sity 



indmdu 



Table 28-1 Vertical zonation of mollusks on a rocky shore at Cape Ann, Massachusetts. Numbers given represent the 

 per square meter (after Dexter 1945). 



Littorina Littorina Mytilus arenaria Littorina Thais Acmaea Anomia Crepidula 



2one saxatilis littorea edulis (seed) obtusata lapillus testudinalis aculeata fomicata 



High tide level 



105 cm lower 



115 cm lower 



131 cm lower 



140 cm lower 



156 cm lower 



174 cm lower 



184 cm lower 



199 cm lower (near 



Spring low-tide 



level) 



that the yield of commercial fish of economic interest 

 to man is the greatest. 



BALANOID-GASTROPOD- 

 THALLOPHYTE BIOME 



This community extends from high to be- 

 low low tide levels on rocky shores. Benthic animals 

 and attached algal plants are conspicuous and im- 

 portant. The benthos is mostly epifauna as the sub- 

 stratum is too hard to permit development of exten- 

 sive infauna. When the tide is out, the organisms are 

 subjected to drying, the occasional inflow of fresh 

 water, higher temperatures, and greater light inten- 

 sities. Organisms avoid desiccation when the tide is 

 out by variously crawling under stones or thick algal 

 growths, closing thick shells or operculae, retreating 

 into crevices, or secreting a mucous seal. Most or- 

 ganisms are also faced with the pounding action of 

 waves. Various holdfast or anchoring devices have 

 developed, and many species protect their more deli- 

 cate structures with a hard shell. The adaptations for 

 life on the seashore are many and varied (Yonge 

 1949). The plankton and nekton associated with the 

 benthos include many species not common to the 

 oceanic biome. 



Zonation 



Vertical zonation of species on rocky shores is 

 usually conspicuous (Table 28-1), although individ- 

 ual species may extend widely into adjacent areas 

 (He watt 1937, Yonge 1949, Stephenson 1949, South- 

 ward 1958). 



Beginning on the landward side there is a supra- 

 littoral zone mostly above the action of tides and in- 

 habited as much by land as by marine animals. This 



is followed seaward by a supralittoral or Littorina 

 fringe which is wetted by the highest tides and by 

 the splashing of waves. Because of the presence of 

 either Myxophyceae or lichens, this zone is often 

 discolored ; commonly, black. The fringe is especially 

 characterized by large numbers of small snails and 

 sometimes isopods. 



Next below this fringe is the midlittoral or 

 balanoid zone. It is strictly inter-tidal, being covered 

 and uncovered every day, and is occupied character- 

 istically by acorn barnacles. This zone is often di- 

 vided into subzones with the barnacles predominant 

 in the upper portion, while polychaets, colonial hy- 

 droids, or other forms are relatively more important 

 in the lower part. The subzonation of algae is often 

 also well marked. 



The lowest zone ever exposed, and then only at 

 extreme low tides, is called the injralittoral fringe. 

 It is a transition area. The entire area between 

 extreme high and low tides, including the mid- 

 littoral zone and its supralittoral and infralittoral 

 fringes, when considered as a unit, may be referred 

 to as the littoral, eulittoral, or tidal aone to distin- 

 guish it from the infralittoral or siiblittoral zone 

 that extends from the lowest of low tides to the edge 

 of the continental shelf. 



Zonation is brought about in large part by dif- 

 ferences between species in tolerance to length of 

 exposure and submergence. Animals get into the 

 proper zones by one of several ways (McDougall 

 1943). Motile species move in and out of favorable 

 areas in direct response to stimuli. In sessile forms, 

 however, it is the motile larvae which are dispersed 

 uniformly, but die off in unfavorable microhabitats. 

 In some forms the larvae become aggregated into a 

 certain area before settling because of response to 

 environmental factors, but the exact nature of the 

 factors responsible for the aggregation of these pe- 

 lagic larvae remains obscure. The presence of or- 

 ganisms already there may exert an influence on 



362 Geographic distribution of communities 



