Observe the rocky shore at low tide and you will notice the distinct horizontal color bands on the rocks. One of the obvious features noticed by students visiting the coast is that intertidal organisms tend to occur at certain levels or zones. These bands show that organisms are not randomly distributed, but tend to occupy distinct and predictable zones within the intertidal. The intertidal zone can be divided into four biologically recognizable zones; the spray zone, the high-tide zone, the broad mid-tide zone, and the low-tide zone. While intertidal zonation occurs in both sandy beach and rocky environments, it is most observable in rocky environments.
The Splash zone is the upper most region above the high tide that is reached only by wave splash. The width of this zone varies greatly depending upon the local wave activity and the time of the year.
Inhabiting the Splash zone is the Rock Louse (Ligia sp.), a dark gray isopod about an inch long which is active at night or during cloudy days. Rock Lice scavenge upon both animal and plant material cast upon the shore. Also found living here are two species of Littorines, commonly known as the Gray Periwinkle (Littoriiia planaxis) and the Checkered Periwinkle (Littoiiiia scutulata). These dark 1/2-inch long colored snails are unable to withstand neither prolonged immersion nor exposure to total terrestrial conditions. Periwinkles roam the narrow strip of the spray zone environment feeding by scraping minute algae from the rock surfaces. They are only active when their gills are moist from the humidity of the spray. During dry conditions, they clamp down tightly against the rock surface. Withdraw in their shell, they close their protective door (operculum) and seal the shell to the rock with glue-like mucus.
The High-Tide ZoneThe high-tide zone extends from the level of the highest tides down to about +5 feet on the tidal scale. This zone is uncovered at nearly every low tide. Common inhabitants of this zone are the tufted red algae, the acorn barnacle, and the High-tide-zone limpets. The Tufted Red Algae (Endocladia muricata) is a bushy, dark red or blackish-brown plant with stiff, short branches. These densely intertwined branches are 1-2 inches long and 1/16 inch in diameter. Also known as the nail brush seaweed, this algae grows from Alaska to southern California in areas where the surf beats strongly. The acorn barnacles (Balanus spp.) are characterized by having their shells directly cemented to the rock surface, not elevated on a stalk. Among these volcano-shaped barnacles is the common Acorn Barnacle (Balailus glandula), whose ability to survive prolonged exposure to air has allowed it to avoid competition with numerous other organisms less tolerant of exposure and which must therefore live in the lower zones. 
This sessile crustacean is covered with numerous calcareous plates. Modified feather-like thoracic legs are used to filter food by rhythmically sweeping the surrounding water or are held extended as nets for several minutes before being pulled back inside the shell, after which the captured plankton and small animals are moved to the mouth and swallowed. The Finger Limpet (Colliselia digitalis) and the Rough Limpet (Colliselia scabra) inhabit the area from the high-tide zone up to the splash zone. These limpets move down into the water at high tide to feed then retreat back to their upper-zone level home. This behavior reduces the amount of time spent in the more crowded and competitive lower zones. After a feeding foray, the Rough Limpet is known to return to its "home" spot. Each limpet forms a smooth scar on the rock surface into which its shell fits perfectly, allowing a tight seal at low tide. The Mid-tide ZoneThe mid-tide zone ranges from approximately the +5 feet to +1 foot level. This zone is completely uncovered during the spring low tides, and at least partially uncovered by most other low tides. Because of this exposure, the organisms in this highly colonized zone are those most commonly observed by students.
The Frilled Dogwinkle (Nijcella lamellosa) and the Emarginate Dogwinkle (Nucelia emarginata) are the two common species of rock snails on the northern California coast. Both the snails are predators, feeding mostly on small mussels and barnacles by using their radula as a drill.
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The Goose Barnacle (Pollicipes polymerus) is supported above many of its competitors by a fleshy stock which allows it to twist to face the flow of water for more effective filter feeding and for added flexibility during heavy wave action.
The Aggregating Anemone (Anthopleura elegantissima) is one of the most conspicuous animals of the mid-tide zone. Covering themselves with bits of shell and gravel, these grayish to greenish colored anemones often occur in large colonies. The Aggregating Anemone reproduces asexually, forming a squishy carpet over the rocks consisting of clones from one parent anemone.
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The Lined Shore Crab (Pachygrapsus crassipes) is found in cracks and crevices at the level of or among mussel beds. Many fine red and green lines mark the top of this crab's carapace. 
The California Mussel (Mytiluss californianus) is one of the most common inhabitants of this state's rocky coastline. Large numbers of these animals may be found on nearly any wave swept area with a suitable rocky environment. While they are commonly associated with the stalked barnacle and the acorn barnacle, the dense mats of these creatures are home for an incredible number of less obvious creatures. Numerous worms, small sea cucumbers, limpets, and small crustaceans live beneath, on, and between the mussels. 
Ochre Stars (Pisaster ochraceus) are active predators of mussels, and during low tides they may be seen on the rocks just below the mussel beds. As the tide rises, Ochre Stars move up into the mussels to feed