328 MARINE BIOTIC COMMUNITIES 



nations of tides, cloudiness, temperature, rainfall, etc., over short 

 periods control the arrangement of dominant species and the conse- 

 quent local variations in community composition. Such local differ- 

 ences have no significance except in the light of knowledge of condi- 

 tions at the time of setting and later survival. Other locations such 

 as tide pools are controlled by conditions evident at the time and 

 place. The usual rock-bottomed tide pool is an example that contains 

 Balanus of two species, both species of Littorina, Mytilus, and limpets, 

 as well as hermit crabs and snails that frequent the intertidal area 



TABLE 13 

 Faciations of the Balanus-M. Californianus Association (Shelford, 1935) 



The term faciation is applied to minor subdivisions of communities characterized 

 by the presence or absence of some of the characteristic species. Here they occur in 

 a very short vertical space. 



No. per 

 Square 

 Meter 

 Balanus-M. californianus Association (190 cm. wide): 



1. Littorina-B. glandula Faciation (20 cm. wide) 



Balanus glandula Darw., barnacle 2,400 



Littorina scutulata Gould, snail 200 



2. Littorina-B. cariosus Faciation (45 cm. wide) 



Balanus cariosus (Pall.), barnacle 3,140 



Littorina scutulata Gould, snail 338 



Ac7naea digitalis umhonata (Nutt.) Reeve, limpet 70 



Thais emarginata Desh., snail or whelk 38 



3. Mitella-Mytilus Faciation (65 cm. wide) 



Mytilus californianus Conrad, Calif oi-nia mussel 1,945 



Mitella -polymer us (Sow.), gooseneck barnacle 1,506 



Balanus cariosus (Pall.), barnacle 1,363 



Acrnaea cassis Esch., limpet 380 



Littorina scutulata Gould, snail 225 



Thais emarginata Desh., snail or whelk 180 



Red sea anemone 120 



Acmaea digitalis umhonata (Nutt.) Reeve, limpet 110 



Littorina sitchana Phil., snail 50 



4. Cribrina Faciation (60 cm. wide) 



Cribrina xanthogrammica Brandt, green sea anemone 3,140 



Mytilus californianus Conrad, California mussel 3,000 



Chthamalus dalli Pils. 1,* small barnacle 3,000 



Chitons 40 



Pisaster ochraceus (Brandt), com.mon starfish 6 



Balanus cariosus (Pall.) 3 



* Chthamalus does not average this den.sity over the area, but occurs in local clans ha\ing 

 about this number per square meter. 



