COMMUNITIES OF THE SEA BOTTOM 325 



the barnacle-gastropod and the bivalve-annelid community types. 

 The latter type, being subtidal, is not subdivisible, while the former 

 is divisible into tidal and subtidal type (cf. Kirsop, 1922). 



Barnacle-Gastropod Tidal Communities 



Wherever the substratum materials are not moved by wave action, 

 and the area of the seashore alternately exposed and submerged by 

 the rise and fall of the tide, a tidal community exists. Such com- 

 munities appear to occur on all stable shores, except the icebound 

 ones. They are made up of species tolerating or requiring exposure 

 to the atmosphere at daily intervals. The tidal community proper 

 is a barnacle-gastropod-mussel one, which in the Puget Sound region 

 of the Pacific begins near mean low tide and reaches to a vertical 

 meter or meter and one-half above the upper limit of the large bi- 

 valve-worm community. Its presence may be governed almost as 

 much by water movement as by substratum, since mussels may form 

 a bed in still water, or on the upper portions of a sand beach, and 

 constitute a substratum for the attachment of barnacles. These two 

 species may be followed by others and thus build up the entire 

 barnacle-gastropod-mussel community. The general principles gov- 

 erning this community type may be illustrated in the North Pacific. 

 The community has been sufficiently studied to show its climax na- 

 ture, permanency, etc., and may be termed a biome. 



Balanus-Littorina Biome 



The most important dominants include three species of barnacles 

 {Balanus cariosus [Pall.], B. glandula Darw., and Chthamalus dalli 

 [Pil.]) and two species of mussels {Mytilus ediilis L. and M. cali- 

 fornianus Conrad). The gooseneck barnacle [Mitella polymerus 

 [Sow.] ) plays an important role in some parts of the community. A 

 few others, such as the green anemone {Cribrina xanthogrammica 

 Brandt), are local and less important. There is evidently competition 

 for space among the dominants, for, when one is seated on a rock 

 surface through some favorable condition, the others are excluded or 

 may attach to the shells of the true dominants in sparing numbers. 



The most characteristic motile forms are gastropods of the genus 

 Littorina (L. sitchana Phil., L. scutulata Gould, and other species) 

 and of limpets {Acmaea digitalis Esch., A. cassis Esch., etc.). The 

 food relations of these are not well known but evidently are based 

 upon microscopic plants and animals. Other mobile influents of ir- 



