COMMUNITIES OF THE SEA BOTTOM 



337 



term biocenosis, meaning a social community, have been much studied. 

 Generally speaking, those of the coast of northern Europe and of the 

 Atlantic and Pacific coasts of northern North America occupy areas 

 in the IN'Iacoma-Mya community or its ecological equivalent, but are 

 restricted to a portion not ordinarily exposed at low tides. 



Communities of Ostrea edulis L. have been described in terms most 

 easily interpreted from our point of view. Their arrangement in rcla- 



OYSTER BEDS 



Fig. 77a and b. — Showing the relative abundance of oysters in the reduced 

 saHnity of Chesapeake Bay (77a) as compared with the outer Atlantic waters 

 (77b). (Drawn from maps by the Maryland Dept. of Conservation; courtesy 



R. V. fruitt.) 



tion to shore, tides, and currents is shown in Mobius's illustration 

 (1883). They occur on areas swept clean of the very fine materials, 

 which have coarse sands and pebbles to which attachment may be 

 made. According to Blcgvad (Petersen, 1908), they are not piled up 

 in close masses, but scattered about in groups. They can reproduce 

 in the conditions resulting from their own presence and are capable 

 of widening their area by lateral extension due to the growth of shells 

 attached at the periphery. The oysters are true dominants, which 

 build up the communities and habitats which they control. They are 



