Professor R. V. 

 Truitt points out in a personal communication the small numbers of 

 oyster enemies, such as starfishes and drills, between the tide lines in 

 waters of high salinity. In these waters, exposures between tides are 

 numerous, which results from their occurrence at a higher level, where 

 their enemies do not flourish. In the brackish waters of Chesapeake 

 Bay, the oyster builds up a community which includes a number 

 of species that are of uncommon occurrence outside the oyster-domi- 

 nated areas. Ostrea lurida Carp, of the North Pacific coast is smaller 

 than the other two and occurs in bottoms with considerable mud. 

 Thompson (1913) pointed out the close dependence of this oyster on 

 the shells of clams and cockles as a base for attachment. Depleted 

 beds are said to have been destroyed by the earth thrown up by the 

 large burrowing crustacean Eupogebia (cf. Stevens 1926). 



deeper- water communities 

 (habitat not exposed at low tide) 



The community characterized by bivalves of the genera Pandora 

 and Yoldia in the north Pacific and asymmetrical sea urchin-bivalve 

 community of the northeast Atlantic are similar in many respects. 

 Both are typical bivalve-annelid communities of the kind always sub- 

 merged, differing chiefly in the type of large echinoderms, which is 

 added among the abundant constituents of characteristic life form 

 such as bivalves, annelids, and brittle stars. In the northeast Pacific 

 large starfishes (Asteroidea) take the place of the asymmetrical sea 

 urchins of the northeast Atlantic. 



