326 MARINE BIOTIC COMMUNITIES 



regular occurrence are gastropods of the genera Thais, Purpurea, and 

 Amphissa, the first two of which feed upon barnacles locally and de- 

 stroy considerable numbers. 



Rhythmic migrants and ecotone species play a considerable role. 

 The purple shore crab [Hemigrapsis nudis Dana) , the place of which 

 is taken by an equivalent species on the California coast, is signifi- 

 cant. Several fishes, especially blennies such as Xiphister mucosus 

 (Gir.) and Epigcichthys atro-purpiireiis (Kitt), have habits similar 

 to those of the shore crabs. These motile species move into the com- 

 munity and feed when the tide is in and retreat as it falls, stopping 

 under stones near the average low-tide line. They are not present 

 on vertical cliffs, but are numerous on boulder and loose rock-covered 

 slopes and are much more abundant in the more protected waters. A 

 few ecotone species, unable to live long when exposed to air, inhabit 

 the upper portion of the community below and the lower portion of the 

 Balanus-Littorina biome. The starfish, Pisaster ochraceus Brandt, is 

 an example; it may be very destructive of barnacles and mussels lo- 

 cally. Other less influent species such as the six-rayed star are more 

 generally distributed. 



Extent, Rank, and Boundaries of the Balanus-Littorina Biome. 

 This community occupies an area bounded roughly at the lower limit 

 by the mean of one-half the lowest tides in each month and at its 

 upper limit by the average high tides. It is, therefore, from 2 to 4 

 meters wide (vertically) in the area studied. The difference in taxo- 

 nomic composition between this and subtidal communities is sharp. 

 Balanus cariosus and glandula cease to be present at a distinct boun- 

 dary, as do all other important species. Subtidal barnacles are almost 

 as definitely distributed and only occasionally overlap the lowest por- 

 tion for a few centimeters. Rasmussen (in Shelford et al., 1935) 

 found a subtidal barnacle overlapping the intertidal species in south- 

 ern California, but waves and constant ocean swells furnish the prob- 

 able explanation. Gislen (1930, a, h) indicates a similar possibility 

 on the Swedish coast (cf. also Hewatt, 1937). 



In a horizontal direction, the Balanus-Littorina biome is appar- 

 ently widely distributed around the North Pacific. It is narrow be- 

 cause of its dependence upon the rise and fall of the tide, but the 

 sharp difference in 1 meter of height within the belt occupied by the 

 biome may easily be the equivalent of 1,000 meters on a^ mountain 

 side. For example, in the month of August at 48° 30' north latitude 

 where the biome occupies 3 meters' vertical height, its lower edge is 

 exposed to the air about 1 per cent and the upper edge about 96 per 

 cent of the time; hence differences in physical conditions are very 



