COMMUNITY ORGANIZATION: STRATIFICATION 



455 



shores the world over, although the taxo- 

 nomic composition may change with 

 locality. 



A recent study of vertical distribution 

 of macroscopic marine algae is that of Doty 

 (1946) along the coast of northern Cali- 

 fornia and Oregon. Here, as elsewhere, 

 such algae are distributed in a series of 

 intertidal zones. The vertical widths of 

 these strata are directly correlated with the 

 tide range, and the stratal limits with cer- 

 tain critical tide levels. 



Doty found pecuharities in the variations 

 of the tide, and termed them tide factors. 

 He found that the operation of such factors 

 generally provided sudden twofold or three- 

 fold increases in exposure of the algae to 

 other environmental influences (Fig. 158). 



n 



Fig. 158. Tide levels in feet above or below 

 mean lower low water (MLLW) at San Fran- 

 cisco. LLLW = lowest lower low water; 

 LHLW = lowest higher low water; HHLW =. 

 highest higher low water; LLHW = lowest 

 lower high water; LHHW = lowest higher 

 high water; HHHW = highest higher high 

 water. (After Doty.) 



Changes of such magnitude are considered 

 sufBcient to account for the abrupt restric- 

 tions in the observed vertical zonation. So 

 far, the nature of the restriction is not 

 understood, that is, whether the restriction 

 results from light, temperature, gas tensions, 

 or other direct factors, or is brought about 

 by other influences directly affected by tide 



factors, or by desiccation and consequent 

 osmotic changes. 



Furthermore, the variation in vertical sea- 

 weed belts appears to be correlated with 

 diel, lunar, and annual variation in the 

 levels at which tidal phenomena occur, as 

 well as with variation in the reproductive 

 periods of the algae, and with local topog- 

 raphy. 



The global influence of tidal activity 

 has induced an apparent vertical distribu- 

 tion of intertidal organisms in the marine 

 httoral. In general, the intertidal zonation 

 of animals and plants is both directly and 

 indirectly induced, after account is taken 

 of the obviously less marked zonation of the 

 motile animals. 



The resident animals react to the flora in 

 part, and to the environmental gradient in 

 part. They may be roughly divided into 

 four categories on the basis of their most 

 abundant habitat. In the first place, there 

 are those that hve exposed on the rocks 

 or upon the algae. These include sessile 

 acorn-barnacles (Balanui;), often forming a 

 "Balanus zone" especially near high-water 

 mark and thus associated with the green 

 seaweeds; abundant marine moUusks that 

 browse upon the rock-attached green sea- 

 weeds, such as the common hmpet 

 (Patella), periwinkles (Littorina) , and top- 

 shells {Gibbula and Calliostoma) . The dis- 

 tribution pattern in this zone is highly 

 varied. For example, there may be a ter- 

 tiary stratification within the Littorina pop- 

 ulation: on British coasts Littorina rudis 

 lives high on the rocky shore, often un- 

 touched by sea water for several weeks at a 

 time; L. littorea is lower on the shore, but 

 always on the rocks; and L. obtusata is 

 zoned with littorea, but always on the 

 brown fronds of Fucus (cf. Verrill, 1873, 

 and also Allee, 1923, for Littorina zones 

 on the Massachusetts coast). These sea- 

 weeds support a large population of 

 hydroids and other organisms, while over 

 the upper half of the shore the limpets and 

 top-shells are preyed upon by a tertiary 

 resident, the dog- whelk (Purpurea), and 

 nearer low-water mark the rocks become 

 covered with sponges (Halichondria, Gran- 

 tia) , tunicates (Botnjllus) , bryozoans, 

 mussels (Mtjtilus) , and scallops (Pecten). 



A second rocky littoral habitat develops 

 in the sheltered niche beneath loose stones. 

 Beneath these stones, near high-water mark, 



