THE ENVIRONMENT 



313 



Boy and Esfuary. In this habitat there is almost 

 complete protection from surf, but life in these 

 waters is subject to greater temperature and salinity 

 variations than in any other intertidal habitat. How- 

 ever, many of the animals are in other ocean habi- 

 tats, especially deep water. Four subdivisions are 

 generally recognized: Rocky Shores with jingle shells 

 (clams) and slipper shells; Sand Flats with sand 

 dollars, heart urchins, and snails; Eelgrass (not 

 seaweed but a vascular plant) with snails, hydroids, 

 and sessile jellyfish; and Mud Flats with various 

 snails, sea pens, ghost shrimps, and clams. 



Wharf Piling. Here are found an assemblage of 

 animals often found elsewhere. Animals are hy- 

 droids, crabs, sea anemones, sea blubber, boring 

 snails, and mussels. 



INTERTIDAL ZONATION AND TIDAL EXPOSURE 



Normally the intertidal habitats are subdivided 

 into four zones that reflect tidal exposure. Zone I, 

 the Uppermost Beach or Spray Zone, extends from 

 the highest reach of spray and storm waves to about 

 average level of all high tides. Zone II, the High 

 Tide Region or Upper Horizon, extends from the 

 average high tide line to about the average level of 

 the higher of the two daily low tides. Zone III, the 



Mid-tide Region or Middle Horizon, extends from 

 the average higher low tide line to the average lower 

 low tide line. Zone IV, the Low Tide Region or 

 Lower Horizon, extends from the average lower low 

 tide level to the lowest low tide level. 



Fortunately the tidal definitions of exposure zones 

 need not always be applied in the field. In rocky 

 areas, and especially along the Pacific Coast, living 

 creatures tend to indicate the zonation (Figure 17.7); 

 elsewhere, the zones often are obscure. 



The following characteristic organisms regularly 

 are sufficient to diagnose particular rocky-area zones: 



Zone I is mostly bare rock but contains scattered 

 green algae (Chlorophyta), lichens, limpets, and 

 periwinkles (snails. Gastropoda), and acorn 

 barnacles (Cirripedia). 



Zone II often has a type of green algae called sea 

 lettuce, various branching brown algae (Phaeophyta) 

 called rockweeds, a new and diverse group of limpets 

 and snails (Gastropoda), more acorn barnacles but 

 often added to the species in Zone I, various crabs 

 (Decapoda), colonial hydroids (Hydrozoa), and 

 marine worms (Polychaeta). 



Zone III has many kinds of seaweeds; the greater 

 diversity alone often is sufficient to separate this zone 

 from Zone II. Here, often, are silky, tufted types of 

 green algae, additional kinds of brown algae, and the 



ZONE LIFE 



Spray sparse filamentous green 



algae; acorn barnacles, snails 

 and sowbugs 



High Tide mostly branching brown 



algae (rockweeds) but also 

 membranous green (sea 

 lettuce); many snails and 

 crabs 



Mid-Tide many red algae including 



coral kelps; sponges, sea 

 anemones, segmented worms, 

 chitons, sea slugs, crabs and 

 starfishes 



Low Tide most algae are long and 



strap-like but the brown 

 sea palm occurs; chitons, 

 abalone, sea urchins and sea 

 cucumbers 



Figure 17.7 Intertidal zonation of life on a roci^y shore. 



