COMMUNITIES OF THE SEA BOTTOM 329 



and are regularly found out of water at low tide. In addition, there 

 are fishes that stay near the water margin and hence are quasi-resi- 

 dents of the biome. To these are added a few subtidal animals, such 

 as a Cucumaria, serjuilids, and occasional snails and chitons. 



Relationship of the Associations. Locally, the change from one 

 association to the other maj' be found in passing from the outside of 

 an open coast island to the inner or protected side. At a point on 

 the south shore of the Strait of Juan de Fuca, all the principal species 

 of both associations of the biome appeared on the same shore and 

 were quite generally mixed together. This is the transition between 

 the communities which may contain ]\Iitella and Mytilus calif ornianus 

 as important general dominants, and those in which the former oc- 

 curs in clans and the latter is not abundant. This arrangement simu- 

 lates that of the dominants of the deciduous forest which occur to- 

 gether in certain parts of the Appalachians (Braun, 1935), though in 

 other places they are separated into three associations: oak-hickory, 

 beech-maple, and oak-chestnut, each covering a large area. 



Community Development or Succession. Pierron and Huang 

 (1926), in a brief study of the Balanus-M. edulis association, con- 

 cluded that all the dominant species were present as juvenile stages 

 on denuded rocks after a few weeks. An examination of pilings of 

 known age in the Balanus-M. edulis area showed that the three prin- 

 cipal dominants, Mytilus edulis, Balanus cariosus, and B. glandida, 

 were all present on piles six months old. Piles one year old merely 

 showed more and larger specimens of the same species. But Rice se- 

 cured suggestions of non-survival of barnacles on planted rocks taken 

 from land. These studies, however, were carried on only in the 

 summer. 



Hewatt (1935, 1937) has found true succession in the Mytilus 

 calif ornianus area, which he describes as follows: "The results of this 

 investigation seem to indicate that ecological succession in the INIytilus 

 habitat progresses in the following manner: (1) a clean area first 

 becomes covered with a film of algae; (2) those forms which feed on 

 this algal growth, such as the limpets, are the first animals to appear 

 in the area; (3) during their respective spawning seasons, the mussels, 

 gooseneck barnacles and rock barnacles attach themselves to the 

 cleaned surface; (4) these sessile forms gradually come to occupy the 

 greater part of the surface and make the habitat unfavorable for the 

 larger specimens of limpets; (5) the limpets thus move to a higher 

 zone in which the mussels and barnacles cannot exist. The upward 

 migration of the limpets becomes quite evident soon after the appear- 

 ance of the rock barnacles. The concentration of the larger limpets 



